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, SlUVR8^W289A, 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 SlUVR8^W289A, 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, SlUVR8^W289A failed to activate UV-B-induced early transcriptional responses in seedlings. In mature plants, SlUVR8^W289A significantly increased chlorophyll accumulation in fruits by regulating key genes involved in chlorophyll biosynthesis. Collectively, our results demonstrate that SlUVR8^W289A 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 SlUVR8^W289A as a valuable genetic tool for both fundamental UV-B research and practical crop improvement.