VSF-1 helps tomatoes build firmness before ripening

Fruit firmness is usually treated as a trait that is lost during ripening, but new evidence shows that part of the story begins much earlier, while the fruit is still building its structure. The study identifies the basic leucine zipper (bZIP) transcription factor VSF-1 as a positive regulator of tomato firmness during development. Removing the VSF-1 gene softened fruit, whereas increasing VSF-1 activity strengthened it. The effect was linked to changes in cell-wall architecture and to direct control of the β-galactosidase gene TBG6. By revealing how developing fruit establishes mechanical strength before ripening begins, the work opens a new route for improving texture quality while preserving the normal transition to a softer ripe fruit.

Tomato texture depends heavily on the primary cell wall, where cellulose, hemicellulose and pectin form a dynamic framework that supports growing fruit. Most molecular studies have focused on ripening, when enzymes and transcription factors drive wall disassembly and softening. Far less is known about how firmness is established during the earlier expansion phase, even though this strength helps fruit maintain tissue integrity and withstand mechanical and biological stress. Previous work connected VSF-1 with vascular gene regulation and showed that the softening regulator LOB1 can activate VSF-1, but its role in fruit texture remained unresolved. Because of these gaps, deeper investigation is needed into the transcriptional networks that build and maintain firmness during early fruit development.

Researchers from Zhejiang University, together with collaborators from the Laboratoire de Recherche en Sciences Végétales–Génomique et Biotechnologie des Fruits in Toulouse, France, reported (DOI: 10.1093/hr/uhag181) the study in Horticulture Research on April 30, 2026. The article, published as an open-access accepted manuscript, examines how VSF-1 shapes tomato fruit firmness before and during ripening. Using genetic, biomechanical, microscopic and genome-wide approaches, the team connected VSF-1 activity with cell-wall remodeling and identified TBG6 as a direct downstream target, providing a mechanistic link between a basic leucine zipper (bZIP) transcription factor and the physical strength of developing tomato fruit.

The researchers first used clustered regularly interspaced short palindromic repeats (CRISPR)–CRISPR-associated protein 9 (Cas9) genome editing to create two VSF-1 knockout lines and generated two overexpression lines in the ‘Ailsa Craig’ tomato background. Texture measurements showed that knockout fruit were consistently softer in whole-fruit compression tests, with the largest effects at the mature-green stage; cell-wall material fell by 13–18%, and subepidermal cell walls were 32–37% thinner than in wild type. In contrast, VSF-1 overexpression increased firmness, raised pericarp cell-wall material by 27–30%, and increased the number of cell layers. Light microscopy and transmission electron microscopy linked these texture changes to tissue organization and cell-wall ultrastructure, rather than simply to changes in fruit size. Ribonucleic acid sequencing (RNA-seq) showed strong enrichment of cell-wall-related processes. DNA affinity purification sequencing (DAP-seq) identified 59,371 high-confidence VSF-1 binding peaks and 5,952 promoter-associated genes; integrating both datasets produced 218 candidate direct targets. The team then focused on TBG6, which encodes a β-galactosidase. Yeast one-hybrid analysis, electrophoretic mobility shift assays and dual-luciferase tests showed that VSF-1 binds to and activates the TBG6 promoter. Knocking out TBG6 also reduced whole-fruit firmness during early development, confirming that this regulatory link contributes to the phenotype.

The authors said the findings shift fruit-texture research from softening alone to the earlier process of building firmness. They said VSF-1 appears to coordinate a broader cell-wall program rather than acting through a single enzyme, with TBG6 providing one experimentally validated route and LOB1 suggesting an indirect feedback mechanism. The results also suggest that firmness must be balanced across development: fruit needs sufficient structural strength while growing, but it must still soften appropriately as ripening proceeds. They said this developmental timing may be especially important for crop improvement, because pushing the regulator too strongly can affect other fruit traits.

For breeding and biotechnology, the study provides candidate molecular targets for tuning tomato texture at an earlier stage than conventional ripening-focused strategies. Rather than simply maximizing VSF-1 activity, future approaches could aim to adjust when, where or how strongly the pathway operates, potentially strengthening developing fruit without blocking desirable ripening. That caution is supported by the overexpression lines, which were firmer but also showed greater postharvest water loss and increased cuticle permeability. The work therefore offers both an opportunity and a design constraint: improving firmness may require coordinated control of cell walls, cuticle properties and developmental timing. The VSF-1–TBG6 module now provides a defined entry point for testing such precision-breeding strategies in tomato and, potentially, related fleshy-fruit crops.

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References

DOI

10.1093/hr/uhag181

Original Source URL

https://doi.org/10.1093/hr/uhag181

Funding information

This work was supported by the National Natural Science Foundation of China (32422080) and the 111 Project (B17039).

About Horticulture Research

Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2023. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.