Differential Expression of Organic Acid Degradation-Related Genes During Fruit Development of Navel Oranges (Citrus sinensis) in Two Habitats

Organic acids as well as soluble sugars contribute highly to flavor and overall quality of citrus fruit. Citric acid level in fruit is influenced by several factors including environmental conditions. In this study, it was observed that different environments in two habitats (Ganzhou, Jiangxi; Songyang, Zhejiang) had minor effects on total soluble solids and citrus color index but had significant effects on organic acids levels, particularly on citric acid level, in fruit of "Newhall" and "SkaggsBonanza" navel oranges (Citrus sinensis). Expression of genes involved in citric acid biosynthesis and degradation (CitCS1, CitCS2, CitAco1, CitAco2, CitAco3, CitIDH1, CitIDH2, CitIDH3, CitGAD4, CitGAD5, and CitGS2) was analyzed in fruit grown in each of the two habitats. Citric acid biosynthesis-related citrate synthase genes were steadily expressed during navel orange fruit development, while degradation-related genes were differentially expressed. These findings suggested that the influence of different environments on fruit quality traits was predominant on the regulation of organic acids level, particularly on the degradation of citric acid. A cascade of CitAco3-CitIDH1-CitGS2 might be involved in citric acid degradation in response to different environments during fruit growth and development.