The transcription factor CmERFI-2 represses CmMYB44 expression to increase sucrose levels in oriental melon fruit

Soluble sugar accumulation in fruit ripening determines fleshy fruit quality. However, the molecular mechanism for this process is not yet understood. Here, we showed a transcriptional repressor, CmMYB44 regulates sucrose accumulation and ethylene synthesis in oriental melon (Cucumis. melo var. makuwa Makino) fruit. Overexpressing CmMYB44 suppressed sucrose accumulation and ethylene production. Furthermore, CmMYB44 repressed the transcriptional activation of CmSPS1 (sucrose phosphate synthase 1) and CmACO1 (ACC oxidase 1), two key genes in sucrose and ethylene accumulation, respectively. During the later stages of fruit ripening, the repressive effect of CmMYB44 on CmSPS1 and CmACO1 could be released by overexpressing CmERFI-2 (ethylene response factor I-2) and exogenous ethylene in HS fruit (high sucrose accumulation fruit). CmERFI-2 acted upstream of CmMYB44 as a repressor by directly binding the CmMYB44 promoter region, indirectly stimulating the expression level of CmSPS1 and CmACO1. Taken together, we provided a molecular regulatory pathway mediated by CmMYB44, which determines the degree of sucrose and ethylene accumulation in oriental melon fruit and sheds light on transcriptional responses triggered by ethylene sensing that enable the process of fruit ripening. An ethylene response factor derepresses expression of ethylene and sucrose synthesis genes to improve sucrose accumulation during oriental melon fruit ripening.

Automated summary

In this study, a transcriptional repressor, CmMYB44, was found to regulate sucrose and ethylene accumulation in oriental melon fruit by repressing the expression of key genes CmSPS1 and CmACO1. The inhibitory effect of CmMYB44 on these genes could be relieved by overexpressing CmERFI-2 and exogenous ethylene. The findings revealed a molecular regulatory pathway mediated by CmMYB44 and provided insights into the transcriptional responses triggered by ethylene sensing during fruit ripening.