Mitogen-Activated Protein Kinases and Phosphatases Synergistically Regulate 3-Hydroxy-3-Methylglutaryl-CoA Reductase To Enhance Squalene Biosynthesis in Camellia oleifera Fruits under Heat Stress Conditions

Heat stress substantially influences lipid metabolism and triterpenoid biosynthesis in oil-tea ( Camellia oleifera ) seeds, yet the regulatory coordination between protein phosphorylation signaling and mevalonate (MVA) flux remains poorly understood. Here, we combined transcriptomic, metabolomic, and enzymatic analyses with RT-qPCR validation to elucidate how MAPK–phosphatase cascades modulate HMGR-dependent squalene biosynthesis during thermal exposure. Fully mature seed kernels harvested after Shuangjiang (late October to early November) were incubated at 35°C for 0, 12, and 24 h. Heat stress significantly enhanced HMGR activity and squalene accumulation, accompanied by transcriptional activation of HMGR-1 , which showed the most pronounced induction among the four HMGR isoforms. Multi-omics integration revealed that fatty-acid elongation and desaturation modules were positively correlated with oleic acid levels, while pyridoxine-associated genes in vitamin B₆ metabolism formed a strong co-expression subnetwork, reflecting enhanced membrane remodeling and coenzyme turnover under heat. RT-qPCR analyses further confirmed that MAPK (MPK3/MPK6-like) and phosphatase ( PP1c / PP2A ) genes exhibited synchronized transcriptional patterns with HMGR-1 , supporting a reversible phosphorylation mechanism that dynamically regulates carbon flux through the MVA pathway. Collectively, these findings establish a mechanistic framework in which MAPK–PP–HMGR signaling enhances triterpenoid synthesis and lipid homeostasis, thereby contributing to thermal resilience in C. oleifera seeds. This work provides mechanistic insights and candidate targets for metabolic engineering toward improved squalene productivity and heat tolerance in oil-tea germplasm.