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

Abstract

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 photomorphogenesis in the dark. We identified a suppressor of the BR hypersensitive mutant bzr1-1D and named it bzr1-1D suppressor1-Dominant (bzs1-D). The bzs1-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 BZS1 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 BZS1-OX plants and in the cop1 mutants, and BZS1 interacts with COP1 in vitro, suggesting that light regulates BZS1 through COP1-mediated ubiquitination and proteasomal degradation. These results demonstrate that BZS1 mediates the crosstalk between BR and light pathways.

Arabidopsis MICROTUBULE DESTABILIZING PROTEIN40 Is Involved in Brassinosteroid Regulation of Hypocotyl Elongation

“…Studies have detailed the mechanisms involved in hypocotyl cell elongation regulated by light, phytohormones, and transcription factors (Wang et al, 2002;Niwa et al, 2009;Luo et al, 2010;Fan et al, 2012). In addition, a recent study showed that pectin-dependent cell wall homeostasis is important for BR regulation of hypocotyl growth (Wolf et al, 2012).…”

The BBX subfamily IV

“…65,71 Independent experiments demonstrate that BBX21, BBX22 and BBX24 have transactivating capacity in transcriptional systems, 44,45,51,60 although no DNA binding ability has been described. The proposed mechanism of action uses the interaction with other transcription factors that bind to DNA, such as HY5 and HYH (Fig.…”

“…However, it is still not known if BBX20 has a role in karrikindependent germination responses. Fan et al 71 described the function of BBX20 as a negative regulator of brassinosteroid (BR) signaling, but a positive regulator of photomorphogenesis. The authors found that BBX20 is regulated transcriptionally via BR signaling (by BZR1), and posttranscriptionally by means of a COP1-dependent mechanism, 71 similarly to GATA2 regulation.…”

“…Fan et al 71 described the function of BBX20 as a negative regulator of brassinosteroid (BR) signaling, but a positive regulator of photomorphogenesis. The authors found that BBX20 is regulated transcriptionally via BR signaling (by BZR1), and posttranscriptionally by means of a COP1-dependent mechanism, 71 similarly to GATA2 regulation. 72 Confocal microscopy analyses localize BBX20:YFP in the nucleolus and cytoplasm, making this feature unique for this protein in sub-family IV.…”

“…72 Confocal microscopy analyses localize BBX20:YFP in the nucleolus and cytoplasm, making this feature unique for this protein in sub-family IV. 71 Much less is known for BBX23. Genevestigator transcriptomic data mining showed that the and with common partners in order to fine-tune light-related responses.…”