Alleviation Effect And Mechanism Of Exogenous Dopamine On Cucumber Downy Mildew

Cucumber is one of the top ten vegetable cultivation crops in the world and ranks fourth in Asia.In cucumber production,downy mildew is one of the main diseases causing cucumber yield reduction.At present,the application of pesticides is still the main measure to prevent downy mildew,however,there are disadvantages such as increased drug resistance of pathogenic bacteria,environmental pollution,and pesticide residues on fruits.As a strong antioxidant,dopamine participates in a variety of physiological activities in plants resisting adversity stress,and effectively alleviating the damage caused by adversity.Therefore,exploring the mitigation effect and mechanism of exogenous dopamine on cucumber downy mildew is of great significance to seek safe and efficient control measures for downy mildew.In this study,the pot experiment was carried out with susceptible cucumber cultivar‘Changchun Mi Ci’ and the disease-resistant cultivar ‘Jinchun NO.4’.Different concentrations of dopamine were used to irrigate roots to explore the alleviating effect of exogenous dopamine on cucumber downy mildew.And the results are as follows:1.Under downy mildew infection,exogenous dopamine significantly increased the activities of cucumber superoxide dismutase(SOD),catalase(CAT),peroxidase(POD),and ascorbate peroxidase(APX),and up-regulated expression of CsSOD,CsPOD,CsCAT,and CsAPX.Exogenous dopamine also significantly reduced the accumulation of superoxide anion and hydrogen peroxide in leaves and decreased the content of malondialdehyde(MDA)and relative electrical(REL)conductivity in leaves,which significantly improved the antioxidant capacity of cucumber.At the same time,exogenous dopamine significantly increased the wax content and trichome density of cucumber leaves,increased the cucumber polyphenol oxidase(PPO),chitinase,and β-1,3-glucanase activities,and up-regulated CsPR1 gene expression so that the disease index significantly reduced.2.Under downy mildew infection,exogenous dopamine maintained cucumber chloroplast morphology and function,significantly increased leaf chlorophyll content,downregulated the expression of CsPAO and CsRCCR,improved leaf Chlorophyll fluorescence parameters and photosynthetic parameters,up-regulated the expressions of Csrbc L and Csrbc S and increased the activity of Rubisco.At the same time,exogenous dopamine significantly increased the activities of sucrose synthase(SS)and sucrose phosphate synthase(SPS),upregulated the expressions of CsSUS3 and CsSPS4,thereby increasing the soluble sugar,reducing sugar,starch,and cellulose content in cucumber leaves;it can also increase invertase activity in cucumber leaves to increase the sucrose,fructose and glucose content.All in all,strengthen the photosynthesis of cucumber leaves and ensures the function of carbon metabolism3.Under downy mildew infection,exogenous dopamine significantly increased activities of cucumber nitrate reductase(NR),glutamine synthase(GOGAT),glutamate synthase(GS),and glutamate dehydrogenase(GDH),up-regulated the expression of CsNRT1.1,CsNR1,CsGS1,CsGOGAT1.1,and CsGDH1,also significantly increased the nitric nitrogen content in leaves and decreased the ammonium nitrogen accumulation.So that the function of nitrogen metabolism in cucumber can be ensured.4.Under downy mildew infection,exogenous dopamine significantly increased cucumber plant height,stem diameter,fresh weight and dry weight of shoots,improved cucumber seedling index,and significantly increased cucumber fruit yield;at the same time,exogenous dopamine significantly increased fruit transverse diameter,longitudinal diameter,and single fruit weight,and significantly increased fruit VC,soluble sugar and soluble protein content,so that guaranteed the fruit quality.5.Under downy mildew infection,100 μmol/L exogenous dopamine has the most significant effect on the disease resistance and growth of cucumber.It can significantly increase the content of endogenous dopamine in cucumber leaves and up-regulate the expression of tyrosine decarboxylase regulatory gene CsTyDc.