Most morphogenic events in plants occur in the sporophyte following seed germination. However, the embryonic phase is crucial as it is then the meristems and the shoot-root plant body pattern are specified. Mutagenesis and the subsequent screening for and analysis of mutants, mainly in Arabidopsis, has been elucidating the genetic regulation of embryonic pattern formation in angiosperms. In contrast, hardly anything is known about embryo development in gymnosperms. Gymnosperms have several disadvantages as exprerimental organisms. They have large genomes, about 200 to 400 times bigger than in Arabidopsis, larger size and a long generation time. Molecular data suggest that extant seed plants (gymnosperms and angiosperms) shared a last common ancestor about 300 million years ago.
Therefore, characterization of genes regulating embryo development, their expression pattern and their functions in gymnosperms is interesting from an evolutionary point of view. We have developed somatic embryogenesis in Norway spruce as a model for studying embryology in conifers. This model system includes a stereotyped sequence of developmental stages, resembling zygotic embryogeny, which can be synchronized, by specific treatments, making it possible to collect a large number of somatic embryos at specific developmental stages. Important processes during embryo development which we have been studying include: embryo polarization, differentiation and degradation of the suspensor, differentiation and functioning of the protoderm, differentiation of cotyledons and meristems. Ongoing research include establishment and maintenance of stem cells and totipotence.
Somatic embryogenesis is also an attractive method to propagate conifers vegetatively both as a tool in the breeding programme and for reforestation. Many spruce species can today be propagated in large scale via somatic embryos. However, before somatic embryogenesis can be used for clonal propagation of many pine species, further studies are required in order to control the differentiation of early embryos.
Zhu, T., Moschou, P. N., Alvarez J.M., Sohlberg, J. and Von Arnold, S (2014) WUSCHEL-RELATED HOMEOBOX 8/9 is important for proper embryo patterning in the gymnosperm Norway spruce. Journal of Experimental Botany 65:6543-6552.
Uddenberg D, Reimgård J, Clapham D, Almqvist C, von Arnold S, Emanuelsson O, Sundström J (2013) Early cone-setting in Picea abies var. acrocona is associated with increased transcriptional activity of MADS-box transcription factor. Plant Physiol 161: 813-823.
Larsson E, Sundström J, Sitbon F, von Arnold S (2012) Expression of PaNAC01, a Picea abies CUP-SHAPED COTYLEDON orthologue, is regulated by polar auxin transport and associated with differentiation of the shoot apical meristem and formation of separated cotyledons. Annals of Botany 110: 923-934.
Uddenberg D, Valladares S, Abrahamsson M, Sundström JF, Sundås-Larsson A, et al. (2011) Embryogenic potential and expression of embryogenesis-related genes in conifers are affected by treatment with a histone deacetylase inhibitor. Planta: 1–13-13.
Vestman D, Larsson E, Uddenberg D, Cairney J, Clapham D, et al. (2010) Important processes during differentiation and early development of somatic embryos of Norway spruce as revealed by changes in global gene expression. Tree Genetics & Genomes 7: 347–362.
Wadenbäck J, von Arnold S, Egertsdotter U, Walter MH, Grima-Pettenati J, et al. (2008) Lignin biosynthesis in transgenic Norway spruce plants harboring an antisense construct for cinnamoyl CoA reductase (CCR). Transgenic research 17: 379–392.
Larsson E, Sitbon F, Ljung K, von Arnold S (2008) Inhibited polar auxin transport results in aberrant embryo development in Norway spruce. The New phytologist 177: 356–366.