Photoperiodic signals repress the transcription of SVP to control gibberellin biosynthesis during floral transition in Arabidopsis

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Andrés, Fernando | Porri, Aimone | Torti, Stefano | Mateos, Julieta | Gregis, Veronica | Kater, Martin M. | Coupland, George

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In plants the transition from vegetative growth to flowering is regulated by several environmental stimuli and by the age of the individual. This complexity is conferred by a network of genetic pathways. In Arabidopsis, these pathways include the photoperiodic pathway that promotes flowering in response to long days (LDs) and the response pathway to the growth regulator gibberellins (GA), which has its strongest effect under short days (SDs). FLOWERING LOCUS T (FT) is a positive regulator of flowering that acts in the photoperiodic pathway. Under LDs the FT protein is transported from the leaves to the shoot apical meristem (SAM) where it induces the expression of several floral promoter genes, such as SQUAMOSA PROMOTER BINDING PROTEIN LIKE (SPL) transcription factors. Interestingly, recent studies have shown that GA also induces flowering and affects the expression of the SPLs at the SAM under LDs. However, how the gibberellin pathway is activated in response to photoperiod to promote flowering is still unclear. We have found that mutations in the MADS box transcription factor encoding gene SHORT VEGETATIVE PHASE (SVP) – a well-known repressor of flowering – lead to an increase in the GA content, demonstrating that SVP acts as a negative regulator of the biosynthetic pathway of this hormone. Moreover, our results show that SVP regulates the expression of the SPL genes and flowering time by repressing the transcription of GA20ox2, a rate-limiting enzyme in the biosynthesis of GA. We also show that SVP transcription is repressed by the photoperiodic signals mainly represented by FT, TWIN SISTER OF FT (TSF), and the two MADS box transcription factors SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) and FRUITFULL (FUL), so that inductive LD conditions contribute to the reduction of SVP expression in the shoot apex. Taking our results together, we propose a model to explain how during the floral transition the GA biosynthetic pathway is activated in the SAM in response to inductive photoperiods.

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