Genome-wide identification of the peach LOB/LBD genes and the positive role of the PpNAP4–PpLOB1 module in peach fruit softening

Softening of fleshy fruits during ripening and postharvest is a programmed physiological process that substantially impacts fruit quality and shelf life. However, the molecular mechanism underlying peach softening remains largely unknown. Lateral organ boundary (LOB) domain (LBD) proteins are pivotal regulators of plant growth and development. To date, certain LOB/LBD transcription factors are seemingly implicated in fruit softening. In this study, we identified 42 LOB/LBD genes in the peach genome. Expression analysis showed a significant upregulation of PpLOB1 transcripts toward peach fruit ripening. PpLOB1 was classified into Class Ⅱ subgroup, and showed high sequence similarity to several softening-related LOB/LBD transcription factors. Transient transformation assays showed that PpLOB1 positively modulates peach softening. Further experiments demonstrated that PpLOB1 directly targets and activates the promoters of pectate lyase 1 (PpPL1) and PpPL15, thereby contributing to the regulation of fruit softening. Additionally, PpNAP4 up-regulates PpLOB1 expression by binding to its promoter. Meanwhile, our findings revealed that PpNAP4 and PpNAP6 cooperatively modulate the expression of PpLOB1. Taken together, our findings revealed a novel regulatory module involving PpNAP4 and PpLOB1 that modulates peach fruit softening.