Exogenous dopamine and overexpression of the dopamine synthase gene MdTYDC alleviated apple replant disease.

Apple replant disease (ARD) is a soil-borne disease that leads to economic losses due to reduced plant growth and diminished fruit yields. Dopamine is involved in interactions between plants and pathogens. However, it remains unclear whether dopamine can directly stimulate defense responses to ARD. In this study, an exogenous dopamine treatment and dopamine synthetase MdTYDC (tyrosine decarboxylase) transgenic plants were used to verify the role of dopamine in treating ARD. First, 2-year-old apple trees (Malus domestica cv. Fuji), grafted onto rootstock M26, were grown in replant soils. The addition of dopamine (100 µM) to the soil promoted seedling growth and changed the accumulation of mineral elements in plants in replant soils. Such supplementation improved the activity of invertase, urease, proteinase and phosphatase under replant conditions. Sequencing analysis of 16S rDNA and internal transcribed spacer (ITS) rDNA revealed that dopamine had a slight influence on bacterial diversity but had an obvious effect on the fungal diversity in replant soils. The application of dopamine to replant soil changed the composition of bacterial and fungal communities. Second, overexpression of MdTYDC in apple plants alleviated the effects of ARD. MdTYDC transgenic lines exhibited mitigated ARD through inhibited degradation of photosynthetic pigment, maintaining the stability of photosystems I and II and improving the antioxidant system. Furthermore, overexpression of MdTYDC improved arbuscular mycorrhizal fungi colonization by improving the accumulation of soluble sugars under replant conditions. Together, these results demonstrated that dopamine enhances the tolerance of apples to ARD.