Contribution d'une "Calmodulin-like protein" CML9, et d'un facteur de transcription de type GARP PRR2, à la mise en place des réactions de défense chez arabidopsis thaliana

Chez les plantes, le calcium (Ca2+) est considéré comme un messager secondaire important permettant d'intégrer à l'échelle cellulaire les fluctuations de l'environnement. Des protéines senseurs du Ca2+, telle la calmoduline (CaM), permettent de décoder ce message. A côté de cette famille de protéines présentes chez tous les Eucaryotes, une famille de protéines apparentées à la CaM mais spécifiques aux plantes, les CMLs pour Calmodulin-like proteins, existent. A l'heure actuelle, peu de données sont disponibles sur cette famille de protéines. L'objectif de ce travail de thèse a concerné CML9, une CML d'Arabidopsis thaliana. Des approches génétiques nous ont permis de montrer que CML9 contribue à la défense des plantes en réponse à la bactérie P. syringae DC3000 (Pst DC3000). La recherche des partenaires de CML9 a permis d'identifier PRR2, un régulateur de réponse atypique portant un motif GARP de liaison à l'ADN. L'analyse fonctionnelle réalisée sur différentes lignées perte ou gain de fonction a montré que PRR2 est un régulateur positif de la défense en réponse à l'infection par Pst DC3000. La combinaison de différentes approches transcriptomiques et de phénotypage a révélé que PRR2 contribue à la mise en place des réactions de défense dans une voie de signalisation dépendante de l'acide salicylique. Ces travaux indiquent que PRR2 est un régulateur positif de l'accumulation de callose et de camalexine. L'ensemble de ces données ont permis de mettre en évidence une nouvelle voie de signalisation impliquant une calmodulin-like protein et un facteur de transcription spécifiques au règne végétal au cours de l'immunité.Calcium signals are one of the primary mediators of plant defence responses against pathogens. Although rises in cytosolic calcium levels are well described during the plant-pathogen interactions, how calcium signals are sensed and relayed into downstream events is poorly understood. Studies on calcium sensors such as calmodulin and calmodulin-like (CML) proteins have demonstrated a critical role of these calcium signalling components in plants. In this work, evidences were provided indicating that the calmodulin-like protein CML9 is involved in plant immunity in Arabidopsis. CML9 expression is up-regulated during infection with the virulent pathogen Pseudomonas Syringae pv. tomato DC3000 (Pst DC3000). Recently, using cml9 knock-out mutants and CML9 overexpressors lines, we have demonstrated that CML9 contributes to plant defence mechanisms against Pst DC3000. Search for CML9 interacting partners was performed and among the CML9 binding proteins, a putative GARP transcription factor PRR2 was identified. Then, in my work, a genetic analysis was carried-out and shows that PRR2 is a positive regulator of defence responses when challenged with Pst DC3000. The combination of transcriptomic and phenotyping analyses have demonstrated that PRR2 contributes to plant immunity through a salicylic acid signalling pathway. Moreover, our analyses indicate that PRR2 is a positive regulator of camalexin and callose production in response to Pst DC3000. These findings contribute to decipher a novel calcium signalling pathway mediated by a plant specific calcium sensor and a potential transcription factor during biotic stress

Chitin perception in plasmodesmata characterizes submembrane immune-signaling specificity in plants

The plasma membrane (PM) is composed of heterogeneous subdomains, characterized by differences in protein and lipid composition. PM receptors can be dynamically sorted into membrane domains to underpin signaling in response to extracellular stimuli. In plants, the plasmodesmal PM is a discrete microdomain that hosts specific receptors and responses. We exploited the independence of this PM domain to investigate how membrane domains can independently integrate a signal that triggers responses across the cell. Focusing on chitin signaling, we found that responses in the plasmodesmal PM require the LysM receptor kinases LYK4 and LYK5 in addition to LYM2. Chitin induces dynamic changes in the localization, association, or mobility of these receptors, but only LYM2 and LYK4 are detected in the plasmodesmal PM. We further uncovered that chitin-induced production of reactive oxygen species and callose depends on specific signaling events that lead to plasmodesmata closure. Our results demonstrate that distinct membrane domains can integrate a common signal with specific machinery that initiates discrete signaling cascades to produce a localized response

A calmodulin-like protein regulates plasmodesmal closure during bacterial immune responses

Plants sense microbial signatures via activation of pattern recognition receptors (PPRs), which trigger a range of cellular defences. One response is the closure of plasmodesmata, which reduces symplastic connectivity and the capacity for direct molecular exchange between host cells. Plasmodesmal flux is regulated by a variety of environmental cues but the downstream signalling pathways are poorly defined, especially the way in which calcium regulates plasmodesmal closure. Here, we identify that closure of plasmodesmata in response to bacterial flagellin, but not fungal chitin, is mediated by a plasmodesmal-localized Ca2+ -binding protein Calmodulin-like 41 (CML41). CML41 is transcriptionally upregulated by flg22 and facilitates rapid callose deposition at plasmodesmata following flg22 treatment. CML41 acts independently of other defence responses triggered by flg22 perception and reduces bacterial infection. We propose that CML41 enables Ca2+ -signalling specificity during bacterial pathogen attack and is required for a complete defence response against Pseudomonas syringae.Bo Xu, Cecilia Cheval, Anuphon Laohavisit, Bradleigh Hocking, David Chiasson, Tjelvar S. G. Olsson, Ken Shirasu, Christine Faulkner and Matthew Gilliha

Involvement of RD20, a member of caleosin family, in ABA-mediated regulation of germination in Arabidopsis thaliana

The RD20 gene encodes a member of the caleosin family, which is primarily known to function in the mobilization of seed storage lipids during germination. In contrast to other caleosins, RD20 expression is early-induced by water deficit conditions and we recently provided genetic evidence for its positive role in drought tolerance in Arabidopsis. RD20 is also responsive to pathogen infection and is constitutively expressed in diverse tissues and organs during development suggesting additional roles for this caleosin. This addendum describes further exploration of phenotypic alterations in T-DNA insertional rd20 mutant and knock-out complemented transgenic plants in the context of early development and susceptibility to a phytopathogenic bacteria. We show that the RD20 gene is involved in ABA-mediated inhibition of germination and does not play a significant role in plant defense against Pseudomonas syringae