Nitric Oxide and Phytohormone Crosstalk Mediates Soybean Growth and Defence Against Aphis glycines.

Aphid infestation, particularly by Aphis glycines, poses a major threat to soybean (Glycine max) production, necessitating sustainable defence strategies. This study evaluated the role of nitric oxide (NO) donor S-nitrosocysteine (CySNO) in enhancing soybean resistance to aphids through hormonal crosstalk. A dose optimisation assay revealed that 0.1 mM CySNO significantly improved seed germination, shoot and root growth, while higher concentrations inhibited development. Greenhouse trials were then conducted by applying 0.1 mM CySNO directly to the roots, followed by foliar applications of salicylic acid (SA), jasmonic acid (JA) and their combination. Physiological parameters showed that CySNO pre-treatment enhanced shoot length and leaf area, particularly under SA + JA application. Although chlorophyll content did not differ significantly, a slight positive trend was observed in all treated plants. Aphid population dynamics monitored over 20 days revealed that CySNO combined with SA + JA significantly suppressed aphid colonisation compared to individual treatments or controls. Quantitative real-time PCR analysis demonstrated that CySNO upregulated key defence genes, including GmGSNOR1, GmNOX1, GmPR1, GmLOX2 and GmLOX10, indicating enhanced activation of SA- and JA-mediated pathways. Together, these results demonstrate that NO enhances soybean growth and strengthens defence responses via hormonal signalling, highlighting its potential as an effective strategy for aphid management within integrated pest management frameworks.