Lysin motif–containing proteins LYP4 and LYP6 play dual roles in peptidoglycan and chitin perception in rice innate immunity
Bing Liu^§, Jian-Feng Li^§, Ying Ao^§, Jinwang Qu, Zhangqun Li, Jianbin Su, Yang Zhang, Jun Liu, Dongru Feng, Kangbiao Qi, Yanming He, Jinfa Wang and Hong-Bin Wang*

State Key Laboratory of Biocontrol, Key Laboratory of Gene Engineering of the Ministry of Education and Guangdong Key Laboratory of Plant Resources, School of Life Sciences, Sun Yat-sen University, 510275 Guangzhou, People’s Republic of China.
^§These authors contributed equally to this work.
*Corresponding Author：Tel/ Fax: (+8620) 8403 9179; E-mail: wanghb@mail.sysu.edu.cn

Abstract
Plant innate immunity relies on successful detection of microbe-associated molecular patterns (MAMPs) of invading microbes via pattern recognition receptors (PRRs) at the plant cell surface. Here, we report two homologous rice (Oryza sativa) lysin motif–containing proteins, LYP4 and LYP6, as dual functional PRRs sensing bacterial peptidoglycan (PGN) and fungal chitin. Live cell imaging and microsomal fractionation consistently revealed the plasma membrane localization of these proteins. Transcription of these two genes could be induced rapidly upon exposure to bacterial pathogens or diverse MAMPs. Both proteins selectively bound PGN and chitin but not lipopolysaccharide (LPS) in vitro. Accordingly, silencing of either LYP specifically impaired PGN- or chitin- but not LPS-induced defense responses in rice, including reactive oxygen species generation, defense gene activation, and callose deposition, leading to compromised resistance against bacterial pathogen Xanthomonas oryzae and fungal pathogen Magnaporthe oryzae. Interestingly, pretreatment with excess PGN dramatically attenuated the alkalinization response of rice cells to chitin but not to flagellin; vice versa, pretreatment with chitin attenuated the response to PGN, suggesting that PGN and chitin engage overlapping perception components in rice. Collectively, our data support the notion that LYP4 and LYP6 are promiscuous PRRs for PGN and chitin in rice innate immunity.

Key Words: LYP4; LYP6; Lysin Motif; Peptidoglycan; Chitin; Rice Innate Immunity