| I have used reverse-genetic analysis to investigate the function of MAP3K&egr;1 and MAP3K&egr;2, a pair of closely-related Arabidopsis thaliana genes that encode protein kinases. Plants homozygous for insertional mutations of either map3k&egr;1 or map3k&egr;2 displayed no apparent mutant phenotype, whereas the double-mutant combination caused pollen lethality. Through the use of transmission electron microscopy it was determined that double-mutant pollen grains develop plasma membrane irregularities following pollen mitosis I. Use of confocal microscopy and biochemical fractionation indicated that MAP3K&egr;1 protein is localized to the plasma membrane.;Analysis of the expression of a YFP-MAP3K&egr;1 fusion protein revealed strong expression in actively dividing tissues, lateral roots, and embryos. In order to investigate the function of these kinases in the sporophyte, we used a conditional-rescue strategy based on an ethanol-inducible system to construct map3k&egr;1-/- map3k&egr;2-/- double-mutant plants. The conditionally-rescued double-mutants had decreased cell expansion and proliferation, and their embryo development was delayed and arrested. By contrast, MAP3K&egr;1 over-expression caused ectopic cell division in dark-grown hypocotyls, increased cell expansion in light-grown hypocotyls, stimulated auxin-responsive gene expression, and supported auxin-independent growth of suspension cells. My work has indicated that MAP3K&egr;1 and MAP3K&egr;2 regulate cell division and cell expansion via auxin signal transduction in Arabidopsis, and that this pair of genes is required for embryo development. Future studies will investigate the precise mechanisms by which MAP3K&egr;1 and MAP3K&egr;2 regulate cell division, expansion, and auxin signaling. |
