Lipidomics reveals divergent root and shoot responses to combined nitrogen deficiency and heat stress.

INTRODUCTION: Soybean productivity is highly sensitive to heat and insufficient nitrogen supply, stresses that can occur simultaneously. However, organ-specific lipid responses to this stress combination remain poorly resolved. METHODS: We exposed hydroponically grown soybean plants to nitrogen deficiency, heat stress (HS) and their combination and compared physiological, lipidomic and gene-expression responses in roots and leaves. RESULTS: Combined stress caused the largest reductions in biomass and photosynthetic performance and the highest H₂O₂ and MDA accumulation and relative total abundance of candidate oxylipin features. Untargeted LC-MS lipid profiling coupled with WGCNA revealed distinct organ-specific lipid abundance patterns. HS produced the strongest root lipidomic response, characterized by increased abundances of features putatively annotated as polyunsaturated diacylglycerols, lysophospholipids and a gibberellin A12-related compound. Leaves showed their largest lipidomic shift under combined stress, with decreased abundances of photosynthetic membrane-lipid features, increased linoleic-acid- and digalactosylmonoacylglycerol-related features and divergent changes among jasmonate-related features, including cis-jasmone. RT-qPCR showed higher PLD1 and lower GA20ox expression in roots under HS. In leaves, FAD2, LOX2, OPR3 and MYC2 expression increased under all stress treatments, whereas PLA2 expression increased only under HS and combined stress. DISCUSSION: These findings reveal pronounced tissue specificity in soybean lipid responses to combined nitrogen deficiency and HS and highlight candidate membrane-lipid- and oxylipin-associated responses for further investigation. However, they do not establish direct root-shoot lipid transport or metabolic coordination.