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

Hans E Holtan,Simona Bandong,Sasha Preuss,T Lynne Reuber,Colleen M Marion,Oliver J Ratcliffe,Julin N Maloof,Marie Petracek,Peter P Repetti,Rajnish Khanna,Rob Meister,Luc Adam,Yu Shen,Shiv Tiwari,Don R Maszle,Masa-Aki Ohto

doi:10.1104/pp.111.177139

Abstract

A B-box zinc finger protein, B-BOX32 (BBX32), was identified as playing a role in determining hypocotyl length during a large-scale functional genomics study in Arabidopsis (Arabidopsis thaliana). Further analysis revealed that seedlings overexpressing BBX32 display elongated hypocotyls in red, far-red, and blue light, along with reduced cotyledon expansion in red light. Through comparative analysis of mutant and overexpression line phenotypes, including global expression profiling and growth curve studies, we demonstrate that BBX32 acts antagonistically to ELONGATED HYPOCOTYL5 (HY5). We further show that BBX32 interacts with SALT TOLERANCE HOMOLOG2/BBX21, another B-box protein previously shown to interact with HY5. Based on these data, we propose that BBX32 functions downstream of multiple photoreceptors as a modulator of light responses. As such, BBX32 potentially has a native role in mediating gene repression to maintain dark adaptation.

Highlights

A B-box zinc finger protein, B-BOX32 (BBX32), was identified as playing a role in determining hypocotyl length during a largescale functional genomics study in Arabidopsis (Arabidopsis thaliana)
Over 40% and 25% of the genes responding early to the light signal are predicted to encode different classes of transcription factors and accessory proteins that assist transcription factor function, which in turn may regulate the light-mediated transcriptional network (Tepperman et al, 2001, 2004, 2006). This early-light-responsive set of transcriptional regulators includes proteins known to be involved in light signaling, such as ELONGATED HYPOCOTYL5 (HY5), and factors involved in circadian clock function, such as CIRCADIAN CLOCK-ASSOCIATED PROTEIN1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY; Tepperman et al, 2001)
BBX32 (AT3G21150; Khanna et al, 2009) was identified as a putative regulator of hypocotyl growth in a large-scale functional genomics project performed in Arabidopsis

Summary

Introduction

A B-box zinc finger protein, B-BOX32 (BBX32), was identified as playing a role in determining hypocotyl length during a largescale functional genomics study in Arabidopsis (Arabidopsis thaliana). Exposure to light triggers deetiolation, a coordinated inhibition of hypocotyl elongation, initiation of cotyledon expansion, and stimulation of chlorophyll accumulation and chloroplast development This developmental transition (photomorphogenesis) leads to photoautotrophic growth. Over 40% and 25% of the genes responding early to the light signal (within 1 h; far-red and red light, respectively) are predicted to encode different classes of transcription factors and accessory proteins that assist transcription factor function, which in turn may regulate the light-mediated transcriptional network (Tepperman et al, 2001, 2004, 2006). In 35S::HA:HY5 transgenic plants, HY5 was found to bind its target sites constitutively, in darkness and in different light conditions, revealing that binding of HY5 alone is not sufficient for regulating the transcriptional activities of these light-responsive genes (Lee et al, 2007). Loss of function of HY5 HOMOLOG (HYH) results in weak impairment in the inhibition of hypocotyl elongation in blue light, suggesting that HYH functions redundantly with HY5 under specific light conditions (Holm et al, 2002)

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