Exogenous Serotonin Increases the Tolerance of Apple to Drought Stress by Mitigating Oxidative Damage and Sustaining Photosynthetic Capacity.

Drought stress is one of the important factors limiting the growth of the apple industry. Although 5-hydroxytryptamine (5-HT), an ancient indole amine, is widely present in plants, the role of 5-HT in drought stress in apples is unknown. We tested the impact of 5-HT on long-term drought stress in apple seedlings. Our findings revealed that 1 μM of exogenous 5-HT could improve the resistance of apple seedlings to short- and long-term drought stress. These plants experienced better root growth, more efficient water use, and less reactive oxygen species (ROS). The higher ABA levels mediated by 5-HT in leaves increased stomatal density and improved the photosynthetic capacity of leaves. Furthermore, exogenous 5-HT boosted the expression of drought stress-related genes, enabling apple plants to adapt to drought stress.