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

E3 ubiquitin ligases play important roles in plant immunity, but their role in soybean has not been investigated previously. Here, we used Bean pod mottle virus (BPMV)-mediated virus-induced gene silencing (VIGS) to investigate the function of Gm SAUL1 (Senescence-Associated E3 Ubiquitin Ligase 1) homologs in soybean. When two closely related SAUL1 homologs were silenced simultaneously, the soybean plants displayed autoimmune phenotypes, which were significantly alleviated by high temperature, suggesting that Gm SAUL1a/1b might be guarded by an R protein. Interestingly, silencing GmSAUL1a/1b resulted in the decreased activation of Gm MPK6, but increased activation of Gm MPK3 in response to flg22, suggesting that the activation of Gm MPK3 is most likely responsible for the activated immunity observed in the Gm SAUL1a/1b-silenced plants. Furthermore, we provided evidence that Gm SAUL1a is a bona fide E3 ligase. Collectively, our results indicated that Gm SAUL1 plays a negative role in regulating cell death and immunity in soybean.