Pyrimidine nucleotide and nucleic acid synthesis in embryos and megagametophytes of white spruce (Picea glauca) during germination

Pyrimidine nucleotide synthesis was investigated in isolated germinating zygotic embryos and separated megagametophytes of white spruce by following the metabolic fate of ¹⁴C-labelled orotic acid, uridine, and uracil, as well as by measuring the activities of the major enzymes participating in nucleotide synthesis. The rate of nucleic acid synthesis in these tissues was also examined by tracer experiments and autoradiographic studies conducted with labelled thymidine, and by conventional light microscopy. From our results, it emerges that changes in the contribution of the de novo and salvage pathways of pyrimidines play an important role during the initial stages of zygotic embryo germination. Preferential utilization of uridine for nucleic acid synthesis, via the salvage pathway, was observed at the onset of germination, before the restoration of a fully functional de novo pathway. Similar metabolic changes, not observed in the gametophytic tissue, were also documented in somatic embryos previously. These alterations of the overall pyrimidine metabolism may represent a strategy for ensuring the germinating embryos with a large nucleotide pool. Utilization of ¹⁴C-thymidine for nucleic acid synthesis increased in both dissected embryos and megagametophytes during germination. Autoradiographic and light microscopic studies indicated that soon after imbibition, DNA synthesis was preferentially initiated along the embryonic axis, especially in the cortical cells. Apical meristem reactivation was a later event, and the root meristem became activated before the shoot meristem. Taken together, these results indicate that precise changes in nucleotide and nucleic acid metabolism occur during the early phases of embryo germination.