The XA21-associated Kinase 1 (XAK1) regulates immunity mediated by the pattern-recognition receptor XA21
Xuewei Chen1,2,3,a, Shimin Zuo1,3,a, Mawsheng Chern1,3, Patrick E. Canlas1,3, Deling Ruan1,3, Benjamin Schwessinger1, Pamela C. Ronald1,2,3,4,5*
1 Department of Plant Pathology, University of California, Davis, California 95616, USA.
2 Rice Research Institute, Sichuan Agricultural University at Wenjiang, Chengdu, Sichuan 611130, China .
3 Joint Bioenergy Institute, Emeryville, California 94710, USA.
4 Department of Plant Molecular Systems Biotechnology and Crop Biotech Institute, Kyung Hee University, Yongin 446-701, Korea.
5 Department of Genetics, Scripps Research Institute, La Jolla, California 92037, USA.
*Corresponding author: Pamela C. Ronald (email@example.com) ; Phone: 530-752-1654, Fax: 530-752-6088.
a, These authors contribute to this work equally.
Plants sense potential microbial invaders through pattern-recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs). The early molecular events occurring to the PAMP-triggered immunity (PTI) via host PRRs are largely unknown. In rice, the model of monocot species, the leucine-rich repeat receptor kinase XA21 acts as a PRR to recognize the bacterial PAMP Ax21 (Activator of XA21-mediated immunity) and contribute to innate immunity against Xanthomonas oryzae pv. oryzae (Xoo), the causal bacterial pathogen to rice. Here we report that XAK1 (XA21-associated kinase 1), one of two rice somatic embryogenesis receptor kinases (SERKs), is essential for XA21-mediated innate immunity. Xa21 plants silenced for Xak1 are strongly compromised in XA21-mediated resistance to Xoo indicating that XAK1 acts as a positive regulator in XA21-mediated PTI signaling. XAK1 is a function kinase capable of the abilities of auto-phosphorylation and trans-phosphorylation on XA21. We provide evidence supporting that XAK1 interacts with XA21 in yeast and in vivo of rice. We also found that XAK1 is critical for the innate immunity mediated by XA3, potential rice PRR which confers resistance to Xoo. In addition, we found that XAK1 is involved in developmental regulation, likely through interaction with OsBRI1 to affect the brassinolide signaling. These results suggest that XAK1 plays dual roles in innate immunity by regulating PRR-dependent signaling and in development through the BL hormone in rice. Most importantly, the association of XAK1 with the PRR XA21 is ligand-independent but kinase-dependent which is different from the associations of BAK1 (the ortholog of XAK1 in Arabidposis) with PRRs are ligand-independent but kinase-independent in Arabidposis. Thus, the early molecular events occurring for XAK1 regulating XA21-mediated PAMP-triggerred immunity (PTI) in rice is different from that for BAK1 regulating PRRs-mediated PTI in dicots. These findings shed light on the mechanism of PTI-mediated immunity in rice.
Key words: Rice, innate immunity, Xoo, development, brassinolide