Auxin Controls Grape Berry Firmness Through a Hierarchical Gene Cascade

Grape berry softening is a crucial physiological transition that affects postharvest storage, transport, and flavor. During ripening, the cell wall components-pectin, cellulose, and hemicellulose-undergo enzymatic degradation, leading to texture loss. Although auxin and abscisic acid are known to participate in ripening regulation, the exact molecular mechanism linking auxin signaling to cell wall metabolism has remained unclear. Earlier studies suggested that auxin delays ripening onset by inhibiting polysaccharide breakdown, but the transcriptional regulators and signal networks involved were unknown. Due to these gaps in understanding, a comprehensive investigation into the auxin-dependent regulatory network controlling grape berry softening was needed.

Researchers from Henan Agricultural University have uncovered how auxin regulates grape berry firmness through a hierarchical gene cascade. The study, published (DOI: 10.1093/hr/uhae322) on February 1, 2025, in Horticulture Research, reports that auxin-responsive factor VvARF19 represses the expression of VvLBD13, which otherwise activates genes encoding cell wall–degrading enzymes. By suppressing VvLBD13, auxin effectively halts the breakdown of hemicellulose and pectin, delaying softening and maintaining berry structure. The research provides a detailed molecular model explaining how auxin signaling influences grape texture during ripening.

The researchers examined berry firmness and cell wall dynamics throughout development in the grape cultivar 'Shine Muscat'. Both auxin levels and firmness peaked at 4–6 weeks after flowering, then declined sharply as softening began. When berries were sprayed with synthetic auxin (NAA), the softening process was significantly delayed, maintaining higher firmness and propectin content while preventing hemicellulose degradation. Microscopic observations revealed that auxin treatment preserved cell wall integrity and regular cell arrangement.

RNA-seq and weighted gene co-expression network analysis (WGCNA) identified VvARF19 and VvLBD13 as key regulatory nodes linking auxin signaling and cell wall metabolism. Functional assays demonstrated that VvARF19 binds to the VvLBD13 promoter, repressing its transcription. In turn, VvLBD13 directly activates VvXTH10 and VvEXPLA1, encoding xyloglucan endotransglucosylase/hydrolase and expansin-like proteins responsible for hemicellulose degradation. Overexpression of VvARF19 enhanced firmness, whereas VvLBD13 overexpression accelerated softening. These results reveal a signaling cascade where auxin-triggered VvARF19 delays fruit softening by suppressing VvLBD13-mediated activation of cell wall–loosening genes.

Our research deciphers how auxin maintains grape firmness at the molecular level. The identification of the VvARF19–VvLBD13–VvXTH10/VvEXPLA1 module provides new insights into how hormonal signals coordinate cell wall remodeling during fruit ripening. By fine-tuning this regulatory cascade, we can better control softening in table grapes and other fruit crops, improving storage and transportability without compromising natural ripening processes."

Professor Jiancan Feng, corresponding author of the study

Understanding how auxin regulates fruit firmness offers practical implications for viticulture and postharvest management. Manipulating the VvARF19–VvLBD13 pathway could enable the development of grape cultivars with longer shelf life and superior texture quality. The study also highlights potential molecular targets for genetic or hormonal interventions to delay softening in other nonclimacteric fruits. Beyond commercial applications, this auxin-centered model deepens our understanding of hormone–cell wall interactions and opens new avenues for breeding fruits that combine enhanced flavor with improved mechanical resilience during handling and storage.