The storability of fresh longan is negatively correlated with the accumulation of organic acids throughout storage. Our previous work reported that the expressions of six genes (DlACL, DlLDH, DlPEPC, DlSCS, DlAST, DlMDH) related to organic acid synthesis increased during storage. Moreover, compared to the good storability ‘Dongbi’ longan, these genes maintained higher expression levels in the poor storability ‘Fuyan’ longan. Here, one bHLH transcription factor, DlbHLH66, was identified. It was upregulated in parallel with fruit deterioration in postharvest longan, and also kept higher levels in poor storability ‘Fuyan’ longan, compared with ‘Dongbi’ longan. Subcellular localization and transactivation assays showed that DlbHLH66 is a globally localized transcriptional activator. Furthermore, electrophoretic mobility shift assay (EMSA), dual-luciferase reporter (DLR) assay and chromatin immunoprecipitation quantitative PCR (ChIP-qPCR) analysis demonstrated that DlbHLH66 directly bound to the promoters of those six organic acid synthesis genes, and activated their transcription. Additionally, transient overexpression of DlbHLH66 in the pulp of longan fruit upregulated the expression of organic acid synthesis genes, accelerated pulp breakdown and acidity increase, thereby promoting fruit deterioration. In summary, DlbHLH66 acts as an activator to accelerate fruit deterioration by upregulating organic acid synthesis genes. These data provide novel perspectives regarding the regulatory network associated with the storability difference of fresh longan cv. ‘Fuyan’ and ‘Dongbi’ during storage.
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