Rice <scp>RING E3</scp> Ligase <scp>OsRMT1</scp> Negatively Regulates Salt Tolerance by Degrading the Canonical Salt‐Responsive Protein <scp>OsMBL1</scp>

Excessive salinity is a major non-biological stressor that hinders plant development and reduces agricultural productivity. In rice, a jacalin-related mannose-binding lectin (OsMBL1) is known to be a canonical salt-responsive factor, but its biological function and regulatory mechanism in response to salt stress remain unclear. In the current study, we show that OsMBL1 is a positive regulator of salt tolerance in rice plants. The OsMBL1 knock out (KO) lines had lower survival rates and higher H₂O₂ levels under salinity stress than did the WT (wild type), whereas the OsMBL1 overexpression lines presented a positive regulatory phenotype opposite to that of mbl1 lines. Interestingly, heterologous overexpression of OsMBL1 also conferred salt tolerance to Solanum lycopersicum, Brassica napus, and Arabidopsis. A RING E3 ubiquitin ligase, OsRMT1, was screened by Y2H, and its physical interaction with OsMBL1 was then verified both in vivo and in vitro. Knocking out the expression of OsRMT1 enhanced salt tolerance, while overexpressing OsRMT1 decreased it. Further examination revealed that OsRMT1 negatively regulates salt tolerance by degrading OsMBL1 in a process mediated by ubiquitination. Moreover, genetic analyses confirmed that OsRMT1 operates as an upstream factor of OsMBL1. At the same time, the experiments further revealed that OsRMT1 might affect the interaction between OsMBL1 and OsERF040 by degrading OsMBL1, thereby influencing the expression of downstream salt-tolerant genes. Collectively, our findings demonstrate that the OsRMT1-OsMBL1 regulatory module is essential in the rice response to salt stress.