Molecular Regulation of Embryo Development in Norway Spruce Polar Auxin Transport and Transcription Factors

Early events in embryo development are critical for the plant body formation. During this phase the apical-basal axis, the radial symmetry, and the primary meristems are specified. The shoot apical meristem (SAM) and the root apical meristem (RAM) will subsequently give rise to all above-ground and below-ground tissues. Despite the environmental and economical importance of conifers, the regulation of their embryonic development is still relatively unknown. We are using somatic embryos of Norway spruce as a model system for studying embryology in conifers. In this thesis, I show that polar auxin transport (PAT) is essential for proper patterning of both apical and basal parts of conifer embryos. Blocked PAT caused increased auxin levels, increased differentiation of early somatic embryos, a skewed balance between the embryonal mass and the suspensor cells in late embryos, and an abnormal morphology with irregular RAM, fused or aborted cotyledons and absence of a functional SAM in mature embryos. Two NAC gene family members (PaNAC01 and PaNAC02), closely related to the Arabidopsis CUP-SHAPED COTYLEDON (CUC) genes, were characterized. PaNAC01 harbors previously characterized functional motifs, and could complement the cuc1cuc2 double mutant. The temporal expression of PaNAC01 was dependent on PAT, and coincided with the formation of separated cotyledons and a functional SAM. Furthermore, the expression profiles of four KNOXI genes (HBK1 to 4) showed that also the expression of HBK2 and HBK4 depend on PAT, and indicated their role in embryo differentiation and SAM formation, while HBK1 and HBK3 seemingly have a more general role during embryo development. In addition, a global gene expression analysis revealed important processes during early somatic embryogenesis in Norway spruce. Taken together the results show that central parts of the regulatory network for embryo development are still conserved between angiosperms and gymnosperms, despite their separation 300 million years ago.