SlSPA3 regulates the nuclear abundance of SlUVR8 in tomato.

SUMMARY Tomato (Solanum lycopersicum L.) is an important model plant species in photomorphogenesis research. Ultraviolet B (UV‐B) induces the dissociation of homodimers of the photoreceptor UV RESISTANCE LOCUS8 (UVR8) into monomers, which translocate into the nucleus. Nuclear accumulation of UVR8 is a prerequisite for its signaling function. Previous studies have reported that SUPPRESSOR OF PHYTOCHROME A‐105 (SPA) family members may regulate UV‐B signaling in Arabidopsis (Arabidopsis thaliana); however, the underlying mechanism is unknown. Here, we show that the tomato genome encodes four SPA (SlSPA) orthologs. Genome‐edited Slspa3 mutants exhibited enhanced photomorphogenic responses in white light, suggesting that SlSPA3 inhibits general photomorphogenesis. By contrast, UVR8‐mediated gene expression in response to UV‐B was compromised in Slspa3 mutants, suggesting that SlSPA3 promotes UV‐B signaling. UV‐B‐induced nuclear accumulation of UVR8, which is essential for UV‐B signaling, was reduced in the Slspa3 mutants. Moreover, UV‐B‐induced nuclear accumulation of UVR8 was also reduced in the Arabidopsis spa1 spa2 spa3 and spa1 spa2 spa4 triple mutants, indicating a conserved mechanism in these two species. Notably, spa1 spa2 spa4 exhibited normal UV‐B‐induced interaction between UVR8 and the plant morphogenesis repressor CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1). This suggests that the well‐established mechanisms of UVR8 nuclear retention remained unaffected in spa1 spa2 spa4. Thus, our work uncovered a potentially unrecognized mechanism by which SPA proteins regulate UV‐B signaling through the promotion of UVR8 nuclear abundance in land plants. [ABSTRACT FROM AUTHOR]