GmHMGR6 enhances salt stress tolerance in soybean through modulation of nitrogen metabolism

GmHMGR6 coordinates a regulatory network linking nodulation, nitrogen metabolism, and photosynthesis, therebyimproving nitrogen utilization and sustaining carbon assimilation under salt stress in soybean. 3-Hydroxy-3-methylglutaryl-CoA reductase (HMGR) functions in the mevalonate pathway and is essential for plant development and stress adaptation. We identified GmHMGR6 as the most salt-responsive HMGR isoform in soybean, with predominant expression in roots. To elucidate its function in salt tolerance, we generated GmHMGR6-overexpressing hairy roots and subjected these composite plants to NaCl treatment. Physiological assays, metabolite measurements, chlorophyll fluorescence and gas-exchange analyses, together with RNA-seq of roots and leaves, were performed to characterize the GmHMGR6-dependent responses. GmHMGR6 overexpression markedly reduced salt-induced DEGs in roots relative to wild type and primarily affected nitrogen-related metabolic pathways. Leaf DEGs were enriched in photosynthesis-associated processes, including antenna proteins, electron transport, and CO₂ assimilation. GmHMGR6 also regulated key nodulation genes, thereby promoting nodule formation and enhancing nitrogen assimilation through higher ammonium levels and increased glutamine synthetase (GS) and glutamine oxoglutarate aminotransferase (GOGAT) activities. Moreover, GmHMGR6 overexpression alleviated NaCl-induced photosynthetic inhibition by maintaining photosystem function and reducing photoinhibition and oxidative damage. These findings demonstrate that GmHMGR6 enhances soybean salt tolerance through coordinated regulation of nitrogen metabolism, nodulation, and photosynthetic performance.