Exogenous Spermidine Alleviated Low-Temperature Damage by Affecting Polyamine Metabolism and Antioxidant Levels in Apples

Low-temperature stress significantly limits the growth, development, and geographical distribution of apple cultivation. Spermidine (Spd), a known plant growth regulator, plays a vital role in the plant's response to abiotic stress. Yet, the mechanisms by which exogenous Spd enhances cold resistance in apples remain poorly understood. Therefore, the present study analyzed the effects of exogenous Spd on antioxidant enzyme activity, polyamine metabolism, and related gene expression levels of 1-year-old apple branches under low-temperature stress. Treatment with exogenous Spd was found to stabilize branch tissue biofilms and significantly reduce the levels of reactive oxygen species by elevating proline content and boosting the activity of antioxidants such as superoxide dismutase. It also upregulated the activities of arginine decarboxylase, S-adenosylmethionine decarboxylase, and spermidine synthase and the expression levels of MdADC1, MdSAMDC1, and MdSPDS1 under low-temperature stress and led to the accumulation of large amounts of Spd and spermine. Moreover, compared with the 2 mmol<middle dot>L-1 Spd treatment, the 1 mmol<middle dot>L-1 Spd treatment increased the expression levels of cold-responsive genes MdCBF1/2/3, MdCOR47, and MdKIN1, significantly. The findings suggest that exogenous Spd can enhance cold resistance in apple branches significantly. This enhancement is achieved by modulating polyamine metabolism and improving antioxidant defense mechanisms, which could be exploited to improve apple cultivation under cold stress conditions.