Abstract
Chitin is commonly found in fungal cell walls and is one of the well-studied microbe/pathogen-associated molecular patterns. Previous studies showed that lysin motif (LysM)-containing proteins are essential for plant recognition of chitin, leading to the activation of plant innate immunity. In Arabidopsis (Arabidopsis thaliana), the LYK1/CERK1 (for LysM-containing receptor-like kinase1/chitin elicitor receptor kinase1) was shown to be essential for chitin recognition, whereas in rice (Oryza sativa), the LysM-containing protein, CEBiP (for chitin elicitor-binding protein), was shown to be involved in chitin recognition. Unlike LYK1/CERK1, CEBiP lacks an intracellular kinase domain. Arabidopsis possesses three CEBiP-like genes. Our data show that mutations in these genes, either singly or in combination, did not compromise the response to chitin treatment. Arabidopsis also contains five LYK genes. Analysis of mutations in LYK2, -3, -4, or -5 showed that LYK4 is also involved in chitin signaling. The lyk4 mutants showed reduced induction of chitin-responsive genes and diminished chitin-induced cytosolic calcium elevation as well as enhanced susceptibility to both the bacterial pathogen Pseudomonas syringae pv tomato DC3000 and the fungal pathogen Alternaria brassicicola, although these phenotypes were not as dramatic as that seen in the lyk1/cerk1 mutants. Similar to LYK1/CERK1, the LYK4 protein was also localized to the plasma membrane. Therefore, LYK4 may play a role in the chitin recognition receptor complex to assist chitin signal transduction and plant innate immunity.
Highlights
Chitin is commonly found in fungal cell walls and is one of the well-studied microbe/pathogen-associated molecular patterns
We used reverse transcription (RT)-PCR to investigate whether the expression levels of chitin-responsive genes (CRGs) were changed in these mutants after treatment with chitin
The results showed that the mutants responded in a similar fashion to the wild type (Supplemental Fig. S1), suggesting that, contrary to the case of the rice OsCEBiP (Kaku et al, 2006), these genes are not involved in chitin signaling in Arabidopsis
Summary
Chitin is commonly found in fungal cell walls and is one of the well-studied microbe/pathogen-associated molecular patterns. In rice (Oryza sativa), CEBiP (for chitin elicitor-binding protein) was shown to be important in the activation of plant innate immunity upon chitin addition (Kaku et al, 2006) This protein has an extracellular domain containing two LysMs and a single transmembrane domain. In addition to these plant receptors, fungal pathogens appear to employ secreted LysM domain-containing proteins to either compete for binding chitin with the plant chitin receptors or to coat the fungal cell wall to prevent the release of elicitor-active chitin fragments by plant chitinases (van den Burg et al, 2006; de Jonge and Thomma, 2009; Stergiopoulos and de Wit, 2009; de Jonge et al, 2010) These plant and fungal studies point to a central role for LysM proteins in chitin recognition and modulating plant innate immunity in response to fungal infection. Proteins similar to rice CEBiP (i.e. CEBiP-like1, -2, and -3) are encoded by the Arabidopsis genome, but so far their role in chitin signaling has not been established
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