Involvement of Polyamine Metabolism in Melatonin-Induced Salt Tolerance in Rice Seedlings

Salt stress is a major constraint on rice production, threatening global food security. Polyamines (PAs) play diverse roles in plant growth, development, and stress responses. Although melatonin (MT) is known to enhance plant stress resistance, its specific role in regulating rice PAs under salt stress remains unclear. This study investigated the relationship between MT-mediated changes in PA metabolism and growth tolerance under salt stress in rice seedlings. Our results demonstrated that MT application was associated with partial alleviation of salt-induced growth reductions, as indicated by increases in plant height, dry weight and chlorophyll content. Moreover, MT application was correlated with increased proline accumulation and changes in ion homeostasis, including increased K⁺ content, reduced Na⁺ content, and an elevated K⁺/Na⁺ ratio. In terms of PA metabolism, MT application was correlated with higher activities of biosynthetic enzymes (arginine decarboxylase, ADC; ornithine decarboxylase, ODC) and lower activities of catabolic enzymes (diamine oxidase, DAO; polyamine oxidase, PAO). MT-treated seedlings showed reduced free Put but elevated free Spd and Spm levels, alongside reduced conjugated PAs and increased bound Spd and Spm. These shifts were accompanied by an increased (Spd + Spm)/Put ratio in both free and bound PA fractions. Furthermore, application of the PA synthesis inhibitor D-arginine was associated with exacerbated growth inhibition under salt stress and reduced free Put levels, while subsequent MT application was associated with partial recovery of both free Put levels and the associated growth and biochemical changes. These results provide a physiological basis for further mechanistic investigation into the role of PA metabolism in MT-associated growth tolerance under salt stress, and suggest a potential avenue for improving rice resilience under saline conditions.