5-Aminolevulinic acid promotes cell division and young fruit development by suppressing the PpARF9–PpGH3.1 auxin-conjugation module in peach

5-Aminolevulinic acid (ALA) is an emerging plant growth regulator, yet its mechanisms in fruit development remain unclear. Here, we show that ALA promotes early growth and ultimate size in peach (Prunus persica) fruits. ALA treatment significantly enhanced the mesocarp area and the cell number, accompanied by elevated levels of indole-3-acetic acid (IAA) and trans-zeatin riboside. Transcriptomic analysis identified PpARF9 as a key transcriptional factor, regulating auxin homeostasis and responsive to ALA treatment. Functional studies with yeast-one-hybrid (Y1H), dual-luciferase (DLR), electrophoretic mobility shift assay (EMSA) and homogenous genetic transformation revealed that PpARF9 directly transcriptionally activates PpGH3.1, a crucial gene which encodes chloroplastic auxin amide synthetase to conjugate free IAA and repress cell division. Overexpression of PpGH3.1 reduced free IAA with more IAA conjugate accumulation, thereby suppressing cell cycle. Conversely, silencing PpGH3.1 with RNA interference had opposite effects. Notably, exogenous ALA alleviated the inhibitory effects of PpARF9 and PpGH3.1 on cell proliferation. These findings indicate that ALA promotes cell division by downregulating the PpARF9-PpGH3.1 module to maintain higher levels of active IAA, facilitating cell division, increasing cell number and fruit size. These provide a mechanistic insight into ALA-regulated fruit development and support its application in improving fruit production.