The inositol-phosphate synthase gene, MdMIPS1, enhances cold stress tolerance in transgenic apple plants

Extreme temperature stress has severely restricted the development of the global apple industry. Myo-inositol (MI) is a small-molecule polyol in animals, plants, and microorganisms. In plants, MI and its derivatives function as osmolytes that participate in plant metabolism and as antioxidants to enhance plant stress tolerance. Previously, we showed that MI plays a role in the development of apple plants, as well as in the response to salt, drought, and pathogen infection. In this study, we analyzed the biological role of MI and its synthesized gene, MdMIPS1, in the response of apple plants to cold stress. Exogenous MI effectively alleviated stress inhibition caused by cold, and the expression of MdMIPS1 was gradually induced by cold stress. We then generated transgenic apple plants with different expression levels of MdMIPS1 and treated them with cold stress. Overexpression of MdMIPS1 in transgenic apple (OE) plants alleviated damage caused by both chilling and freezing stress, and suppression of MdMIPS1 (Ri) increased sensitivity to chilling and freezing stress. Under cold stress, the content of MI was higher in OE plants than in wild-type (WT) plants, and several amino acids, including arginine, accumulated more in OE plants than in WT plants. Further analysis suggested that OE plants had higher levels of putrescine (Put). Exogenous Put prevented Ri plants from dying, and its inhibitor, D-arginine, abolished the freezing stress tolerance of OE plants. In conclusion, our study demonstrates that both MI and MdMIPS1 positively regulate apple's response to cold stress. The MdMIPS1 mediated synthesis of MI could act as an osmoprotectant to maintain cellular osmotic homeostasis, and also as a signaling molecule to activate both the ROS scavenging and the Put biosynthesis.