STERILE APETALA (SAP)

From Purdue Genomics Database Facility

Victor A. Albert

STERILE APETALA (SAP) is a unique single-copy gene of Arabidopsis that encodes a protein without obvious relationships. The mutant phenotype is highly pleiotropic, with homeotic floral alterations and disrupted megasporogenesis. In wild tpye, SAP apparently has a cadastral function by preventing the expression of AGAMOUS (AG) in sepals and petals. Moreover, it works together with AG to preserve floral meristem identity. A cooperation with APETALA2 (AP2) seems clear in establishing proper megasporogenesis. A serine-rich region followed by a glycine-rich one near the N terminus led SAP's discoverers to suggest that it is a transcriptional regulator.

In the mutant state, sap ovules show meiotic arrest at a very early stage, such that tetrads are never observed. As such, SAP has been implicated as an important gene in Arabidopsis megagametophyte development.

I have found SAP homologs -- as single-copy genes -- only in tracheophytes; no copy appears to exist in Physcomitrella. Interestingly, a SAP homolog is found in banana (Musa; a monocot), but not in rice (or any other monocot grass-crop EST sets) -- perhaps representing a loss in the grass lineage. In addition to the sequences in the preliminary tree below, I have found probable homologs in Medicago and Lotus (and perhaps Zamia, a cycad).

There are two Selaginella sequences with substantial sequence differences outside the regions conserved with seed plants; these might be interpreted as distinct genes. The Selaginella sequences (boxed in the alignment below) differ from those of seed plants in having long N- and especially C-terminal tails. It is possible that the tails represent gene prediction problems (i.e., false extensions), in which case Selaginella would be best interpreted as having a single SAP homolog with 2 alleles.

The absence of a SAP homolog in Physcomitrella suggests that the gene may be unique to vascular plants. A possible interpretation is that SAP is involved with heterosporous gametogenesis. It will be interesting to look for SAP homologs in Lycopodium (a homosporous lycopod) and Ceratopteris (a homosporous fern), for example.

The lack of a SAP homolog in rice and other grasses is quite mysterious, given that single-copy genes can play hub-like (pleiotropic!), dosage-dependent roles during development (e.g., LEAFY).