Allelochemical P-hydroxybenzoic Acid Inhibits Root Growth Via Regulating Ros Accumulation In Cucumber(Cucumis Sativus L.)

Many crops,including cucumber,are often grown repeatedly in the same soil in intensive agriculture,result in soil sickness,such as severely poor growth and crop yield decline.Allelopathy has been believed to be responsible for the continuous agricultural obstacle faced by cucumber?Cucumis sativus L.?.When cucumber plants have been continuously cropped for 3 years,p-hydroxybenzoic acid?pHBA?was one of the main phenolic compounds in rhizospheric soil.However,the mechanism by which pHBA regulating root growth in cucumber is still unclear.In order to elucidate this issue,we analyzed the effect of pHBA on cucumber root growth.The main results and conclusions are as follows:?1?pHBA treatment could inhibit cucumber root growth in a dosage–dependent manner due to a decrease in growth rate.?2?The meristem size of pHBA-treated cucumbers was smaller than untreated cucumbers by about 28%.The root mature cortical cell length of pHBA-treated cucumbers was also slightly shorter than untreated cucumbers by about 21%.Furthermore,the expression level of three cell cycle-related genes?CDKB1;2,CYCB1;1,CYCB2;4?in pHBA-treated cucumbers were significantly lower than those in untreated cucumbers.These results indicated that the pHBA treatment inhibit root growth by decreasing meristem activity and cell elongation.?3?pHBA treatment reduced levels of H₂O₂ and O₂.^-in the root tips.Application of NADPH oxidase inhibitor diphenyleneiodonium?DPI?or the antioxidants ascorbic acid reduced ROS accumulation in root tips and inhibited root growth,suggesting that ROS play an important role in maintain cucumber root growth.Furthermore,application of pHBA together with DPI or ascorbic acid further inhibited ROS accumulation and root growth,indicating that reduced ROS levels in roots tips is responsible for the root growth defect caused by pHBA treatment.?4?The expression levels of Peroxidases?POD?genes were upregulated upon pHBA treatment.The expression levels of Catalase?CAT??metallothioneins?MTs?and Superoxide dismutase?SOD?also slightly were increased after pHBA treatment.However,our expression analysis showed that none of the NADPH oxidase genes displayed reduced expression after pHBA treatment.Therefore,pHBA treatment reduce ROS levels in the root tip likely through modulating the expression level of ROS scavenging-related genes.Application of salicylhydroxamate?SHAM,a peroxidase inhibitor?could partially rescued the ROS accumulation and root growth defects caused by pHBA treatment.These results suggest that root growth defect caused by pHBA treatment was partially due to the evaluated ROS scavenging activity which reduced the ROS accumulation in the root tips.?5?The sensitivity of root growth of different cucumber accessions to pHBA treatment was different.pHBA less sensitive cucumber?HX25?accumulated higher levels of H₂O₂ in the root tip and showed lower expression levels of ROS scavenging-related genes than pHBA sensitive cucumber?ZN16?after pHBA treatment.These results suggest that ROS scavenging capacity paly crucial role in modulating root growth during allelopathic stress in cucumber.The higher the ROS scavenging capacity,the more severely inhibited the root elongation.