A gain-of-function mutation in SlUVR8 reveals critical roles of UV-B signaling in tomato development and stress tolerance.

Studying the functions of the UV-B photoreceptor UVR8 in large, indeterminate-growth tomato plants is challenging due to the difficulty of applying uniform UV-B irradiation. To overcome this limitation, we generated a constitutively active photoreceptor variant, SlUVR8W289A, by substituting tryptophan 289 with alanine. This variant enables functional investigation of SlUVR8 in tomato without the need for UV-B treatment. Size exclusion chromatography coupled with western blot analysis revealed that the W289A substitution destabilizes SlUVR8 homodimers and alters protein conformation. In transgenic tomato lines expressing SlUVR8W289A, the variant enhanced photomorphogenesis and suppressed thermomorphogenesis at the seedling stage in the absence of UV-B, and also conferred substantial tolerance to UV-B stress without prior acclimation. Notably, SlUVR8W289A failed to activate UV-B-induced early transcriptional responses in seedlings. In mature plants, SlUVR8W289A significantly increased chlorophyll accumulation in fruits by regulating key genes involved in chlorophyll biosynthesis. Collectively, our results demonstrate that SlUVR8W289A represents a constitutively active UVR8 photoreceptor for long-term UV-B responses, allowing the dissection of UV-B signaling pathways independently of UV-B exposure. This study establishes intrinsic UV-B signaling as a critical regulator of seedling establishment, fruit development, and stress adaptation in tomato. Our findings provide a mechanistic basis for harnessing UV-B signaling to improve horticultural crop performance, and position SlUVR8W289A as a valuable genetic tool for both fundamental UV-B research and practical crop improvement.