Journal of Experimental Botany

Mycorrhizal and non-mycorrhizal Lactuca sativa plants exhibit contrasting responses to exogenous ABA during drought stress and recovery.

Publication Date: 2008 May 9 PMID: 18469324
Authors: Aroca, R. - Vernieri, P. - Ruiz-Lozano, J. M.
Journal: J Exp Bot

The arbuscular mycorrhizal (AM) symbiosis enhances plant tolerance to water deficit through the alteration of plant physiology and the expression of plant genes. These changes have been postulated to be caused (among others) by different contents of abscisic acid (ABA) between AM and non-AM plants. However, there are no studies dealing with the effects of exogenous ABA on the expression of stress-related genes and on the physiology of AM plants. The aim of the present study was to evaluate the influence of AM symbiosis and exogenous ABA application on plant development, physiology, and expression of several stress-related genes after both drought and a recovery period. Results show that the application of exogenous ABA had contrasting effects on AM and non-AM plants. Only AM plants fed with exogenous ABA maintained shoot biomass production unaltered by drought stress. The addition of exogenous ABA enhanced considerably the ABA content in shoots of non-AM plants, concomitantly with the expression of the stress marker genes Lsp5cs and Lslea and the gene Lsnced. By contrast, the addition of exogenous ABA decreased the content of ABA in shoots of AM plants and did not produce any further enhancement of the expression of these three genes. AM plants always exhibited higher values of root hydraulic conductivity and reduced transpiration rate under drought stress. From plants subjected to drought, only the AM plants recovered their root hydraulic conductivity completely after the 3 d recovery period. As a whole, the results indicate that AM plants regulate their ABA levels better and faster than non-AM plants, allowing a more adequate balance between leaf transpiration and root water movement during drought and recovery.

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A field-grown transgenic tomato line expressing higher levels of polyamines reveals legume cover crop mulch-specific perturbations in fruit phenotype at the levels of metabolite profiles, gene expression, and agronomic characteristics.

Publication Date: 2008 May 9 PMID: 18469323
Authors: Neelam, A. - Cassol, T. - Mehta, R. A. - Abdul-Baki, A. A. - Sobolev, A. P. - Goyal, R. K. - Abbott, J. - Segre, A. L. - Handa, A. K. - Mattoo, A. K.
Journal: J Exp Bot

Genetic modification of crop plants to introduce desirable traits such as nutritional enhancement, disease and pest resistance, and enhanced crop productivity is increasingly seen as a promising technology for sustainable agriculture and boosting food production in the world. Independently, cultural practices that utilize alternative agriculture strategies including organic cultivation subscribe to sustainable agriculture by limiting chemical usage and reduced tillage. How the two together affect fruit metabolism or plant growth in the field or whether they are compatible has not yet been tested. Fruit-specific yeast S-adenosylmethionine decarboxylase (ySAMdc) line 579HO, and a control line 556AZ were grown in leguminous hairy vetch (Vicia villosa Roth) (HV) mulch and conventional black polyethylene (BP) mulch, and their fruit analysed. Significant genotypexmulch-dependent interactions on fruit phenotype were exemplified by differential profiles of 20 fruit metabolites such as amino acids, sugars, and organic acids. Expression patterns of the ySAMdc transgene, and tomato SAMdc, E8, PEPC, and ICDHc genes were compared between the two lines as a function of growth on either BP or HV mulch. HV mulch significantly stimulated the accumulation of asparagine, glutamate, glutamine, choline, and citrate concomitant with a decrease in glucose in the 556AZ fruits during ripening as compared to BP. It enables a metabolic system in tomato somewhat akin to the one in higher polyamine-accumulating transgenic fruit that have higher phytonutrient content. Finally, synergism was found between HV mulch and transgenic tomato in up-regulating N:C indicator genes PEPC and ICDHc in the fruit.

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Determination of leaf fresh mass after storage between moist paper towels: constraints and reliability of the method.

Publication Date: 2008 May 9 PMID: 18469322
Authors: Ryser, P. - Bernardi, J. - Merla, A.
Journal: J Exp Bot

To ensure comparability among leaf fresh mass measurements it is important to handle the leaves in a standardized manner. In the present work constraints of a commonly used method to achieve full turgor, storage between damp paper towels, were investigated. After overnight rehydration in a saturated atmosphere, the fresh mass of leaves of 14 species was measured, and the leaves were stored between paper towels (two treatments: moist and wet) at 4 degrees C. Their mass was measured after 24, 48, and 72 h. Leaf fresh mass increased during the first 24 h of storage between moist paper towels by an average of 1.8%, between wet towels by 3.3%. Among the species, the increase of leaf fresh mass between moist towels correlated with the species' desiccation propensity, indicating that it was rehydration from water loss during initial handling. On the other hand, between wet towels the fresh mass increase was associated with the species' leaf tissue structure, and it continued to increase beyond 24 h, indicating that the increase was a result of water penetration into the leaf air spaces. It is concluded that storage between moist paper towels results in reliable values of leaf fresh mass, and that desiccated leaves rehydrate well between moist towels. However, care has to be taken to avoid too wet conditions as they may lead to erroneously high fresh mass values, especially in species with large air spaces. Furthermore, exposure to unsaturated atmospheric conditions during handling has to be minimized.

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Modulation of heat shock factors accompanies salicylic acid-mediated potentiation of Hsp70 in tomato seedlings.

Publication Date: 2008 May 8 PMID: 18468986
Authors: Snyman, M. - Cronje, M. J.
Journal: J Exp Bot

In plants, salicylic acid (SA) is a signalling molecule regulating disease resistance responses such as systemic acquired resistance (SAR) and the hypersensitive response (HR), and has been implicated in both basal and acquired thermotolerance. It has been shown that SA enhances heat-induced Hsp/Hsc70 accumulation in plants. To investigate the mechanism of how SA influences the heat shock response (HSR) in plants, tomato seedlings were treated with SA alone, heat shock, or a combination of both before analyses of hsp70 mRNA, heat shock factor (Hsf)-DNA binding, and gene expression of hsp70, hsfA1, hsfA2, and hsfB1. SA alone led to activation of Hsf-DNA binding, but not induction or transcription of hsp70 mRNA. SA had no significant effect on hsfA2 and hsfB1 gene expression, but potentiated the basal levels of hsfA1. In heat-shocked plants, Hsf-DNA binding was established, and increased hsfA1, hsfA2, and hsfB1 expression was followed by accumulation of Hsp70. SA plus heat shock showed enhanced Hsf-DNA binding, enhanced induction of hsp70 mRNA transcription, and gene expression of hsfA1, hsfA2, and hsfB1, resulting in potentiated levels of Hsp/Hsc70. Since increased hsp70 and hsf gene expression coincide with increased levels of Hsp70 accumulation, it is concluded that SA-mediated potentiation of Hsp70 is due to modulation of these Hsfs by SA. In our efforts to understand the role of Hsp70 in heat-related disease susceptibility, the degree of the complexity of the cross-talk between the pathways in which SA is involved, inter alia, the plant defence response, the HSR and thermotolerance, was further underscored.

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Arabidopsis thaliana leaves with altered chloroplast numbers and chloroplast movement exhibit impaired adjustments to both low and high light.

Publication Date: 2008 May 8 PMID: 18468985
Authors: Koniger, M. - Delamaide, J. A. - Marlow, E. D. - Harris, G. C.
Journal: J Exp Bot

The effects of chloroplast number and size on the capacity for blue light-dependent chloroplast movement, the ability to increase light absorption under low light, and the susceptibility to photoinhibition were investigated in Arabidopsis thaliana. Leaves of wild-type and chloroplast number mutants with mean chloroplast numbers ranging from 120 to two per mesophyll cell were analysed. Chloroplast movement was monitored as changes in light transmission through the leaves. Light transmission was used as an indicator of the ability of leaves to optimize light absorption. The ability of leaves to deal with 3 h of high light stress at 10 degrees C and their capacity to recover in low light was determined by measuring photochemical efficiencies of PSII using chlorophyll a fluorescence. Chloroplast movement was comparable in leaves ranging in chloroplast numbers from 120 to 30 per mesophyll cell: the final light transmission levels after exposure to 0.1 (accumulation response) and 100 mumol photons m(-2) s(-1) (avoidance response) were indistinguishable, the chloroplasts responded quickly to small increases in light intensity and the kinetics of movement were similar. However, when chloroplast numbers per mesophyll cell decreased to 18 or below, the accumulation response was significantly reduced. The avoidance response was only impaired in mutants with nine or fewer chloroplasts, both in terms of final transmission levels and the speed of movement. Only mutants lacking both blue light receptors (phot1/phot2) or those with drastically reduced chloroplast numbers and severely impacted avoidance responses showed a reduced ability to recover from high light stress.

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Overexpression of phytochelatin synthase in tobacco: distinctive effects of AtPCS1 and CePCS genes on plant response to cadmium.

Publication Date: 2008 May 7 PMID: 18467325
Authors: Wojas, S. - Clemens, S. - Hennig, J. - Sklodowska, A. - Kopera, E. - Schat, H. - Bal, W. - Antosiewicz, D. M.
Journal: J Exp Bot

Phytochelatins, heavy-metal-binding polypeptides, are synthesized by phytochelatin synthase (PCS) (EC 2.3.2.15). Previous studies on plants overexpressing PCS genes yielded contrasting phenotypes, ranging from enhanced cadmium tolerance and accumulation to cadmium hypersensitivity. This paper compares the effects of overexpression of AtPCS1 and CePCS in tobacco (Nicotiana tabacum var. Xanthi), and demonstrates how the introduction of single homologous genes affects to a different extent cellular metabolic pathways leading to the opposite of the desired effect. In contrast to WT and CePCS transformants, plants overexpressing AtPCS1 were Cd-hypersensitive although there was no substantial difference in cadmium accumulation between studied lines. Plants exposed to cadmium (5 and 25 muM CdCl(2)) differed, however, in the concentration of non-protein thiols (NPT). In addition, PCS activity in AtPCS1 transformants was around 5-fold higher than in CePCS and WT plants. AtPCS1 expressing plants displayed a dramatic accumulation of gamma-glutamylcysteine and concomitant strong depletion of glutathione. By contrast, in CePCS transformants, a smaller reduction of the level of glutathione was noticed, and a less pronounced change in gamma-glutamylcysteine concentration. There was only a moderate and temporary increase in phytochelatin levels due to AtPCS1 and CePCS expression. Marked changes in NPT composition due to AtPCS1 expression led to moderately decreased Cd-detoxification capacity reflected by lower SH:Cd ratios, and to higher oxidative stress (assessed by DAB staining), which possibly explains the increase in Cd-sensitivity. The results indicate that contrasting responses to cadmium of plants overexpressing PCS genes might result from species-dependent differences in the activity of phytochelatin synthase produced by the transgenes.

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Transgenic wheat expressing a barley class II chitinase gene has enhanced resistance against Fusarium graminearum.

Publication Date: 2008 May 7 PMID: 18467324
Authors: Shin, S. - Mackintosh, C. A. - Lewis, J. - Heinen, S. J. - Radmer, L. - Dill-Macky, R. - Baldridge, G. D. - Zeyen, R. J. - Muehlbauer, G. J.
Journal: J Exp Bot

Fusarium head blight (FHB; scab), primarily caused by Fusarium graminearum, is a devastating disease of wheat worldwide. FHB causes yield reductions and contamination of grains with trichothecene mycotoxins such as deoxynivalenol (DON). The genetic variation in existing wheat germplasm pools for FHB resistance is low and may not provide sufficient resistance to develop cultivars through traditional breeding approaches. Thus, genetic engineering provides an additional approach to enhance FHB resistance. The objectives of this study were to develop transgenic wheat expressing a barley class II chitinase and to test the transgenic lines against F. graminearum infection under greenhouse and field conditions. A barley class II chitinase gene was introduced into the spring wheat cultivar, Bobwhite, by biolistic bombardment. Seven transgenic lines were identified that expressed the chitinase transgene and exhibited enhanced Type II resistance in the greenhouse evaluations. These seven transgenic lines were tested under field conditions for percentage FHB severity, percentage visually scabby kernels (VSK), and DON accumulation. Two lines (C8 and C17) that exhibited high chitinase protein levels also showed reduced FHB severity and VSK compared to Bobwhite. One of the lines (C8) also exhibited reduced DON concentration compared with Bobwhite. These results showed that transgenic wheat expressing a barley class II chitinase exhibited enhanced resistance against F. graminearum in greenhouse and field conditions.

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Expression pattern and activity of six glutelin gene promoters in transgenic rice.

Publication Date: 2008 May 7 PMID: 18467323
Authors: Qu, L. Q. - Xing, Y. P. - Liu, W. X. - Xu, X. P. - Song, Y. R.
Journal: J Exp Bot

The shortage of strong endosperm-specific expression promoters for driving the expression of recombinant protein genes in cereal endosperm is a major limitation in obtaining the required level and pattern of expression. Six promoters of seed storage glutelin genes (GluA-1, GluA-2, GluA-3, GluB-3, GluB-5, and GluC) were isolated from rice (Oryza sativa L.) genomic DNA by PCR. Their spatial and temporal expression patterns and expression potential in stable transgenic rice plants were examined with beta-glucuronidase (GUS) used as a reporter gene. All the promoters showed the expected spatial expression within the endosperm. The GluA-1, GluA-2, and GluA-3 promoters directed GUS expression mainly in the outer portion (peripheral region) of the endosperm. The GluB-5 and GluC promoters directed GUS expression in the whole endosperm, with the latter expressed almost evenly throughout the whole endosperm, a feature different from that of other rice glutelin gene promoters. The GluB-3 promoter directed GUS expression solely in aleurone and subaleurone layers. Promoter activities examined during seed maturation showed that the GluC promoter had much higher activity than the other promoters. These promoters are ideal candidates for achieving gene expression for multiple purposes in monocot endosperm but avoid promoter homology-based gene silencing. The GluC promoter did not contain the endosperm specificity-determining motifs GCN4, AACA, and the prolamin-box, which suggests the existence of additional regulatory mechanism in determining endosperm specificity.

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Identification of woolliness response genes in peach fruit after post-harvest treatments.

Publication Date: 2008 May 3 PMID: 18453640
Authors: Gonzalez-Aguero, M. - Pavez, L. - Ibanez, F. - Pacheco, I. - Campos-Vargas, R. - Meisel, L. A. - Orellana, A. - Retamales, J. - Silva, H. - Gonzalez, M. - Cambiazo, V.
Journal: J Exp Bot

Woolliness is a physiological disorder of peaches and nectarines that becomes apparent when fruit are ripened after prolonged periods of cold storage. This disorder is of commercial importance since shipping of peaches to distant markets and storage before selling require low temperature. However, knowledge about the molecular basis of peach woolliness is still incomplete. To address this issue, a nylon macroarray containing 847 non-redundant expressed sequence tags (ESTs) from a ripe peach fruit cDNA library was developed and used. Gene expression changes of peach fruit (Prunus persica cv. O'Henry) ripened for 7 d at 21 degrees C (juicy fruit) were compared with those of fruit stored for 15 d at 4 degrees C and then ripened for 7 d at 21 degrees C (woolly fruit). A total of 106 genes were found to be differentially expressed between juicy and woolly fruit. Data analysis indicated that the activity of most of these genes (>90%) was repressed in the woolly fruit. In cold-stored peaches (cv. O'Henry), the expression level of selected genes (cobra, endopolygalacturonase, cinnamoyl-CoA-reductase, and rab11) was lower than in the juicy fruit, and it remained low in woolly peaches after ripening, a pattern that was conserved in woolly fruit from two other commercial cultivars (cv. Flamekist and cv. Elegant Lady). In addition, the results of this study indicate that molecular changes during fruit woolliness involve changes in the expression of genes associated with cell wall metabolism and endomembrane trafficking. Overall, the results reported here provide an initial characterization of the transcriptome activity of peach fruit under different post-harvest treatments.

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Low temperature and light regulate delta 12 fatty acid desaturases (FAD2) at a transcriptional level in cotton (Gossypium hirsutum).

Publication Date: 2008 May 2 PMID: 18453533
Authors: Kargiotidou, A. - Deli, D. - Galanopoulou, D. - Tsaftaris, A. - Farmaki, T.
Journal: J Exp Bot

Lipid modifying enzymes play a key role in the development of cold stress tolerance in cold-resistant plants such as cereals. However, little is known about the role of the endogenous enzymes in cold-sensitive species such as cotton. Delta 12 fatty acid desaturases (FAD2), known to participate in adaptation to low temperatures through acyl chain modifications were used in gene expression studies in order to identify parameters of plant response to low temperatures. The induction of microsomal delta 12 fatty acid desaturases at an mRNA level under cold stress in plants is shown here for first time. Quantitative PCR showed that though both delta 12 omega 6 fatty acid desaturase genes FAD2-3 and FAD2-4 identified in cotton are induced under cold stress, FAD2-4 induction is significantly higher than FAD2-3. The induction of both isoforms was light regulated, in contrast a third isoform FAD2-2 was not affected by cold or light. Stress tolerance and light regulatory elements were identified in the predicted promoters of both FAD2-3 and FAD2-4 genes. Di-unsaturated fatty acid species rapidly increased in the microsomal fraction isolated from cotton leaves, following cold stress. Expression analysis patterns were correlated with the observed increase in both total and microsomal fatty acid unsaturation levels suggesting the direct role of the FAD2 genes in membrane adaptation to cold stress.

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Is the onset of senescence in leaf cells of intact plants due to low or high sugar levels?

Publication Date: 2008 May 2 PMID: 18453532
Authors: van Doorn, W. G.
Journal: J Exp Bot

This review examines the hypotheses that developmental programmed cell death in leaves is mediated (i) by sugar starvation in the leaf cells or (ii) by sugar accumulation in these cells. Experimental evidence for both hypotheses is critically discussed and found to be lacking. For example, some papers show that sugars prevent senescence of cut leaves placed in darkness, and prevent low sugar levels in the leaves. In these tests, the sugars seem to replace photosynthesis, hence the results have little relevance to leaf senescence in intact plants in the light. Low nitrogen nutrition and high light results in earlier senescence than the low nitrogen treatment alone. This is accompanied by high sugar levels in the leaves. The results have led to the idea that accumulation of sugars is the cause of the additional effect, or more generally, that sugar accumulation is always the direct cause of leaf senescence. Results from over-expressing, or knocking out, hexokinase genes tend to support the high sugar hypothesis, but pleiotropic effects confound this conclusion. In addition, several experiments show the effects of treatments on senescence without the increase in leaf sugar levels. Nonetheless, sugar levels are usually measured in whole leaves. Such an overall level does not reflect the differences in the onset of senescence between tissues and cells, and can therefore not be used as an argument for or against either of the two hypotheses. It is argued that future work should determine the time line of the concentrations of various sugars in various cells and cellular compartments, in relation to senescence processes in the same cells. Taken together, the data are not decisive. It is possible that neither of the two hypotheses is correct.

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The rice StMADS11-like genes OsMADS22 and OsMADS47 cause floral reversions in Arabidopsis without complementing the svp and agl24 mutants.

Publication Date: 2008 May 2 PMID: 18453531
Authors: Fornara, F. - Gregis, V. - Pelucchi, N. - Colombo, L. - Kater, M.
Journal: J Exp Bot

During floral induction and flower development plants undergo delicate phase changes which are under tight molecular control. MADS-box transcription factors have been shown to play pivotal roles during these transition phases. SHORT VEGETATIVE PHASE (SVP) and AGAMOUS LIKE 24 (AGL24) are important regulators both during the transition to flowering and during flower development. During vegetative growth they exert opposite roles on floral transition, acting as repressor and promoter of flowering, respectively. Later during flower development they act redundantly as negative regulators of AG expression. In rice, the orthologues of SVP and AGL24 are OsMADS22, OsMADS47, and OsMADS55 and these three genes are involved in the negative regulation of brassinosteroid responses. In order to understand whether these rice genes have maintained the ability to function as regulators of flowering time in Arabidopsis, complementation tests were performed by expressing OsMADS22 and OsMADS47 in the svp and agl24 mutants. The results show that the rice genes are not able to complement the flowering-time phenotype of the Arabidopsis mutants, indicating that they are biologically inactive in Arabidopsis. Nevertheless, they cause floral reversions, which mimic the SVP and AGL24 floral overexpressor phenotypes. Yeast two-hybrid analysis suggests that these floral phenotypes are probably the consequence of protein interactions between OsMADS22 and OsMADS47 and other MADS-box proteins which interfere with the formation of complexes required for normal flower development.

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Rice-arsenate interactions in hydroponics: whole genome transcriptional analysis.

Publication Date: 2008 May 2 PMID: 18453530
Authors: Norton, G. J. - Lou-Hing, D. E. - Meharg, A. A. - Price, A. H.
Journal: J Exp Bot

Rice (Oryza sativa) varieties that are arsenate-tolerant (Bala) and -sensitive (Azucena) were used to conduct a transcriptome analysis of the response of rice seedlings to sodium arsenate (AsV) in hydroponic solution. RNA extracted from the roots of three replicate experiments of plants grown for 1 week in phosphate-free nutrient with or without 13.3 muM AsV was used to challenge the Affymetrix (52K) GeneChip Rice Genome array. A total of 576 probe sets were significantly up-regulated at least 2-fold in both varieties, whereas 622 were down-regulated. Ontological classification is presented. As expected, a large number of transcription factors, stress proteins, and transporters demonstrated differential expression. Striking is the lack of response of classic oxidative stress-responsive genes or phytochelatin synthases/synthatases. However, the large number of responses from genes involved in glutathione synthesis, metabolism, and transport suggests that glutathione conjugation and arsenate methylation may be important biochemical responses to arsenate challenge. In this report, no attempt is made to dissect differences in the response of the tolerant and sensitive variety, but analysis in a companion article will link gene expression to the known tolerance loci available in the BalaxAzucena mapping population.

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Rice-arsenate interactions in hydroponics: a three-gene model for tolerance.

Publication Date: 2008 May 2 PMID: 18453529
Authors: Norton, G. J. - Nigar, M. - Williams, P. N. - Dasgupta, T. - Meharg, A. A. - Price, A. H.
Journal: J Exp Bot

In this study, the genetic mapping of the tolerance of root growth to 13.3 muM arsenate [As(V)] using the BalaxAzucena population is improved, and candidate genes for further study are identified. A remarkable three-gene model of tolerance is advanced, which appears to involve epistatic interaction between three major genes, two on chromosome 6 and one on chromosome 10. Any combination of two of these genes inherited from the tolerant parent leads to the plant having tolerance. Lists of potential positional candidate genes are presented. These are then refined using whole genome transcriptomics data and bioinformatics. Physiological evidence is also provided that genes related to phosphate transport are unlikely to be behind the genetic loci conferring tolerance. These results offer testable hypotheses for genes related to As(V) tolerance that might offer strategies for mitigating arsenic (As) accumulation in consumed rice.

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Genetic variability for leaf growth rate and duration under water deficit in sunflower: analysis of responses at cell, organ, and plant level.

Publication Date: 2008 Apr 29 PMID: 18448477
Authors: Pereyra-Irujo, G. A. - Velazquez, L. - Lechner, L. - Aguirrezabal, L. A.
Journal: J Exp Bot

Plants under water deficit reduce leaf growth, thereby reducing transpiration rate at the expense of reduced photosynthesis. The objective of this work was to analyse the response of leaf growth to water deficit in several sunflower genotypes in order to identify and quantitatively describe sources of genetic variability for this trait that could be used to develop crop varieties adapted to specific scenarios. The genetic variability of the response of leaf growth to water deficit was assessed among 18 sunflower (Helianthus annuus L.) inbred lines representing a broad range of genetic diversity. Plants were subjected to long-term, constant-level, water-deficit treatments, and the response to water deficit quantified by means of growth models at cell-, leaf-, and plant-scale. Significant variation among lines was found for the response of leaf expansion rate and of leaf growth duration, with an equal contribution of these responses to the variability in the reduction of leaf area. Increased leaf growth duration under water deficit is usually suggested to be caused by changes in the activity of cell-wall enzymes, but the present results suggest that the duration of epidermal cell division plays a key role in this response. Intrinsic genotypic responses of rate and duration at a cellular scale were linked to genotypic differences in whole-plant leaf area profile to water deficit. The results suggest that rate and duration responses are the result of different physiological mechanisms, and therefore capable of being combined to increase the variability in leaf area response to water deficit.

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Release of sunflower seed dormancy by cyanide: cross-talk with ethylene signalling pathway.

Publication Date: 2008 Apr 30 PMID: 18448476
Authors: Oracz, K. - El-Maarouf-Bouteau, H. - Bogatek, R. - Corbineau, F. - Bailly, C.
Journal: J Exp Bot

Freshly harvested sunflower (Helianthus annuus L.) seeds are considered to be dormant because they fail to germinate at relatively low temperatures (10 degrees C). This dormancy results mainly from an embryo dormancy and disappears during dry storage. Although endogenous ethylene is known to be involved in sunflower seed alleviation of dormancy, little attention had been paid to the possible role of cyanide, which is produced by the conversion of 1-aminocyclopropane 1-carboxylic acid to ethylene, in this process. The aims of this work were to investigate whether exogenous cyanide could improve the germination of dormant sunflower seeds and to elucidate its putative mechanisms of action. Naked dormant seeds became able to germinate at 10 degrees C when they were incubated in the presence of 1 mM gaseous cyanide. Other respiratory inhibitors showed that this effect did not result from an activation of the pentose phosphate pathway or the cyanide-insensitive pathway. Cyanide stimulated germination of dormant seeds in the presence of inhibitors of ethylene biosynthesis, but its improving effect required functional ethylene receptors. It did not significantly affect ethylene production and the expression of genes involved in ethylene biosynthesis or in the first steps of ethylene signalling pathway. However, the expression of the transcription factor Ethylene Response Factor 1 (ERF1) was markedly stimulated in the presence of gaseous cyanide. It is proposed that the mode of action of cyanide in sunflower seed dormancy alleviation does not involve ethylene production and that ERF1 is a common component of the ethylene and cyanide signalling pathways.

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OsBIRH1, a DEAD-box RNA helicase with functions in modulating defence responses against pathogen infection and oxidative stress.

Publication Date: 2008 Apr 25 PMID: 18441339
Authors: Li, D. - Liu, H. - Zhang, H. - Wang, X. - Song, F.
Journal: J Exp Bot

DEAD-box proteins comprise a large protein family with members from all kingdoms and play important roles in all types of processes in RNA metabolism. In this study, a rice gene OsBIRH1, which encodes a DEAD-box RNA helicase protein, was cloned and characterized. The predicted OsBIRH1 protein contains a DEAD domain and all conserved motifs that are common characteristics of DEAD-box RNA helicases. Recombinant OsBIRH1 protein purified from Escherichia coli was shown to have both RNA-dependent ATPase and ATP-dependent RNA helicase activities in vitro. Expression of OsBIRH1 was activated in rice seedling leaves after treatment with defence-related signal chemicals, for example benzothiadiazole, salicylic acid, l-aminocyclopropane-1-carboxylic acid, and jasmonic acid, and was also up-regulated in an incompatible interaction between a resistant rice genotype and the blast fungus, Magnaporthe grisea. Transgenic Arabidopsis plants that overexpress the OsBIRH1 gene were generated. Disease resistance phenotype assays revealed that the OsBIRH1-overexpressing transgenic plants showed an enhanced disease resistance against Alternaria brassicicola and Pseudomonas syringae pv. tomato DC3000. Meanwhile, defence-related genes, for example PR-1, PR-2, PR-5, and PDF1.2, showed an up-regulated expression in the transgenic plants. Moreover, the OsBIRH1 transgenic Arabidopsis plants also showed increased tolerance to oxidative stress and elevated expression levels of oxidative defence genes, AtApx1, AtApx2, and AtFSD1. The results suggest that OsBIRH1 encodes a functional DEAD-box RNA helicase and plays important roles in defence responses against biotic and abiotic stresses.

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Expression of the {beta}-oxidation gene 3-ketoacyl-CoA thiolase 2 (KAT2) is required for the timely onset of natural and dark-induced leaf senescence in Arabidopsis.

Publication Date: 2008 Apr 30 PMID: 18441338
Authors: Castillo, M. C. - Leon, J.
Journal: J Exp Bot

The onset of leaf senescence is regulated by a complex mechanism involving positive and negative regulators. Among positive regulators, jasmonic acid (JA) accumulates in senescing leaves and the JA-insensitive coi1-1 mutant displays delayed leaf senescence in Arabidopsis. A strong activated expression of the gene coding for the JA-biosynthetic beta-oxidation enzyme 3-ketoacyl-CoA thiolase 2 (KAT2) in natural and dark-induced senescing leaves of Arabidopsis thaliana is reported here. By using KAT2::GUS and KAT2::LUC transgenic plants, it was observed that dark-induced KAT2 activation occurred both in excised leaves as well as in whole darkened plants. The KAT2 activation associated with dark-induced senescence occurred soon after a move to darkness, and it preceded the detection of symptoms and the expression of senescence-associated gene (SAG) markers. Transgenic plants with reduced expression of the KAT2 gene showed a significant delayed senescence both in natural and dark-induced processes. The rapid induction of the KAT2 gene in senescence-promoting conditions as well as the delayed senescence phenotype and the reduced SAG expression in KAT2 antisense transgenic plants, point to KAT2 as an essential component for the timely onset of leaf senescence in Arabidopsis.

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The organ-dependent abundance of a Solanum lipid transfer protein is up-regulated upon osmotic constraints and associated with cold acclimation ability.

Publication Date: 2008 Apr 25 PMID: 18441337
Authors: Kielbowicz-Matuk, A. - Rey, P. - Rorat, T.
Journal: J Exp Bot

The expression of a gene isolated from cDNA differential screening and encoding a lipid transfer protein, designated as SsLTP1, was analysed at the protein level in two groups of Solanum species and lines differing in cold acclimation capacity. Under control conditions, the SsLTP1 was localized in all aerial organs of S. sogarandinum and S. tuberosum plants. Western analysis of subcellular extracts indicated that the protein possesses an intracellular localization. The protein abundance was found to vary as a function of organ type, the highest levels being observed in flowers, stems, and young leaves. During low temperature treatment, no change in protein level was noticed in either the S. tuberosum cv. Irga, which displays a low capacity for cold acclimation, or in a S. sogarandinum line which has lost its cold acclimation capacity. By contrast, low temperature induced a noticeable increase in SsLTP1 level in stems and leaves of S. sogarandinum and S. tuberosum cv. Ursus plants, which are able to acclimate to cold, indicating that SsLTP1 could participate in the processes leading to freezing tolerance. In other respects, SsLTP1 accumulation was observed both in cold-acclimating and in non-acclimating Solanum species when subjected to water deficit or to salt treatment. These data indicate that SsLTP1 gene expression is regulated in an organ-dependent manner and through distinct pathways under non-freezing low temperature and during osmotic treatments.

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Occurrence and forms of Kranz anatomy in photosynthetic organs and characterization of NAD-ME subtype C4 photosynthesis in Blepharis ciliaris (L.) B. L. Burtt (Acanthaceae).

Publication Date: 2008 Apr 24 PMID: 18440932
Authors: Akhani, H. - Ghasemkhani, M. - Chuong, S. D. - Edwards, G. E.
Journal: J Exp Bot

Blepharis (Acanthaceae) is an Afroasiatic genus comprising 129 species which occur in arid and semi-arid habitats. This is the only genus in the family which is reported to have some C(4) species. Blepharis ciliaris (L.) B. L. Burtt. is a semi-desert species with distribution in Iran, Oman, and Pakistan. Its form of photosynthesis was investigated by studying different organs. C(4)-type carbon isotope composition, the presence of atriplicoid type Kranz anatomy, and compartmentation of starch all indicate performance of C(4) photosynthesis in cotyledons, leaves, and the lamina part of bracts. A continuous layer of distinctive bundle sheath cells (Kranz cells) encircle the vascular bundles in cotyledons and the lateral vascular bundles in leaves. In older leaves, there is extensive development of ground tissue in the midrib and the Kranz tissue becomes interrupted on the abaxial side, and then becomes completely absent in the mature leaf base. Cotyledons have 5-6 layers, and leaves 2-3 layers, of spongy chlorenchyma beneath the veins near the adaxial side of the leaf, indicating bifacial organization of chlorenchyma. As the plant matures, bracts and spines develop and contribute to carbon assimilation through an unusual arrangement of Kranz anatomy which depends on morphology and exposure to light. Stems do not contribute to carbon assimilation, as they lack chlorenchyma tissue and Kranz anatomy. Analysis of C(4) acid decarboxylases by western blot indicates B. ciliaris is an NAD-malic enzyme type C(4) species, which is consistent with the Kranz cells having chloroplasts with well-developed grana and abundant mitochondria.

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The peripheral xylem of grapevine (Vitis vinifera). 1. Structural integrity in post-veraison berries.

Publication Date: 2008 Apr 24 PMID: 18440931
Authors: Chatelet, D. S. - Rost, T. L. - Shackel, K. A. - Matthews, M. A.
Journal: J Exp Bot

During the development of many fleshy fruits, water flow becomes progressively more phloemic and less xylemic. In grape (Vitis vinifera L.), the current hypothesis to explain this change is that the tracheary elements of the peripheral xylem break as a result of berry growth, rendering the xylem structurally discontinuous and hence non-functional. Recent work, however, has shown via apoplastic dye movement through the xylem of post-veraison berries that the xylem should remain structurally intact throughout berry development. To corroborate this, peripheral xylem structure in developing Chardonnay berries was investigated via maceration and plastic sectioning. Macerations revealed that, contrary to current belief, the xylem was comprised mostly of vessels with few tracheids. In cross-section, the tracheary elements of the vascular bundles formed almost parallel radial files, with later formed elements toward the epidermis and earlier formed elements toward the centre of the berry. Most tracheary elements remained intact throughout berry maturation, consistent with recent reports of vascular dye movement in post-veraison berries.

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The peripheral xylem of grapevine (Vitis vinifera) berries. 2. Anatomy and development.

Publication Date: 2008 Apr 24 PMID: 18440930
Authors: Chatelet, D. S. - Rost, T. L. - Matthews, M. A. - Shackel, K. A.
Journal: J Exp Bot

It has been hypothesized that the substantial reductions in xylemic water flow occurring at veraison are due to physical disruption (breaking) of the xylem as a result of renewed berry growth. In a companion paper, evidence was presented that the vast majority of xylem tracheary elements remained intact despite the growth of the berry, and it was proposed that existing tracheary elements stretch to accommodate growth and that additional elements may also differentiate after veraison. Measurements of the intergyre distance of tracheary elements in macerated tissue were used to test for stretching, and the numbers of tracheary elements per vascular bundle and of branch points of the peripheral xylem network were analysed to test for continued differentiation from 18 to 120 d after anthesis in Chardonnay berries. The distance between the epidermis and the vasculature increased substantially from pre- to post-veraison, potentially increasing the amount of skin available for analysis of compounds important for winemaking. Tracheary elements continued to differentiate within the existing vascular bundles throughout berry development. Additional vascular bundles also appeared until after veraison, thereby increasing the complexity of the peripheral vascular network. The results also confirmed that tracheary elements stretched by approximately 20%, but this was not as much as that predicted based on the growth of the vascular diameter (40%). These results complete a comprehensive evaluation of grape berry peripheral xylem during its development and show that tracheary development continues further into berry maturation than previously thought.

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Expression of gibberellin 20-oxidase1 (AtGA20ox1) in Arabidopsis seedlings with altered auxin status is regulated at multiple levels.

Publication Date: 2008 Apr 24 PMID: 18440929
Authors: Desgagne-Penix, I. - Sponsel, V. M.
Journal: J Exp Bot

Bioactive gibberellins (GAs) affect many biological processes including germination, stem growth, transition to flowering, and fruit development. The location, timing, and level of bioactive GA are finely tuned to ensure that optimal growth and development occur. The balance between GA biosynthesis and deactivation is controlled by external factors such as light and by internal factors that include auxin. The role of auxin transport inhibitors (ATIs) and auxins on GA homeostasis in intact light-grown Arabidopsis thaliana (L.) Heynh. seedlings was investigated. Two ATIs, 1-N-naphthylthalamic acid (NPA) and 1-naphthoxyacetic acid (NOA) caused elevated expression of the GA biosynthetic enzyme AtGA20-oxidase1 (AtGA20ox1) in shoot but not in root tissues, and only at certain developmental stages. It was investigated whether enhanced AtGA20ox1 gene expression was a consequence of altered flow through the GA biosynthetic pathway, or was due to impaired GA signalling that can lead to enhanced AtGA20ox1 expression and accumulation of a DELLA protein, Repressor of ga1-3 (RGA). Both ATIs promoted accumulation of GFP-fused RGA in shoots and roots, and this increase was counteracted by the application of GA(4). These results suggest that in ATI-treated seedlings the impediment to DELLA protein degradation may be a deficiency of bioactive GA at sites of GA response. It is proposed that the four different levels of AtGA20ox1 regulation observed here are imposed in a strict hierarchy: spatial (organ-, tissue-, cell-specific) > developmental > metabolic > auxin regulation. Thus results show that, in intact auxin- and auxin transport inhibitor-treated light-grown Arabidopsis seedlings, three other levels of regulation supersede the effects of auxin on AtGA20ox1.

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On the nature of facultative and constitutive CAM: environmental and developmental control of CAM expression during early growth of Clusia, Kalanchoe, and Opuntia.

Publication Date: 2008 Apr 24 PMID: 18440928
Authors: Winter, K. - Garcia, M. - Holtum, J. A.
Journal: J Exp Bot

The capacity to induce crassulacean acid metabolism developmentally (constitutive CAM) and to up-regulate CAM expression in response to drought stress (facultative CAM) was studied in whole shoots of seven species by measuring net CO(2) gas exchange for up to 120 day-night cycles during early growth. In Clusia rosea, CAM was largely induced developmentally. Well-watered seedlings began their life cycle as C(3) plants and developed net dark CO(2) fixation indicative of CAM after the initiation of the fourth leaf pair following the cotyledons. Thereafter, CAM activity increased progressively and drought stress led to only small additional, reversible increases in dark CO(2) fixation. In contrast, CAM expression was overwhelmingly under environmental control in seedlings and mature plants of Clusia pratensis. C(3)-type CO(2) exchange was maintained under well-watered conditions, but upon drought stress, CO(2) exchange shifted, in a fully reversible manner, to a CAM-type pattern. Clusia minor showed CO(2) exchange reponses intermediate to those of C. rosea and C. pratensis. Clusia cretosa operated in the C(3) mode at all times. Notably, reversible stress-induced increases of dark CO(2) fixation were also observed during the developmental progression to pronounced CAM in young Kalanchoe daigremontiana and Kalanchoe pinnata, two species considered constitutive CAM species. Drought-induced up-regulation of CAM was even detected in young cladodes of a cactus, Opuntia ficus-indica, an archetypal constitutive CAM species. Evidently, the defining characteristics of constitutive and facultative CAM are shared, to variable degrees, by all species.

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Evolutionary physiology: the extent of C4 and CAM photosynthesis in the genera Anacampseros and Grahamia of the Portulacaceae.

Publication Date: 2008 Apr 24 PMID: 18440927
Authors: Guralnick, L. J. - Cline, A. - Smith, M. - Sage, R. F.
Journal: J Exp Bot

The Portulacaceae is one of the few terrestrial plant families known to have both C(4) and Crassulacean acid metabolism (CAM) species. There may be multiple origins of the evolution of CAM within the Portulacaceae but the only clear evidence of C(4) photosynthesis is found in members of the genus Portulaca. In the Portulaca, CAM succulent tissue is overlaid with the C(4) tissue in a unique fashion where both pathways are operating simultaneously. Earlier reports have shown that the clade containing the genera Anacampseros and Grahamia may also contain C(4) photosynthetic species similar to the Portulaca, which would indicate multiple origins of C(4) photosynthesis within the family. The aim of the present study was to ascertain the true photosynthetic nature of these genera. An initial survey of the carbon isotope composition of the Anacampseros ranged from -12.6 per thousand to -24.0 per thousand, indicating very little CAM activity in some species, with other values close to the C(4) range. Anacampseros (=Grahamia) australiana which had been previously identified as a C(4) species had a carbon isotope composition value of -24.0 per thousand, which is more indicative of a C(3) species with a slight contribution of CAM activity. Other Anacampseros species with C(4)-like values have been shown to be CAM plants. The initial isotope analysis of the Grahamia species gave values in the range of -27.1 per thousand to -23.6 per thousand, placing the Grahamia species well towards the C(3) photosynthetic range. Further physiological studies indicated increased night-time CO(2) uptake with imposition of water stress, associated with a large diurnal acid fluctuation and a marked increased phosphoenolpyruvate carboxylase activity. This showed that the Grahamia species are actually facultative CAM plants despite their C(3)-like carbon isotope values. The results indicate that the Grahamia and Anacampseros species do not utilize the C(4) photosynthetic pathway. This is the first to identify that the Grahamia species are facultative CAM plants where CAM can be induced by water stress. This work supports earlier physiological work that indicates that this clade containing Anacampseros and Grahamia species comprises predominantly facultative CAM plants. This report suggests there may be only one clade which contains C(4) photosynthetic members with CAM-like characteristics.

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Clusia: Holy Grail and enigma.

Publication Date: 2008 Apr 23 PMID: 18436546
Authors: Luttge, U.
Journal: J Exp Bot

Clusia is the only genus with bona fide dicotyledonous trees performing Crassulacean acid metabolism (CAM). Clusia minor L. is extraordinarily flexible, being C(3)/CAM intermediate and expressing the photosynthetic modes C(3), CAM, CAM-cycling, and CAM-idling. C(3) photosynthesis and CAM can be observed simultaneously in two opposite leaves on a node and possibly even within the same leaf in the interveinal lamina and the major vein tissue, respectively. The relative activity of photosystem II (PhiPSII) indicating photosynthetic energy use, is larger under photorespiratory than under non-photorespiratory conditions due to the particular energy demand of photorespiration. The heterogeneity of PhiPSII over the leaves as visualized by chlorophyll fluorescence imaging in the C(3) mode is larger under non-photorespiratory conditions than under photorespiratory conditions. These observations indicate that photorespiration, presumably by its particular energy demand, synchronizes photosynthetic activity over the leaves. In the CAM mode, the heterogeneity of PhiPSII is more dependent on the transitions between CAM phases. Free-running circadian oscillations of photosynthesis are strongly dampened in both the C(3) and the CAM mode. Photorespiration is under circadian clock control in both the C(3) and the CAM mode. PhiPSII and the heterogeneity of PhiPSII oscillate in phase with CO(2) uptake and photorespiration only under non-photorespiratory conditions. Under photorespiratory conditions, PhiPSII does not oscillate and there is no heterogeneity, again indicating the stabilizing function of photorespiration. Plants acclimatized to perform CAM switch to C(3) photosynthesis during free-running oscillations while subjected to constant illumination.

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Curiosity and context revisited: crassulacean acid metabolism in the Anthropocene.

Publication Date: 2008 Apr 23 PMID: 18436545
Authors: Osmond, B. - Neales, T. - Stange, G.
Journal: J Exp Bot

Having gained some understanding of the consequences of the CO(2)-concentrating mechanisms in crassulacean acid metabolism (CAM) that internalize the photosynthetic environment of the Cretaceous on a daily basis, it may be time to consider potential long-term effects of the planetary CO(2)-concentrating mechanism on growth and ecology of these plants in the Anthropocene. This paper emphasizes our limited understanding of the carbohydrate economy of CAM in relation to growth processes and briefly reviews recent studies of the diel cycles of growth in these plants. An inadvertent long-term, regional-scale experiment from the past is revisited in which an Opuntia monoculture grew to occupy >25 million hectares of farmland in central eastern Australia, producing a total biomass of about 1.5 billion tonnes in about 80 years. Although at the time it does not seem to have been recognized that this invasion involved CAM, a botanist from the University of Melbourne, Jean White-Haney emerges as a heroic pioneer in the control of the invader by poison and pioneered its biological control. The Opuntia population was expanding at 10-100 ha h(-1) when it was brought to a halt within a decade by the voracious appetite of Cactoblastis cactorum larvae. It is now known that the female parent moth of this predator detects CAM in O. stricta prior to oviposition by deploying the most sensitive CO(2) detector system yet found in the Lepidoptera. The O. stricta invasion is a dramatic demonstration of the capacity of CAM plants to attain and sustain high biomass; to sequester and retain atmospheric CO(2). In conclusion, experiments are reviewed that show stimulation of CO(2) assimilation, growth, and biomass of CAM plants by elevated atmospheric [CO(2)], and the proposition that these plants may have a role in atmospheric CO(2) sequestration is re-examined. This role may be compromised by predators such as Cactoblastis. However the moth CO(2) sensors are adapted to pre-industrial atmospheric [CO(2)] and FACE (free-air CO(2) enrichment) experiments show this exquisite system of biological control is also compromised by rising global [CO(2)] in the Anthropocene.

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Rubisco, Rubisco activase, and global climate change.

Publication Date: 2008 Apr 23 PMID: 18436544
Authors: Sage, R. F. - Way, D. A. - Kubien, D. S.
Journal: J Exp Bot

Global warming and the rise in atmospheric CO(2) will increase the operating temperature of leaves in coming decades, often well above the thermal optimum for photosynthesis. Presently, there is controversy over the limiting processes controlling photosynthesis at elevated temperature. Leading models propose that the reduction in photosynthesis at elevated temperature is a function of either declining capacity of electron transport to regenerate RuBP, or reductions in the capacity of Rubisco activase to maintain Rubisco in an active configuration. Identifying which of these processes is the principal limitation at elevated temperature is complicated because each may be regulated in response to a limitation in the other. Biochemical and gas exchange assessments can disentangle these photosynthetic limitations; however, comprehensive assessments are often difficult and, for many species, virtually impossible. It is proposed that measurement of the initial slope of the CO(2) response of photosynthesis (the A/C(i) response) can be a useful means to screen for Rubisco activase limitations. This is because a reduction in the Rubisco activation state should be most apparent at low CO(2) when Rubisco capacity is generally limiting. In sweet potato, spinach, and tobacco, the initial slope of the A/C(i) response shows no evidence of activase limitations at high temperature, as the slope can be accurately modelled using the kinetic parameters of fully activated Rubisco. In black spruce (Picea mariana), a reduction in the initial slope above 30 degrees C cannot be explained by the known kinetics of fully activated Rubisco, indicating that activase may be limiting at high temperatures. Because black spruce is the dominant species in the boreal forest of North America, Rubisco activase may be an unusually important factor determining the response of the boreal biome to climate change.

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Rubisco regulation: a role for inhibitors.

Publication Date: 2008 Apr 23 PMID: 18436543
Authors: Parry, M. A. - Keys, A. J. - Madgwick, P. J. - Carmo-Silva, A. E. - Andralojc, P. J.
Journal: J Exp Bot

In photosynthesis Rubisco catalyses the assimilation of CO(2) by the carboxylation of ribulose-1,5-bisphosphate. However, the catalytic properties of Rubisco are not optimal for current or projected environments and limit the efficiency of photosynthesis. Rubisco activity is highly regulated in response to short-term fluctuations in the environment, although such regulation may not be optimally poised for crop productivity. The regulation of Rubisco activity in higher plants is reviewed here, including the role of Rubisco activase, tight binding inhibitors, and the impact of abiotic stress upon them.

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Is internucleosomal DNA fragmentation an indicator of programmed death in plant cells?

Publication Date: 2008 Apr 23 PMID: 18436542
Authors: Kuthanova, A. - Opatrny, Z. - Fischer, L.
Journal: J Exp Bot

Specific DNA fragmentation into oligonucleosomal units occurs during programmed cell death (PCD) in both animal and plant cells, usually being regarded as an indicator of its apoptotic character. This internucleosomal DNA fragmentation is demonstrated in tobacco suspension and leaf cells, which were killed immediately by freezing in liquid nitrogen, and homogenization or treatment with Triton X-100. Although these cells could not activate and realize the respective enzymatic processes in a programmed manner, the character of DNA fragmentation was similar to that in the cells undergoing typical gradual PCD induced by 50 muM CdSO(4). This internucleosomal DNA fragmentation was connected with the action of cysteine proteases and the loss of membrane, in particular tonoplast, integrity. The mechanisms of DNase activation in the rapidly killed cells, hypothetical biological relevance, and implications for the classification of cell death are discussed.

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The influence of epidermal windows on the light environment within the leaves of six succulents.

Publication Date: 2008 Apr 23 PMID: 18436541
Authors: Egbert, K. J. - Martin, C. E. - Vogelmann, T. C.
Journal: J Exp Bot

An omni-directional fibre optic microprobe was used to measure the quantity and quality of light within the leaves of six succulents having epidermal windows, three species having a subterranean growth habit (Haworthia truncata, Lithops olivacea, and Opthalmophyllum longum) and three growing above ground (Peperomia dolabriformis, P. graveolens, and the sprawling vine Senecio rowleyanus). Although light levels at most locations inside the leaves of all species were high, near those incident on the window surfaces, light levels inside the leaves of the two species of Peperomia often greatly exceeded incident light levels, indicating considerable light scattering and focusing by the leaf tissue. The spectral quality of light inside the leaves of all taxa reflected the absorption properties of chlorophyll, with most of the photons in the green wavelengths. Light quality and quantity inside the leaves did not correlate with the growth habit of the plants, the size of the window (as a proportion of the total leaf area), or location inside the leaf, although light levels generally declined and wavelengths increased deeper in the leaves. Application of reflective tape to the windows reduced internal light levels in L. olivacea and S. rowleyanus, although reductions were not always statistically significant. Although light levels throughout the leaves of P. graveolens were substantially and significantly reduced as a result of the application of reflective tape to its windows, the light levels even at the basal chlorenchyma on the abaxial side of the leaf remained high. In all species investigated, the levels of near-infrared radiation inside the leaves were surprisingly high, yet also declined deeper inside the succulent leaves. This near-infrared radiation may add to the heat load of these plants. Furthermore, application of reflective tape to the windows also reduced the amount of near-infrared radiation inside the leaves of the three succulents examined. These results led to a novel, testable hypothesis that may help to explain previous findings that application of reflective tape to the windows of the leaves of these succulents did not effect a reduction in photosynthetic activity.

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Root hydraulic conductivity measured by pressure clamp is substantially affected by internal unstirred layers.

Publication Date: 2008 Apr 16 PMID: 18420591
Authors: Knipfer, T. - Steudle, E.
Journal: J Exp Bot

Using the root pressure probe in the pressure clamping (PC) mode, the impact of internal unstirred layers (USLs) was quantified for young corn roots, both in experiments and in computer simulations applying the convection/diffusion model of Knipfer et al. In the experiments, water flows (J(Vr)s) during PC were analysed in great detail, showing that J(Vr)s (and the apparent root hydraulic conductivity) were high during early stages of PC and declined rapidly during the first 80 s of clamping to a steady-state value of 40-30% of the original. The comparison of experimental results with simulations showed that, during PC, internal USLs at the inner surface of the endodermis substantially modify the overall force driving the water. As a consequence, J(Vr) and Lp(r) were inhibited. Effects of internal USLs were minimized when using the pressure relaxation mode, when internal USLs had not yet developed. Additional stop-clamp experiments and experiments where the endodermis was punctured to reduce the effect of internal USLs verified the existence of internal USLs during PC. Data indicated that the role of pressure propagation along the root xylem for both PC and pressure relaxation modes should be small, as should the effects of filling of the capacities during root pressure probe experiments, which are discussed as an alternative model. The results supported the idea that concentration polarization effects at the endodermis (internal USLs) cause a serious problem whenever relatively large amounts of water (xylem sap) are radially moved across the root, such as during PC or when using the high-pressure flow meter technique.

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Low temperature effects on leaf physiology and survivorship in the C3 and C4 subspecies of Alloteropsis semialata.

Publication Date: 2008 Apr 24 PMID: 18403381
Authors: Osborne, C. P. - Wythe, E. J. - Ibrahim, D. G. - Gilbert, M. E. - Ripley, B. S.
Journal: J Exp Bot

The species richness of C(4) grasses is strongly correlated with temperature, with C(4) species dominating subtropical ecosystems and C(3) types predominating in cooler climates. Here, the effects of low temperatures on C(4) and C(3) grasses are compared, controlling for phylogenetic effects by using Alloteropsis semialata, a unique species with C(4) and C(3) subspecies. Controlled environment and common garden experiments tested the hypotheses that: (i) photosynthesis and growth are greater in the C(4) than the C(3) subspecies at high temperatures, but this advantage is reversed below 20 degrees C; and (ii) chilling-induced photoinhibition and light-mediated freezing injury of leaves occur at higher temperature thresholds in the C(4) than the C(3) plants. Measurements of leaf growth and photosynthesis showed the expected advantages of the C(4) pathway over the C(3) type at high temperatures. These declined with temperature, but were not completely lost until 15 degrees C, and there was no evidence of a reversal to give a C(3) advantage. Chronic chilling (5-15 degrees C) or acute freezing events induced a comparable degree of photodamage in illuminated leaves of both subspecies. Similarly, freezing caused high rates of mortality in the unhardened leaves of both subtypes. However, a 2-week chilling treatment prior to these freezing events halved injury in the C(3) but not the C(4) subspecies, suggesting that C(4) leaves lacked the capacity for cold acclimation. These results therefore suggest that C(3) members of this subtropical species may gain an advantage over their C(4) counterparts at low temperatures via protection from freezing injury rather than higher photosynthetic rates.

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RuBisCO-like proteins as the enolase enzyme in the methionine salvage pathway: functional and evolutionary relationships between RuBisCO-like proteins and photosynthetic RuBisCO.

Publication Date: 2008 Apr 16 PMID: 18403380
Authors: Ashida, H. - Saito, Y. - Nakano, T. - de Marsac, N. T. - Sekowska, A. - Danchin, A. - Yokota, A.
Journal: J Exp Bot

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is the key enzyme in the fixation of CO(2) in the Calvin cycle of plants. Many genome projects have revealed that bacteria, including Bacillus subtilis, possess genes for proteins that are similar to the large subunit of RuBisCO. These RuBisCO homologues are called RuBisCO-like proteins (RLPs) because they are not able to catalyse the carboxylase or the oxygenase reactions that are catalysed by photosynthetic RuBisCO. It has been demonstrated that B. subtilis RLP catalyses the 2,3-diketo-5-methylthiopentyl-1-phosphate (DK-MTP-1-P) enolase reaction in the methionine salvage pathway. The structure of DK-MTP-1-P is very similar to that of ribulose-1,5-bisphosphate (RuBP) and the enolase reaction is a part of the reaction catalysed by photosynthetic RuBisCO. In this review, functional and evolutionary relationships between B. subtilis RLP of the methionine salvage pathway, other RLPs, and photosynthetic RuBisCO are discussed. In addition, the fundamental question, 'How has RuBisCO evolved?' is also considered, and evidence is presented that RuBisCOs evolved from RLPs.

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