Nitrogen-Mediated Transcriptional Reprogramming Balances Carbon Metabolism and Flavonoid Biosynthesis to Enhance Drought Resilience in Camellia oleifera

Camellia oleifera has constantly been threatened by drought and insufficient soil nutrients. Our study used RNA sequencing (RNA-Seq) to investigate the molecular responses to nitrogen application under drought conditions. Concurrently, we also analyzed associated leaf functional traits. The results showed that supplemental nitrogen effectively alleviated drought-induced stress in C. oleifera. Fertilization increased leaf chlorophyll and flavonoid concentrations, restored non-structural carbohydrate balance and enhanced antioxidant capacity under drought conditions under drought, thereby enhancing drought resistance. RNA-Seq identified differentially expressed genes predominantly engaged in drought stress response mechanisms such as light harvesting, starch and sucrose metabolic pathways, and flavonoid biosynthesis. Under drought conditions, nitrogen application activated CoHEMA, CoHEMB, CoCHI and CoLAR while repressing CoSGR, CoUFGT, CoSPS and CoInv expression, thereby enhancing chlorophyll content and maintaining flavonoid-sucrose homeostasis to meet the metabolic demands of C. oleifera survival. Co-expression network analysis revealed two highly interconnected modules (pink and blue), primarily enriched in carbon metabolism, nitrogen metabolism and secondary metabolite metabolism. The two modules strongly correlated with opposite effects on physiological indicators. In addition, nitrogen fertilizer treatment identified numerous transcription factors associated with drought response. Heterologous expression in Nicotiana tabacum confirmed that CoWHY1 promoted chlorophyll accumulation by regulating the expression of HEMA1 and SGR. This study provides molecular insights into the impact of soil nutrients on the drought response of C. oleifera foliage, setting the groundwork for nutrient management in economic trees under drought conditions.