Mutation of a Nucleotide-Binding Leucine-Rich Repeat Immune Receptor-Type Protein Disrupts Immunity to Bacterial Blight

Most characterized plant resistance (R) proteins belong to the nucleotide-binding (NB) domain and leucine-rich repeat-containing (NLR) family. NLRs are present in an auto-inhibited state in the absence of specific pathogens, while gain-of-function mutations in NLRs usually cause autoimmunity. Here, we show that a gain-of-function mutation, weaker defense (wed), which caused a phenylalanine-to-leucine substitution in the NB domain of a typical NLR in rice (Oryza sativa), led to enhanced susceptibility to Xoo (Xanthomonas oryzae pv. oryzae). The unexpected accumulation of salicylic acid (SA), along with downregulation of NONEXPRESSOR OF PR1 (NPR1), in wed indicates the potential presence of a feedback regulation loop of SA biosynthesis in rice. Epistasis analyses illustrated that SA accumulation and the NLR-associated components RAR1, OsRac1, and PhyB are indispensable for the wed phenotypes. Intriguingly, besides pattern-triggered immunity, effector-triggered immunity conferred by different R proteins, including Xa3/Xa26, Xa4, and Xa21, was also disturbed by wed to a certain extent, indicating the existence of shared regulatory mechanisms for various defense systems. The identification of wed therefore provides a unique system for genetic dissection of shared immune signaling pathways activated by different types of immune receptors.