Receptor-like kinases and pathogen-associated molecular patterns perception in Arabidopsis

Comparable to the innate immune response in mammals and insects, plants possess highly specific and sensitive recognition systems for pathogen-associated molecular patterns (PAMPs), such as bacterial flagellin. In Arabidopsis, perception of flagellin occurs via recognition of the most conserved domain in its N-terminus, represented by the peptide flg22. Perception of this elicitor-active domain depends on the LRR receptor kinase FLS2. Here, using transcriptome analysis in Arabidopsis, we first present evidences that treatment with flg22, bacteria or avirulence proteins are similar, and that virulent bacteria are likely to suppress flg22-induced responses (Chapter 1). Second, we demonstrated that flagellin perception contributes to the basal resistance against bacteria, as fls2 mutants are more susceptible to bacterial infection, proving for the first time that perception of a single PAMP is sufficient to restrict bacterial growth (Chapter 2). This study also showed that PAMPs other than flagellin trigger resistance against virulent bacteria. Wild-type and fls2 mutants both display enhanced resistance when treated with crude bacterial extracts, even devoid of elicitor-active flagellin, indicating the existence of functional perception systems for PAMPs other than flagellin. One of these novel PAMPs was just identified as the elongation factor EF-Tu, and the corresponding active peptidic epitope identified (Appendix 1). FLS2 constituted so far the only PAMP receptor identified in Arabidopsis. Using ATH1 Affymetrix arrays, we identified about 1000 genes whose expression was induced 30 minutes after flg22 treatment (Chapter 2). Strikingly, among these induced genes there were 106 RLK out of the 610 RLK genes present in Arabidopsis. We hypothesize that the perception of a single PAMP (e.g. flg22) could enhance the sensitivity of the plant to microbial stimuli sensing the presence of invading microorganisms. In particular, some of the induced RLKs might be involved in the recognition of other PAMPs. We generated a collection of mutants for the induced LRR-RLKs to test the implication of the corresponding proteins in innate immune responses (Chapter 3.1). This approach led to the discovery of the EF-Tu receptor (Chapter 3.2) and suggested a new role for the previously described BAK1 gene in flg22- and probably elf18-signaling (Chapter 3.3).