Search

Zymoseptoria tritici is the causal agent of Septoria tritici blotch (STB) disease of wheat. Z. tritici is an apoplastic fungal pathogen, which does not penetrate plant cells at any stage of infection, and has a long initial period of symptomless leaf colonisation. During this phase it is unclear to what extent the fungus can access host plant nutrients or communicate with plant cells. Several important primary and secondary metabolite pathways in fungi are regulated by the post-translational activator phosphopantetheinyl transferase (Ppt) which provides an essential co-factor for lysine biosynthesis and the activities of non-ribosomal peptide synthases (NRPS) and polyketide synthases (PKS). To investigate the relative importance of lysine biosynthesis, NRPS-based siderophore production and PKS-based DHN melanin biosynthesis, we generated deletion mutants of ZtPpt. The ∆ZtPpt strains were auxotrophic for lysine and iron, non-melanised and non-pathogenic on wheat. Deletion of the three target genes likely affected by ZtPpt loss of function (Aar- lysine; Nrps1-siderophore and Pks1- melanin), highlighted that lysine auxotrophy was the main contributing factor for loss of virulence, with no reduction caused by loss of siderophore production or melanisation. This reveals Ppt, and the lysine biosynthesis pathway, as potential targets for fungicides effective against Z. tritici.

W975W975; Septoria tritici blotch (STB) is a devastating foliar disease of wheat encountered in most wheat growing areas of the world resulting from infection by the ascomycete fungus Zymoseptoria tritici. Emergence and dispersal of fungicide resistance in Z. tritici field populations seriously threatens wheat production, therefore STB resistance breeding is considered a high priority. To date, 21 major genes conferring isolate-specific resistance as well as 89 resistance QTLs have been characterized genetically. However, none of these have as yet been cloned and mechanisms of resistance remain poorly understood. Here, we report isolation of the Stb6 gene controlling resistance to Z. tritici isolates carrying the matching AvrStb6 gene. This wheat gene confers a semi-dominant resistance to Z. tritici, which does not involve induction of HR. Fine-mapping using a large F2 mapping population derived from a cross between the resistant wheat Chinese Spring (Stb6) and the susceptible wheat Courtot and subsequent BAC and genomic DNA contigs sequence analysis revealed a cluster of wall-associated receptor kinase (WAK) like genes. Using VIGS, TILLING and genetic complementation, we demonstrated that one of these candidates corresponds to Stb6. Disease susceptibility in Courtot results most likely from a point-mutation in the Stb6 kinase domain leading to a loss of defense signaling. Stb6 allele mining revealed an ancient origin of this gene and the predominance of a major resistance haplotype. Furthermore, genotyping a large wheat collection using diagnostic markers showed that Stb6 is present in nearly half of European cultivated wheat. Stb6 is the first gene to be cloned that confers resistance to the STB disease. Isolation of this gene is the first step toward in depth characterization of the wheat - Z. tritici molecular interactions.

Many race- or isolate-specific disease resistance responses of plants toward pathogens (incompatible interactions) invoke hypersensitive response (HR)-like programmed cell death (PCD) and the coordinated activation of mitogen-activated protein kinases homologous with Arabidopsis (Arabidopsis thaliana) AtMPK6 and AtMPK3 (or tobacco [Nicotiana tabacum] SIPK and WIPK), respectively. Resistance of wheat (Triticum aestivum) leaves to the necrotrophic fungal pathogen Mycosphaerella graminicola can also operate at an isolate/cultivar-specific level. We confirm here that resistance is achieved without any sign of HR-like PCD during the incompatible interaction. Instead, PCD is strictly associated with the compatible interaction and is triggered during disease symptom expression. A strong transcriptional activation of TaMPK3, the wheat homolog of Arabidopsis AtMPK3, was observed immediately preceding PCD and symptom development in the compatible interaction. Generation and use of TaMPK3- and TaMPK6-specific antibodies on western blots and in coupled immunoprecipitation-protein kinase assays demonstrated that the TaMPK3 protein also accumulated, and was subsequently posttranslationally activated, during the compatible interaction in parallel to PCD. In contrast, no increase in expression, protein levels, or posttranslational activation of TaMPK6 was observed at any stage of either compatible or incompatible interactions. However, the protein levels of TaMPK6 became markedly reduced during the compatible interaction coincident with the onset of TaMPK3 protein accumulation. These data highlight the emerging similarity between the signaling pathways triggered in a host plant during successful infection by a necrotrophic fungal pathogen and the resistance responses normally effective against biotrophs.

La réponse immunitaire innée est une forme ancienne de défense anti-microbienne très bien caractérisée chez les vertébrés et les insectes et, également suggérée jouer un rôle important chez les plantes. La perception du non-soi est médiée par des récepteurs du type toll (TLRs) qui reconnaissent des motifs microbiens (PAMPs) absents chez l'hâte. Ces motifs (lipopolysaccharide bactérien, peptidoglycane, mannanes etc..) sont hautement conservés chez les microbes, fonctionellement indispensables et peu assujettis aux mutations. Le genre phytophthora (oomycètes) regroupe environ 70 espèces d'organismes qui sont pour la plupart de redoutables phytopathogènes. Certaines plantes comme le persil sont non-hôtes: Toutefois, les bases moléculaires de la résistance non-hôte sont mal connues à l'heure actuelle. Un peptide de 13 amino acides (Pep-13) dérivé d'une glycoprotéine de 42 kDa (GP42), localisée dans la paroi hyphale de Phytophthora sojae élicite des réponses de défense chez le persil; incluant des modifications de la perméabilité membranaire, un stress oxydatif ainsi que l'activation des gènes de défense et la production de phytoalexines (Nürnberger et al., 1994). GP42 est une transglutaminase. Les transglutaminases (R-glutaminyl-peptide: amine g-glutamyltransferase, EC 2.3.2.13) sont des enzymes Ca2+ -dependents qui catalysent la formation de liaisons isopeptidiques intra- ou inter-protéines. Les transglutaminases ont été très bien décrites au sein des mammifères mais ne présentent pas d'homologies de séquences avec GP42 qui appartient à l'évidence à une nouvelle famille de ces enzymes. Des mutations au sein de la séquence du Pep-13 ont montré que les mêmes amino acides sont à la fois importants pour l'activité élicitrice et l'activité enzymatique de GP42. L'existence de protéines homologues à GP42 chez toutes les espèces de Phytophthora testées peut expliquer, ou du moins contribuer, à la résistance non-hôte chez le persil. Récemment, des études faites chez la pomme de terre et la tomate ont montré que Pep-13 était capable d'induire l'expression de gènes de défense avec les mêmes spécificités que celles décrites pour le persil. Ces résultats montrent que l'aptitude à reconnaître Phytophthora spp. est partagée par de nombreuses espèces végétales appartenant à différentes familles et que Pep-13 rempli les caractéristiques d'un PAMP pour l'activation des réponses de défense innées chez les plantes. (Texte intégral)