Effect of stimulating maize germination on cell cycle proteins

The germination process can be accelerated if seeds are stimulated either by adding cytokinins or by osmopriming. Under these conditions, cells in maize (Zea mays) embryo axes shorten the time at which the first round of DNA replication and mitosis lakes place, thus advancing the cell cycle. Using heterologous antibodies against different cell cycle proteins, we have followed the behaviour of several markers for G1 phase (cyclin D, E2F and p53) and a marker of G2 phase (cyclin B) under either control or "accelerated" germination conditions. The results showed two classes of behaviour: either there was no variation in the amount of the protein present under control or accelerated germination conditions, represented by cyclin Band E2F-type proteins, or the amount of the proteins was drastically reduced, more rapidly under accelerated germination, as was the case for cyclin D- and p53-type proteins. Although the cyclin D-type protein was synthesized de novo during germination, the balance was towards degradation so that there was no cyclin D detected 15 h after germination in benzyladenine-treated and osmoprimed seeds. A Cdk4-type protein seemed to be present in cyclin D immunoprecipitates and its kinase activity paralleled the fluctuations of the cyclin amount during germination. These data are discussed in the context of early seed germination.