Molecular components of PAMP-triggered oxidative burst in plant immunity

Abstract

Reactive oxygen species (ROS) are important molecules that are rapidly generated in response to abiotic and biotic stimuli and which regulate diverse physiological processes such as stomatal aperture and cell death. Plant immunity involves the detection of pathogen-associated molecular patterns (PAMPs) through cognate pattern recognition receptors (PRRs). Perception of PAMPs induces an extracellular oxidative burst, which requires the function of the NADPH oxidase AtRbohD. However, little is known about the regulation of PAMP-elicited ROS and its role in plant PAMP-triggered immunity. We investigated ROS production mediated by the FLS2 receptor kinase responsible for the detection of bacterial flagellin (flg22) in Arabidopsis to elucidate components of early flg22 signaling. Rboh proteins are assumed to predominantly mediate extracellular ROS production in plants. We observed AtRbohD to be rate limiting for flg22-elicited ROS production. Moreover, bacterial multiplication monitored at early stages of infection with a disarmed but pathogenic Pseudomonas syringae strain indicated ROS accumulation important for plant immunity. A forward genetic screen led to the isolation of rio1 to rio5 mutants, all of which were severely reduced in flg22-induced ROS production. Rio1, rio2 and rio3 carry mutations in FLS2 and its co-receptor BAK1, respectively. The responsible gene mutations of rio4 and rio5 remain to be identified in a subsequent mapping approach. A candidate gene approach revealed that flg22-stimulated oxidative burst was specifically inhibited in ethylene signaling mutants. Impaired ROS production in etr1 and ein2 mutants could be partially rescued by chemical interference with ethylene accumulation. Notably, wounding partially complemented the ROS reduced phenotype. Furthermore, accumulation of FLS2 but not BAK1 was in part dependent on ethylene signaling. Bacterial multiplication at early time points was significantly enhanced in ethylene signaling mutants indicating the importance of PAMP-triggered ROS production in plant defence responses