Functional Characterization of CEBiP and CERK1 Homologs in Arabidopsis and Rice Reveals the Presence of Different Chitin Receptor Systems in Plants

Abstract

Chitin is a representative microbe-associated molecular pattern (MAMP) molecule for various fungi and induces immune responses in many plant species. It has been clarified that the chitin signaling in rice requires a receptor kinase OsCERK1 and a receptor-like protein (Os)CEBiP, which specifically binds chitin oligosaccharides. On the other hand, Arabidopsis requires a receptor kinase (At)CERK1 for chitin signaling but it is not clear whether the plant also requires a CEBiP-like molecule for chitin perception/signaling. To clarify the similarity/difference of the chitin receptor in these two model plants, we first characterized CEBiP homologs in Arabidopsis. Only one of three CEBiP homologs, AtCEBiP (LYM2), showed a high-affinity binding for chitin oligosaccharides similar to rice CEBiP. AtCEBiP also represented the major chitin-binding protein in the Arabidopsis membrane. However, the single/triple knockout (KO) mutants of Arabidopsis CEBiP homologs and the overexpressor of AtCEBiP showed chitin-induced defense responses similar to wild-type Arabidopsis, indicating that AtCEBiP is biochemically functional as a chitin-binding protein but does not contribute to signaling. Studies of the chitin binding properties of the ectodomains of At/OsCERK1 and the chimeric receptors consisting of ecto/cytosolic domains of these molecules indicated that AtCERK1 is sufficient for chitin perception by itself.

LYM2-dependent chitin perception limits molecular flux via plasmodesmata

“…2). These results support previous work that demonstrated that chitin-triggered, CERK1-dependent increases in gene expression are normal in a lym1,2,3 triple knockout (11). Chitin-induced PD flux reduction occurs in cerk1-2 mutant plants, indicating that as LYM2 is not required for CERK1-mediated responses, neither is CERK1 required for LYM2-mediated responses (Fig.…”

“…In rice, chitin perception employs the receptor-like protein CEBiP, but a similar function for its Arabidopsis homolog, LYM2, has been elusive despite evidence that LYM2 is a chitin-binding protein (7,11). This study has identified that LYM2 has a specific role that mediates a reduction in molecular flux through PD in the presence of chitin.…”

“…The observation that CERK1 is a target for the bacterial effector AvrPtoB (9) and functions as part of a peptidoglycan receptor system (10) indicates that it also plays a role in responses triggered by pathogens that do not contain chitin. The receptor-like proteins LYM1 and LYM3 are close relatives of LYM2 and have been identified as additional components of the peptidoglycan receptor system in Arabidopsis (10); however, unlike CERK1, they do not bind chitin (11). LYM2 was identified in chitin pull-down assays, suggesting that it either binds chitin itself or is a component of a chitin-binding protein complex (7).…”

“…LYM2 was identified in chitin pull-down assays, suggesting that it either binds chitin itself or is a component of a chitin-binding protein complex (7). Despite this affinity for chitin (11), there has been no direct evidence that LYM2 functions in chitin perception.…”

Chitin-induced activation of immune signaling by the rice receptor CEBiP relies on a unique sandwich-type dimerization

“…In Arabidopsis, CERK1 was shown to bind chitin and trigger immune responses as a kind of "all-in-one" receptor. On the other hand, CEBiP seems to play a major role in the perception of chitin in rice, as the knockdown of CEBiP almost abolished the binding of a radio-labeled chitin oligosaccharide to the plasma membrane, whereas OsCERK1 was shown not to bind chitin (6,12). Liu …”