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E, 1986, Histones and the first cell cycle in maize germination. -Physiol. Plantarum 68: 259-264.The timing of the onset of cell division during seed germination in maize and the role of histones for this process have been studied. Embryonic axes of maize seeds [Zea mays L, hybrid H-30) were incubated in a sterile nutrient medium for different periods of time. For some experiments putresdne was also added, Mesocotyl, root tip and scutellar node were dissected at specific periods after incubation and the mitotic indices were determined in these tissues. Embryonic axes were incubated in the same medium either with ["Cj-lysine or ["P]-phosphate. The incorporation of either 'C or "P into histones was followed, both in postribosomal supernatant and in nuclei, it was found that during germination, there is specific timing for meristematic cells entering into cell division. Among the tissues tested, the mesocotyl meristem was the first to initiate this process De novo synthesis of histones was detected as early as after 6 h of imbibition and the rate increased up to 12 h. Putresdne stimulated cell division and phosphorylation of the histones. The implications of these findings are discussed.Additional key words -Histone phosphoryiation, histone synthesis, mitotic index, putrescine, Zea mays.

Bai'za, A. M. and Sanchez-de-Jimenez, E. 1989. Effect of the auxin 2-(2-methyl-4chloro-phenoxy)propionic acid, on cell cycle regulation in maize embryonic tissues. [261][262][263][264][265][266] During seed maturation, cells from embryonic tissues stop division at different phases of the cell cycle. In maize, neither these phases nor the effect of exogenous auxin on them are known. Disinfected whole maize {Zea mays L. Mexican commercial hybrid H30) seeds or sectioned embryonic axes were incubated in Murashige and Skoog medium, with or without 2-(2-methyl-4-chlorophenoxy)propionic acid (MCPP), a synthetic auxin. For some in vitro experiments, radioactive ['H]-thymidine was also added. After the stated incubation period, meristems of mesocotyl, primary and seminal roots from embryonic axes were dissected, fixed, and analyzed under a microscope. The percentage of mitotic indices was recorded. In the labeling experiments, labeled and non-labeled percentage of mitotic figures (MI %) were determined. It was found that cell division is a programmed event in the meristematic tissues of maize embryonic axes. Populations of cells entering cell division were obseved during the germination process. The mesocotyl was the first tissue to divide, followed by seminal and primary roots. Meristematic cells from dry embryos are arrested during the G2 and G, phases of the cell cycle. MCPP has a differential effect, stimulating G2 cells to enter cell division. It is concluded that MCPP might regulate the cell cycle at specific points.

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