The cell cycle is regulated, at least partially, by protein phosphorylation, activity that is carried out by a series of calcium-independent protein kinases among which the p34cdc2 kinase plays a preponderant role. The behaviour of p34cdc2-like protein kinase(s) has been followed during germination of maize in the presence or absence of the synthetic cytokinin benzyladenine (BA). Whereas there is an increase in p34cdc2-like kinase(s) activity at 15 h of germination in the presence of BA, coinciding with a peak of Ca2+-independent kinase activity, the amount of either the transcript for p34cdc2 or of the corresponding protein(s) does not show much variation during the 0–24 h period of germination studied, whether the phytohormone is present or not. Benzyladenine stimulates the cell cycle and promotes an early mitosis during maize germination. We have found evidence that BA promotes the movement of the p34cdc2-like protein(s) to nuclei several hours before this takes place during germination of control seeds, and this may constitute part of the mechanism by which the phytohormone promotes cytokinesis in plants.

In the present study the effects of roscovitine on the in vitro nuclear maturation of porcine oocytes were investigated. Roscovitine, a specific inhibitor of cyclin-dependent protein kinases, prevented chromatin condensation in a concentration-dependent manner. This inhibition was reversible and was accompanied by non-activation of p34cdc2/histone H1 kinase. It also decreased enzyme activity of MAP kinase, suggesting a correlation between histone H1 kinase activation and the onset of chromatin condensation. The addition of roscovitine (50 μM) to extracts of metaphase II oocytes revealed that the MAP kinase activity was not directly affected by roscovitine, which indicates a possible link between histone H1 and MAP kinase. Chromatin condensation occurred between 20 and 28 h of culture of cumulus-oocyte complexes (COCs) in inhibitor-free medium (germinal vesicle stage I, GV1: 74.6% and 13.7%, respectively). Nearly the same proportion of chromatin condensation was detected in COCs incubated initially in inhibitor-free medium for 20-28 h and subsequently in roscovitine-supplemented medium (50 μM) for a further 2-10 h (GV I: 76.2% and 18.8%, respectively). This observation indicates that roscovitine prevents chromatin condensation even after an initial inhibitor-free cultivation for 20 h. Extending this initial incubation period to ≥22 h led to an activation of histone H1 and MAP kinase and increasing proportions of oocytes exhibiting chromatin condensation in the presence of roscovitine. It is concluded that histone H1 kinase is involved in the induction of chromatin condensation during in vitro maturation of porcine oocytes.