Natural variation in the NAC transcription factor NONRIPENING contributes to melon fruit ripening

The NAC transcription factor NONRIPENING (NOR) is a master regulator of climacteric fruit ripening. Melon (Cucumis melo L.) has climacteric and non-climacteric fruit ripening varieties and is an ideal model to study fruit ripening. Two natural CmNAC-NOR variants, the climacteric haplotype CmNAC-NOR^S,N and the non-climacteric haplotype CmNAC-NOR^A,S , have effects on fruit ripening; however, their regulatory mechanisms have not been elucidated. Here, we report that a natural mutation in the transcriptional activation domain of CmNAC-NOR^S,N contributes to climacteric melon fruit ripening. CmNAC-NOR knockout in the climacteric-type melon cultivar "BYJH" completely inhibited fruit ripening, while ripening was delayed by 5-8 d in heterozygous cmnac-nor mutant fruits. CmNAC-NOR directly activated carotenoid, ethylene, and abscisic acid biosynthetic genes to promote fruit coloration and ripening. Furthermore, CmNAC-NOR mediated the transcription of the "CmNAC-NOR-CmNAC73-CmCWINV2" module to enhance flesh sweetness. The transcriptional activation activity of the climacteric haplotype CmNAC-NOR^S,N on these target genes was significantly higher than that of the non-climacteric haplotype CmNAC-NOR^A,S . Moreover, CmNAC-NOR^S,N complementation fully rescued the non-ripening phenotype of the tomato (Solanum lycopersicum) cr-nor mutant, while CmNAC-NOR^A,S did not. Our results provide insight into the molecular mechanism of climacteric and non-climacteric fruit ripening in melon.