CRISPR/Cas9 engineering: Insights into tomato fruit development, stress resilience via brassinosteroid pathways.

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.