Homeostasis of Arabidopsis R protein RPS2 is negatively regulated by the RING-type E3 ligase MUSE16

The homeostasis of resistance (R) proteins in plants must be tightly regulated to ensure precise activation of plant immune responses upon pathogen infection, while avoiding autoimmunity and growth defects when plants are uninfected. It is known that CPR1, an F-box protein in the SCF E3 complex, functions as a negative regulator of plant immunity through targeting the resistance (R) proteins SNC1 and RPS2 for degradation. However, whether these R proteins are also targeted by other E3 ligases is unclear. Here, we isolated Arabidopsis MUSE16, which encodes a RING-type E3 ligase, from a forward genetic screen and suggest that it is a negative regulator of plant immunity. Unlike CPR1, knocking out MUSE16 alone in Arabidopsis is not enough to result in defense-related dwarfism, since only RPS2 out of the tested R proteins accumulated in the muse16 mutants. Thus, our study identifies a novel E3 ligase involved in the degradation of nucleotide-binding and leucine-rich repeat (NLR) R proteins, support the idea that ubiquitin-mediated degradation is a fine-tuned mechanism for regulating the turnover of R proteins in plants, and that the same R protein can be targeted by different E3 ligases for regulation of its homeostasis.