Floral initiation in Arabidopsis plants, particularly as the days become shorter, is promoted by the phytohormone gibberellin (GA). GA overcomes the repressive effects of DELLA proteins to allow transcription of the floral meristem identity gene LEAFY. On of this issue, Achard et al. investigate the role of a highly conserved GA- and DELLA-regulated microRNA known as miR159 during floral initiation. The authors show that miR159 cleaves the mRNA transcript of a GAMYB protein called AtMYB33. GAMYB proteins are GA-specific transcriptional regulators that activate the transcription of LEAFY. By making transgenic Arabidopsis plants, the researchers confirmed that overexpression of miR159 results in the reduction in LEAFY transcript levels and delayed flowering during short-day photoperiods. They go on to discuss a model for the modulation of floral development by this microRNA, whereby it acts,within a negative-feedback loop, as a homeostatic regulator of GAMYB function.
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