PEBP gene family

From Purdue Genomics Database Facility

Victor A. Albert

The PEBP gene family is defined by the phosphatidylethanolamine-binding protein domain. Well known PEBP genes in Arabidopsis are TERMINAL FLOWER1 (TFL1) and FLOWERING LOCUS T (FT). FT, the so-called "florigen", is a promoter of flowering that acts in a photoperiod-dependent manner, integrating thermal signalling. TFL1, on the other hand, is a flowering inhibitor. A third gene, MOTHER OF FT AND TFL1 (MFT) may have inductive function similar to FT.

Previously, the gene family has been characterized as having 3 subfamilies in plants, the TFL1, FT, and MFT subfamilies.

Jody Banks has already annotated 2 different Selaginella PEBP genes as TFL1-2 and TFL2-2. My preliminary phylogenetic analysis on protein data from NJ with BLOSUM62 in Jalview indicates that these two loci are not orthologous to either TFL1 or FT, but rather (at least in part) to MFT. Importantly, the 2 loci do not form a monophyletic group.

NOTE FROM JODY: I didn't make a tree to demonstrate orthology, so feel free to change the names or demote my gene model.

In my tree, the FT and TFL1 lineages contain only angiosperm sequences, whereas the MFT lineage (at least when viewed as a single unrooted group) contains gymnosperm and Physcomitrella sequences in addition to the 2 Selaginella loci. TFL1-2 is sister to the rest of this MFT lineage, and is rather sequence divergent compared to all other proteins in the tree. A rooting for the tree cannot be provided given the sampling, although it is tempting to place one on the lineage leading to TFL1-2. However, this would imply that Physcomitrella, and all other tracheophytes, may have lost a TFL1-2 ortholog.

One functional hypothesis could be that one or both of the Selaginella proteins are inducers of the reproductive state. It has been reported that a FT (MFT?) homolog in spruce is a promoter of reproduction based on photoperiod. However, the work was based on unpublished sequences (Plant Physiology 144:248-257, 2007). Perhaps TFL1-2 and TFL2-2 act redundantly, as has been hypothesized for some PEBP proteins in Arabidopsis, or perhaps the sequence-deviant TFL1-2 has a novel function.

Regardless, it is important in the context of the Selaginella genome to underscore that Selaginella lacks true FT and TFL homologs, and therefore that reproductive inhibition (as with TFL1) may not occur via a PEBP protein in lycophytes. Finally, both Arabidopsis FT and TFL1 are mobile proteins (the former being the so-called florigen), so it will be interesting to see whether Selaginella paralogs are mobile as well.

NB -- TDF should read TSF (TWIN SISTER OF FT); I'll correct if/when a final figure is needed