Soybean SAUL1, a Bona Fide U-Box E3 Ligase, Negatively Regulates Immunity Likely through Repressing the Activation of MPK3.

E3 ubiquitin ligases play important roles in plant immunity, but their role in soybean has not been investigated previously. Here, we used (BPMV)-mediated virus-induced gene silencing (VIGS) to investigate the function of SAUL1 (Senescence-Associated E3 Ubiquitin Ligase 1) homologs in soybean. When two closely related homologs were silenced simultaneously, the soybean plants displayed autoimmune phenotypes, which were significantly alleviated by high temperature, suggesting that SAUL1a/1b might be guarded by an R protein. Interestingly, silencing resulted in the decreased activation of MPK6, but increased activation of MPK3 in response to flg22, suggesting that the activation of MPK3 is most likely responsible for the activated immunity observed in the SAUL1a/1b-silenced plants. Furthermore, we provided evidence that SAUL1a is a bona fide E3 ligase. Collectively, our results indicated that SAUL1 plays a negative role in regulating cell death and immunity in soybean.