STRESS INDUCED FACTOR 2, a Leucine-Rich Repeat Kinase Regulates Basal Plant Pathogen Defense

Abstract

Protein kinases play fundamental roles in plant development and environmental stress responses. Here, we identified the <i>STRESS INDUCED FACTOR</i> (<i>SIF</i>) gene family, which encodes four leucine-rich repeat receptor-like protein kinases in Arabidopsis (<i>Arabidopsis thaliana</i>). The four genes, <i>SIF1</i> to <i>SIF4</i>, are clustered in the genome and highly conserved, but they have temporally and spatially distinct expression patterns. We employed Arabidopsis <i>SIF</i> knockout mutants and overexpression transgenics to examine SIF involvement during plant pathogen defense. <i>SIF</i> genes are rapidly induced by biotic or abiotic stresses, and SIF proteins localize to the plasma membrane. Simultaneous knockout of <i>SIF1</i> and <i>SIF2</i> led to improved plant salt tolerance, whereas <i>SIF2</i> overexpression enhanced PAMP-triggered immunity and prompted basal plant defenses, significantly improving pathogen resistance. Furthermore, <i>SIF2</i> overexpression plants exhibited up-regulated expression of the defense-related genes <i>WRKY53</i> and <i>flg22-INDUCED RECEPTOR-LIKE KINASE1</i> as well as enhanced MPK3/MPK6 phosphorylation upon pathogen and elicitor treatments. The expression of the calcium signaling-related gene <i>PHOSPHATE-INDUCED1</i> also was enhanced in the <i>SIF2</i>-overexpressing lines upon pathogen inoculation but repressed in the <i>sif2</i> mutants. Bimolecular fluorescence complementation demonstrates that the BRI1-ASSOCIATED RECEPTOR KINASE1 protein is a coreceptor of the SIF2 kinase in the signal transduction pathway during pathogen invasion. These findings characterize a stress-responsive protein kinase family and illustrate how SIF2 modulates signal transduction for effective plant pathogenic defense.