Functional Study of Transgenic Tomato BBX8 in Response to UV-B Irradiation

Tomato is an economically important vegetable crop worldwide. B-box (BBX) zinc-finger proteins serve as core transcriptional regulators that widely participate in plant growth, development, and abiotic stress responses. However, the biological function and molecular regulatory mechanism of SlBBX8 in mediating tomato responses to UV-B radiation remain poorly characterized. In this study, SlBBX8 loss-of-function mutants bbx8-79 and bbx8-282 were generated using the CRISPR/Cas9 gene-editing system. The expression of SlBBX8 was significantly induced by UV-B radiation and peaked at 120 μW/cm2. Under UV-B stress, bbx8 mutants exhibited more severe growth inhibition than wild-type plants, with significantly reduced plant height, leaf number, root length, and fresh weight. Moreover, the promotive effects of moderate UV-B radiation (100 μW/cm2) on flowering acceleration and fruit enlargement were markedly weakened in bbx8 mutants. UV-B stress induced excessive accumulation of thiobarbituric acid-reactive substances (TBARS) in bbx8 mutants, while the contents of antioxidant substances, including glutathione, anthocyanins, and flavonoids, displayed an initial increase followed by a decrease, implying that SlBBX8 may be involved in modulating oxidative stress and photoprotective responses in tomato. Multiple UV-B-responsive genes, including ELIP2, KRP1, HY5, F3H, CHS2, and CCS52A, were significantly downregulated in bbx8 mutants under UV-B treatment, confirming that SlBBX8 positively modulates the transcription of these downstream genes. Collectively, this study suggests that SlBBX8 acts as a positive regulator in tomato responses to UV-B stress by modulating photomorphogenesis, antioxidant defense, and reproductive development. These findings provide candidate genes and phenotypic evidence for future investigations into the molecular mechanisms of plant adaptation to UV-B radiation.