MdGRF10 phosphorylation stabilizes MdASMT1 for melatonin-mediated salt tolerance in apple

Salt stress, especially the increasing secondary salt stress, severely compromises apple production worldwide. Mitigation of oxidative damage caused by salt stress is critical for salt tolerance in apple plants. However, it remains unclear how the salt signal triggers the excessive reactive oxygen species (ROS) mitigation system in apple. In this study, we identified a salt-induced gene MdGRF10 (encoding a 14-3-3 protein), whose overexpression conferred transgenic apple plants reduced oxidative damage and enhanced salt tolerance. Furthermore, a salt-activated receptor-like cytoplasmic kinase MdPBL34 was found to interact with and phosphorylate the C-terminal of MdGRF10. This phosphorylation promoted the interaction between MdGRF10 and a melatonin rate-limiting synthetase MdASMT1 (N-acetylserotonin methyltransferase). Its overexpression or knockdown by CRISPR/Cas9 in transgenic apple plants demonstrated that MdASMT1 is critical in melatonin-mediated ROS scavenging for salt tolerance. Their interaction stabilizes MdASMT1 by decreasing its ubiquitin-mediated degradation for increased melatonin level, decreased oxidative damage and therefore promoted salt tolerance. Our findings revealed that 14-3-3 protein could integrate the salt signal in a phosphorylation-dependent manner. Moreover, MdPBL34 was also identified for the first time to be involved in salt signaling. Our research uncovered a novel MdPBL34-MdGRF10-MdASMT1 regulatory module in response to salt stress in apple, which will contribute to the molecular breeding of melatonin-enriched salt-tolerant apple trees.