The Change And Mechanism Of Fruit Coloration Of ’Ruixue’ Apple After Bagging Treatment

‘Ruixue’,newly bred by Northwest A&F University,is a late maturing yellow-green apple cultivar.It was found that‘Ruixue’could colored after bagging treatment.‘Ruixue’and other 10 non-red varieties as plant materials to explore the effect of bagging on the color of non-red varieties.We detected the coloring difference of‘Fuji’and‘Ruixue’after bag removal and analyzed the expression of genes related to anthocyanin synthesis and the methylation level change of MdMYB1 promoter.The main results are as follows:1.Eleven non-red varieties could colored after bag removal,but the coloring speed and degree are quite different.Yellow varieties,such as‘Golden Delicious’,have little change in L~*and a~*,but the b~*value keeps increasing,and the content of anthocyanin is low(1～4 mg/g FW).In green varieties such as‘India’and‘Granny Smith’,L~*decreased,a~*increased,b~*decreased,anthocyanin content even reached 100 mg/g FW,The chlorophyll content and carotenoid content of‘India’were significantly higher than those of other varieties.The anthocyanin synthesis rate of lightly colored fruits,such as‘Golden Delicious’,was significantly lower than that of‘Granny Smith’and‘India’,indicating that green varieties were easier to color than yellow varieties.2.qRT-PCR was used to detect the expression level of genes related to anthocyanin synthesis in three representative varieties of‘Golden Delicious’,‘Wanglin’and‘Granny Smith’.It was found that the expression level of‘Golden Delicious’MdMYB1 increased by1.5 times on the 7th day after bagging,but decreased on the 14th day.However,‘Granny Smith’increased by 6 times on the 7th day and increased by 9 times on the 14th day after bagging,The expression of MdMYB1 promoted the accumulation of anthocyanin in‘Granny Smith’,which led to the rapid and obvious coloring of‘Granny Smith’.3.qRT-PCR was used to detect the expression level of genes related to anthocyanin synthesis.It was found that the expression level of MdMYB1 in‘Ruixue’increased steadily after bagging,reached the peak on the third day after bagging,and then began to decline.The expression level of MdMYB1 in‘Fuji’increased on the first day,and remained unchanged after bagging.The expression of MdCHS、MdF3H、MdANS and MdUFGT peaked on the second day after bagging,while the expression of‘Fuji’peaked on the third day.4.The analysis of promoter methylation level showed that,the methylation level of MdMYB1 promoter of‘Ruixue’was higher in-1901～-1648bp and-1670～-1179bp regions;after bagging,the methylation levels of-1901～-1179bp,-1200～-820bp and-354～-43bp of MdMYB1 promoter in‘Ruixue’decreased;after bag removal,the methylation level of‘Ruixue’was significantly decreased in-1200～-820bp and-354～-43bp regions,while‘Fuji’remained unchanged.