Molecular cloning and functional characterization of AcGST1, an anthocyanin-related glutathione S-transferase gene in kiwifruit (Actinidia chinensis)

AcGST1, an anthocyanin-related GST, may functions as a carrier to transport anthocyanins from ER to tonoplast in kiwifruit. It was positively regulated by AcMYBF110 through directly binding to its promoter. Anthocyanins are synthesized in the cytoplasmic surface of the endoplasmic reticulum but accumulate predominantly in the vacuole. Previous studies in model and ornamental plants have suggested that a member of the glutathione S-transferase (GST) gene family is involved in sequestration of anthocyanins into the vacuole. However, little is known about anthocyanin-related GST protein in kiwifruit. Here, four putative AcGSTs were identified from the genome of the red-fleshed Actinidia chinensis cv 'Hongyang'. Expression analyses reveal only the expression of AcGST1 was highly consistent with anthocyanin accumulation. Molecular complementation of Arabidopsis tt19 demonstrates AcGST1 can complement the anthocyanin-less phenotype of tt19. Transient expression in Actinidia arguta fruits further confirms that AcGST1 is functional in anthocyanin accumulation in kiwifruit. In vitro assays show the recombinant AcGST1 increases the water solubility of cyanidin-3-O-galactoside (C3Gal) and cyanidin-3-O-xylo-galactoside (C3XG). We further show that AcGST1 protein is localized not only in the ER but also on the tonoplast, indicating AcGST1 (like AtTT19) may functions as a carrier protein to transport anthocyanins to the tonoplast in kiwifruit. Moreover, the promoter of AcGST1 can be activated by AcMYBF110, based on results from transient dual-luciferase assays and yeast one-hybrid assays. EMSAs show that AcMYBF110 binds directly to CAGTTG and CCGTTG motifs in the AcGST1 promoter. These results indicate that AcMYBF110 plays an important role in transcriptional regulation of AcGST1 and, therefore, in controlling accumulation of anthocyanins in kiwifruit.

Phenolic compounds and antioxidant activity in red- and in green-fleshed kiwifruits

Red-fleshed kiwifruits are receiving increasing attention because of their high phenolic contents. However, detailed information on their phenolic compounds and antioxidant capacities remains scarce. Here, six red-fleshed and six green-fleshed kiwifruits were investigated to determine their contents of phenolic compounds and their antioxidant capacities. The results showed chlorogenic acid, p-coumaric acid and ferulic acid were the main phenolic compounds found in kiwifruit. Most of red-fleshed kiwifruits contain higher amounts of total phenolics and anthocyanins, as well as higher activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD). Moreover, they exhibited stronger antioxidant capacities than green-fleshed kiwifruits in ABTS, DPPH and FRAP assays. Furthermore, the reactive oxygen species (ROS) inhibition assay showed the phenolics extracted from red-fleshed kiwifruit can better protect tobacco leaves against hydrogen peroxide (H2O2)-induced oxidative damage. This is because of their abundant anthocyanins which in vitro contribute more to H2O2 scavenging than the other phenolic compounds. Based on these findings, it is fair to conclude the red-fleshed kiwifruits are promising sources of antioxidants in human nutrition.