Exogenous spermidine and the CaSPDS gene regulate salt tolerance by maintaining ROS homeostasis and photosynthetic function in pepper seedlings

Salt stress severely restricts pepper ( Capsicum annuum L.) growth and productivity by disrupting osmotic balance, damaging the photosynthetic system, and inducing excessive reactive oxygen species (ROS) accumulation. Spermidine (Spd), an important polyamine, plays critical roles in plant stress adaptation; however, the physiological and molecular mechanisms underlying Spd-mediated salt tolerance in pepper remain insufficiently understood. This study investigated the effects of exogenous Spd on salt tolerance in pepper seedlings and further examined the function of the polyamine biosynthesis gene CaSPDS in the salt stress response. Salt stress significantly inhibited pepper seedling growth; reduced relative water content, chlorophyll content, Fv / Fm , and photosynthetic gas-exchange parameters; and increased relative electrical conductivity (REC), malondialdehyde (MDA) content, H 2 O 2 and O 2 · − accumulation, and antioxidant enzyme activities. Exogenous Spd application markedly alleviated these adverse effects by improving membrane stability, maintaining photosynthetic performance, reducing ROS accumulation, and modulating antioxidant defense responses. Spd treatment also reduced the excessive accumulation of proline and soluble sugars while restoring soluble protein content under salt stress. Expression analysis showed that salt stress strongly induced polyamine biosynthesis-related genes, particularly CaSPDS and CaSPMS , and exogenous Spd further enhanced CaSPMS expression. Moreover, virus-induced gene silencing (VIGS) of CaSPDS caused more severe wilting, higher REC and MDA contents, lower relative water content, and increased expression of ROS-related genes under salt stress, confirming the essential role of CaSPDS in salt tolerance. Exogenous Spd enhances salt tolerance in pepper seedlings by maintaining ROS homeostasis, protecting membrane integrity and photosynthetic capacity, and regulating osmotic adjustment and antioxidant defense. CaSPDS also plays a crucial role in salt tolerance and redox balance in pepper. These results indicate a close association between CaSPDS and Spd function.