The apple palmitoyltransferase MdPAT16 influences sugar content and salt tolerance via an MdCBL1-MdCIPK13-MdSUT2.2 pathway.

Protein S-acyltransferases (PATs) are a category of eukaryotic transmembrane proteins that mediate the S-acylation of their target proteins. S-acylation, commonly known as palmitoylation, is a reversible protein modification that regulates the membrane association and function of target proteins. However, the functions and mechanisms of PATs in apple (Malus domestica) remain poorly understood. In this study, an MdPAT family member, MdPAT16, was identified and shown to have palmitoyltransferase activity. We demonstrated that this gene responds to salt stress and that its expression improves plant salt stress resistance. In addition, its overexpression significantly promotes the accumulation of soluble sugars. The same phenotypes were observed in transgenic tissue culture seedlings, transgenic roots, and Arabidopsis thaliana that ectopically expressed MdPAT16. MdPAT16 was shown to interact with MdCBL1 and stabilize MdCBL1 protein levels through palmitoylation. The N-terminal sequence of MdCBL1 contains a palmitoylation site, and its N-terminal deletion led to changes in MdCBL1 protein stability and subcellular localization. The phenotypes of MdCBL1 transgenic roots and transiently injected apple fruits were fully consistent with the sugar accumulation phenotype of MdPAT16. Mutation of the palmitoylation site interfered with this phenotype. These findings suggest that MdPAT16 palmitoylates its downstream target proteins, improving their stability. This may be a missing link in the plant salt stress response pathway and have an important impact on fruit quality.