Effects of exogenous abscisic acid on the expression of citrus fruit ripening-related genes and fruit ripening
Despite exogenous abscisic acid (ABA) treatment is a widely-used effective method to prove its role in plants, this method has been limitedly explored in citrus fruits. Here, we examined the effects of exogenous ABA and nordihydroguaiaretic acid (NDGA) on the color index H value and the contents of soluble sugars, organic acids and endogenous hormones in the fruits of Citrus reticulata Blanco cv. Ponkan. Additionally, the expression profiles of genes involved in the ABA, sugar and organic acid metabolism and signal transduction pathway in fruits were also investigated. The results indicated that exogenous ABA could accelerate fruit coloring, significantly decrease the organic acid content, and affect the ripening of citrus fruit, while treatment with NDGA could restrain fruit coloring and acid degradation. Exogenous ABA treatment produced minor effect on the accumulation of sugars but largely regulated the expression of most sugar- and acid-related genes, indicating a coordinate interaction between the ABA signaling pathway and sucrose signaling pathway to regulate citrus fruit ripening. Especially, CsSUC 3 plays a vital role in accumulation of sugars during the last ripening stage of citrus fruit, which could be induced by ABA treatment. The transcript levels of CsACO1 and CsNADP-IDH in ABA-treated fruits were significantly higher than in the control fruits during the late ripening stages, which was conducive to the degradation of organic acids. Moreover, ABA could restrict its own accumulation at 14 DAY not only by inducing the expression of ABA 8′-hydroxylase 1 but also by reducing the expression of CsNCED1, and the transcriptional levels of other ABA-related genes were largely triggered by exogenous ABA. Overall, ABA is a positive regulator of ripening and could be used to regulate citrus fruit ripening effectively.