FT Modulates Genome-Wide DNA-Binding of the bZIP Transcription Factor FD.

Silvio Collani,Levi Yant,Manuela Neumann,Markus Schmid

doi:10.1104/pp.18.01505

Abstract

The transition to flowering is a crucial step in the plant life cycle that is controlled by multiple endogenous and environmental cues, including hormones, sugars, temperature, and photoperiod. Permissive photoperiod induces the expression of FLOWERING LOCUS T (FT) in the phloem companion cells of leaves. The FT protein then acts as a florigen that is transported to the shoot apical meristem, where it physically interacts with the Basic Leucine Zipper Domain transcription factor FD and 14-3-3 proteins. However, despite the importance of FD in promoting flowering, its direct transcriptional targets are largely unknown. Here, we combined chromatin immunoprecipitation sequencing and RNA sequencing to identify targets of FD at the genome scale and assessed the contribution of FT to DNA binding. We further investigated the ability of FD to form protein complexes with FT and TERMINAL FLOWER1 through interaction with 14-3-3 proteins. Importantly, we observed direct binding of FD to targets involved in several aspects of plant development. These target genes were previously unknown to be directly related to the regulation of flowering time. Our results confirm FD as a central regulator of floral transition at the shoot meristem and provide evidence for crosstalk between the regulation of flowering and other signaling pathways, such as pathways involved in hormone signaling.

Highlights

The transition to flowering is a crucial step in the plant life cycle that is controlled by multiple endogenous and environmental cues, including hormones, sugars, temperature, and photoperiod
FD is normally expressed at the shoot apical meristem (SAM) whereas its interaction partner FLOWERING LOCUS T (FT) is expressed in leaf phloem companion cells (PCCs)
As most 14-3-3 proteins are ubiquitously expressed at moderate to high levels and have been detected in PCCs (Schmid et al, 2005; Deeken et al, 2008), we reasoned that expression of FD from the PCC-specific SUC2 promoter would maximize florigen-activation complex (FAC) complex formation and enable us to investigate the role of FT in modulating FD transcriptional activity

Summary

Introduction

The transition to flowering is a crucial step in the plant life cycle that is controlled by multiple endogenous and environmental cues, including hormones, sugars, temperature, and photoperiod. We observed direct binding of FD to targets involved in several aspects of plant development These target genes were previously unknown to be directly related to the regulation of flowering time. At the SAM, FT interacts with FD and 14-3-3 proteins and the resulting florigen-activation complex (FAC) is thought to control the correct expression of flowering time and floral homeotic genes to promote the transition of the vegetative meristem into a reproductive inflorescence meristem (Abe et al, 2005; Wigge et al, 2005; Taoka et al, 2011). BROTHER OF FT, which like ATC is strongly expressed in the leaf vasculature, can interact with FD in the nucleus, interfering with FT function under high salinity by inhibiting AP1 expression, thereby delaying flowering (Yoo et al, 2010; Ryu et al, 2014)

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Conclusion