Proliferating cell nuclear antigen expression in maize seed development and germination: Regulation by phytohormones and its association with putative cell cycle proteins

The proliferating cell nuclear antigen (PCNA) plays a fundamental role in D NA replication and repair and recently, it has been found associated to pro teins that control the G1 phase of the cell cycle, such as cyclin D. Maize PCNA cDNA has been cloned and overexpressed in order to raise antibodies. T he expression of PCNA has been followed during seed development and seed ge rmination using the homologous antibodies. The protein was found at a const ant level during seed development up to 48 days after pollination (DAP) and then the amount declined to very low levels, similar to those found in dry seeds. Upon germination, PCNA levels rose gradually reaching a peak by 20 h germination. Imbibition in the presence of cytokinins (Benzyladenine, BA) produced a sharp increase in amount during the first 3-6 h germination, wh ereas imbibition in the presence of abscisic acid (ABA) did not alter the p attern of expression as compared with control seeds, Immunoprecipitation ex periments showed that PCNA was associated to a putative cyclin D protein du ring germination and this association was altered by phytohormones. While t he complex PCNA-cyclin D-like protein was present along the first 15 h of g ermination under control conditions, it was dissociated after 6 h if embryo axes germinated in the presence of BA or ABA. However, complex dissociatio n in the presence of BA was due to degradation of the putative cyclin D pro tein while in the presence of ABA the putative cyclin D was still present. These results are discussed in the context of seed germination and the cell cycle.