Ovule Development Mutants

WT

WT ovule

Arabidopsis ovules are amphitropous, bitegmic and tenuinucellate. The funiculus of an Arabidopsis ovule curves toward the base of the gynoecium. The base of the nucellus (at the chalaza) bends toward the top of the gynoecium and the nucellus (and the embryo sac) curve in this same direction. Prior to anthesis the nucellus degenerates leaving the embryo sac in direct contact with the inner layer of the inner integument. This layer differentiates into an endothelium (integumentary tapetum). For more details see Robinson-Beers et al. (1992) and Schneitz et al. (1995). The embryo sac of Arabidopsis is of the polygonium type and is curved in shape. Details of embryo sac development can be found in Mansfield et al. (1991) and in Webb and Gunning (1990).

ant

ant ovule

aintegumenta mutants are characterized by a lack of both the inner and outer integuments; these two structures are replaced by a single ridge of cells. Flower development is also affected, resulting in narrow petals and the absence of one or both lateral stamens (Klucher et al., 1996; Elliott et al., 1996; Baker et al., 1997). The ANT gene product is homologous to the AP2 gene product (Klucher et al., 1996; Elliott et al., 1996). ANT mRNA is detected in essentially all emerging primordia of the plant. In ovule development ANT mRNA is initially present in the entire ovule primordium but the zone of mRNA is refined to the chalazal region by the time integument primordia emerge (Elliott et al., 1996). Meiosis usually doesn't occur in ant ovules and an embryo sac is not formed (Klucher et al., 1996; Elliott et al., 1996; Baker et al., 1997).

ap2

Certain alleles of apetala2have been shown to result in variable alterations of ovules into carpel-like structures (Modrusan et al., 1994). However, because these plants also produce some normal ovules and other apparently strong ap2 alleles (e. g. ap2-2) do not appear to have these same effects on ovule development the role of this gene in ovule development is not yet clear. ap2 mutations uniformly affect seed development leading to an absence of columellae on the surface of the testa (Leon-Kloosterziel et al., 1994, Jofuku et al., 1994).

ats

ats ovule

The aberrant testa shape ovules have a single integument, rather than the two integuments in wild type ovules (Leon-Kloosterziel et al., 1994). Microscopic analyses of ats mutants reveal that the integument consists of only three cell layers, two layers fewer than wt ovules. In ats ovules the inner integumentary layer forms the endothelium and the outer layer produces normal columella. Thus the single integument of Ats- ovules shares properites of both the inner and outer integuments. It is hypothesized that the unitegmic structure in ats results from a fusion of the inner and outer integuments (Leon-Kloosterziel et al., 1994; Baker et al., 1997). ats voules contain normal embryo sacs and produce fertile seeds with altered shape (Leon-Kloosterziel et al., 1994).

bel1

bel ovule

Ovules of bel1 mutants initiate only a single structure in place of the two integuments. This structure may represent only the outer integument primordium, or may derive from the region which normally gives rise to both integuments. This "integument-like structure" (ILS) develops into a relatively amorphous collar of tissue which does not exhibit the normal cellular organization of the integuments (Robinson-Beers et al., 1992). A normal embryo sac does not form in ovules of bel1 mutants. In a subset of ovules the ILS can go on to expand and differentiate into a complete ectopic carpel (Ray et al., 1994; Modrusan et al., 1994). Carpel formation is associated with aberrant expression of AG in the ILS (Ray et al., 1994; Modrusan et al., 1994). BEL1 mRNA is initially present in the entire ovule primordium, but by the time of emergence of the ovule primordia the mRNA is confined to the chalazal region (Reiser et al., 1995). BEL1 mRNA is also detected in vegetative parts of plants, although no mutant phenotype is detected outside of flowers (Reiser et al., 1995). The BEL1 gene product is a putative transcription factor which includes a homeodomain (Reiser et al., 1995).

ino

ino ovule

In inner no outer mutant ovules the outer integument primordium initiates on the opposite side of the ovule primordium from its normal location. There is little subsequent development of the outer integument, but the inner integument appears to develop normally (Gaiser et al., 1995, Baker et al., 1997). ino ovules develop normal embryo sacs and can form seeds which exhibit aberrant shape and surface topography (Baker et al., 1997). Pollen tube guidance appears to be disrupted in ino mutants indicating a possible role for the ouer integument in pollen tube guidance (Baker et al., 1997).

sin1

sin1 ovule

In an erecta mutant background (such as Ler) the short integuments1 (sin1) mutation leads to production of ovules in which both integuments are short due to a decrease in cell elongation (Robinson-Beers et al., 1992; Lang et al., 1994). The integuments fail to cover the nucellus and an embryo sac does not develop. The internodes of the inflorescence are also shorter in such mutants (Robinson-Beers et al., 1992; Lang et al., 1994). In an ERECTA background the sin1mutations affect only ovule development and the ovule phenotype is dramatically altered (Lang et al., 1994). Ovules of these plants are variable. The outer integument is still reduced, but the inner integument varies from being reduced to exhibiting extreme hypertrophy (Lang et al., 1994). The sin1 mutation also has maternal effects on the embryo (Ray et al., 1996a) and affects flowering time (Ray et al., 1996b).

sup

sup ovule

The superman (sup, also referred to as floral mutant 10 [flo10]) mutant was originally identified on the basis of its effects on flower development where it causes the production of supernumery stamens at the expense of production of gynoecial tissue (Bowman et al., 1992; Schultz et al., 1991). sup mutant ovules were also found to be aberrant (Gaiser et al., 1995). In wt ovules the outer integument primordium initiates asymmetrically and continues to grow asymmetrically, with only the adaxial side of the primordium extending significantly. In contrast, in sup ovules asymmetric formation of the outer integument primordium is followed by uniform extension of the integument around the circumference of the ovules (Gaiser et al., 1995). This results in ovules with an elongate shape and a micropyle that is not adjecent to the funiculus. supovules include normal embryo sacs, are fertile, and develop in seeds with aberrant shape but normal surface topography (Gaiser et al., 1995). While the role of SUP in stamen and carpel development appears to be to exclude activity of B-class floral homeotic genes from the gynoecial whorl (Bowman et al., 1992; Schultz et al., 1991), the effects of mutations in this gene on ovule development are independent of B-class gene activity (Gaiser et al., 1995). The SUP gene has been cloned and encodes a protein which includes a single putative zinc finger domain (Sakai et al., 1995).