Synthesis patterns of vitamin C in Actinidia and identification of key genes

We investigated the developmental patterns of vitamin C (Vc) in hybrid Actinidia species to explore key genes regulating Vc synthesis and provide references and data for the creation of Actinidia germplasms with high Vc content. The fruits of three different hybrid kiwifruit populations were used as test material to investigate Vc synthesis patterns by measuring Vc content in fruit at different developmental stages. Key genes for Vc synthesis in Actinidia were determined using quantitative real-time polymerase chain reaction (qRT-PCR). The protein structure, physicochemical properties, and hydrophilicity of the GGP gene, and the regulation of Vc synthesis in fruits from inbred populations were analyzed. Results showed that Vc inheritance from different cross combinations differed:when Actinidia species with high Vc were used as parents, Vc content in hybrid offspring reached a maximum in the early stage (60 d after flowering) and then decreased gradually with fruit development. In contrast, Vc content in the fruit of offspring from parents with low Vc content (A. rufa cv. 'Guihai') was low and the development pattern was similar to that of the female (A. rufa). The relative expression level of the GGP gene among other Vc synthesis-related genes was significantly higher in the high Vc inbred population than in the low Vc inbred population. Thus, as a key gene for Vc synthesis, GGP was selected for further analysis, with three homologous genes identified (GGP1, GGP3, and GGP2). The α-helix site region in the secondary structure of GGP1 and GGP3 was relatively conserved, and the tertiary spatial structure, physical and chemical properties, and hydrophilicity were also highly similar. In contrast, GGP2 was quite different. Association analysis was used to verify the functions of GGP1 and GGP3 in the offspring fruit of the three hybrid populations, which suggested that GGP3 may play the most important role in Vc synthesis. Our results indicated that the genetic trends of Vc content in the different Actinidia hybrid populations differed. Furthermore, Vc synthesis in the hybrid fruit from middle and high Vc populations reached a maximum in the early and middle stages of fruit development. GGP3 may have an important effect on Vc synthesis in the fruit of offspring from different hybrid populations.