CRISPR/Cas9 Engineering: Insights into Tomato Fruit Development, Stress Resilience via Brassinosteroid Pathways

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Tomato ( Solanum lycopersicum ), a cornerstone of global agriculture and nutrition, has undergone decades of breeding focused on yield and stress resilience. Yet, consumer-driven traits such as fruit uniformity, flavor complexity, and nutritional value remain suboptimal. The emerging convergence of hormonal biology and precision genome editing presents a transformative approach to addressing this gap. Brassinosteroids (BRs), a class of steroidal phytohormones, act as central regulators of cell expansion, tissue patterning, and developmental plasticity. Their signaling cascade, initiated by perception at the BRI1–BAK1 receptor complex and transduced via BES1/BZR1 transcriptional modules, intersects with networks controlling fruit set, morphology, ripening, and stress adaptation. Gene editing through CRISPR/Cas9 technology now enables targeted dissection and manipulation of these BR-regulated nodes with unprecedented accuracy. CRISPR/Cas9 studies have directly characterized BR signaling regulators such as SlBZR1 , SlBIN2 , and SlBES1 , while complementary genetic and transgenic studies have provided functional evidence for other BR-related components, including SlBRI1 and the BR-biosynthetic gene SlDWF4 , further supporting the roles of BR signaling in tomato fruit development, ripening, and carotenoid accumulation. Beyond improvement of traits, CRISPR/Cas9 offers the potential to tune BR pathways and their crosstalk with auxin and ethylene, providing a systems-level framework for engineering climate-resilient and nutritionally superior cultivars. This review integrates mechanistic insights into BR signaling with cutting-edge CRISPR/Cas9 applications, positioning tomato as a model for reprogramming fruit development and as a paradigm for next-generation crop improvement.