Effects Of Exogenous Brassinolide On Root Development Of Kentucky Bluegrass Under Cadmium Stress

Phytoremediation technology is a hot issue in the study of soil heavy metal remediation,and its application prospects are broad.Compared with some cadmium hyperaccumulator plants,which have the characteristics of annual and low biomass,turfgrass has the advantages of strong adaptability,perennial,low pruning resistance,etc.,can quickly cover the ground surface and prevent heavy metals from migrating to other places through water or wind erosion.The turfgrass has significant advantages in repairing Cd-contaminated soil compared with other super-enriched plants.Previous studies have shown that Kentucky bluegrass has strong Cd accumulation ability and Cd tolerance,but high-concentration Cd stress will severely inhibit the growth of roots.2,4-Epibrassinolide is one of the most widely used brassinosteroids in plants.The exogenous EBR can regulate the growth and development of plants,and is resistant to plant low temperature,high temperature,drought,salt and heavy metal stress all have a certain mitigation effect.At present,there are relatively few studies on the regulation of EBR on plant root growth and development under Cd stress,and its mechanism of action is still unclear.In order to explore the effect of exogenous EBR on the root growth of Kentucky bluegrass under Cd stress,this study used Cd-tolerant variety‘Midnight’and Cd-sensitive variety‘RugbyⅡ’as experimental materials,and explored different concentrations of EBR（5,50,100 n M）on the root growth and BR synthesis and signal pathway gene expression of Kentucky bluegrass under Cd stress;analysis of root antioxidant enzyme activity and gene expression,endogenous hormone content,endogenous hormone synthesis and signal transduction pathway genes expression level was designed to determine the role of BRs in regulating the growth of Kentucky bluegrass roots under Cd stress,and to initially reveal the mechanism of BRs regulating the growth of Kentucky bluegrass roots under Cd stress.The main findings are as follows:（1）The root length of‘Midnight’and‘RugbyⅡ’were reduced by 40.52%and33.11%on the sixth day of Cd stress,respectively.Applying 5 n M EBR under Cd stress can significantly increase the root length of Kentucky bluegrass compared with Cd stress,the root lengths of‘Midnight’and‘RugbyⅡ’increased by 62.62%and35%,respectively.Applying different concentrations of EBR under Cd stress can significantly increase the root crossing number of Kentucky bluegrass,but the root crossing number under 5 n M EBR treatment is the highest.In addition,exogenous 5n M EBR can significantly promote the development of lateral root primordium.Therefore,5 n M EBR can significantly promote the root growth of Kentucky bluegrass under Cd stress.In the early stage of Cd stress,the BR synthesis genes PpCPD and PpDET in the root system of Kentucky bluegrass‘Midnight’were all up-regulated,while the expression of PpCPD and PpDET in the root system of‘RugbyⅡ’was down-regulated.PpCPD and PpDET were up-regulated in the roots of the two cultivars in the late stage of stress.The expression levels of PpCPD and PpDET in the roots of‘RugbyⅡ’at the initial stage of Cd stress under 5 and 100 n M EBR treatments were greater than those of the Cd stress control.The BR signaling pathway of Kentucky bluegrass regulates gene expression,such as PpBZR,PpCDG and PpBIN,and so on,which have significant changes under Cd stress and exogenous EBR treatment.Therefore,BR synthesis and signal transduction pathway genes may also be involved in the regulation of Cd-stressed Kentucky bluegrass root growth.（2）Compared with Cd stress,applying 5 n M EBR under Cd stress,the CAT activity of‘Midnight’and‘RugbyⅡ’increased by 18.29%and 29.51%,respectively;POD activity increased by 51.00%and 58.16%,respectively;SOD activity increased by 11.43%and 7.91%,respectively;APX activity increased by 21.55%and 22.68%,respectively;GR activity increased by 37.83%and 2.35%;PpCAT gene expression in‘Midnight’roots increased significantly,as The Cd stress treatment was 3.49 times,the expression of PpCAT gene in‘RugbyⅡ’did not change significantly;the expression of PpGR and PpAPX genes were also significantly up-regulated.In addition,compared with Cd stress,applying 5 n M EBR can reduce the O₂^·-production rate and H₂O₂accumulation of Kentucky bluegrass roots,indicating that exogenous EBR can alleviate the Cd stress caused by promoting the antioxidant defense system of Kentucky bluegrass roots of oxidative damage.（3）Cd stress treatment reduced the content of IAA,t ZR,IPA and JA in the roots of Kentucky bluegrass,and increased the content of ABA.Under Cd stress,exogenous EBR treatment can increase the content of IAA and EBR in the roots of Kentucky bluegrass.In addition,exogenous EBR can regulate the up-regulation of the root system under Cd stress.The expression of the‘Midnight’IAA synthetic genes PpTAR2 and PpYUCCA,the IAA transport gene PpPIN,the‘RugbyⅡ’IAA synthetic gene PpYUCCA,and the IAA signal gene PpIAA6 are up-regulated The expression indicates that exogenous EBR can alleviate the inhibitory effect of Cd stress on the root growth of Kentucky bluegrass by coordinating regulation of auxin synthesis,transport and signal transduction.