Activation and negative feedback regulation of SlHY5 transcription by the SlBBX20/21-SlHY5 transcription factor module in UV-B signaling

In tomato (Solanum lycopersicum) and other plants, the photoreceptor UV-RESISTANCE LOCUS 8 regulates plant UV-B photomorphogenesis by modulating the transcription of many genes, the majority of which depends on the transcription factor ELONGATED HYPOCOTYL 5 (HY5). HY5 transcription is induced and then rapidly attenuated by UV-B. However, neither the transcription factors that activate HY5 transcription nor the mechanism for its attenuation during UV-B signaling is known. Here, we report that the tomato B-BOX (BBX) transcription factors SlBBX20 and SlBBX21 interact with SlHY5 and bind to the SlHY5 promoter to activate its transcription. UV-B-induced SlHY5 expression and SlHY5-controlled UV-B responses are normal in slbbx20 and slbbx21 single mutants, but strongly compromised in the slbbx20 slbbx21 double mutant. Surprisingly, UV-B responses are also compromised in lines overexpressing SlBBX20 or SlBBX21. Both SlHY5 and SlBBX20 bind to G-box1 in the SlHY5 promoter. SlHY5 outcompetes SlBBX20 for binding to the SlHY5 promoter in vitro, and inhibits the association of SlBBX20 with the SlHY5 promoter in vivo. Overexpressing 35S:SlHY5-FLAG in the WT background inhibits UV-B-induced endogenous SlHY5 expression. Together, our results reveal the critical role of the SlBBX20/21-SlHY5 module in activating the expression of SlHY5, the gene product of which inhibits its own gene transcription under UV-B, forming an autoregulatory negative feedback loop that balances SlHY5 transcription in plants. Tomato transcription factor module BBX20/21-HY5 activates HY5 transcription, and HY5 protein inhibits its own gene transcription, forming an autoregulatory negative feedback loop in UV-B signaling.