Effects of dopamine on growth, carbon metabolism, and nitrogen metabolism in cucumber under nitrate stress

Dopamine, which is a catecholamine neurotransmitter, is widely present in plant organs. The accumulation of soil nitrate limits the growth of plants. We investigated the role of dopamine in regulating the growth, carbon metabolism, and nitrogen metabolism in cucumber (Cucumis sativus L. "Jin You No. 1") under nitrate stress. Under nitrate stress, plants showed significant reductions in growth, chlorophyll concentrations, chlorophyll fluorescence and gas exchange parameters. However, dopamine markedly alleviated such reductions. Excess nitrate led to an increase in sucrose phosphate synthase, acid invertase, and neutral invertase activity, but decreased sucrose synthase activity. Similarly, the expression of the CsSPS4 gene was also up-regulated and the expression of the CsSUS3 gene was down-regulated under nitrate stress. Dopamine significantly increased the activity and gene expression of these enzymes. The CsNRT1.1 gene was up-regulated under nitrate conditions. Dopamine significantly alleviated the accumulation of nitrate nitrogen and ammonium nitrogen under nitrate conditions. Nitrate reductase activity and the expression of the CsNR1 gene were significantly inhibited due to exposure to excess nitrate. We found that glutamine synthetase activity decreased and glutamate dehydrogenase activity increased after nitrate induction, which indicated that ammonium assimilation relied mainly on glutamate dehydrogenase pathway rather than glutamine synthetase/glutamate synthase cycle under nitrate stress. Dopamine significantly enhanced the activity of enzymes and gene expression of nitrogen metabolism. Our results indicated that dopamine plays an important role in mediating plant growth, carbon metabolism, and nitrogen metabolism.