The minimum cell length and a threshold level of accumulated CBL proteins are both determining factors by which the nucleus attains commitment to induce an asynchronous course of chromosome condensation. Replication stress-induced alterations in an orderly route of the cell cycle events probably reflect a considerable reprogramming of metabolic functions of chromatin combined with gradients of morphological changes spread along the nucleus.

Excised, carbohydrate-starved root meristems of Vicia faba subsp. minor have been used to investigate the impact of the auxin indole-3-acetic acid (IAA) and the cytokinin benzyl-6-aminopurine (BAP) on (1) the expression of Principal Control Points (PCPs) during the G1- and G2-phases of the cell cycle, and (2) the dynamics of sucrose-mediated resumption of DNA replication and mitosis (G1-to-S and G2-to-M transitions). Compared with the excised root tips starved in mineral medium without hormones, stationary phase meristems induced during continuous treatment with BAP, IAA, or a mixture of BAP+IAA, increased the number of G2 cells, producing characteristic profiles of nuclear DNA content. In medium containing 2% sucrose, BAP accelerated PCP1-->S and PCP2-->M, whereas continuous treatment with IAA resulted in marked prolongation of both transitions. Since the PCPs regulate progression through the key events of interphase and mitosis by interacting with cyclin dependent kinases (CDKs), these results seem to correspond with current data indicating functional connections between phytohormones, nutritional signals, gene expression and the cell division cycles in plants.

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Histone H2A variant H2AX is rapidly phosphorylated on the induction of DNA double-strand breaks by ionizing radiation and hydroxyurea-mediated replication arrest, resulting in the formation of gamma-H2AX foci along megabase chromatin domains nearby the sites of incurred DNA damage. In an attempt to establish a relationship between species-specific nuclear architecture and H2AX phosphorylation in S/G(2) phase-arrested root meristem cells, immunocytochemical comparisons using an antibody raised against human gamma-H2AX were made among three plants differing with respect to DNA contents: Allium porrum, representing a reticulate type of DNA package, Vicia faba, having semireticulate cell nuclei, and Raphanus sativus, characterised by a chromocentric type of chromatin. Another approach was aimed at determining possible correlations between the extent of hydroxyurea-induced phosphorylation of H2AX histones and the quantities of root meristem cells induced by caffeine to enter aberrant mitotic division (premature chromosome condensation). It was concluded that the higher-order structure of chromatin may contribute to the accessibility of molecular factors engaged in the recognition and repair of genetic lesions. Consequently, in contrast to A. porrum and V. faba, a diffuse chromatin in chromocentric cell nuclei of R. sativus may become more vulnerable both to generate DNA double-strand breaks and to recruit molecular elements needed to arrange the cell cycle checkpoint functions, and thus, more resistant to factors which allow the cells to enter premature chromosome condensation spontaneously. On the other hand, however, caffeine-mediated overriding of the S-M checkpoint control system resulted in the typical appearance of premature chromosome condensation, irrespective of the genomic content of DNA.