Embryology in conifers

Conifers, which grow in all climate zones, are important both ecologically and economically. An understanding of the regulation of the development of coniferous trees from single cells to mature trees is critical for scientific and biotechnological applications. Early embryogenesis is a crucial developmental phase during which the basic features of the plant body are established: the apical-basal axis of polarity, the various tissue layers, and the root and the shoot poles. Conifer somatic embryogenesis is of special interest both as a model system for zygotic embryogenesis and because of its potential for mass propagation of selected genotypes. The cell and molecular biology of higher plant embryogenesis has been most studied in the angiosperm Arabidopsis. Like seeds in Arabidopsis, conifer seeds contain a simple mature embryo. However, the patterning of cell division and cell differentiation leading to the final shape are dramatically different in the two groups of species. This is not surprising since the common ancestor of gymnosperms and angiosperms dates from ca. 300Myr ago. The aim of this review is to give an updated survey of the developmental patterning and its regulation during zygotic and somatic embryogenesis in conifers, particularly the genera Picea and Pinus. Somatic embryogenesis biotechnology is much affected by the occurrence of cleavage polyembryony in Pinus and its absence in Picea.