Tissue-Level Defense-Response Of Plants To Phytophthora Melonis Induced By 24-Epibrassinolide In Cucumber Seedling

Cucumber is a worldwide vegetable.Phytophthora blight is one of the main soil-borne diseases in cucumber production in South China.The pathogens of the disease are numerous and the genetic variation is fast,and the prevention and treatment is very difficult.Brassinosteroids(BRs)have important regulatory roles in plant growth and development and biotic and abiotic stress responses.In this study,moderately resistant varieties?Swatow Cucumber?(R1),?Improved Changchun Mi Ci?(R2),and two susceptible varieties?Zhongnong 20?(S1)and?Zaoqing No.2?(S2)were used as test materials.Pretreatment of Different Resistant Cucumber Seedlings by Application of 24-Epibrassinolide(EBR).After 2 days,the roots were inoculated with 5*10~3 Phytophthora cinerea zoospore suspension.Comprehensive analysis of the microstructural changes of hypocotyls in cucumber seedlings was carried out to determine the mechanism of plant tissue defense response induced by EBR to resist cucumber disease.The research results are as follows:(1)Investigation of disease index after 5 days of vaccination.The disease index of R1and R2 were 25.83%and 34.90%,and the disease resistance evaluation was medium resistance.The disease indexes of S1 and S2 were 51.62%and 58.86%,and the disease resistance evaluation was susceptible.Exogenous EBR pretreatment significantly reduced the disease index of different resistant varieties.The disease index of cucumber R1 and R2seedlings in the resistant group decreased by 12.64%and 17.39%.In the susceptible group,cucumber S1 and S2 seedlings decreased by 17.20%and 30.73%.(2)Compared with S1 and S2,the hypocotyl of R1 variety shows short and thick features.Further investigation of the growth of hypocotyls of plants after inoculation with pathogens.Whether not vaccinated or inoculated with pathogens.Exogenous EBR promoted the increase of the diameter of hypocotyls of cucumber and cucumber,but had no significant effect on the growth of hypocotyls.(3)Using the paraffin sectioning technique and the red-solid green staining method to observe the cross section and longitudinal section of the hypocotyl of cucumber,it was found that:(1)Exogenous EBR treatment and Phytophthora infestans promoted the division of epidermal cells and cortical cells in hypocotyls of cucumber seedlings.Shows an increase in the number of cells and a more compact cell arrangement;The parenchyma cell wall has different degrees of thickening,and the cell morphology is clearer;Increased number of vascular bundles in the hypocotyl cross section.Compared with CK treatment,EBR+P treatment was the most obvious after EBR pretreatment,followed by single hormone(EBR)treatment,and finally single Phytophthora inoculation(P)treatment.(2)It can be seen from the staining of bast fiber and ductal red in the hypocotyl vascular bundle.Compared with CK treatment,the phloem fiber and the duct red are the deepest in the EBR+P treatment,followed by the single EBR treatment,and finally the single P treatment.The performance of the resistant and susceptible varieties is basically the same.(3)Measurement of the cross-sectional structure of cucumber hypocotyl.Compared with CK,the area of hypocotyl medullary cavity of cucumber seedlings treated with P,EBR and EBR+P was significantly reduced,but there was no significant difference between the three treatments.(4)Observing the changes of callose in the hypocotyls of cucumber seedlings by paraffin sectioning and aniline blue staining.After the disease-resistant cucumber seedlings were infected by Phytophthora,the accumulation of callose in the hypocotyls was not obvious,and the exogenous EBR pretreatment could promote the accumulation of callose.On the contrary,after the infection of susceptible cucumber seedlings,the accumulation of callose in the hypocotyls of seedlings increased significantly.EBR+P accumulated the most,while EBR treatment had little effect on the accumulation of callose in the hypocotyls of susceptible cucumber seedlings.(5)The relative expression of some cucumber tannin synthase-related genes was detected by qRT-PCR.Cal S5(Csa7G429540,Csa2G302180,Csa2G302250)and Cal S1(Csa7G429560)were induced by EBR in the hypocotyls of cucumber seedlings of two resistant varieties,and the relative expression was significantly higher than that of CK.The Cal S12(Csa7G036450)gene was only induced by EBR in the resistant variety R2,and the relative expression was significantly higher than that of CK.The Cal S3(Csa3G073810)gene was only induced by EBR in the resistant variety R1,and the relative expression was significantly higher than that of CK.