Purine and pyrimidine nucleotide synthesis and degradation during in vitro morphogenesis of white spruce (Picea glauca).

In the last few years, somatic embryogenesis and organogenesis of white spruce (Picea glauca) have been used as model systems to investigate biochemical and physiological events related to morphogenesis. This has been possible because studies conducted in vitro allow for manipulations of the culture conditions, in which changes in morphogenetic events can be easily related to physiological alterations. De-novo synthesis, salvage, and degradation of both purine and pyrimidine nucleotides are operative at all stages of somatic embryo maturation and germination. Fluctuations in the activity of these pathways delineate important morphogenic events. The early phases of embryo development are accompanied by a decreased salvage activity of purine nucleotides, which reflects a reduction of cell proliferation and the initiation of organized growth. Activities of the salvage enzymes are present throughout the maturation period, and also during the imposition of the drying period, which is required for successful embryo germination. The operative salvage pathway in dried embryos is needed for the enlargement of the nucleotide pool necessary to sustain the reactivation of the overall cellular metabolism at germination, before the reactivation of the de-novo pathway, which is a later event. Manipulations of the culture medium which improve the germination frequency of the embryos also result in increased salvage activity. Similar changes in nucleotide synthesis were also observed during the initiation of shoot development from epicotyl explants of white spruce and cotyledons of radiate pine. Results from these studies can be used for improving growth and development in culture.