| The female gametophyte is a haploid, seven-celled structure that develops within ovules and comprises the female germ unit. Following fertilization, the female gametophyte's egg cell and central cell develop into the embryo and endosperm, respectively, of the seed. In addition to giving rise to a seed, the female gametophyte plays a role in pollen tube guidance, fertilization, and the induction of seed development. In spite of its important role in plant reproduction, very little is known on a molecular or genetic level about female gametophyte development or reproductive function. I have taken a genetic approach in Arabidopsis to identify and study the genes that are required for these processes. Female gametophyte mutations result in the failure to produce a viable seed and, as a consequence, the failure to transmit the mutant allele through the female. Based on these two criteria, I developed a genetic screen and identified 39 mutants. To lay a foundation for the analysis of mutant phenotypes, I characterized wild-type female gametophyte development using confocal laser scanning microscopy. Next, I analyzed the phenotypes and found that these mutations affected a variety of cellular and developmental processes. Because the mutations were generated by insertional mutagenesis, I was able to identify the mutated locus in 22 of the mutants. The mutated genes represent a variety of functional categories. Two of the mutations, gfa2 and gfa3, affected the polar nuclei fusion step of development. GFA2 encodes a DnaJ family member that is targeted to the mitochondrion and is probably the Arabidopsis ortholog of S. cerevisiae (yeast) MDJ1. Mdj1p functions as a chaperone in the mitochondrial matrix and the mdj1 mutation in yeast results in a loss of mitochondrial function. However, the apparent metabolic activity of gfa2 female gametophytes suggests that the polar nuclei fusion defect is due to more than energy depletion. GFA3 encodes a AAA-type ATPase that is similar to yeast CDC48 and AFG2. AAA-type ATPases, including Cdc48p, are required for many membrane fusion events within a cell. Thus, the nuclear fusion defect in gfa3 mutants is consistent with a defect in a AAA family member. |
