Identification And Charaterization Of MdMYB88/124 In Root Drought Tolerance Of Apple Rootstock

Apple production in China plays an important role both nationally and internationally.The two main producing area in China are loess plateau and Circum-Bohai Sea region.Various biotic and abiotic stresses restrict the apple production in these two areas;among which,drought is a very important abiotic stress factor in the loess plateau region.In this project,we comprehensively evaluated the drought resistance of two apple rootstocks,M.sieversii and R3,and identified the MdMYB88/124 transcription factors which are responsive to drought in M.sieversii root.We then thoroughly analyzed the role of MdMYB88/124 transcription factors in root drought response of apple.The main results are listed:1.Root morphology of M.sieversii and R3 rootstocks differes under long-term drought conditions.Under drought stress,the dry weight of roots,root-to-shoot ratio,total root length,total root surface area and total root volume of M.sieversii were higher than those of R3 under long-term drought conditions.Under drought stress,we also found vessel diamater is larger in M.sieversii compared with R3.These differences indicated that the roots of M.sieversii had stronger drought resistance than R3.2.RNA-seq analysis was preformed in roots of M.sieversii to discover the droughtresponsive genes.Under drought conditions,the differentially expressed genes in roots of M sieversii were mainly involved in genes regulating hormone transcriptional regulation,signal transduction or biosynthesis and oxidative stress.Consistent with the gene expression profile,roots of M.sieversii had higher activities of peroxidase(POD)and superoxide dismutase(SOD)under drought,as well as higher contents of abscisic acid(ABA)and lower content of auxin.3.From the RNA-seq results,we identified two atypical R2R3 MYB transcription factors,MdMYB88/124.,MdMYB88/124 are drought inducible in roots of M.sieversii.We then examined the root phenotype of MdMYB88/124 transgenic lines under drought stress.We found that MdMYB88 and MdMYB124 play important roles in maintaining root hydraulic conductivity under long-term drought conditions and therefore contribute towards adaptive drought tolerance.Further investigation revealed that MdMYB88 and MdMYB124 regulate root xylem development by directly binding MdVND6 and MdMYB46 promoters and thus influence expression of their target genes under drought conditions.In addition,MdMYB88 and MdMYB124 were shown to regulate the deposition of cellulose and ligninroot cell walls in response to drought.Taken together,our results provide novel insights into the importance of MdMYB88 and MdMYB124 in root architecture,root xylem development,and secondary cell wall deposition in response to drought in apple trees.4.MdMYB88/124 regulate the acitvity of Malate mutase in apple roots,the content of phenylalanine,flavone and flavonoid,and expression of related genes by directly regulating expression of MdCM2 and indirectly modulating other MdCMs genes.Thus,MdMYB88/124 might regulate resistance to fungal disease.