In seed plants, the apical-basal axis of the plant body is established during early embryogenesis. Major regulatory genes of the apical-basal axis formation belong to the WUSCHEL-RELATED HOMEOBOX (WOX) gene family of transcription factors. The spatiotemporal expression pattern and the molecular role of the WOX genes has mainly been studied in the angiosperm model plant Arabidopsis (Arabidopsis thaliana). Similar information in conifers is limited. The aim of my thesis has been to characterize WOX genes in Norway spruce (Picea abies) and to elucidate the function of WOX genes expressed during embryo development.
We cloned 11 WOX homologs from Norway spruce and examined their phylogenetic relationship to WOX genes from other species. The phylogenetic analyses showed that the major diversification within the WOX gene family took place before the gymnosperm-angiosperm split. PaWOX8/9, PaWOX2 and PaWOX3, which are expressed in embryos, were selected for further studies.
PaWOX8/9 and PaWOX2 are highly expressed in early and late embryos, and PaWOX3 is highly expressed in mature embryos. Functional studies were performed in RNAi lines where the genes were down-regulated. Embryos in PaWOX8/9 RNAi lines showed a disturbed apical-basal patterning caused by abnormal orientation of the cell division plane at the basal part of the embryonal mass. In PaWOX2 RNAi lines, vacuolated cells differentiated on the surface of the embryonal mass and the embryos failed to form a proper protoderm. Down-regulation of PaWOX3 disturbed lateral margin outgrowth in cotyledons and needles.
Taken together, our results indicate that WOX8/9, WOX2 and WOX3 exert evolutionarily conserved functions during embryo development. We can therefore conclude that the regulatory networks of embryo development are at least partly conserved between angiosperms and gymnosperms
