Plant Journal

Multiple origins of aluminium resistance in hexaploid wheat are derived from Aegilops tauschii and from more recent cis mutations to TaALMT1.

Publication Date: 2010 Aug 23 PMID: 20804458
Authors: Ryan, P. - Raman, H. - Gupta, S. - Sasaki, T. - Yamamoto, Y. - Delhaize, E.
Journal: Plant J

Acid soils limit plant production worldwide because their high concentrations of soluble aluminium cations (Al(3+) ) inhibit root growth. Major food crops like wheat (Triticum aestivum L.) have evolved mechanisms to resist Al(3+) toxicity thus enabling their wider distribution. The origins of Al(3+) resistance in wheat are perplexing because all progenitors of this hexaploid species are reportedly sensitive to Al(3+) stress. The large genotypic variation for Al(3+) resistance in wheat is largely controlled by the expression of an anion channel, TaALMT1, which releases malate anions from the root apices. A current hypothesis proposes that the malate anions protect this sensitive growing zone by binding with Al(3+) in the apoplasm. We investigated the evolution of this trait in wheat and demonstrated that it has multiple independent origins which enhance Al(3+) resistance by increasing TaALMT1 expression. One origin appears to be derived directly from the D-genome donor Aegilops tauschii while others have arisen more recently from a series of cis mutations that have generated tandemly-repeated elements in the TaALMT1 promoter. We generated transgenic plants to directly compare these promoter alleles and demonstrate that the tandemly-repeated elements act to enhance gene expression. This study provides an example from higher eukaryotes which links perfect tandem repeats with transcriptional regulation and phenotypic change in the context of evolutionary adaptation to a major abiotic stress.

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Intragenic allele pyramiding combines different specificities of wheat Pm3 resistance alleles.

Publication Date: 2010 Aug 23 PMID: 20804457
Authors: Brunner, S. - Hurni, S. - Streckeisen, P. - Mayr, G. - Albrecht, M. - Yahiaoui, N. - Keller, B.
Journal: Plant J

Some plant resistance genes occur as allelic series, with each member conferring specific resistance against a subset of pathogen races. In wheat, there are 17 alleles of the Pm3 gene. They encode nucleotide-binding (NB-ARC) and leucine-rich repeat (LRR) domain proteins, which mediate resistance to distinct race spectra of powdery mildew. It is not known if specificities from different alleles can be combined to create resistance genes with broader specificity. Here, we used an approach based on avirulence analysis of pathogen populations to characterise the molecular basis of Pm3 recognition spectra. A large survey of mildew races for avirulence on the Pm3 alleles revealed that Pm3a has a resistance spectrum that completely contains the one of Pm3f, but extends it towards additional races. The same is true for the Pm3b and Pm3c gene pair. The molecular analysis of these allelic pairs revealed a role of the NB-ARC protein domain in the efficiency of effector-dependent resistance. Analysis of the wildtype and chimeric Pm3 alleles identified single residues in the C-terminal LRR motifs as the main determinant of allele specificity. Variable residues of the N-terminal LRRs are necessary, but not sufficient, to confer resistance specificity. Based on these data, we constructed a chimeric Pm3 gene by intragenic allele pyramiding of Pm3d and Pm3e that showed the combined resistance specificity and, thus, a broader recognition spectrum compared with the parental alleles. Our findings support a model of stepwise evolution of Pm3 recognition specificities.

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The PAM1 Gene of Petunia, Required for Intracellular Accommodation and Morphogenesis of Arbuscular Mycorrhizal Fungi, Encodes a Homologue of VAPYRIN.

Publication Date: 2010 Aug 23 PMID: 20804456
Authors: Feddermann, N. - Muni, R. R. - Zeier, T. - Stuurman, J. - Ercolin, F. - Schorderet, M. - Reinhardt, D.
Journal: Plant J

Most terrestrial plants engage into the arbuscular mycorrhizal (AM) symbiosis with fungi of the phylum Glomeromycota. The initial recognition of the fungal symbiont results in the activation a symbiosis signaling pathway that is shared with the root nodule symbiosis (common SYM pathway). The subsequent intracellular accommodation of the fungus, and the elaboration of its characteristic feeding structures, the arbuscules, depends on a genetic program in the plant that has recently been shown to involve the VAPYRIN gene in Medicaco truncatula. We have previously identified a mutant in Petunia hybrida, penetration and arbuscule morphogenesis1 (pam1), that is defective in the intracellular stages of AM development. Here, we report the cloning of PAM1, which encodes a VAPYRIN homologue. PAM1 protein localizes to the cytosol and the nucleus, with a prominent affinity to mobile spherical structures that are associated with the tonoplast, and therefore are referred to as tonospheres. In mycorrhizal roots, tonospheres were observed in the vicinity of intracellular hyphae, where they may play an essential role in accommodation and morphogenesis of the fungal endosymbiont.

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Multiple Features that Distinguish Unilateral Incongruity and Self-incompatibility in the Tomato Clade.

Publication Date: 2010 Aug 23 PMID: 20804455
Authors: Covey, P. A. - Kondo, K. - Welch, L. - Frank, E. - Sianta, S. - Kumar, A. - Nunez, R. - Lopez-Casado, G. - Van Der Knaap, E. - Rose, J. K. - McClure, B. A. - Bedinger, P. A.
Journal: Plant J

Wild tomato species in Solanum section Lycopersicon often exhibit two types of reproductive barriers: self incompatibility (SI) and unilateral incompatibility, or incongruity (UI), wherein the success of an interspecific cross depends on the direction of the cross. UI pollen rejection often follows the "SI x SC" rule, i.e. pistils of SI species reject pollen of SC (self compatible) species but not vice versa, suggesting that SI and UI pollen rejection mechanisms may overlap. In order to address this question, pollen tube growth was measured following interspecific crosses using wild tomato species as the female parents and pollen from cultivated tomato (Solanum lycopersicum). Two modes of UI pollen rejection, early and late, were observed, both differing from SI pollen rejection. The structure and expression of known stylar SI genes were evaluated. We found that S-RNase expression is not required for either (early or late) mode of UI pollen rejection. However, two HT-family genes, HT-A and HT-B, map to a UI QTL. Surprisingly, we found that a gene previously implicated in SI, HT-B, is mutated in both SI and SC S. habrochaites accessions and no HT-B protein could be detected. HT-A genes were detected and expressed in all species examined and may therefore function in both SI and UI. We conclude that there are significant differences between SI and UI in the tomato clade, in that pollen tube growth differs in these two rejection systems, and some stylar SI factors, including S-RNase and HT-B, are not required for UI.

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