RALF22 promotes plant immunity and amplifies Pep3 immune signal

The rapid alkalinization factors (RALFs) have been reported to dampen pathogen-associated molecular pattern (PAMP)-triggered immunity via suppressing PAMP-induced complex formation between pattern recognition receptor (PRR) and its co-receptor BAK1. However, the direct and positive role of RALFs in plant immunity remains largely unknown. Here we report the direct and positive roles of a typical RALF, RALF22, in plant immunity. RALF22 alone directly elicited a variety of typical immune responses and triggered resistance against the devastating necrotrophic Fungal pathogen Sclerotinia sclerotiorum in a FERONIA (FER) dependent manner. LORELEI (LRE)-like glycosylphosphatidylinositol (GPI)-anchored protein 1 (LLG1) and NADPH Oxidase RBOHD were required for RALF22-elicited Reactive Oxygen Species (ROS) generation. Mutation of cysteines conserved in C terminus of RALFs abolished while constitutive formation of two disulfide bridges between these cysteines promoted the RALF22-elicited ROS production and resistance against S. sclerotiorum, demonstrating the requirement of these cysteines in the functions of RALF22 in plant immunity. Additionally, RALF22 amplified Pep3-induced immune signal by dramatically increasing the PROPEP3 transcript and protein abundance. Supply with RALF22 induced resistance against S. sclerotiorum in Brassica crop Plants. Collectively, our results revealed that RALF22 triggered immune responses and augmented Pep3-induced immune signal in a FER-dependent manner, and exhibited the potential to be exploited as an immune elicitor in crop protection. This article is protected by copyright. All rights reserved.

Sclerotinia sclerotiorum; FER; Pep3; RALF22; plant immunity; reactive oxygen species.