Auxin response factors (ARFs) are important transcription factors to relay auxin signaling. From the Genome Database for Rosaceae (GDR), we identified 17 peach ARF genes (PpARFs) encoding the proteins with three conserved domains. Their gene structure and functional domains were analyzed. Their transcriptional response to exogenous auxin treatment was tested and confirmed. We also expressed PpARF-GFP fusion reporters in tobacco leaves and observed their nuclear localization by fluorescence microscopy. It has been known that ARFs are widely involved in fruit development. We compared the expression pattern of all PpARFs in different tissues including the fruits at different developmental stages of two peach cultivars, “melting” and “stony hard”. We found eight PpARFs were more highly expressed in the “melting” peaches compared to “stony hard” peaches, while three PpARFs were more highly expressed in “stony hard” peaches. Among them, the expression difference of PpARF4, PpARF7 and PpARF12 was large, and their function in regulating fruit development and fruit quality was discussed. Our work provides a basis for further exploring the mechanisms underlying auxin regulated peach fruit ripening.

The Auxin/indole-3-acetic acid (Aux/IAA) repressor genes down-regulate the auxin response pathway during many stages of plant and fruit development. In order to determine if and how Aux/IAAs participate in governing texture and hardness in stone fruit maturation, we identified 23 Aux/IAA genes in peach, confirmed by the presence of four conserved domains. In this work, we used fluorescence microscopy with PpIAA-GFP fusion reporters to observe their nuclear localization. We then conducted PCR-based differential expression analysis in “melting” and “stony hard” varieties of peach, and found that in the “melting” variety, nine PpIAAs exhibited peak expression in the S4-3 stage of fruit maturation, with PpIAA33 showing the highest (>120-fold) induction. The expression of six PpIAAs peaked in the S4-2 stage, with PpIAA14 expressed the most highly. Only PpIAA15/16 showed higher expression in the “stony hard” variety than in the “melting” variety, both peaking in the S3 stage. In contrast, PpIAA32 had the highest relative expression in buds, flowers, young and mature leaves, and roots. Our study provides insights into the expression patterns of Aux/IAA developmental regulators in response to auxin during fruit maturation, thus providing insight into their potential development as useful markers for quantitative traits associated with fruit phenotype.

Fruit softening is an important characteristic of peach fruit ripening. The auxin receptor TIR1 (Transport Inhibitor Response 1) plays an important role in plant growth and fruit maturation. Still, little research has been conducted on the relation of TIR1 to the softening of peach fruits. In this study, the hardness of isolated peach fruits was reduced under exogenous NAA treatment at low concentrations. At the same time, the low concentration of NAA treatment reduced the transcription level of PpPG and Ppβ-GAL genes related to cell wall softening and PpACS1 genes related to ethylene synthesis. The transient overexpression of the PpTIR1 gene in peach fruit blocks caused significant down-regulation of the expression of early auxin-responsive genes, ethylene synthesis, and cell wall metabolic genes related to fruit firmness. Through yeast two-hybrid technology, bimolecular fluorescence complementary technology, and a firefly luciferase complementation imaging assay, we were able to unveil an interaction between PpTIR1 and PpIAA1/3/5/9/27 proteins. Furthermore, it was determined that the interaction depended on auxin and its type and concentration. These results show that the PpTIR1-Aux/IAA module has a possible regulatory effect on fruit ripening and softening.

Fruit softening is an important characteristic of peach fruit ripening. The auxin receptor TIR1 (Transport Inhibitor Response 1) plays an important role in plant growth and fruit maturation, but little research has been conducted on the relation of TIR1 to the softening of peach fruits. In this study, the hardness of isolated peach fruits was reduced under exogenous NAA treatment of low concentration, at the same time, the low concentration of NAA treatment reduced the transcription level of PpPG and Ppβ-GAL genes related to cell wall softening, and PpACS1 genes related to ethylene synthesis. The transient over-expression of the PpTIR1 gene in peach fruit blocks caused significant down-regulation of the expression of early auxin-responsive genes, ethylene synthesis and cell wall metabolic genes related to fruit firmness. Through yeast two-hybrid technology, bimolecular fluorescence complementary technology, and firefly luciferase complementation imaging assay, an interaction between PpTIR1 and PpIAA1/3/5/9/27 proteins was revealed, and the interaction depended on auxin and its type and concentration. These results show that PpTIR1-Aux/IAA module has a possible regulatory effect on fruit ripening and softening.