Preliminary Study On R6 Genotypic White Flesh Genetic Variation Mechanism Of Type ? Red-fleshed Apple

Color of apples is an important economic trait.Anthocyanin concentration is the main determinant of red color.Red-fleshed apple appeals more to consumers due to its bright color in cortex and abundance in antioxidant compounds such as anthocyanin.Study on mechanism that causeswhite flesh genetic variation can bring application and significance in apple genetic improvement.We found 4 apple offspring which were characterized by red leaf but white flesh in the nature population of type I red-fleshed apple ‘No.1 Hongxun'.These apple trees had red leaf,red flower but white flesh,denoted aswhite-fleshed in this research.In order to provide information about this genetic variation,we carried out research including phenotype identification,allelic variation detection of MYB10 loci,investigation of flesh color and phenolic compounds variation,q RT-PCR analysis and enzyme activity assay of anthocyanin accumulation related genes.The main results are as follows:1.Anthocyanin contents of flowers and leaves in white-fleshed offspring were similar to red-fleshed apples,however,the content was low in mature cortex(less than 5mg/kg).The coding sequence of MYB10 in 4 white-fleshed offspring was identical to their red-fleshed parent ‘No.1 Hongxun',and with high similarity to the known apple MYB10.Besides,we also showed that the 4 white-fleshed offspring are R1R6 genotype,and had no SNP variation in R6 region when compared with ‘No.1 Hongxun'.2.Further investigation of fruit flesh development showed that the cortex of both white-fleshed and red-fleshed apples had high anthocyanin content(more than 150mg/kg)at 25 DAFB,and therefore were red-fleshed.The anthocyanin concentration then decreased and the red color faded at 50 DAFB.However,from 75 DAFB to 150 DAFB,cortex of white-fleshed offspring remained colorless due to its low anthocyanin concentration,whereas red-fleshed apples pigmented again.Besides,other phenolic compounds such as chlorogenic acid and quercetin derivatives had no significant differences with ‘No.1 Hongxun' in common.3.Expression of anthocyanin biosynthesis and transporter gene such as Md CHS,Md CHI,Md DFR,Md ANS,Md UFGT1,Md UFGT2 and Md GST4 in 4 white-fleshed offspring was significantly lower than that in red-fleshed apples at 75 DAFB and 100 DAFB.Furthermore,enzyme activity of DFR and UFGT in white-fleshed offspring was significantly lower than that in red-fleshed apple during most of fruit development.Transcript level of Md MYB10 in white-fleshed offspring was significantly lower than that in red-fleshed apples only at 100 DAFB,whereas Mdb HLH3?Mdb HLH33 and Md WD40 showed no significant difference.Expression of Md IAA1,whichwas supposed to negatively regulate anthocynin accumulation,was significantly higher in white-fleshed offspring than that in red-fleshed at 75 DAFB,100 DAFB,125 DAFB and 150 DAFB.In the meantime,anthocyanin concentration in cortex of white-fleshed offspring was significantly lower than that in red-fleshed apples.Transcript level of other repressor genes such as Md MYB111 and Md COP1 was more abundant in white-fleshed offspring only at 75 DAFB and 150 DAFB,75 DAFB and 100 DAFB,respectively.In conclusion,we showed that R6 genotypicwhite flesh genetic variationwas spatial and temporal.Flower and leaf of the white-fleshed offspring accumulated high content of anthocyanin,whereas cortex was red at early developmental stage and became white afterwards till maturity.Besides,expression of anthocyanin biosynthesis and transporter genes was repressed in white-fleshed offspring,which was presumed to be related to high transcript level of Md MYB111,Md COP1 and Md IAA1.