Role of bHLH93 in controlling flowering time in Arabidopsis thaliana
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In plants, flowering time is a tightly regulated process where several environmental and endogenous cues fine-tune the time of flowering. In Arabidopsis, four major genetic pathways regulate flowering time, namely photoperiod, vernalization, autonomous, and phytohormone gibberellic acid (GA) pathways. Arabidopsis is a facultative long day (LD) plant. LD promotes flowering whereas flowering is delayed in short day (SD) conditions. Here, we identified a basic-helix-loop-helix (bHLH) transcription factor called bHLH93 that is necessary to promote flowering only in SD. Also, photoperiod plays more critical roles in regulation of flowering time of bhlh93 mutant compared to GA and vernalization pathways. Thus, bHLH93 might represent a novel transcription factor absolutely required for Arabidopsis thaliana to evolve as a facultative LD plant. bhlh93 mutants also show severe adult phenotype such as shorter stature, curly and darker green leaves, and reduced fertility compared to wild type plants. These results suggest that bHLH93 controls plant stature, fertility and chlorophyll content in Arabidopsis. bHLH93 is expressed in a tissue-specific and developmental stage-dependent manner. bHLH93-YFP protein is localized in the nucleus. bHLH93 homodimerizes in yeast, and it has strong transcription activation activity in yeast. These data suggest that, like other bHLH proteins, bHLH93 may function as a transcriptional regulator in the nucleus controlling gene expression. We have identified floral repressor MAF5 as a major target of bHLH93 to promote flowering in SD. bHLH93 binds to MAF5 promoter element in vivo and in vitro. Other than MAF5, FLC and MAF1-2 are also up-regulated in bhlh93 but at a lower level than MAF5. The activation of multiple floral repressors correlates with bhlh93 flowering phenotype. Taken together, these data suggest that bHLH93 may provide selective advantage for evolution of facultative flowering behavior under varying environmental conditions for reproductive success.