SALT TOLERANCE HOMOLOG2, a B-Box Protein in Arabidopsis That Activates Transcription and Positively Regulates Light-Mediated Development

Abstract

CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1) and ELONGATED HYPOCOTYL5 (HY5) are two major regulators of light signaling in plants. Here, we identify SALT TOLERANCE HOMOLOG2 (STH2) as a gene that interacts genetically with both of these key regulators. STH2 encodes a B-box-containing protein that interacts physically with HY5 in yeast and in plant cells. Whereas STH2 is uniformly nuclear by itself, it shows a COP1-dependent localization to speckles when coexpressed with COP1. We identified two independent T-DNA insertion lines in STH2. Both alleles are hyposensitive to blue, red, and far-red light. The sth2 mutant, like hy5, shows an enhanced number of lateral roots and accumulates less anthocyanin. Analysis of double mutants between sth2 and hy5 indicates that STH2 has both HY5-dependent and -independent functions. Furthermore, besides partially suppressing the hypocotyl phenotype of dark-grown cop1 alleles, sth2 also suppresses the reduced number of lateral roots and high anthocyanin levels in light-grown cop1 alleles. Interestingly, we found that STH2 can activate transcription. Transient transfection assays in protoplasts using a LUC reporter driven by the chalcone isomerase promoter show that the B-boxes in STH2 and a functional G-box element in the promoter are required for this activity. In conclusion, we have identified STH2, a B-box protein in Arabidopsis thaliana, as a positive regulator of photomorphogenesis and report that the B-box domain plays a direct role in activating transcription in plants.

BBX32, an Arabidopsis B-Box Protein, Functions in Light Signaling by Suppressing HY5-Regulated Gene Expression and Interacting with STH2/BBX21

“…The anti-His monoclonal antibody did not detect any 6XHIS:NF-YB2 prey protein in the negative control reaction, but it did detect the 6XHIS:HY5 fusion protein in the pellet with STH2:3XHA, confirming the previously reported interaction between STH2 and HY5 (Fig. 5A, right top panel; Datta et al, 2007). The anti-MBP monoclonal antibody detected a band only in the reaction containing MBP:BBX32 but not in the two controls lacking either the bait or the prey protein (Fig.…”

“…HY5 is known to interact with other B-box proteins like STH2/BBX21 and LZF1/STH3/BBX22, which function positively in light signaling with HY5; null mutations in STH2/BBX21 and LZF1/STH3/BBX22 genes cause phenotypes similar to the hy5 mutants (Datta et al, 2007(Datta et al, , 2008Chang et al, 2008). BBX32 might either directly interact with HY5 and/or bind one or more of the other B-box proteins that interact with HY5.…”

“…STH2/ BBX21 and LZF1/STH3/BBX22 are positive regulators of red, far-red, and blue light signaling. They physically interact with HY5 through the B-box motifs, promote anthocyanin accumulation in concert with HY5, and LZF1/STH3/BBX22 was shown to be ubiquitinated by COP1 (Datta et al, 2007(Datta et al, , 2008Chang et al, 2008). Some other double B-box proteins, DBB1a/ BBX18, DBB1b/BBX19, and STO/BBX24, have been shown to function negatively in light signaling (Indorf et al, 2007;Kumagai et al, 2008;Khanna et al, 2009).…”

“…Several other B-box proteins have been implicated in light-regulated development. STH2/BBX21 and LZF1/STH3/BBX22 contain two B-box motifs in the N terminus but lack the CCT domain (Datta et al, 2007(Datta et al, , 2008Chang et al, 2008;Kumagai et al, 2008;Khanna et al, 2009). STH2/ BBX21 and LZF1/STH3/BBX22 are positive regulators of red, far-red, and blue light signaling.…”

Arabidopsis STO/BBX24 negatively regulates UV-B signaling by interacting with COP1 and repressing HY5 transcriptional activity

“…It has been shown previously that two BBX24 homologs, STH2/BBX21 and STH3/BBX22, interact with HY5 in yeast [21,22]. The essential role of HY5 in UV-B signaling [23] encouraged us to examine whether BBX24 physically interacts with HY5.…”