Nitric oxide affects melatonin mediates enrichment of isoflavones and physiological biochemistry in germinated soybeans under Ultraviolet-B stress

The role of nitric oxide (NO) in exogenous melatonin (MT)-induced isoflavone accumulation under ultraviolet-B (UV-B) stress in germinated soybeans was investigated. Compared to UV-B stress alone, exogenous MT (25 µM) and sodium nitroprusside (SNP), an NO releaser, significantly enhanced NO content by improving nitrate reductase (NR) activity and upregulated the gene expression of NR 1, NR 2, and nitric oxide synthase in UV-B-stressed soybeans. However, exogenous MT combined with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), an NO scavenger, markedly decreased the NO content; it decreased by 9.76% and 18.42% in 2- and 4-day-old soybeans, respectively. Meanwhile, MT-triggered NO generation promoted isoflavone accumulation by enhancing phenylalanine amination (PAL) and cinnamic acid 4-hydroxylase activities and by upregulating the gene expression of key enzymes ( PAL1, 4-coumarate coenzyme A ligase, RNA decarboxylase 3, Chalcone isomerase 1 A, Chalcone synthase ). Furthermore, it alleviated the UV-B stress inhibition by enhancing the antioxidant system and reducing the accumulation of reactive oxygen species. These effects were further enhanced by the application of SNP but inhibited by the application of cPTIO. Therefore, NO is an essential downstream-signaling molecule that mediates MT-induced isoflavone accumulation and growth improvement under UV-B stress in germinated soybeans.