Autosuppression of MdNAC18.1 endowed by a 61-bp promoter fragment duplication delays maturity date in apple

Abstract

Maturity date considerably influences fruit marketing period and commercial value and it is of particular importance in apple due to its association with fruit firmness that determines storage and shelf life, but the underlying mechanism remains unclear. In this study, we report a 61-bp fragment duplication in the MdNAC18.1 promoter that underpins maturity date variation in apple. MdNAC18.1 is the crucial major gene for maturity date and was found to regulate fruit ripening by activating transcription of ethylene biosynthetic genes and ripening-related transcription factors, including the MdNAC18.1 homologue MdNAC72 and the main regulator of JA signalling MdMYC2. Interestingly, MdNAC18.1 was capable of binding to the promoter itself containing an additional NAC recognition site that arose from the 61-bp duplication to repress its own expression, but could not bind to its own promoter without the 61-bp duplication. Thus, the MdNAC18.1 allele with autosuppression function produces a phenotype of delayed maturity date and slower softening of fruit compared to that without autoregulation function. Our results demonstrate an autosuppression module that regulates the overall tempo of fruit ripening through fine-tuning ethylene biosynthesis.