Restraint of Bagging on Fruit Skin Coloration in on-Tree Kiwifruit (Actinidia arguta)

Fruit skin color is an important characteristic of fruit quality. The light-mediated regulation on fruit skin coloration in Actinidia arguta remains unclear. To better understand the role of light in fruit skin coloration, we performed bagging treatments in both on-tree and off-tree ‘Hongbaoshixing’, which is a kind of all-red-typed A. arguta cultivar. Non-bagging kiwifruits were used as control. For off-tree fruits, there was no difference between non-bagging and bagging treatments. For on-tree fruits, physiological and molecular changes were investigated during fruit development in non-bagging and bagging treatments. Phenotypic identification and the hue angle measurement showed that the stage of most significant color difference between non-bagging and bagging treatments was 130 days (after full bloom). Determination of five anthocyanin components suggested cyanidin-3-O-galactoside and cyanidin-3-O-xylose-galactoside made a main contribution to the fruit skin coloration. Gene expression profiles and cluster analysis showed AaLDOX and AaUFGT were highly expressed at 130 days and obviously clustered into the same class in non-bagging treatment, respectively. Correlation analysis suggested only AaLDOX expression was significantly correlated with anthocyanin content in non-bagging treatment while no correlation in bagging treatment. Similar results was observed for MYB1 transcription factor. The result of subcellular localization showed that AaLDOX was located in the cytoplasm, indicating AaLDOX is indeed structural gene that encodes leucoanthocyanidin dioxygenase participated in anthocyanin biosynthesis. All results were used to establish a possible working model, showing that light is indispensable for normal fruit skin coloration, and bagging treatment suppresses anthocyanin biosynthesis and accumulation mainly by inhibiting AaMYB1 and AaLDOX expression in A. arguta .