Maize embryo germination

The cell-cycle progression of germinating embryos of maize (Zea mays L.) was studied from 0 to 72 h after the start of imbibition using DNA flow cytometry on isolated nuclei, and analyses of thymidine kinase activity, histone biosynthesis and levels of proliferating cell nulcear antigen (PCNA). At the start of germination, 75% of the cells were in G1, but this population had decreased to 25% by 72 h. The concomitant increase of cells in S-phase did not occur continuously, but stepwise, indicating that during germination most of the cells enter S-phase as a partially synchronized population. Within the initial 60 h of embryo germination the cells passed through one S-phase; the start and duration of this period of replicative DNA synthesis was further substantiated by the analysis of S-phase-associated events, the biosynthesis of core histones and the enzyme activity of thymidine kinase, which both began to increase at about 12 h after the start of differentiation. Thymidine kinase fluctuated periodically during germination with a transient maximum at 30 h and a second peak at 72 h; histone biosynthesis was not detectable until 12 h after the start of germination. The levels of PCNA protein closely resembled the pattern of thymidine kinase during germination. Together with the cytometric data this allows a clear assignment of cell cycle events to different times of embryo differentiation.Abbreviation PCNA proliferating-cell nuclear antigen Dedicated to Prof. Walter Larcher on the occasion of his 65th birthdayThe authors wish to thank Prof. G. Mikuz (Department of Pathology, University of Innsbruck, Austria) and Prof. G. Stöffler (Department of Microbiology, University of Innsbruck, Austria) for their interest and support. The technical assistance of Mrs. R. Gantschnig is gratefully acknowledged. E.I. Georgieva was recipient of short-term fellowships from the Austrian Academy of Sciences, the Austrian Forschungsgemeinschaft and the Austrian Akademischer Austauschdienst. G. López-Rodas was recipient of a postdoctoral fellowship from the Programa sectorial de Formación de Profesorado y Personal investigador del Ministerio de Educación y Ciencia (Spain). This work was supported in part by Grant SO6011 (to P.L.) from the Austrian Fonds zur Förderung der wissenschaftlichen Forschung and the Dr. Legerlotz-Foundation.