VvARF19 represses VvLBD13-mediated cell wall degradation to delay softening of grape berries

The fruit softening directly impacts its storage life, transportability, and customer acceptance. Auxin plays a key role during fruit ripening, but the underlying mechanisms of how auxin regulates fruit softening remain unclear. In this study, we investigated the regulatory roles of auxin on berry cell wall degradation during grape (Vitis vinifera L.) softening. During grape berry development, berry firmness and auxin content are both firstly increasing and then decreasing, and peaks occur at 4-6 WAFB (weeks after full blooming). Exogenous NAA (α-naphthalene acetic acid, a synthetic auxin) treatment inhibits berry softening by delaying propectin, cellulose and hemicellulose degradation, which maintains cell wall integrity in the grape flesh. Weighted gene co-expression network analysis (WGCNA) shows that VvLBD13 correlated with VvARF19 could be a key gene in this delaying of berry softening, which involved in auxin signal transduction and cell wall degradation metabolism. Over-expression and transient over-expression of VvLBD13 in tomato or in grape berry indicate that VvLBD13 accelerates hemicellulose degradation by binding the promoters of VvXTH10 (the xyloglucan endotransglucosylase/hydrolase 10) and VvEXPLA1 (expansion-like A1), which results in rapid softening after veraison. Collectively, this research furnishes an exhaustive understanding of the auxin- driven regulatory mechanisms of grape berry softening.