A novel NAC transcription factor, PpNAP6, is involved in peach ripening by activating ethylene synthesis

Peach (Prunus persica) is a typical climacteric fruit, having an obvious peak of ethylene release during ripening. Transcriptional regulators play key roles in fruit ripening. NAP (NAC-like, activated by AP3/P1) protein is a subfamily of the NAC (NAM, ATAF1/2 and CUC2) transcription factor (TF) family. Here, we discovered the function of peach NAP subfamily NAC TF, PpNAP6, homologous to the tomato ripening-related TF NOR. In the pulp of 'Zaofengwang' peach, PpNAP6 is highly expressed, and its transcript level decreased gradually throughout the whole development period, however, it is up-regulated at onset of peach ripening. Transient transformation experiments showed that PpNAP6 over-expression in peach fruit increased ethylene release and accelerated fruit ripening, while PpNAP6 down-expression inhibited ethylene biosynthesis. Dual-luciferase reporter assay (DLR) and electrophoretic mobility shift assay (EMSA) proved that PpNAP6 can directly bind to the promoters of key ethylene biosynthesis genes PpACS1 and PpACO1, and activate their transcription. Moreover, PpNAP6 was heterologously overexpressed in tomato nor mutant, and the transgenic lines showed the overexpression of PpNAP6 could partially recover its ripening-inhibited phenotype. Interestingly, PpNAP6 activates ethylene biosynthesis during peach ripening, whereas its transcription was inhibited treated with ethephon and enhanced with 1-MCP treatment. In summary, our work revealed that PpNAP6 regulates peach fruit ripening through ethylene biosynthesis, and may be involved in the fine regulation of ethylene via a feedback mechanism.