Investigation of upstream and downstream events in CERK1-mediated signaling in plant immunity

Abstract

As one part of a two-tiered pathogen-detection system in plants, PTI is not only sufficient to ward off most microbes, but also contributes to basal immunity during infection. Although PGN and the LYM1-LYM3-CERK1 receptor complex have been identified as one PAMP-PRR pair in Arabidopsis, upstream and downstream events of PGN perception remain elusive. Specifically, it is unclear whether and, if so, how PGN supermolecules are processed prior to perception by the LYM1-LYM3-CERK1 receptor complex. Moreover, the mechanisms linking PGN perception with diverse downstream immune responses are little understood. In this study, a lysozyme-like hydrolase (lysozyme 1, LYS1) was identified as an enhancer in PGN-induced immunity. Upon bacterial infection or exposure to bacterial patterns, Arabidopsis produces LYS1 to release soluble PGN fragments from insoluble bacterial cell walls. LYS1-released soluble PGNs trigger typical immunity-associated responses, such as medium alkalinization and up-regulation of resistance-related genes. Importantly, these immune responses are dependent on the PGN receptor complex. LYS1 mutants exhibit super-susceptibility to bacterial infection similar to that oberved in PGN receptor mutants. We propose that plants employ hydrolytic activities for the decomposition of complex bacterial structures and that the subsequent generation of soluble patterns might aid PRR-mediated immune activation in cell layers adjacent to infection sites. In addition, we identified the calcium-dependent protein kinase CPK15 as an interactor of CERK1, which is not only involved in PGN perception but also in chitin recognition. CPK15 was shown to interact with CERK1 in the yeast-two-hybrid system and in plant tissue, and cpk15 mutants displayed reduced ROS accumulation upon chitin treatment. These findings indicate that CPK15 is involved in CERK1-mediated PTI signaling. Summing up, this study aimed at improving our understanding of PGN- and chitin-triggered immunity by identifying and characterizing critical components of the plant immune system, such as LYS1, involved in upstream events of PGN perception, and CPK15, involved in dowstream events of CERK1-mediated glucan perception.