Firmness Variation in Tomato Fruit: Driving Factors, Signaling Regulation, and Genetic Basis

Fruit firmness is a pivotal agronomic and commercial trait that determines the storability, transport tolerance and edible quality of tomato (Solanum lycopersicum L.). Fruit softening severely restricts postharvest performance and economic benefits of tomato products worldwide. This review summarizes the latest research progress on factors and regulatory mechanisms governing tomato fruit firmness. Multiple environmental factors including temperature, light, moisture and atmospheric composition jointly affect cell wall structure and metabolism, thereby altering fruit firmness. Mineral nutrition, postharvest handling and pathogen infection also exert profound impacts on texture characteristics by modulating physiological activities and cell wall integrity. Phytohormones such as gibberellin, ethylene, abscisic acid, jasmonic acid and salicylic acid form complex signaling crosstalk to mediate fruit softening processes. Furthermore, we elaborate on the functions of transcription factors, quantitative trait loci, and key functional genes, as well as research advances in transcriptomics and metabolomics, including transcriptome analyses identifying differentially expressed genes related to cell wall remodeling, and metabolomic profiling revealing hydroxyproline and galacturonic acid as firmness-associated markers. Cell wall metabolism is confirmed as the core pathway controlling fruit softening. Finally, future research directions are proposed, focusing on single-cell and spatial transcriptomics to map softening regulatory networks, AI-driven predictive modeling for softening kinetics and shelf-life optimization, and CRISPR/Cas9-based gene editing for precise trait improvement.