The phenotype ofArabidopsisovule mutants mimics the morphology of primitive seed plants

Abstract

In seed plants, the ovule is the female reproductive structure, which surrounds and nourishes the gametophyte and embryo. This investigation describes the PRETTY FEW SEEDS2 (PFS2) locus, which regulates ovule patterning. The pfs2 mutant exhibited developmental defects in the maternal integuments and gametophyte. This mutation was inherited as a maternal trait, indicating that gametophyte defects resulted from ovule patterning aberrations. Specifically, the boundary between the chalaza and the nucellus, two regions of the ovule primordia, shifted towards the distal end of pfs2 ovule primordia. Results indicated that the PFS2 locus could: (i) be involved in the development of either the nucellus or the chalaza; or (ii) establish a boundary between these two regions. Examination of genetic interactions of the pfs2 mutation with other well-characterized ovule loci indicates that this locus affects integument morphogenesis. Interestingly, the pfs2 inner no outer and pfs2 strubbelig double mutants had inner integuments that appeared similar to their ancestral precursor. The fossil record indicates that the inner integument evolved by fusion of sterilized sporangia or branches around a central megasporangium. The question of whether the structures observed in these double mutants are homologous or merely analogous to the ancestral precursors of the inner integument is discussed.

Haploinsufficiency after successive loss of signaling reveals a role forERECTA-family genes inArabidopsisovule development

“…In pfs2-1, premature differentiation of the cells contributing to the gametophyte or integument primordia could account for decreased integument length and defects in the embryo sac (Park et al, 2004). The addition of pfs2-1 to er-105 erl1-2 erl2-1/+ mutations led to increased variability in both integument and gametophyte development, similar to pfs2-1 alone, indicating that the pfs2-1 mutation is epistatic to the er-family mutations.…”

“…pfs2-1 ovules have aberrant integument growth after initiation and embryo sac arrest (Park et al, 2004) (Fig. 7A).…”

“…Interaction of pfs2 with er erl1-2 erl2-1/+ PFS2, a WUS-related homeobox gene also known as WOX6, affects ovule patterning and regulates integument cell proliferation and gametophyte differentiation (Park et al, 2004;Park et al, 2005). It has been well-documented that the competing actions of the transcription factor WUS, and the CLAVATA (CLV) LRR-RLK signaling pathway, maintain meristem size in Arabidopsis (Brand et al, 2000).…”

“…Several genes, including PRETTY FEW SEEDS 2 (PFS2), SHORT INTEGUMENTS 1 and 2 (SIN1 and 2) and TSO1, affect cell proliferation or expansion of the integuments, and their loss-of-function mutations result in reduction or loss of fertility (Hauser et al, 2000;Park et al, 2004;Park et al, 2005;Schneitz et al, 1997). Embryo sac failure is a secondary consequence of the absence of integuments; the gametophyte fails to develop in cases where the nucellus is not enclosed .…”

ThePRETTY FEW SEEDS2gene encodes anArabidopsishomeodomain protein that regulates ovule development

“…We previously reported that the PRETTY FEW SEEDS2 (PFS2) gene regulates ovule patterning (Park et al, 2004). Based on genetic analyses, we report here that PFS2 encodes a homeodomain gene that is a member of the WUS clade of transcription factors.…”