Exogenous dopamine promotes photosynthesis and carbohydrate metabolism of downy mildew-infected cucumber

Abstract

Downy mildew causes great damage to cucumber photosynthesis and is a major threat to cucumber production. The increase in drawbacks associated with chemical treatments has prompted the need for alternative protecting agents. We evaluated the physiological effects of different exogenous dopamine concentrations on the photosynthetic capacity and carbohydrate metabolism of downy mildew-infected cucumber. The results showed that applying 100 µmol L−1 exogenous dopamine significantly promoted the growth of cucumbers under downy mildew stress and decreased the disease index, thereby contributing to improved disease resistance in cucumber. Exogenous dopamine effectively increased the photosynthetic pigment content, chlorophyll fluorescence parameters, net leaf photosynthetic rate and stomatal conductance and transpiration rate but reduced the intercellular CO2 concentration. Exogenous dopamine application also increased the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase during carbon assimilation, upregulated the expression of Rubisco regulatory genes (CsrbcL and CsrbcS) and downregulated the expression of chlorophyll degradation related genes (CsPAO and CsRCCR). The contents of total soluble and reducing sugars were increased, thereby maintaining the osmotic adjustment function. At the same time, applying 100 µmol L−1 dopamine significantly increased the activity of sucrose synthase, sucrose phosphate synthase, acid invertase and neutral invertase in carbohydrate metabolism. Further, dopamine application upregulated the expression of CsSUS3 and CsSPS4 and increased cucumber starch, cellulose, fructose, sucrose and glucose contents. Therefore, exogenous dopamine can effectively alleviate the damage to cucumber photosynthetic and carbohydrate metabolism rates caused by downy mildew, thereby maintaining the normal growth of cucumber under stress.