Spatially distinct regulatory roles for gibberellins in the promotion of flowering of Arabidopsis under long photoperiods.

The plant growth regulator gibberellin (GA) contributes to many developmental processes, including the transition to flowering. In Arabidopsis, GA promotes this transition most strongly under environmental conditions such as short days (SDs) when other regulatory pathways that promote flowering are not active. Under SDs, GAs activate transcription of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and LEAFY (LFY) at the shoot meristem, two genes encoding transcription factors involved in flowering. Here, the tissues in which GAs act to promote flowering were tested under different environmental conditions. The enzyme GIBBERELLIN 2 OXIDASE 7 (GA2ox7), which catabolizes active GAs, was overexpressed in most tissues from the viral CaMV 35S promoter, specifically in the vascular tissue from the SUCROSE TRANSPORTER 2 (SUC2) promoter or in the shoot apical meristem from the KNAT1 promoter. We find that under inductive long days (LDs), GAs are required in the vascular tissue to increase the levels of FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF) mRNAs, which encode a systemic signal transported from the leaves to the meristem during floral induction. Similarly, impairing GA signalling in the vascular tissue reduces FT and TSF mRNA levels and delays flowering. In the meristem under inductive LDs, GAs are not required to activate SOC1, as reported under SDs, but for subsequent steps in floral induction, including transcription of genes encoding SQUAMOSA PROMOTER BINDING PROMOTER LIKE (SPL) transcription factors. Thus, GA has important roles in promoting transcription of FT, TSF and SPL genes during floral induction in response to LDs, and these functions are spatially separated between the leaves and shoot meristem.