Exogenous spermidine improves water stress tolerance of white clover (Trifolium repens L.) involved in antioxidant defence, gene expression and proline metabolism

Spermidine (Spd) may be involved in the regulation of plant adaptation to drought stress. The objectives of the study were to identify the physiological effect and elucidate the possible mechanism caused by exogenous Spd (0.05 mM) in white clover under water stress induced by 20% polyethylene glycol 6000 for 12 days. Water stress elevated significantly the accumulation of reactive oxygen species and malonaldehyde, and resulted in the decrease of cell membrane stability, relative water content and relative growth mte. Spd effectively alleviated the damage effect from water stress. Spd-treated plants showed a promoted the ascorbate-glutathione cycle and maintained greater antioxidant enzyme activities (superoxide dismutase, peroxidase and catalase), as well as higher transcript level of genes encoding antioxidant enzymes than untreated plants. Additionally, the plants treated with Spd under water stress exhibited more accumulated organic solutes including soluble sugar, reducing sugar, betaine and free proline. Spd also accelemted proline catabolism and biosynthesis proceeding from glutamate pathway during water stress, but had no effect on the omithine pathway of proline biosynthesis. These results suggest that exogenous Spd under water stress may directly or indirectly regulate antioxidant defense system, organic solutes accumulation and proline metabolism.