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The SVP transcription factor acts at the shoot meristem to co-ordinate GA biosynthesis with floral induction during photoperiodic flowering
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In plants, the indeterminate growth of the vegetative shoot apical meristem (SAM) is maintained by small groups of pluripotent cells. The balance between the re-renewal of these cells and the floral organ initiation from the lateral anlagen is sustained during the vegetative growth by complex regulatory feedbacks and the hormone signaling. During the floral initiation this balance is broken in favor of floral organogenesis triggering the transition from vegetative to inflorescence meristem. The floral transition is regulated by several environmental and endogenous stimuli and by the age of the individual. In Arabidopsis, these pathways include the photoperiodic pathway that promotes flowering in response to long days (LDs) characteristic of summer, and the response pathway to the growth regulator gibberellin, which has its strongest effect under short days (SDs). LDs promote the transcription of FLOWERING LOCUS T (FT) in the leaf vascular tissue. Then, FT protein moves through the phloem to the shoot apex, where it causes changes in gene expression and promotes flowering. Interestingly, recent studies have shown that gibberellins also induce flowering by affecting the expression of floral promoter genes at the shoot apex under LDs. However, how the gibberellin pathway is activated in response to photoperiod to promote flowering is still unclear. Mutations in the MADS box transcription factor encoding gene SHORT VEGETATIVE PHASE (SVP) cause a number of pleiotropic developmental effects, such as early flowering and floral homeotic changes. These pleiotropic phenotypes suggest that SVP is involved in different genetic pathways. In order to identify the pathways affected by SVP we have performed several phenotypic, genetic and genomic studies. Surprisingly, our results show that SVP represses a key gene of the gibberellin biosynthetic pathway. In addition, we found that SVP transcription is repressed by the photoperiodic signals, mainly represented by FT protein, so that inductive LD conditions contribute to the reduction of SVP expression in the shoot apex. In turn, SVP reduction allows the de-repression of the gibberellin pathway and therefore accelerates the floral transition process. Taking our results together, we propose a model to explain how the floral transition is accelerated by the accumulation of gibberellins in the shoot apex in response to inductive photoperiods.
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