UNUSUAL FLORAL ORGANS (UFO)
From Purdue Genomics Database Facility
Victor A. Albert
UNUSUAL FLORAL ORGANS (UFO) is an F-box-containing protein in Arabidopsis that interacts with ASK1, a Skp1 homolog. The B-function MADS box genes APETALA3 (AP3) and PISTILLATA (PI) are positively regulated by both LEAFY (LFY) and UFO. UFO expresses in a mid-radial pattern in the developing floral apex. Functional research on UFO shows that it inhibits an AG-dependent pathway during petal development, thereby prohibiting 'stamenization' of the 2nd whorl. Interestingly, UFO also plays a role in normal floral primordium initiation before AP1 activation. Moreover, UFO and LFY together are necessary to inhibit vegetative bract growth.
Fascinatingly, UFO expresses mid-radially in the Arabidopsis globular stage embryo, which gives rise to the cotyledons and shoot apical meristem (SAM).
UFO is single-copy in Arabidopsis, and apparently so in other angiosperms.
Very recent research has shown that Arabidopsis UFO physically interacts with LFY, and forms part of a transcriptional complex at the AP3 promoter.
Selaginella, and Physcomitrella, have UFO homologs (re: the latter, note incongruity of opinion with Table 3 in Evolution of developmental genes). Selaginella appears to harbor a single locus, with two alleles (fgenesh2_pg.C_scaffold_56000172 and fgenesh2_pg.C_scaffold_6000300). See the preliminary tree and Jalview alignment below (Arabidopsis UFO is boxed; note also that the tree includes two identical Arabidopsis UFOs extracted from different accession numbers). Selaginella and Physcomitrella also have LFY homologs (cf. UFO/LFY interaction above).
The finding that Arabidopsis UFO and some ABC genes were expressed similarly in SAMs (and embryos) led some to conclude that the floral ABC pattern was evolutionarily coopted from the SAM. I pointed out in 1999 ([1]; see ref. below) that this way of thinking neglects to highlight the primacy of reproductive over vegetative development during plant evolution. To quote (p. 84, reference numbers excluded):
"... flowering plants are but one lineage of seed-bearing plants, and only in one small extant lineage, the Gnetales, and in one extinct clade, the Bennettitales, are the male and female organs arrayed in a radial, ABC-like pattern that might be homologous to that of flowers. Other seed plants, such as cycads, conifers and Ginkgo, bear unisexual reproductive shoots, either on separate plant individuals (cycads, Ginkgo) or on spatially separated branches within the same individuals (conifers). In cycads and conifers the male and female sporophylls (functionally equivalent to angiosperm stamens and carpels) are borne in spiral cycles, and in female conifer cones each sporophyll is subtended by a bract-like organ. These non-flowering seed plants do express some ABC genes; they also have complex, zonal SAMs, and although SAM gene expression might be involved in sex expression in their simpler reproductive shoots, a fully ABC-like patterning function is not evident morphologically. Therefore, the unique mid-radial aspects of ABC patterning probably evolved by the Triassic, at least 200 million years BP and no earlier than the separation of angiosperms from other seed plants with hermaphroditic reproductive shoots. Although the vast majority of the current emphasis on plant developmental evolution is centered on angiosperms, this phylogenetic dichotomy represents, at least in structural terms, a relatively insignificant event in the greater scheme of land-plant reproductive diversification."
Further (pp. 84-85; emphasis is mine at present),
"...most sporing vascular plants – ferns, horsetails, and the resurrection plant, Selaginella, have SAMs with single apical cells [but see ref. to Harrison et al. in BRK1 and apical growth, these pages], and where roots are clearly present (as in ferns and horsetails), a single apical cell accounts for root growth as well. Because single cells are the progenitors of all apical meristematic activity in such plants, these cells probably cannot by themselves produce the requisite inner, outer and mid-radial expression domains ostensibly coopted by the flower developmental program. It is possible, however, that differentiating daughter cells that cut off from apical cells might express UFO and other angiosperm SAM gene orthologs, but this pattern would be posterior to apical growth. Fern, horsetail and Selaginella leaves do initiate some distance from the apical cell, so it will be interesting to look for UFO-like activity there. Additionally, it is possible that overtly different cell division patterns (as in complex versus apical-celled SAMs) might poorly reflect undiscovered but potentially homologous cell signaling patterns."
Reference:
Albert, V. A. 1999. Shoot apical meristems and floral patterning: an evolutionary perspective. Trends in Plant Science 4: 84-86. [2]
Note GI numbers in tree and below.
