Arabidopsis heterotrimeric G proteins regulate immunity by directly coupling to the FLS2 receptor.

Xiangxiu Liang,Miaomiao Ma,Pingtao Ding,Yuelin Zhang,Xiaojuan Zhang,Jian-Min Zhou,She Chen,Kehui Lian,Jinlong Wang,Lin Li,Meng Li,Lei Li

doi:10.7554/elife.13568

Abstract

The Arabidopsis immune receptor FLS2 perceives bacterial flagellin epitope flg22 to activate defenses through the central cytoplasmic kinase BIK1. The heterotrimeric G proteins composed of the non-canonical Gα protein XLG2, the Gβ protein AGB1, and the Gγ proteins AGG1 and AGG2 are required for FLS2-mediated immune responses through an unknown mechanism. Here we show that in the pre-activation state, XLG2 directly interacts with FLS2 and BIK1, and it functions together with AGB1 and AGG1/2 to attenuate proteasome-mediated degradation of BIK1, allowing optimum immune activation. Following the activation by flg22, XLG2 dissociates from AGB1 and is phosphorylated by BIK1 in the N terminus. The phosphorylated XLG2 enhances the production of reactive oxygen species (ROS) likely by modulating the NADPH oxidase RbohD. The study demonstrates that the G proteins are directly coupled to the FLS2 receptor complex and regulate immune signaling through both pre-activation and post-activation mechanisms.

Highlights

As an intensely studied Pattern Recognition Receptor (PRR) in plants, FLS2 serves as an excellent model understanding plant innate immune signaling and receptor kinases in general (Macho and Zipfel, 2014)
A comparison of xlg2 and agb1 mutant showed that the two mutants were compromised in flg22-induced reactive oxygen species (ROS) burst and resistance against Pst, supporting that they act together to regulate FLS2 immunity
Because AGG1/2, but not AGG3 and GPA1, are required for flg22-induced defense responses and Pst resistance (Liu et al, 2013; Torres et al, 2013), these results confirmed that the heterotrimeric G proteins required for FLS2 signaling and Pst resistance include the non-canonical Ga proteins XLG2/3, Gb protein AGB1, and Gg proteins AGG1/2

Summary

Introduction

As an intensely studied Pattern Recognition Receptor (PRR) in plants, FLS2 serves as an excellent model understanding plant innate immune signaling and receptor kinases in general (Macho and Zipfel, 2014). It forms a dynamic complex with the co-receptor BAK1 and the receptor-like cytoplasmic kinase BIK1 to perceive a conserved bacterial flagellar epitope, flg, to activate a variety of defense responses (Chinchilla et al, 2007; Heese et al, 2007; Lu et al, 2010; Zhang et al, 2010; Sun et al, 2013). Plants encode extra-large G proteins (XLGs, encoded by XLG1, XLG2, and XLG3 in Arabidopsis) that carry a variable N-terminal domain and a C-terminal Ga domain

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