StMKK9 positively regulates tuber formation and cold adaptation in potato.

Potato tuberization is controlled by photoperiod, temperature, and kinase-mediated signaling, but the mitogen-activated protein kinase kinase (MAPKK) components involved in the stolon-to-tuber transition remain poorly defined. In this study, a transcriptomic analysis of the potato cultivar 'Favorita' across eight stages of tuber formation identified 6, 2982, 723, 2579, 4673, 5028, and 6015 differentially expressed genes (DEGs) when comparing each of the seven developmental stages (Sto1, Sto2, Tu1, Tu2, Tu3, Tu4, and Tu5) to the early stolon stage (Sto), respectively. Among them, StMKK9 emerged as a candidate regulator of the transition from stolon to tuber. Subcellular localization showed that StMKK9 is predominantly localized in the nucleus. In the mid-late maturing potato cultivar 'Desiree', StMKK9 overexpression accelerated tuber initiation, increased tuber number and weight per plant, and altered the expression of early-maturity-related genes. StMKK9-overexpressing (StMKK9-OE) lines also exhibited alleviated cold-induced oxidative damage, as indicated by reduced ROS accumulation, MDA content, and electrolyte leakage. Yeast two-hybrid (Y2H) and luciferase complementation imaging (LCI) assays further showed that StMKK9 interacts with the cold-responsive transcription factor StCBF3. Together, these results identify StMKK9 as a positive regulator associated with potato tuber development and low-temperature adaptation. They support a presumptive regulatory module in which MAPK-associated signaling may connect StCBF3-mediated cold responses with photoperiod-related tuberization programs, providing a potential target for breeding early-maturing and climate-resilient potato cultivars.