Exogenous Dopamine and MdTyDC Overexpression Enhance Copper Stress Resistance in Apple

Excess copper (Cu) causes significant abiotic stress and toxicity in plants, severely inhibiting the growth of apples. Dopamine (DA) and its key synthesis enzyme, tyrosine decarboxylase (TyDC), are known to play vital roles in alleviating various abiotic stresses; yet, their function in plant heavy metal detoxification is not well understood. To bridge this gap and explore novel strategies for preventing heavy metal pollution in orchards, this study employed seedlings of Malus hupehensis and MdTyDC-overexpressing apple plants as experimental materials. A pot experiment was conducted under controlled greenhouse conditions for 45 days, in which the soil was irrigated with CuSO₄ solution at concentrations of 0 and 500 μmol l^-1, and treated with 100 μmol l^-1 of DA. The results showed that exogenous DA and MdTyDC significantly enhanced plant tolerance to Cu stress and alleviated the inhibition of plant growth and root development caused by Cu stress, improved photosynthetic efficiency, enhanced antioxidant enzyme activity, and reduced reactive oxygen species accumulation. In addition, DA treatment significantly reduced the accumulation of Cu in both the roots and leaves of apple plants, upregulated endogenous hormone levels and free amino acid content. Furthermore, exogenous DA and MdTyDC overexpression enhanced apple adaptation to Cu stress by regulating the expression of multiple genes involved in Cu uptake, transport, and detoxification. In conclusion, exogenous DA and MdTyDC overexpression can effectively mitigate the adverse effects of Cu stress on apple plants, providing a novel solution for the prevention and control of heavy metal pollution in orchards.