The microRNA ppe-miR393 mediates auxin-induced peach fruit softening by promoting ethylene production

Auxin can inhibit or promote fruit ripening, depending on the species. Melting flesh (MF) peach fruit (Prunus persica L. Batsch) cultivars produce high levels of ethylene caused by high concentrations of indole-3-acetic acid (IAA), which leads to rapid fruit softening at the late stage of development. In contrast, due to the low concentrations of IAA, the fruit of stony hard (SH) peach cultivars does not soften and produces little ethylene. Auxin seems necessary to trigger the biosynthesis of ethylene in peach fruit; however, the mechanism is not well understood. In this study, we identified miRNA gene family members ppe-miR393a and ppe-miR393b that are differentially expressed in SH and MF fruits. RNA ligase-mediated 5′ rapid amplification of cDNA ends and transient transformation of Nicotiana benthamiana revealed TRANSPORT INHIBITOR RESPONSE 1 (PpTIR1), part of the auxin perception and response system, as a target of ppe-miR393a and b. Yeast 2-hybrid assay and bimolecular fluorescence complementation assay revealed that PpTIR1 physically interacts with an Aux/IAA protein PpIAA13. The results of yeast 1-hybrid assay, electrophoretic mobility shift assay, and dual-luciferase assay indicated that PpIAA13 could directly bind to and trans-activate the promoter of 1-aminocyclopropane-1-carboxylic acid synthase 1 (PpACS1), required for ethylene biosynthesis. Transient overexpression and suppression of ppe-miR393a and PpIAA13 in peach fruit induced and repressed the expression of PpACS1, confirming their regulatory role in ethylene synthesis. Gene expression analysis in developing MF and SH fruits, combined with postharvest α-naphthalene acetic acid (NAA) treatment, supports a role for a ppe-miR393-PpTIR1-PpIAA13-PpACS1 module in regulating auxin-related differences in ethylene production and softening extent in different types of peach.