Florigen revisited: proteins of the FT/CETS/PEBP/PKIP/YbhB family may be the enzymes of small molecule metabolism

Flowering signals are sensed in plant leaves and transmitted to the shoot apical meristems, where the formation of flowers is initiated. Searches for a diffusible hormone-like signaling entity ("florigen") went on for many decades, until in the 1990s a product of plant gene FT was identified as the key component of florigen, based on genetic evidence and protein localization studies. Sequence homologs of FT protein are found throughout prokaryotes and eukaryotes; some eukaryotic family members appear to bind phospholipids or interact with the components of the signal transduction cascades. We studied molecular features of the FT homologs in prokaryotes and analyzed their genome context, to find tentative evidence connecting the bacterial family members with small molecule metabolism, often involving sugar- or ribonucleoside-containing substrates. Most FT homologs share a constellation of five charged residues, three of which, i.e., two histidines and an aspartic acid, circumfere the rim of a well-defined cavity on the protein surface. We argue that this conserved feature is more likely to be an enzymatic active center than a catalytically inactive ligand-binding site. We propose that most of FT-related proteins are enzymes operating on small diffusible molecules, which may constitute an overlooked essential ingredient of the florigen signal.