Dopamine confers cadmium tolerance in apples by improving growth, reducing reactive oxygen species, and changing secondary metabolite levels

Cadmium (Cd) is an element that is not only unnecessary for plant growth, but can also induce irreversible damage to plants. Dopamine (DA) and tyrosine decarboxylase (TyDC), a key protein in its synthesis, play important roles in responses to biotic and abiotic stress in plants. To explore the role of dopamine in apple under Cd stress, Malus hupehensis seedlings and TyDC transgenic plants were treated with 0 and 300 μmol L−1 CdCl2 solutions. We demonstrated that the application of dopamine alleviated Cd stress-induced growth retardation, and the chlorophyll content increased by 22.5%. The root vigor and photosynthesis increased by 1.10 and 1.01 times, respectively. Dopamine significantly increased the activity of antioxidant enzymes to reduce the reactive oxygen species (ROS) level. The accumulation of Cd2+ in the roots and shoots of apple plants was decreased by 33.7% and 28.6% under dopamine treatment, and the phytohormone levels were noticeably increased. Dopamine and MdTyDC alleviated Cd stress by markedly increasing the content of free amino acids and phenolic compounds following Cd exposure. In addition, exogenous dopamine and MdTyDC overexpression alleviated Cd stress by regulating the expression of genes involved in Cd uptake, transport, and detoxification, including HA7, NRAMP1, NRAMP3, HMA4, PCR2, ABCC1, MHX, NAS1, and MT2. Overall, our findings indicate that dopamine can eliminate ROS by increasing antioxidant enzyme activity and reducing Cd accumulation. In addition, dopamine also increased the levels of phytohormones, amino acids, and phenols under Cd stress and regulated the transcription of Cd transporters to enhance Cd tolerance in apple.