F-box ubiquitin-ligating enzyme ZEITLUPE 1 inhibits the promotion of caffeic acid O-methyltransferase 1 in salt tolerance of apple by the multimeric E3 protein complex SCFZTL1-mediated ubiquitination modification and protein degradation.

Caffeic acid O-methyltransferase (COMT) is involved in melatonin biosynthesis and regulates salt tolerance in plants, but no COMT homologs have been reported in apple. The post-translational mechanisms through which COMT regulates salt tolerance remain elusive in apple. In this study, we identified a novel protein module MdZTL1-MdCOMT1 regulating melatonin biosynthesis and salt tolerance. MdCOMT1 was a key bioactive melatonin synthetase and exhibited positive biology activity in enhancing salt tolerance of apple. MdCOMT1 could interact with MdZTL1, a F-box ubiquitin-ligating enzyme (E3). Interestingly, MdZTL1 negatively regulated melatonin biosynthesis and salt tolerance by forming the multimeric E3 complex SCFZTL1 to target the ubiquitination of MdCOMT1 and its subsequent degradation in the 26S proteasome. These findings suggest that MdZTL1 inhibits the promotion of MdCOMT1 in salt tolerance, with decreasing melatonin biosynthesis and impairing subsequent ROS scavenging. Our study reveals a ubiquitination-mediated post-translational regulation mechanism in salt tolerance, providing novel insights into the improvement of salt-tolerant apple through molecular design breeding.