Reinitiation of cell division and polyamine and aromatic monoamine levels in alfalfa explants during the induction of somatic embryogenesis

The early cellular events in leaf explants of Medicago sativa L. cultured on somatic embryogenesis‐ and callogenesis‐inducing media (EIM and CIM, respectively) were correlated with the endogenous contents of polyamines (PAs) and aromatic monoamines (AMs). On the second day of culture, replication of DNA occurred in epidermal and subepidermal cells on the edges of explants on EIM and was a prerequisite for proembryonal mass and, later, globular proembryo formation. In explants cultured on CIM, replication occurred at least one day later and in fewer cells, which were randomly spread all over the explant. Transition of leaf explant cells to rapidly dividing meristematic‐like cells on EIM or to enlarged, highly vacuolated cells on CIM was observed. The increase in total PA levels in cultured explants was primarily a consequence of increases in putrescine (Put) and spermidine (Spd) contents and was much more pronounced on EIM than on CIM. High Spd levels were characteristic of meristematic cells and might be essential for the development of globular structures. The higher amount of insoluble PA conjugates was determined in explant cells on EIM, as compared with CIM. Proembryogenic cell mass formation was positively correlated with free tyramine and negatively correlated with free phenylethylamine contents.