Journal of Experimental Botany

Activation of the plant mitochondrial potassium channel by free fatty acids and acyl-CoA esters: a possible defence mechanism in the response to hyperosmotic stress.

Publication Date: 2010 Aug 27 PMID: 20801915
Authors: Laus, M. N. - Soccio, M. - Trono, D. - Liberatore, M. T. - Pastore, D.
Journal: J Exp Bot

The effect of free fatty acids (FFAs) and acyl-CoA esters on K(+) uptake was studied in mitochondria isolated from durum wheat (Triticum durum Desf.), a species that has adapted well to the semi-arid Mediterranean area and possessing a highly active mitochondrial ATP-sensitive K(+) channel (PmitoK(ATP)), that may confer resistance to environmental stresses. This was made by swelling experiments in KCl solution under experimental conditions in which PmitoK(ATP) activity was monitored. Linoleate and other FFAs (laurate, palmitate, stearate, palmitoleate, oleate, arachidonate, and the non-physiological 1-undecanesulphonate and 5-phenylvalerate), used at a concentration (10 muM) unable to damage membranes of isolated mitochondria, stimulated K(+) uptake by about 2-4-fold. Acyl-CoAs also promoted K(+) transport to a much larger extent with respect to FFAs (about 5-12-fold). In a different experimental system based on safranin O fluorescence measurements, the dissipation of electrical membrane potential induced by K(+) uptake via PmitoK(ATP) was found to increase in the presence of 5-phenylvalerate and palmitoyl-CoA, both unable to elicit the activity of the Plant Uncoupling Protein. This result suggests a direct activation of PmitoK(ATP). Stimulation of K(+) transport by FFAs/acyl-CoAs resulted in a widespread phenomenon in plant mitochondria from different mono/dicotyledonous species (bread wheat, barley, triticale, maize, lentil, pea, and topinambur) and from different organs (root, tuber, leaf, and shoot). Finally, an increase in mitochondrial FFAs up to a content of 50 nmol mg(-1) protein, which was able to activate PmitoK(ATP) strongly, was observed under hyperosmotic stress conditions. Since PmitoK(ATP) may act against environmental/oxidative stress, its activation by FFAs/acyl-CoAs is proposed to represent a physiological defence mechanism.

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Open and closed inflorescences: more than simple opposites.

Publication Date: 2010 Aug 25 PMID: 20798000
Authors: Bull-Herenu, K. - Classen-Bockhoff, R.
Journal: J Exp Bot

The absence of a terminal flower in inflorescences ('open inflorescences') is currently explained by the maintenance of putative stem-cells in the central zone (CZ) of the inflorescence meristem (IM) governed by the CLAVATA-WUSCHEL regulatory loop. Disruption of this regulatory pathway, as in Arabidopsis TERMINAL FLOWER LOCUS 1 mutants, leads to terminal flower production. However, recent studies in other taxa reveal novel mechanisms of inflorescence termination; for example, the SEPALLATA-like MADS-box floral identity gene GERBERA REGULATOR OF CAPITULUM DEVELOPMENT 2 in Gerbera excludes the retention of a CZ as an ontogenetic cause for the openness of these inflorescences. Moreover, comparative histological studies show that the retention of a CZ in the IM, mostly a feature of the 'typical open families', is absent in open inflorescences of other families. Concerning these groups, new evidence suggests that spatial constraints at the IM could play a role at the time when terminal flower production (or not) is determined. This indicates that the multiple loss and re-gain of the terminal flower in angiosperms is necessarily based on more than one ontogenetic pathway.

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Haem oxygenase delays programmed cell death in wheat aleurone layers by modulation of hydrogen peroxide metabolism.

Publication Date: 2010 Aug 25 PMID: 20797999
Authors: Wu, M. - Huang, J. - Xu, S. - Ling, T. - Xie, Y. - Shen, W.
Journal: J Exp Bot

Haem oxygenase-1 (HO-1) confers protection against a variety of oxidant-induced cell and tissue injury in animals and plants. In this report, it is confirmed that programmed cell death (PCD) in wheat aleurone layers is stimulated by GA and prevented by ABA. Meanwhile, HO activity and HO-1 protein expression exhibited lower levels in GA-treated layers, whereas the hydrogen peroxide (H(2)O(2)) content was apparently increased. The pharmacology approach illustrated that scavenging or accumulating H(2)O(2) either delayed or accelerated GA-induced PCD. Furthermore, pretreatment with the HO-1 specific inhibitor, zinc protoporphyrin IX (ZnPPIX), before exposure to GA, not only decreased HO activity but also accelerated GA-induced PCD significantly. The application of the HO-1 inducer, haematin, and the enzymatic reaction product of HO, carbon monoxide (CO) aqueous solution, both of which brought about a noticeable induction of HO expression, substantially prevented GA-induced PCD. These effects were reversed when ZnPPIX was added, suggesting that HO in vivo played a role in delaying PCD. Meanwhile, catalase (CAT) and ascorbate peroxidase (APX) activities or transcripts were enhanced by haematin, CO, or bilirubin (BR), the catalytic by-product of HO. This enhancement resulted in a decrease in H(2)O(2) production and a delay in PCD. In addition, the antioxidants butylated hydroxytoluene (BHT), dithiothreitol (DTT), and ascorbic acid (AsA) were able not only to delay PCD but also to mimic the effects of haematin and CO on HO up-regulation. Overall, the above results suggested that up-regulation of HO expression delays PCD through the down-regulation of H(2)O(2) production.

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Plant physiology and proteomics reveals the leaf response to drought in alfalfa (Medicago sativa L.).

Publication Date: 2010 Aug 25 PMID: 20797998
Authors: Aranjuelo, I. - Molero, G. - Erice, G. - Avice, J. C. - Nogues, S.
Journal: J Exp Bot

Despite its relevance, protein regulation, metabolic adjustment, and the physiological status of plants under drought is not well understood in relation to the role of nitrogen fixation in nodules. In this study, nodulated alfalfa plants were exposed to drought conditions. The study determined the physiological, metabolic, and proteomic processes involved in photosynthetic inhibition in relation to the decrease in nitrogenase (N(ase)) activity. The deleterious effect of drought on alfalfa performance was targeted towards photosynthesis and N(ase) activity. At the leaf level, photosynthetic inhibition was mainly caused by the inhibition of Rubisco. The proteomic profile and physiological measurements revealed that the reduced carboxylation capacity of droughted plants was related to limitations in Rubisco protein content, activation state, and RuBP regeneration. Drought also decreased amino acid content such as asparagine, and glutamic acid, and Rubisco protein content indicating that N availability limitations were caused by N(ase) activity inhibition. In this context, drought induced the decrease in Rubisco binding protein content at the leaf level and proteases were up-regulated so as to degrade Rubisco protein. This degradation enabled the reallocation of the Rubisco-derived N to the synthesis of amino acids with osmoregulant capacity. Rubisco degradation under drought conditions was induced so as to remobilize Rubisco-derived N to compensate for the decrease in N associated with N(ase) inhibition. Metabolic analyses showed that droughted plants increased amino acid (proline, a major compound involved in osmotic regulation) and soluble sugar (D-pinitol) levels to contribute towards the decrease in osmotic potential (Psi(s)). At the nodule level, drought had an inhibitory effect on N(ase) activity. This decrease in N(ase) activity was not induced by substrate shortage, as reflected by an increase in total soluble sugars (TSS) in the nodules. Proline accumulation in the nodule could also be associated with an osmoregulatory response to drought and might function as a protective agent against ROS. In droughted nodules, the decrease in N(2) fixation was caused by an increase in oxygen resistance that was induced in the nodule. This was a mechanism to avoid oxidative damage associated with reduced respiration activity and the consequent increase in oxygen content. This study highlighted that even though drought had a direct effect on leaves, the deleterious effects of drought on nodules also conditioned leaf responsiveness.

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Misregulation of the LOB domain gene DDA1 suggests possible functions in auxin signalling and photomorphogenesis.

Publication Date: 2010 Aug 25 PMID: 20797997
Authors: Mangeon, A. - Bell, E. M. - Lin, W. C. - Jablonska, B. - Springer, P. S.
Journal: J Exp Bot

The LATERAL ORGAN BOUNDARIES DOMAIN (LBD) gene family encodes plant-specific transcription factors. In this report, the LBD gene DOWN IN DARK AND AUXIN1 (DDA1), which is closely related to LATERAL ORGAN BOUNDARIES (LOB) and ASYMMETRIC LEAVES2 (AS2), was characterized. DDA1 is expressed primarily in vascular tissues and its transcript levels were reduced by exposure to exogenous indole-3-acetic acid (IAA or auxin) and in response to dark exposure. Analysis of a T-DNA insertion line, dda1-1, in which the insertion resulted in misregulation of DDA1 transcripts in the presence of IAA and in the dark revealed possible functions in auxin response and photomorphogenesis. dda1-1 plants exhibited reduced sensitivity to auxin, produced fewer lateral roots, and displayed aberrant hypocotyl elongation in the dark. Phenotypes resulting from fusion of a transcriptional repression domain to DDA1 suggest that DDA1 may act as both a transcriptional activator and a transcriptional repressor depending on the context. These results indicate that DDA1 may function in both the auxin signalling and photomorphogenesis pathways.

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Augmentation of abscisic acid (ABA) levels by drought does not induce short-term stomatal sensitivity to CO2 in two divergent conifer species.

Publication Date: 2010 Aug 25 PMID: 20797996
Authors: McAdam, S. A. - Brodribb, T. J. - Ross, J. J. - Jordan, G. J.
Journal: J Exp Bot

The stomata of conifers display very little short-term response to changes in atmospheric CO(2) concentration (C(a)), whereas the stomatal responses of angiosperms to C(a) increase in response to water stress. This behaviour of angiosperm stomata appears to be dependent on foliar levels of abscisic acid (ABA(f)). Here two alternative explanations for the stomatal insensitivity of conifers to C(a) are tested: that conifers have either low ABA(f) or a higher or absent threshold for ABA-induced sensitivity. The responsiveness of stomatal conductance (g(s)) to a sequence of transitions in C(a) (386, 100, and 600 mumol mol(-1)) was recorded over a range of ABA(f) in an angiosperm and two divergent conifer species. The different ABA levels were induced by a mild drought cycle. Although the angiosperm and conifer species showed similar proportional increases in ABA(f) following drought, conifer stomata remained insensitive to changes in C(a) whereas angiosperm stomata showed enhanced sensitivity with increasing ABA(f). The conifers, however, had much higher ABA(f) prior to drought than the angiosperm species, suggesting that non-sensitivity to C(a) in these conifers was due to an absent or inactive response/signalling pathway rather than insufficient ABA(f).

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Convergence of the 26S proteasome and the REVOLUTA pathways in regulating inflorescence and floral meristem functions in Arabidopsis.

Publication Date: 2010 Aug 26 PMID: 20797995
Authors: Zhang, Z. - Wang, H. - Luo, D. - Zeng, M. - Huang, H. - Cui, X.
Journal: J Exp Bot

The 26S proteasome is a large multisubunit proteolytic complex, regulating growth and development in eukaryotes by selective removal of short-lived regulatory proteins. Here, it is shown that the 26S proteasome and the transcription factor gene REVOLUTA (REV) act together in maintaining inflorescence and floral meristem (IM and FM) functions. The characterization of a newly identified Arabidopsis mutant, designated ae4 (asymmetric leaves1/2 enhancer4), which carries a mutation in the gene encoding the 26S proteasome subunit, RPN2a, is reported. ae4 and rev have minor defects in phyllotaxy structure and meristem initiation, respectively, whereas ae4 rev demonstrated strong developmental defects. Compared with the rev single mutant, an increased percentage of ae4 rev plants exhibited abnormal vegetative shoot apical and axillary meristems. After flowering, ae4 rev first gave rise to a few normal-looking flowers, and then flowers with reduced numbers of all types of floral organs. In late reproductive development, instead of flowers, the ae4 rev IM produced numerous filamentous structures, which contained cells seen only in the floral organs, and then carpelloid organs. In situ hybridization revealed that expression of the WUSCHEL and CLAVATA3 genes was severely down-regulated or absent in the late appearing ae4 rev primordia, but the genes were strongly expressed in top-layer cells of inflorescence tips. Double mutant plants combining rev with other 26S proteasome subunit mutants, rpn1a and rpn9a, resembled ae4 rev, suggesting that the 26S proteasome might act as a whole in regulating IM and FM functions.

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Analysis of the willow root system by electrical impedance spectroscopy.

Publication Date: 2010 Aug 26 PMID: 20797994
Authors: Cao, Y. - Repo, T. - Silvennoinen, R. - Lehto, T. - Pelkonen, P.
Journal: J Exp Bot

Information on plant roots is increasingly needed for understanding and managing plants under various environmental conditions, including climate change. Several methods have been developed to study fine roots but they are either destructive or cumbersome, or may not be suitable for studies of fine root functionality. Electrical impedance, resistance, and capacitance have been proposed as possible non-destructive measures for studying roots. Their use is limited by a lack of knowledge concerning the electrical circuit of the system. Electrical impedance spectroscopy (EIS) was used for hydroponically raised willows (Salix schwerinii) to estimate the root system size. The impedance spectra were investigated in three experimental set-ups and the corresponding appropriate lumped models were formulated. The fit of the proposed lumped models with the measured impedance spectra data was good. The model parameters were correlated with the contact area of the roots and/or stems raised in the hydroponic solution. The EIS method proved a useful non-destructive method for assessing root surface area. This work may be considered to be a new methodological contribution to understanding root systems and their functions in a non-destructive manner.

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Expression profiling of cell cycle genes reveals key facilitators of cell production during carpel development, fruit set, and fruit growth in apple (Malusxdomestica Borkh.).

Publication Date: 2010 Aug 22 PMID: 20732881
Authors: Malladi, A. - Johnson, L. K.
Journal: J Exp Bot

Cell production is an essential facilitator of fruit growth and development. Cell production during carpel/floral-tube growth, fruit set, and fruit growth, and its regulation by cell cycle genes were investigated in apple (Malusxdomestica Borkh.). Cell production was inhibited during late carpel/floral-tube development, resulting in growth arrest before bloom. Fruit set re-activated cell production between 8 d and 11 d after full bloom (DAFB) and triggered fruit growth. The early phase of fruit growth involved rapid cell production followed by exit from cell proliferation at approximately 24 DAFB. Seventy-one cell cycle genes were identified, and expression of 59 genes was investigated using quantitative RT-PCR. Changes in expression of 19 genes were consistently associated with transitions in cell production during carpel/floral-tube growth, fruit set, and fruit growth. Fourteen genes, including B-type cyclin-dependent kinases (CDKs) and A2-, B1-, and B2-type cyclins, were positively associated with cell production, suggesting that availability of G2/M phase regulators of the cell cycle is limiting for cell proliferation. Enhanced expression of five genes including that of the putative CDK inhibitors, MdKRP4 and MdKRP5, was associated with reduced cell production. Exit from cell proliferation at G0/G1 during fruit growth was facilitated by multiple mechanisms including down-regulation of putative regulators of G1/S and G2/M phase progression and up-regulation of KRP genes. Interestingly, two CDKA genes and several CDK-activating factors were up-regulated during this period, suggesting functions for these genes in mediating exit from cell proliferation at G0/G1. Together, the data indicate that cell cycle genes are important facilitators of cell production during apple fruit development.

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Ionic and osmotic relations in quinoa (Chenopodium quinoa Willd.) plants grown at various salinity levels.

Publication Date: 2010 Aug 22 PMID: 20732880
Authors: Hariadi, Y. - Marandon, K. - Tian, Y. - Jacobsen, S. E. - Shabala, S.
Journal: J Exp Bot

Ionic and osmotic relations in quinoa (Chenopodium quinoa Willd.) were studied by exposing plants to six salinity levels (0-500 mM NaCl range) for 70 d. Salt stress was administered either by pre-mixing of the calculated amount of NaCl with the potting mix before seeds were planted or by the gradual increase of NaCl levels in the irrigation water. For both methods, the optimal plant growth and biomass was achieved between 100 mM and 200 mM NaCl, suggesting that quinoa possess a very efficient system to adjust osmotically for abrupt increases in NaCl stress. Up to 95% of osmotic adjustment in old leaves and between 80% and 85% of osmotic adjustment in young leaves was achieved by means of accumulation of inorganic ions (Na(+), K(+), and Cl(-)) at these NaCl levels, whilst the contribution of organic osmolytes was very limited. Consistently higher K(+) and lower Na(+) levels were found in young, as compared with old leaves, for all salinity treatments. The shoot sap K(+) progressively increased with increased salinity in old leaves; this is interpreted as evidence for the important role of free K(+) in leaf osmotic adjustment under saline conditions. A 5-fold increase in salinity level (from 100 mM to 500 mM) resulted in only a 50% increase in the sap Na(+) content, suggesting either a very strict control of xylem Na(+) loading or an efficient Na(+) removal from leaves. A very strong correlation between NaCl-induced K(+) and H(+) fluxes was observed in quinoa root, suggesting that a rapid NaCl-induced activation of H(+)-ATPase is needed to restore otherwise depolarized membrane potential and prevent further K(+) leak from the cytosol. Taken together, this work emphasizes the role of inorganic ions for osmotic adjustment in halophytes and calls for more in-depth studies of the mechanisms of vacuolar Na(+) sequestration, control of Na(+) and K(+) xylem loading, and their transport to the shoot.

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Differences in enzymic properties of five recombinant xyloglucan endotransglucosylase/hydrolase (XTH) proteins of Arabidopsis thaliana.

Publication Date: 2010 Aug 22 PMID: 20732879
Authors: Maris, A. - Kaewthai, N. - Eklof, J. M. - Miller, J. G. - Brumer, H. - Fry, S. C. - Verbelen, J. P. - Vissenberg, K.
Journal: J Exp Bot

Xyloglucan endotransglucosylase/hydrolases (XTHs) are cell wall enzymes that are able to graft xyloglucan chains to oligosaccharides or to other available xyloglucan chains and/or to hydrolyse xyloglucan chains. As they are involved in the modification of the load-bearing cell-wall components, they are believed to be very important in the regulation of growth and development. Given the large number (33) of XTH genes in Arabidopsis and the overlapping expression patterns, specific enzymic properties may be expected. Five predominantly root-expressed Arabidopsis thaliana XTHs belonging to subgroup I/II were analysed here. These represent two sets of closely related genes: AtXTH12 and 13 on the one hand (trichoblast-enriched) and AtXTH17, 18, and 19 on the other (expressed in nearly all cell types in the root). They were all recombinantly produced in the yeast Pichia pastoris and partially purified by ammonium sulphate precipitation before they were subsequently all subjected to a series of identical in vitro tests. The kinetic properties of purified AtXTH13 were investigated in greater detail to rule out interference with the assays by contaminating yeast proteins. All five proteins were found to exhibit only the endotransglucosylase (XET; EC 2.4.1.207) activity towards xyloglucan and non-detectable endohydrolytic (XEH; EC 3.2.1.151) activity. Their endotransglucosylase activity was preferentially directed towards xyloglucan and, in some cases, water-soluble cellulose acetate, rather than to mixed-linkage beta-glucan. Isoforms differed in optimum pH (5.0-7.5), in temperature dependence and in acceptor substrate preferences.

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Subgroup 4 R2R3-MYBs in conifer trees: gene family expansion and contribution to the isoprenoid- and flavonoid-oriented responses.

Publication Date: 2010 Aug 23 PMID: 20732878
Authors: Bedon, F. - Bomal, C. - Caron, S. - Levasseur, C. - Boyle, B. - Mansfield, S. D. - Schmidt, A. - Gershenzon, J. - Grima-Pettenati, J. - Seguin, A. - Mackay, J.
Journal: J Exp Bot

Transcription factors play a fundamental role in plants by orchestrating temporal and spatial gene expression in response to environmental stimuli. Several R2R3-MYB genes of the Arabidopsis subgroup 4 (Sg4) share a C-terminal EAR motif signature recently linked to stress response in angiosperm plants. It is reported here that nearly all Sg4 MYB genes in the conifer trees Picea glauca (white spruce) and Pinus taeda (loblolly pine) form a monophyletic clade (Sg4C) that expanded following the split of gymnosperm and angiosperm lineages. Deeper sequencing in P. glauca identified 10 distinct Sg4C sequences, indicating over-represention of Sg4 sequences compared with angiosperms such as Arabidopsis, Oryza, Vitis, and Populus. The Sg4C MYBs share the EAR motif core. Many of them had stress-responsive transcript profiles after wounding, jasmonic acid (JA) treatment, or exposure to cold in P. glauca and P. taeda, with MYB14 transcripts accumulating most strongly and rapidly. Functional characterization was initiated by expressing the P. taeda MYB14 (PtMYB14) gene in transgenic P. glauca plantlets with a tissue-preferential promoter (cinnamyl alcohol dehydrogenase) and a ubiquitous gene promoter (ubiquitin). Histological, metabolite, and transcript (microarray and targeted quantitiative real-time PCR) analyses of PtMYB14 transgenics, coupled with mechanical wounding and JA application experiments on wild-type plantlets, allowed identification of PtMYB14 as a putative regulator of an isoprenoid-oriented response that leads to the accumulation of sesquiterpene in conifers. Data further suggested that PtMYB14 may contribute to a broad defence response implicating flavonoids. This study also addresses the potential involvement of closely related Sg4C sequences in stress responses and plant evolution.

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Genome-wide identification and analysis of drought-responsive microRNAs in Oryza sativa.

Publication Date: 2010 Aug 20 PMID: 20729483
Authors: Zhou, L. - Liu, Y. - Liu, Z. - Kong, D. - Duan, M. - Luo, L.
Journal: J Exp Bot

In addition to regulating growth and development, the most important function of microRNAs (miRNAs) in plants is the regulation of a variety of cellular processes underlying plant adaptation to environmental stresses. To gain a deep understanding of the mechanism of drought tolerance in rice, genome-wide profiling and analysis of miRNAs was carried out in drought-challenged rice across a wide range of developmental stages, from tillering to inflorescence formation, using a microarray platform. Among the 30 miRNAs identified as significantly down- or up-regulated under the drought stress, 11 down-regulated miRNAs (miR170, miR172, miR397, miR408, miR529, miR896, miR1030, miR1035, miR1050, miR1088, and miR1126) and eight up-regulated miRNAs (miR395, miR474, miR845, miR851, miR854, miR901, miR903, and miR1125) were revealed for the first time to be induced by drought stress in plants, and nine (miR156, miR168, miR170, miR171, miR172, miR319, miR396, miR397, and miR408) showed opposite expression to that observed in drought-stressed Arabidopsis. The most conserved down-regulated miRNAs were ath-miR170, the miR171 family, and ath-miR396, and the most conserved up-regulated miRNAs were ptc-miR474 and ath-miR854a. The identification of differentially expressed novel plant miRNAs and their target genes, and the analysis of cis-elements provides molecular evidence for the possible involvement of miRNAs in the process of drought response and/or tolerance in rice.

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Sodium and chloride exclusion and retention by non-grafted and grafted melon and Cucurbita plants.

Publication Date: 2010 Aug 20 PMID: 20729482
Authors: Edelstein, M. - Plaut, Z. - Ben-Hur, M.
Journal: J Exp Bot

The effects of grafting on Na and Cl(-) uptake and distribution in plant tissues were quantified in a greenhouse experiment using six combinations of melon (Cucumis melo L. cv. Arava) and pumpkin (Cucurbita maxima DuchesnexCucurbita moschata Duchesne cv. TZ-148): non-grafted, self-grafted, melons grafted on pumpkins, and pumpkins grafted on melons. Total Na concentration in shoots of plants with pumpkin or melon rootstocks was <60 mmol kg(-1) and >400 mmol kg(-1), respectively, regardless of the scion. In contrast, shoot Cl(-) concentrations were quite similar among the different scion-rootstock combinations. Na concentrations in exudates from cut stems of plants with a pumpkin rootstock were very low (<0.18 mM), whereas those in the exudates of plants with melon rootstocks ranged from 4.7 mM to 6.2 mM, and were quite similar to the Na concentration in the irrigation water. Root Na concentrations averaged 11.7 times those in the shoots of plants with pumpkin rootstocks, while in plants with melon rootstocks, values were similar. Two mechanisms could explain the decrease in shoot Na concentrations in plants with pumpkin rootstocks: (i) Na exclusion by the pumpkin roots; and (ii) Na retention and accumulation within the pumpkin rootstock. Quantitative analysis indicated that the pumpkin roots excluded approximately 74% of available Na, while there was nearly no Na exclusion by melon roots. Na retention by the pumpkin rootstocks decreased its amount in the shoot by an average 46.9% compared with uniform Na distribution throughout the plant. In contrast, no retention of Na could be found in plants grafted on melons.

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Molecular cloning and characterization of a novel tomato xylosyltransferase specific for gentisic acid.

Publication Date: 2010 Aug 20 PMID: 20729481
Authors: Tarraga, S. - Lison, P. - Lopez-Gresa, M. P. - Torres, C. - Rodrigo, I. - Belles, J. M. - Conejero, V.
Journal: J Exp Bot

The importance of salicylic acid (SA) in the signal transduction pathway of plant disease resistance has been well documented in many incompatible plant-pathogen interactions, but less is known about signalling in compatible interactions. In this type of interaction, tomato plants have been found to accumulate high levels of 2,5-dihydroxybenzoic acid (gentisic acid, GA), a metabolic derivative of SA. Exogenous GA treatments induce in tomato plants a set of PR proteins that differ from those induced by salicylic acid. While SA accumulates in tomato plants mainly as 2-O-beta-D-glucoside, GA has only been found as 5-O-beta-D-xyloside. To characterize this step of the GA signalling pathway further, the present work focuses on the study of the GA-conjugating activity in tomato plants. A gentisate glycosyltransferase (GAGT) cDNA has been isolated and overexpressed in Pichia pastoris, and GA-conjugating activity was confirmed by detecting the xylosylated GA. The purified plant protein is highly specific for GA, showing no activity toward many other phenolic compounds, including SA. In addition, it shows an outstanding selectivity for UDP-xylose as the sugar donor, which differentiates this enzyme from most glycosyltransferases. Both the GA-conjugating activity and the corresponding mRNA show a strong, rapid, and transient induction upon treatment of tomato plants with GA or SA. Furthermore, its expression is rapidly induced by compatible infections. However, neither the gene nor the activity seems to respond to incompatible infections or wounding. The unique properties of this new glycosyltransferase suggest a specific role in regulating the free GA levels in compatible plant-pathogen interactions.

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Modulation of embryo-forming capacity in culture through the expression of Brassica genes involved in the regulation of the shoot apical meristem.

Publication Date: 2010 Aug 20 PMID: 20729480
Authors: Elhiti, M. - Tahir, M. - Gulden, R. H. - Khamiss, K. - Stasolla, C.
Journal: J Exp Bot

Somatic embryogenesis in Arabidopsis is achieved by culturing bending-cotyledon embryos on a 2,4-D-containing induction medium for 14 d followed by a transfer on to a hormone-free development medium. Several genes orthologous to Arabidopsis SHOOTMERISTEMLESS (STM), CLAVATA 1 (CLV1), and ZWILLE (ZLL) were isolated from Brassica oleracea (Bo), B. rapa (Br), and B. napus (Bn), and ectopically expressed in Arabidopsis to assess their effects on somatic embryogenesis. Ectopic expression of BoSTM, BrSTM, and BnSTM increased the number of somatic embryos, whereas a different effect was observed in lines overexpressing BnCLV1 in which somatic embryo formation was severely repressed. The introduction of BnZLL did not have any effects on Arabidopsis somatic embryogenesis. The increased embryo-forming capacity observed in lines overexpressing Brassica STM was associated with a lower requirement for the inductive signal 2,4-D, and a higher expression of WUSCHEL (WUS) which demarcates the formation of embryogenic cells. This was in contrast to the 35S::BnCLV1 lines which showed the highest requirement for exogenous 2,4-D and a reduced WUS expression. Microarray studies were conducted to monitor global changes in transcript levels during Arabidopsis somatic embryogenesis between the wild-type (WT) line and a BoSTM-overexpressing line, which showed the most pronounced enhancement of somatic embryo yield. The introduction of BoSTM affected the expression of many genes involved in hormone perception and signalling, as well as genes encoding DNA methyltransferases and enzymes of glutathione metabolism. Pharmacological experiments performed to confirm some of the microarray results showed that Arabidopsis somatic embryogenesis is encouraged by a global hypomethylation of the DNA during the induction phase and by a switch of the glutathione pool towards an oxidized state during the subsequent development phase. Both events occurred in the 35S::BoSTM line, but not in the WT line. Altered expression of Brassica STM also had profound effects on B. napus microspore-derived embryogenesis. The yield of microspore-derived embryos increased in lines overexpressing BnSTM and significantly decreased in antisense lines down-regulating BnSTM.

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Pollen density on the stigma affects endogenous gibberellin metabolism, seed and fruit set, and fruit quality in Pyrus pyrifolia.

Publication Date: 2010 Aug 16 PMID: 20713466
Authors: Zhang, C. - Tateishi, N. - Tanabe, K.
Journal: J Exp Bot

To clarify the relationship between pollen density and gametophytic competition in Pyrus pyrifolia, gametophytic performance, gibberellin metabolism, fruit set, and fruit quality were investigated by modifying P. pyrifolia pollen grain number and density with Lycopodium spores. Higher levels of pollen density improved seed viability, fruit set, and fruit quality. Treatments with the highest pollen density showed a significantly increased fruit growth rate and larger fruit at harvest. High pollen density increased germination rate and gave a faster pollen tube growth, both in vivo and in vitro. Endogenous gibberellin (GA) concentrations increased in pollen tubes soon after germination and the concentration of two growth-active GAs, GA(3), and GA(4), was positively correlated to final fruit size, cell numbers in the mesocarp, and pollen tube growth rate. These two GAs appear to be biosynthesized de novo in pollen tube and are the main pollen-derived bioactive GAs found after pollen germination. GA(1) levels in the pollen tube appear to be related to a pollen-style interaction that occurred after the pollen grains landed on the stigma.

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Nicotiana attenuata NaHD20 plays a role in leaf ABA accumulation during water stress, benzylacetone emission from flowers, and the timing of bolting and flower transitions.

Publication Date: 2010 Aug 16 PMID: 20713465
Authors: Re, D. A. - Dezar, C. A. - Chan, R. L. - Baldwin, I. T. - Bonaventure, G.
Journal: J Exp Bot

Homeodomain-leucine zipper type I (HD-Zip I) proteins are plant-specific transcription factors associated with the regulation of growth and development in response to changes in the environment. Nicotiana attenuata NaHD20 was identified as an HD-Zip I-coding gene whose expression was induced by multiple stress-associated stimuli including drought and wounding. To study the role of NaHD20 in the integration of stress responses with changes in growth and development, its expression was silenced by virus-induced gene silencing (VIGS), and control and silenced plants were metabolically and developmentally characterized. Phytohormone profiling showed that NaHD20 plays a positive role in abscisic acid (ABA) accumulation in leaves during water stress and in the expression of some dehydration-responsive genes including ABA biosynthetic genes. Moreover, consistent with the high levels of NaHD20 expression in corollas, the emission of benzylacetone from flowers was reduced in NaHD20-silenced plants. Additionally, bolting time and the opening of the inflorescence buds was decelerated in these plants in a specific developmental stage without affecting the total number of flowers produced. Water stress potentiated these effects; however, after plants recovered from this condition, the opening of the inflorescence buds was accelerated in NaHD20-silenced plants. In summary, NaHD20 plays multiple roles in N. attenuata and among these are the coordination of responses to dehydration and its integration with changes in flower transitions.

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Genetic mapping of natural variation in a shade avoidance response: ELF3 is the candidate gene for a QTL in hypocotyl growth regulation.

Publication Date: 2010 Aug 16 PMID: 20713464
Authors: Coluccio, M. P. - Sanchez, S. E. - Kasulin, L. - Yanovsky, M. J. - Botto, J. F.
Journal: J Exp Bot

When plants become shaded by neighbouring plants, they perceive a decrease in the red/far-red (R/FR) ratio of the light environment, which provides an early and unambiguous warning of the presence of competing vegetation. The mechanistic bases of the natural genetic variation in response to shade signals remain largely unknown. This study demonstrates that a wide range of genetic variation for hypocotyl elongation in response to an FR pulse at the end of day (EOD), a light signal that simulates natural shade, exists between Arabidopsis accessions. A quantitative trait locus (QTL) mapping analysis was done in the BayreuthxShahdara recombinant inbred line population. EODINDEX1 is the most significant QTL identified in response to EOD. The Shahdara alleles at EODINDEX1 caused a reduced response to shade as a consequence of an impaired hypocotyl inhibition under white light, and an accelerated leaf movement rhythm, which correlated positively with the pattern of circadian expression of clock genes such as PRR7 and PRR9. Genetic and quantitative complementation analyses demonstrated that ELF3 is the most likely candidate gene underlying natural variation at EODINDEX1. In conclusion, ELF3 is proposed as a component of the shade avoidance signalling pathway responsible for the phenotypic differences between Arabidopsis populations in relation to adaptation in a changing light environment.

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High concentrations of Na+ and Cl- ions in soil solution have simultaneous detrimental effects on growth of faba bean under salinity stress.

Publication Date: 2010 Aug 16 PMID: 20713463
Authors: Tavakkoli, E. - Rengasamy, P. - McDonald, G. K.
Journal: J Exp Bot

Despite the fact that most plants accumulate both sodium (Na(+)) and chloride (Cl(-)) ions to high concentration in their shoot tissues when grown in saline soils, most research on salt tolerance in annual plants has focused on the toxic effects of Na(+) accumulation. There have also been some recent concerns about the ability of hydroponic systems to predict the responses of plants to salinity in soil. To address these two issues, an experiment was conducted to compare the responses to Na(+) and to Cl(-) separately in comparison with the response to NaCl in a soil-based system using two varieties of faba bean (Vicia faba), that differed in salinity tolerance. The variety Nura is a salt-sensitive variety that accumulates Na(+) and Cl(-) to high concentrations while the line 1487/7 is salt tolerant which accumulates lower concentrations of Na(+) and Cl(-). Soils were prepared which were treated with Na(+) or Cl(-) by using a combination of different Na(+) salts and Cl(-) salts, respectively, or with NaCl. While this method produced Na(+)-dominant and Cl(-)-dominant soils, it unavoidably led to changes in the availability of other anions and cations, but tissue analysis of the plants did not indicate any nutritional deficiencies or toxicities other than those targeted by the salt treatments. The growth, water use, ionic composition, photosynthesis, and chlorophyll fluorescence were measured. Both high Na(+) and high Cl(-) reduced growth of faba bean but plants were more sensitive to Cl(-) than to Na(+). The reductions in growth and photosynthesis were greater under NaCl stress and the effect was mainly additive. An important difference to previous hydroponic studies was that increasing the concentrations of NaCl in the soil increased the concentration of Cl(-) more than the concentration of Na(+). The data showed that salinity caused by high concentrations of NaCl can reduce growth by the accumulation of high concentrations of both Na(+) and Cl(-) simultaneously, but the effects of the two ions may differ. High Cl(-) concentration reduces the photosynthetic capacity and quantum yield due to chlorophyll degradation which may result from a structural impact of high Cl(-) concentration on PSII. High Na(+) interferes with K(+) and Ca(2+) nutrition and disturbs efficient stomatal regulation which results in a depression of photosynthesis and growth. These results suggest that the importance of Cl(-) toxicity as a cause of reductions in growth and yield under salinity stress may have been underestimated.

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NMR metabolomics of esca disease-affected Vitis vinifera cv. Alvarinho leaves.

Publication Date: 2010 Aug 13 PMID: 20709726
Authors: Lima, M. R. - Felgueiras, M. L. - Graca, G. - Rodrigues, J. E. - Barros, A. - Gil, A. M. - Dias, A. C.
Journal: J Exp Bot

Esca is a destructive disease that affects vineyards leading to important losses in wine production. Information about the response of Vitis vinifera plants to this disease is scarce, particularly concerning changes in plant metabolism. In order to study the metabolic changes in Vitis plants affected by esca, leaves from both infected and non-affected cordons of V. vinifera cv. Alvarinho (collected in the Vinho Verde region, Portugal) were analysed. The metabolite composition of leaves from infected cordons with visible symptoms [diseased leaves (dl)] and from asymptomatic cordons [healthy leaves (hl)] was evaluated by 1D and 2D (1)H-nuclear magnetic resonance (NMR) spectroscopy. Principal component analysis (PCA) of the NMR spectra showed a clear separation between dl and hl leaves, indicating differential compound production due to the esca disease. NMR/PCA analysis allowed the identification of specific compounds characterizing each group, and the corresponding metabolic pathways are discussed. Altogether, the study revealed a significant increase of phenolic compounds in dl, compared with hl, accompanied by a decrease in carbohydrates, suggesting that dl are rerouting carbon and energy from primary to secondary metabolism. Other metabolic alterations detected comprised increased levels of methanol, alanine, and gamma-aminobutyric acid in dl, which might be the result of the activation of other defence mechanisms.

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Increasing productivity by matching farming system management and genotype in water-limited environments.

Publication Date: 2010 Aug 13 PMID: 20709725
Authors: Kirkegaard, J. A. - Hunt, J. R.
Journal: J Exp Bot

Improvements in water productivity and yield arise from interactions between varieties (G) and their management (M). Most GxM interactions considered by breeders and physiologists focus on in-crop management (e.g. sowing time, plant density, N management). However, opportunities exist to capture more water and use it more effectively that involve judicious management of prior crops and fallows (e.g. crop sequence, weed control, residue management). The dry-land wheat production system of southern Australia, augmented by simulation studies, is used to demonstrate the relative impacts and interactions of a range of pre-crop and in-crop management decisions on water productivity. A specific case study reveals how a novel genetic trait, long coleoptiles that enable deeper sowing, can interact with different management options to increase the water-limited yield of wheat from 1.6 t ha(-1) to 4.5 t ha(-1), reflecting the experience of leading growers. Understanding such interactions will be necessary to capture benefits from new varieties within the farming systems of the future.

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UV radiation reduces epidermal cell expansion in leaves of Arabidopsis thaliana.

Publication Date: 2010 Aug 11 PMID: 20702567
Authors: Hectors, K. - Jacques, E. - Prinsen, E. - Guisez, Y. - Verbelen, J. P. - Jansen, M. A. - Vissenberg, K.
Journal: J Exp Bot

Plants have evolved a broad spectrum of mechanisms to ensure survival under changing and suboptimal environmental conditions. Alterations of plant architecture are commonly observed following exposure to abiotic stressors. The mechanisms behind these environmentally controlled morphogenic traits are, however, poorly understood. In this report, the effects of a low dose of chronic ultraviolet (UV) radiation on leaf development are detailed. Arabidopsis rosette leaves exposed for 7, 12, or 19 d to supplemental UV radiation expanded less compared with non-UV controls. The UV-mediated decrease in leaf expansion is associated with a decrease in adaxial pavement cell expansion. Elevated UV does not affect the number and shape of adaxial pavement cells, nor the stomatal index. Cell expansion in young Arabidopsis leaves is asynchronous along a top-to-base gradient whereas, later in development, cells localized at both the proximal and distal half expand synchronously. The prominent, UV-mediated inhibition of cell expansion in young leaves comprises effects on the early asynchronous growing stage. Subsequent cell expansion during the synchronous phase cannot nullify the UV impact established during the asynchronous phase. The developmental stage of the leaf at the onset of UV treatment determines whether UV alters cell expansion during the synchronous and/or asynchronous stage. The effect of UV radiation on adaxial epidermal cell size appears permanent, whereas leaf shape is transiently altered with a reduced length/width ratio in young leaves. The data show that UV-altered morphogenesis is a temporal- and spatial-dependent process, implying that common single time point or single leaf zone analyses are inadequate.

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A CESA from Griffithsia monilis (Rhodophyta, Florideophyceae) has a family 48 carbohydrate-binding module.

Publication Date: 2010 Aug 11 PMID: 20702566
Authors: Matthews, P. R. - Schindler, M. - Howles, P. - Arioli, T. - Williamson, R. E.
Journal: J Exp Bot

Cellulose synthases form rosette terminal complexes in the plasma membranes of Streptophyta and various linear terminal complexes in other taxa. The sequence of a putative CESA from Griffithsia monilis (Rhodophyta, Floridiophyceae) was deduced using a cloning strategy involving degenerate primers, a cDNA library screen, and 5' and 3' rapid amplification of cDNA ends (RACE). RACE identified two alternative transcriptional starts and four alternative polyadenylation sites. The first translation start codon provided an open reading frame of 2610 bp encoding 870 amino acids and was PCR amplified without introns from genomic DNA. Southern hybridization indicated one strongly hybridizing gene with possible weakly related genes or pseudogenes. Amino acid sequence analysis identified a family 48 carbohydrate-binding module (CBM) upstream of the protein's first predicted transmembrane domain. There are broad similarities in predicted 3D structures of the family 48 modules from CESA, from several glycogen- and starch-binding enzymes, and from protein kinases, but there are substitutions at some residues thought to be involved in ligand binding. The module in G. monilis CESA will be on the cytoplasmic face of the plasma membrane so that it could potentially bind either low molecular weight ligands or starch which is cytosolic rather than inside membrane-bound plastids in red algae. Possible reasons why red algal CESAs have evolved family 48 modules perhaps as part of a system to regulate cellulose synthase activity in relation to cellular carbohydrate status are briefly discussed.

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COI1, a jasmonate receptor, is involved in ethylene-induced inhibition of Arabidopsis root growth in the light.

Publication Date: 2010 Aug 10 PMID: 20699268
Authors: Adams, E. - Turner, J.
Journal: J Exp Bot

Plant response to stress is orchestrated by hormone signalling pathways including those activated by jasmonates (JAs) and by ethylene, both of which stunt root growth. COI1 is a JA receptor and is required for the known responses to this hormone. It was observed that the coi1 mutant, which is largely unresponsive to growth inhibition by JAs, was also partially unresponsive to growth inhibition by ethylene and by its immediate precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), in the light but not in the dark. Although COI1 was required for this response to ACC, other components of the JA signal perception pathway were not. Mutants selected for insensitivity to ethylene, including etr1, ein2, and ein3, showed greater ACC-induced root growth inhibition in the light than in the dark. However, the double mutants etr1;coi1, ein2;coi1, and ein3;coi1, and coi1 seedlings treated with silver ions to block the ethylene receptors showed almost complete unresponsiveness to ACC-induced root growth inhibition in the light. The light requirement for the COI1-mediated growth inhibition by ACC was for long photoperiods, and the ACC response was not abolished by mutations in the known photoreceptors. The complementation assay indicated that SCF complex assembly was not required for COI1 function in the ACC response, in contrast to the JA response. It is concluded that COI1 is required for the light-dependent, JA-independent, root growth inhibition by ethylene.

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Kin recognition in plants: a mysterious behaviour unsolved.

Publication Date: 2010 Aug 8 PMID: 20696656
Authors: Biedrzycki, M. L. - Bais, H. P.
Journal: J Exp Bot

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Floret development of durum wheat in response to nitrogen availability.

Publication Date: 2010 Aug 8 PMID: 20696655
Authors: Ferrante, A. - Savin, R. - Slafer, G. A.
Journal: J Exp Bot

In Mediterranean durum wheat production, nitrogen (N) fertilization may be important to stabilize and increase yields. Wheat yield responses to N fertilization are usually related to grains per m(2), which in turn is the consequence of processes related to floret development (floret initiation followed by floret death/survival) during stem elongation. The literature is rather scarce in terms of the relevance of floret developmental dynamics, determining the final number of grains in general terms and in particular regarding responsiveness to N. The aim of this study was to determine whether durum wheat responses to N under different water regimes are related to the dynamics of development of floret primordia to produce fertile florets. During the 2006-2007 and 2007-2008 growing seasons, experiments with a factorial combination of two N levels (0 and 100 or 250 kg N ha(-1)) and two levels of water availability (rainfed and irrigated) were carried out (although the water regime was only effective in the second season). The response of yield was largely a consequence of that in grain number per spike. Floret initiation was similar for both N levels in each experiment and water regime, for which the survival of a higher proportion of initiated florets was critical in the response of the crop. The diminished rate of floret abortion during the late part of stem elongation in response to N was associated with a slightly accelerated rate of floret development which allowed a higher proportion of the primordia initiated to reach the stage of fertile floret by flowering.

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Spectral reflectance from a soybean canopy exposed to elevated CO2 and O3.

Publication Date: 2010 Aug 8 PMID: 20696654
Authors: Gray, S. B. - Dermody, O. - Delucia, E. H.
Journal: J Exp Bot

By affecting the physiology and structure of plant canopies, increasing atmospheric CO(2) and O(3) influence the capacity of agroecosystems to capture light and convert that light energy into biomass, ultimately affecting productivity and yield. The objective of this study was to determine if established remote sensing indices could detect the direct and interactive effects of elevated CO(2) and elevated O(3) on the leaf area, chlorophyll content, and photosynthetic capacity of a soybean canopy growing under field conditions. Large plots of soybean (Glycine max) were exposed to ambient air ( approximately 380 mumol CO(2) mol(-1)), elevated CO(2) ( approximately 550 mumol mol(-1)), elevated O(3) (1.2x ambient), and combined elevated CO(2) plus elevated O(3) at the soybean free air gas concentration enrichment (SoyFACE) experiment. Canopy reflectance was measured weekly and the following indices were calculated from reflectance data: near infrared/red (NIR/red), normalized difference vegetation index (NDVI), canopy chlorophyll content index (chl. index), and photochemical reflectance index (PRI). Leaf area index (LAI) also was measured weekly. NIR/red and LAI were linearly correlated throughout the growing season; however, NDVI and LAI were correlated only up to LAI values of approximately 3. Season-wide analysis demonstrated that elevated CO(2) significantly increased NIR/red, PRI, and chl. index, indicating a stimulation of LAI and photosynthetic carbon assimilation, as well as delayed senescence; however, analysis of individual dates resolved fewer statistically significant effects of elevated CO(2). Exposure to elevated O(3) decreased LAI throughout the growing season. Although NIR/red showed the same trend, the effect of O(3) on NIR/red was not statistically significant. Season-wide analysis showed significant effects of O(3) on PRI; however, analysis of individual dates revealed that this effect was only statistically significant on two dates. Elevated O(3) had minimal effects on the total canopy chlorophyll index. PRI appeared to be more sensitive to decreased photosynthetic capacity of the canopy as a whole compared with previously published single leaf gas exchange measurements at SoyFACE, possibly because PRI integrates the reflectance signal of older leaves with accumulated O(3) damage and healthy young, upper canopy leaves, enabling detection of significant decreases in photosynthetic carbon assimilation which have not been detected in previous studies which measured gas exchange of upper canopy leaves. When the canopy was exposed to elevated CO(2) and O(3) simultaneously, the deleterious effects of elevated O(3) were diminished. Reflectance data, while less sensitive than direct measurements of physiological/structural parameters, corroborate direct measurements of LAI and photosynthetic gas exchange made during the same season, as well as results from previous years at SoyFACE, demonstrating that these indices accurately represent structural and physiological effects of changing tropospheric chemistry on soybean growing in a field setting.

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Integration of phot1, phot2, and PhyB signalling in light-induced chloroplast movements.

Publication Date: 2010 Aug 6 PMID: 20693413
Authors: Luesse, D. R. - Deblasio, S. L. - Hangarter, R. P.
Journal: J Exp Bot

In Arabidopsis thaliana, chloroplasts move towards the periclinal cell walls upon exposure to low blue light intensities and to anticlinal walls under high light. The regulation of these chloroplast movements involves members of both the phototropin and phytochrome families of photoreceptors. Examination of fluence-rate response dependencies in phot1 and phot2 mutants revealed that although both photoreceptors are capable of inducing chloroplast accumulation under low-light conditions, the signals from these photoreceptors appear to be antagonistic. Chloroplast movements in wild-type plants were intermediate between those of the single phot mutants, consistent with each operating through separate signalling cascades. Mutants in phot2 showed transient chloroplast avoidance responses upon exposure to intense blue light, and slow but sustained chloroplast avoidance under intense white light, indicating that in the absence of phot2, phot1 is capable of generating both a low and a high-light response signal. Mutations in phytochrome B (phyB) caused an enhanced avoidance response at intermediate and high light intensities. Examination of phyB, phot1phyB, and phot2phyB mutants indicated that this enhancement is caused by PhyB inhibition of the high-light avoidance response in wild-type plants. In addition, our results suggest that the inhibition by PhyB is not exclusive to either of the phot1 or phot2 signalling pathways.

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Functional analysis of the structural domain of ARF proteins in rice (Oryza sativa L.).

Publication Date: 2010 Aug 6 PMID: 20693412
Authors: Shen, C. - Wang, S. - Bai, Y. - Wu, Y. - Zhang, S. - Chen, M. - Guilfoyle, T. J. - Wu, P. - Qi, Y.
Journal: J Exp Bot

Auxin response factors (ARFs) are key regulators of plant growth and development. Through interaction with auxin/indole acetic acid (Aux/IAA) proteins, they influence the expression of auxin response genes. An ARF gene family has been predicted in rice, but the functions of the individual structural domains of the OsARFs remain obscure. Bioinformatics was used to analyse the position of the DNA-binding domain (DBD), middle region (MR), and C-terminal dimerization domain (CTD) of OsARFs, and experimentally confirmed the presence of a classical monopartite nuclear localization signal (NLS) in the DBD. The DBD was shown to contribute to nuclear localization of OsARF proteins in addition to its known DNA-binding function. Interactions between 14 integrated OsARFs and 15 OsIAA proteins were tested using yeast two-hybrid assays. It was found that eight OsARF activators interacted with the 15 OsIAA proteins, while six OsARF repressors did not. The interactions between the MR+CTD or CTD of 10 OsARFs and 15 OsIAA proteins were also tested and the results were consistent with those of each intact OsARF, although some slight differences in interaction intensity were observed by alpha-galactosidase quantitative assays.. The truncated CTD of OsARF11 did not interact with any OsIAA, implying that the CTD is required for ARF-IAA dimerization, and that the MR influences the interaction intensity in yeast. A subset of the interactions in yeast were also observed in tobacco plants using firefly luciferase complementation imaging assays, indicating that these interactions are specific in plants, and might have a special role in the auxin signalling response. This study provides new insight into the structure of OsARF proteins and ARF-Aux/IAA interactions.

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Is the remobilization of S and N reserves for seed filling of winter oilseed rape modulated by sulphate restrictions occurring at different growth stages?

Publication Date: 2010 Aug 6 PMID: 20693411
Authors: Dubousset, L. - Etienne, P. - Avice, J. C.
Journal: J Exp Bot

How the remobilization of S and N reserves can meet the needs of seeds of oilseed rape subject to limitation of S fertilization remains largely unclear. Thus, this survey aims to determine the incidence of sulphate restriction [low S (LS)] applied at bolting [growth stage (GS) 32], visible bud (GS 53), and start of pod filling (GS 70) on source-sink relationships for S and N, and on the dynamics of endogenous/exogenous S and N contributing to seed yield and quality. Sulphate restrictions applied at GS 32, GS 53, and GS 70 were annotated LS(32), LS(53), and LS(70). Long-term (34)SO(4)(2-) and (15)NO(3)(-) labelling was used to explore S and N partitioning at the whole-plant level. In LS(53), the sulphur remobilization efficiency (SRE) to seeds increased, but not enough to maintain seed quality. In LS(32), an early S remobilization from leaves provided S for root, stem, and pod growth, but the subsequent demand for seed development was not met adequately and the N utilization efficiency (NUtE) was reduced when compared with high S (HS). The highest SRE (65+/-1.2% of the remobilized S) associated with an efficient foliar S mobilization (with minimal residual S concentrations of 0.1-0.2% dry matter) was observed under LS(70) treatment, which did not affect yield components.

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Quantitative trait loci associated with longevity of lettuce seeds under conventional and controlled deterioration storage conditions.

Publication Date: 2010 Aug 6 PMID: 20693410
Authors: Schwember, A. R. - Bradford, K. J.
Journal: J Exp Bot

Lettuce (Lactuca sativa L.) seeds have poor shelf life and exhibit thermoinhibition (fail to germinate) above approximately 25 degrees C. Seed priming (controlled hydration followed by drying) alleviates thermoinhibition by increasing the maximum germination temperature, but reduces lettuce seed longevity. Controlled deterioration (CD) or accelerated ageing storage conditions (i.e. elevated temperature and relative humidity) are used to study seed longevity and to predict potential seed lifetimes under conventional storage conditions. Seeds produced in 2002 and 2006 of a recombinant inbred line (RIL) population derived from a cross between L. sativa cv. SalinasxL. serriola accession UC96US23 were utilized to identify quantitative trait loci (QTLs) associated with seed longevity under CD and conventional storage conditions. Multiple longevity-associated QTLs were identified under both conventional and CD storage conditions for control (non-primed) and primed seeds. However, seed longevity was poorly correlated between the two storage conditions, suggesting that deterioration processes under CD conditions are not predictive of ageing in conventional storage conditions. Additionally, the same QTLs were not identified when RIL populations were grown in different years, indicating that lettuce seed longevity is strongly affected by production environment. Nonetheless, a major QTL on chromosome 4 [Seed longevity 4.1 (Slg4.1)] was responsible for almost 23% of the phenotypic variation in viability of the conventionally stored control seeds of the 2006 RIL population, with improved longevity conferred by the Salinas allele. QTL analyses may enable identification of mechanisms responsible for the sensitivity of primed seeds to CD conditions and breeding for improved seed longevity.

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ZmMPK5 is required for the NADPH oxidase-mediated self-propagation of apoplastic H2O2 in brassinosteroid-induced antioxidant defence in leaves of maize.

Publication Date: 2010 Aug 6 PMID: 20693409
Authors: Zhang, A. - Zhang, J. - Ye, N. - Cao, J. - Tan, M. - Zhang, J. - Jiang, M.
Journal: J Exp Bot

Brassinosteroids (BRs) have been shown to induce hydrogen peroxide (H(2)O(2)) accumulation, and BR-induced H(2)O(2) up-regulates antioxidant defence systems in plants. However, the mechanisms by which BR-induced H(2)O(2) regulates antioxidant defence systems in plants remain to be determined. In the present study, the role of ZmMPK5, a mitogen-activated protein kinase, in BR-induced anitioxidant defence and the relationship between the activation of ZmMPK5 and H(2)O(2) production in BR signalling were investigated in leaves of maize (Zea mays) plants. BR treatment activated ZmMPK5, induced apoplastic and chloroplastic H(2)O(2) accumulation, and enhanced the total activities of antioxidant enzymes. Such enhancements were blocked by pre-treatment with mitogen-activated protein kinase kinase (MAPKK) inhibitors and H(2)O(2) inhibitors or scavengers. Pre-treatment with MAPKK inhibitors substantially arrested the BR-induced apoplastic H(2)O(2) production after 6 h of BR treatment, but did not affect the levels of apoplastic H(2)O(2) within 1 h of BR treatment. BR-induced gene expression of NADPH oxidase was also blocked by pre-treatment with MAPKK inhibitors and an apoplastic H(2)O(2) inhibitor or scavenger after 120 min of BR treatment, but was not affected within 30 min of BR treatment. These results suggest that the BR-induced initial apoplastic H(2)O(2) production activates ZmMPK5, which is involved in self-propagation of apoplastic H(2)O(2) via regulation of NADPH oxidase gene expression in BR-induced antioxidant defence systems.

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Growth of the C4 dicot Flaveria bidentis: photosynthetic acclimation to low light through shifts in leaf anatomy and biochemistry.

Publication Date: 2010 Aug 6 PMID: 20693408
Authors: Pengelly, J. J. - Sirault, X. R. - Tazoe, Y. - Evans, J. R. - Furbank, R. T. - von Caemmerer, S.
Journal: J Exp Bot

In C(4) plants, acclimation to growth at low irradiance by means of anatomical and biochemical changes to leaf tissue is considered to be limited by the need for a close interaction and coordination between bundle sheath and mesophyll cells. Here differences in relative growth rate (RGR), gas exchange, carbon isotope discrimination, photosynthetic enzyme activity, and leaf anatomy in the C(4) dicot Flaveria bidentis grown at a low (LI; 150 mumol quanta m(2) s(-1)) and medium (MI; 500 mumol quanta m(2) s(-1)) irradiance and with a 12 h photoperiod over 36 d were examined. RGRs measured using a 3D non-destructive imaging technique were consistently higher in MI plants. Rates of CO(2) assimilation per leaf area measured at 1500 mummol quanta m(2) s(-1) were higher for MI than LI plants but did not differ on a mass basis. LI plants had lower Rubisco and phosphoenolpyruvate carboxylase activities and chlorophyll content on a leaf area basis. Bundle sheath leakiness of CO(2) () calculated from real-time carbon isotope discrimination was similar for MI and LI plants at high irradiance. increased at lower irradiances, but more so in MI plants, reflecting acclimation to low growth irradiance. Leaf thickness and vein density were greater in MI plants, and mesophyll surface area exposed to intercellular airspace (S(m)) and bundle sheath surface area per unit leaf area (S(b)) measured from leaf cross-sections were also both significantly greater in MI compared with LI leaves. Both mesophyll and bundle sheath conductance to CO(2) diffusion were greater in MI compared with LI plants. Despite being a C(4) species, F. bidentis is very plastic with respect to growth irradiance.

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The impact of drought on leaf physiology of Quercus suber L. trees: comparison of an extreme drought event with chronic rainfall reduction.

Publication Date: 2010 Aug 4 PMID: 20685731
Authors: Grant, O. M. - Tronina, L. - Ramalho, J. C. - Besson, C. K. - Lobo-do-Vale, R. - Pereira, J. S. - Jones, H. G. - Chaves, M. M.
Journal: J Exp Bot

Understanding the responses of cork oak (Quercus suber L.) to actual and predicted summer conditions is essential to determine the future sustainability of cork oak woodlands in Iberia. Thermal imaging may provide a rapid method for monitoring the extent of stress. The ecophysiology of cork trees was studied over three years. Three treatments were applied by means of rainfall capture and irrigation, with plots receiving 120%, 100%, or 80% of natural precipitation. Despite stomatal closure, detected using both thermal imaging and porometry, leaf water potential fell during the summer, most drastically during the third year of accumulative stress. The quantum efficiency (Phi(PSII)) and the maximum efficiency F(V('))/F(M(')) of photosystem II also fell more intensely over the third summer, while non-photochemical quenching (NPQ) increased. The reduced precipitation treatment sporadically further reduced leaf water potential, stomatal conductance (g(s)), I(G) (an index of g(s) derived from thermal imaging), Phi(PSII), and F(V('))/F(M(')), and increased leaf temperature and NPQ. It is concluded that these are very resilient trees since they were only severely affected in the third year of severe drought (the third year registering 45% less rainfall than average), and removing 20% of rainfall had a limited impact.

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Involvement of brassinosteroid signals in the floral-induction network of Arabidopsis.

Publication Date: 2010 Aug 4 PMID: 20685730
Authors: Li, J. - Li, Y. - Chen, S. - An, L.
Journal: J Exp Bot

The transition to flowering is known to be regulated by numerous interacting endogenous and environmental cues, of which brassinosteroids (BRs), a group of polyhydroxylated steroid phytohormones, appear to be linked to the regulation of flowering time. In Arabidopsis, BR biosynthetic det2 mutants exhibited delayed flowering time by at least 10 d compared with the wild type. The levels of endogenous BRs in det2 were below 10% of the wild type. The timing of flowering was also delayed in the BR biosynthetic dwf4 and cpd mutants and in the BR-insensitive bri1 mutants. Because brassinolide (BL) and different BL precursors were over-accumulated in BR biosynthetic mutants and BR-insensitive bri1 mutants, this showed that alterations in the endogenous BL content and the level of different BL precursors affect flowering time in Arabidopsis. The late-flowering phenotypes of bri1 also showed that components of the BR signal transduction pathway affect flowering time. So far, reports on a connection between BRs and flowering time are limited. This review summarizes recent advances regarding the action of BRs in the transition to flowering.

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Exploiting phytochemicals for developing a 'push-pull' crop protection strategy for cereal farmers in Africa.

Publication Date: 2010 Aug 5 PMID: 20670998
Authors: Khan, Z. R. - Midega, C. A. - Bruce, T. J. - Hooper, A. M. - Pickett, J. A.
Journal: J Exp Bot

Lepidopteran stemborers and parasitic weeds in the genus Striga are major constraints to efficient production of cereals, the most important staple food crops in Africa. Smallholder farmers are resource constrained and unable to afford expensive chemicals for crop protection. Development of a push-pull approach for integrated pest and weed management is reviewed here. Appropriate plants were discovered that naturally emit signalling chemicals (semiochemicals). Plants highly attractive for egg laying by stemborer pests were selected and employed as trap crops (pull), to draw pests away from the main crop. Of these, Napier grass, Pennisetum purpureum (Schumach), despite its attractiveness, supported minimal survival of the pests' immature stages. Plants that repelled stemborer pests, notably molasses grass, Melinis minutiflora P. Beauv., and forage legumes in the genus Desmodium, were selected as intercrops (push). Desmodium intercrops suppress Striga hermonthica (Del.) Benth. through an allelopathic mechanism. Their root exudates contain novel flavonoid compounds, which stimulate suicidal germination of S. hermonthica seeds and dramatically inhibit its attachment to host roots. The companion crops provide valuable forage for farm animals while the leguminous intercrops also improve soil fertility and moisture retention. The system is appropriate as it is based on locally available plants, not expensive external inputs, and fits well with traditional mixed cropping systems in Africa. To date it has been adopted by more than 30 000 smallholder farmers in East Africa where maize yields have increased from approximately 1 t ha(-1) to 3.5 t ha(-1). Future directions for semiochemical delivery by plants including biotechnological opportunities are discussed.

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