Searching for interactions at the membrane interface that confer antagonistic activities on floral regulators FT and TFL1

In plants, the switch from leaf- to flower-producing meristems is controlled by the FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1 (TFL1) proteins. FT is a major component of the so-called "florigen", a systemic signal that moves from leaves to shoot meristems and triggers the floral transition. By contrast, TFL1 is expressed in the shoot meristems and represses flowering, antagonising FT. Despite their opposite functions, FT and TFL1 are both members of the same family, showing homology to PHOSPHATIDYLETHANOLAMINE BINDING PROTEINs (PEBPs). They are unable to bind phosphatidylethanolamines though, but bind phosphatidylcholines that are major phospholipids of plant membranes and whose concentration impacts flowering time (Nakamura et al., 2014). The mechanisms by which FT and TFL1 regulate floral transition have been focused on their ability to complex with the transcription factor FD, which itself regulates floral identity genes. However, the loss of FD function does not abolish flowering and hence other interactors remain to be identified. We are therefore investigating in further detail the ability of Arabidopsis FT and TFL1 proteins to interact with membrane lipids and proteins. Langmuir method and molecular dynamics are currently being undertaken to characterize FT and TFL1 interactions with different phospholipids and to simulate their behaviors in contact with membranes, respectively. In parallel, FT and TFL1 protein interactomes are being investigated. We are performing in vitro pulldown assays using His-FT and His-TFL1 as baits and solubilized membranes proteins from cauliflower meristem as preys, taking advantage of the conservation of flowering genes among Brassicaceae. In parallel, complementation of Arabidopsis ft and tfl1 mutants with FT-GS and TFL1-GS constructs allowed us to perform in vivo co-purification analyses. This work will provide us with a better understanding of FT and TFL1 actions and highlight missing checkpoints of the floral transition.