BZS1, a B-box Protein, Promotes Photomorphogenesis Downstream of Both Brassinosteroid and Light Signaling Pathways

Photomorphogenesis is controlled by multiple signaling pathways, including the light and brassinosteroid （BR） pathways. BR signaling activates the BZR1 transcription factor, which is required for suppressing photomorphogen- esis in the dark, We identified a suppressor of the BR hypersensitive mutant bzrl-lD and named it bzrl-lD suppressorl- Dominant （bzsl-D）. The bzsl-D mutation was caused by overexpression of a B-box zinc finger protein BZS1, which is transcriptionally repressed by BZR1. Overexpression of BZS1 causes de-etiolation in the dark, short hypocotyls in the light, reduced sensitivity to BR treatment, and repression of many BR-activated genes. Knockdown of BZS1 by co-suppression partly suppressed the short hypocotyl phenotypes of BR-deficient or insensitive mutants. These results support that BZSl is a negative regulator of BR response. BZS1 overexpressors are hypersensitive to different wavelengths of light and loss of function of BZS1 reduces plant sensitivity to light and partly suppresses the constitutive photomorphogenesis 1 （cop1） mutant in the dark, suggesting a positive role in light response. BZS1 protein accumulates at an increased level after light treatment of dark-grown BZSl-OXplants and in the cop1 mutants, and BZS1 interacts with COP1 in vitro, suggesting that light regulates BZS1 through COPl-mediated ubiquitination and proteasomal degradation. These results demonstrate that BZS1 mediates the crosstalk between BR and light pathways.

The Arabidopsis B-box protein BZS1/BBX20 interacts with HY5 and mediates strigolactone regulation of photomorphogenesis

Plant growth is controlled by integration of hormonal and light-signaling pathways. BZS1 is a B-box zinc finger protein previously characterized as a negative regulator in the brassinosteroid （BR）-signaling pathway and a positive regulator in the light-signaling pathway. However, the mechanisms by which BZS1/BBX20 integrates light and hormonal pathways are not fully understood. Here, using a quantitative proteomic workflow, we identified several BZSl-associated proteins, including light-signaling compo- nents COP1 and HY5. Direct interactions of BZS1 with COPI and HY5 were verified by yeast two-hybrid and co-immunoprecipitation assays. Overexpression of BZS1 causes a dwarf phenotype that is sup- pressed by the by5 mutation, while overexpression of BZSI fused with the SRDX transcription repressor domain （BZS1-SRDX） causes a long-hypocotyl phenotype similar to by5, indicating that BZSI＇s function requires HY5. BZSI positively regulates HY5 expression, whereas HY5 negatively regulates BZS1 protein level, forming a feedback loop that potentially contributes to signaling dynamics. In contrast to BR, strigolactone （SL） increases BZS1 level, whereas the SL responses of hypocoryl elongation, chlorophyll and HY5 accumulation are diminished in the BZSI-SRDX seedlings, indicating that BZS1 is involved in these SL responses. These results demonstrate that BZS1 interacts with HY5 and plays a central role in integrating light and multiple hormone signals for photomorphogenesis in Arabidopsis.