During Arabidopsis development the shoot apical meristem (SAM) generates lateral primordia which display stage-specific traits. In long days, wild-type Arabidopsis generates leaves in an initial vegetative phase (V). Upon integration of environmental and endogenous signals, the SAM enters the reproductive phase. First it makes an 11 phase, which consists of 2-3 leaves (cauline) subtending secondary shoots (coflorescences). Next it enters the 12 phase and produces flowers on its flanks. The TFL 1 gene is a key component of the phase change machinery as mutations in TFL 1 affect the timing of phase switching. Also ffl1 mutants enter a novel phase whereby the SAM, after 12, is converted into a terminal flower, a phase normally absent in wild type. The molecular mechanism of how TFL 1 protein acts is unclear. In animal systems, TFL 1-like proteins have been shown to be components of signal transduction pathways. To understand the mechanism underlying TFL 1 function I aimed to identify proteins interacting with TFL 1 by introducing into Arabidopsis a functional TAP tag version of TFL 1 under the control of the 35S promoter. I set up conditions which allowed me to isolate and visualize by total protein staining TAPtag TFL 1. However, no obvious proteins appeared to co-purify with TFL 1. To understand how TFL 1 is modified, and to follow TFL 1 protein expression throughout development and in cell fractions, I developed polyclonal antibodies against TFL 1. These antibodies recognized TFL 1 in vivo and were used to characterize TFL 1 biochemically. TFL 1 detection by immunoblots in conjunction with mass-spectrometry analysis showed that TFL 1 was not subjected to obvious modifications unlike animal homologues. Moreover, from cellular fractionation experiments TFL 1 was located in the cytosol. To reveal essential downstream functions required for TFL 1 signaling, I characterized a suppressor mutant, called sof1, of plants ectopically expressing TFL 1. I mapped sof1 within a confined region on the bottom of chromosome 3. Physiological analysis of sof1 led to a model of SOT1 action in controlling phase change. TFL 1 mRNA is found in a unique expression domain which comprises a group of cells in the centre of the SAM and yet TFL 1 affects the identity of lateral primordia. By using affinity purified anti-TFL 1 antibodies I showed that TFL 1 protein moves and is distributed throughout the SAM. This might account for the effect of TFL 1 on controlling overall shoot identity and raises important questions on the role of the TFL 1 protein outside its mRNA expression domain.