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SlNCED1 and SlCYP707A2 are key genes in the regulation of ABA synthesis and catabolism, and are involved in fruit ripening as positive and negative regulators, respectively.
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PacCYP707A2 plays a primary role in regulating ABA levels during the onset of cherry fruit ripening, while PacCYP707A1 regulates the ABA content in response to dehydration.
To further understand the basis of abscisic acid (ABA) signal transduction in regulating sweet cherry fruit development and ripening, and also the interaction between ABA and indole acetic acid (IAA) at the onset of fruit ripening, full-length or partial cDNA of three PaPYLs (ABA receptor), six PaPP2Cs (type 2C protein phosphatase) and six PaSnRK2s (subfamily 2 of SNF1-related kinases) were identified from sweet cherry. Multiple alignments and phylogeny analyses showed that most of the functional residues or domains were well conserved within each gene family; all PaPP2Cs belonged to the group A PP2C; PaSnRK2.4 and PaSnRK2.5 belonged to subclass III of SnRK2. PaPYL2/3, PaPP2C3/4/6 and PaSnRK2.4 were highly expressed at the early stages of fruit development. Moreover, PaPYL2, PaPP2C3/4, PaSnRK2.4 also had a high expression level at the onset of fruit ripening. Exogenous ABA treatment at 28 days after full bloom decreased the IAA level and promoted fruit ripening by increasing anthocyanin and soluble solids content. However, although IAA treatment induced the ABA accumulation, it delayed the onset of fruit ripening. Most of the PaPYLs and PaSnRK2s didn't strongly respond to ABA and IAA treatments at 28 days after full bloom, while expressions of PaPP2C3, PaPP2C5 and PaPP2C6 were significantly induced by exogenous ABA and PaPYL1 was significantly induced by exogenous IAA.
Abscisic acid (ABA) glucose conjugation mediated by uridine diphosphate glucosyltransferases (UGTs) is an important pathway in regulating ABA homeostasis. In the present study, we investigated three tomato SlUGTs that are highly expressed in fruit during ripening, and these SlUGTs were localized to the cytoplasm and cell nucleus. Among these three UGTs, SlUGT75C1 catalyzes the glucosylation of both ABA and IAA in vitro; SlUGT76E1 can only catalyze the conjugation of ABA; and SlUGT73C4 cannot glycosylate either ABA or IAA. Therefore, SlUGT75C1 was selected for further investigation. SlUGT75C1 RNA interference significantly up-regulated the expression level of SlCYP707A2, which encodes an ABA 8'-hydroxylase but did not affect the expression of SlNCED1, which encodes a key enzyme in ABA biosynthesis. Suppression of SlUGT75C1 significantly accelerated fruit ripening by enhancing ABA levels and promoting the early release of ethylene. SlUGT75C1-RNAi altered the expression of fruit ripening genes (genes involved in ethylene release and cell wall catabolism). SlUGT75C1-RNAi seeds showed delayed germination and root growth compared with wild-type as well as increased sensitivity to exogenous ABA. SlUGT75C1-RNAi plants were also more resistant to drought stress. These results demonstrated that SlUGT75C1 plays a crucial role in ABA-mediated fruit ripening, seed germination, and drought responses in tomato.
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