Cell Cycle Regulation in Plants.

Cell division plays a crucial role during a11 phases of plant development. Continuing organogenesis and plastic growth responses to a changing environment require precise spatial, temporal, and developmental regulation of cell division activity in meristems. The molecular analysis of cell division and its regulation in plants lags far behind such studies in yeast and animals. Since the cell theory was proposed by Schleiden and Schwann in 1838, many approaches have been taken to elucidate how cells divide, but insight into the molecular basis of division control developed only after the genetic analysis of cell division was initiated in yeast 25 years ago (Hartwell et al., 1970). Cell division cycle mutants in the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae continue to provide a conceptual framework for analyzing gene interactions controlling the cellular processes leading to division. The convergence of genetic, biochemical, and molecular lines of inquiry during the last decade has shown that key cell-cycle regulators are universally conserved and has triggered the explosive growth of the field. The complex network controlling growth in eukaryotes is hierarchically organized (Fig. 1). Regulatory pathways that communicate environmental constraints, such as nutrient availability, signals such as growth factors or hormones, as well as developmental cues control when and in which cells division occurs. In animals, genes such as ras and p53 are players in these pathways that activate the cell-cycle engine. This engine coordinates the various biochemical processes required for division. Although much is known about the biochemistry of these events, e.g. DNA synthesis, in most cases the question of how these metabolic pathways are activated remains unanswered. Cell-cycle regulation in animals and yeast has been extensively reviewed (Nurse, 1990; Forsburg and Nurse, 1991; Murray, 1992; Norbury and Nurse, 1992). This update will introduce the components of the eukaryotic cell-cycle machinery and their regulation, and then focus on progress in molecular dissection of cell-cycle regulation in plants.