Dopamine and arbuscular mycorrhizal fungi act synergistically to promote apple growth under salt stress

Dopamine is common in animals and plants where it plays a variety of important roles, including roles in stress responses. Salt stress commonly occurs in China and seriously affects plant growth and arbuscular mycorrhizal (AM) symbiosis. So, we studied the influence of dopamine on AM fungi (AMF) colonization in apple trees (Males domestica Borkh.) under salt stress. Apple trees were inoculated with the AM fungi, Funneliforrnis mosseae, and grown for three weeks. Roots were irrigated with 100 mu M dopamine or water and after a 10 day pre-culture period, 200 mM NaCl was introduced (or not). Dopamine improved apple trees salt tolerance by promoting AM symbiosis through increased carbohydrate content. AMF and dopamine worked synergistically to maintain plant cell membrane stability and improve photosynthesis under high salinity, which improved the salt tolerance of the plants. AMF and dopamine also significantly increased root length, surface area, average diameter, and number of root forks, which increased the surface area in contact with soil nutrients and water under salt stress. Dopamine synthetase MdTYDC (tyrosine decarboxylase) transgenic apples were used to study the role of dopamine on AM symbiosis. Transcriptome analyses showed that overexpression of MdTYDC improved AMF symbiosis by promoting synthesis of flavonoids such as luteolin, kaempferol and quercetin. This is the first report that exogenous dopamine promoted AM symbiosis under salt stress, the synergistic effect of dopamine and AMF improved apple salt resistance, and overexpression of MdTYDC promoted AMF symbiosis.