Peter Doerner

Every cell is the product of a cell cycle. Cell proliferation is controlled precisely by the cell division control machinery, which ensures that cells divide only at appropriate times and that crucial components are replicated with adequate fidelity. The basic mechanisms governing cell division arose early in eukaryotic evolution and are highly conserved.
      The cell cycle comprises two alternating events: cell division, during which a cell replicates its genome and partitions it to daughter cells, and cell growth, during which cells increase their mass by synthesizing proteins, membrane lipids, and other essential components. Cell growth and cell division serve different purposes. Cell growth increases the fitness of the individual cell and increases the likelihood of its survival. Controlled cell division perpetuates and spreads genetic information, thereby increasing the likelihood that a species will be successful and survive. To reconcile these divergent purposes, cell division and growth are flexibly coupled such that, in a given environment, most cells divide when reaching a particular size (Fig. 11.1). However, growth alone is generally insufficient to initiate division; cells must be stimulated by growth factor signals to divide.
      In addition to regulating the timing of cell division, cell cycle control mechanisms perform a quality-control function to prevent transmission of incompletely replicated or damaged genomes to daughter cells. Incomplete replication of the genome, rereplication of DNA without division, or division before DNA replication is complete can all wreak havoc on the cell’s progeny. To prevent such errors from occurring, all organisms utilize molecular mechanisms to monitor cell division progress at specific steps in the cell cycle, called checkpoints.
      Cell growth and most biochemical processes are continuous, but the cell cycle proceeds in discrete, incremental steps. Eukaryotic cells have evolved specialized protein kinases, phosphatases, and proteases that function as switches to impose this stepwise mode of progression on the processes of DNA replication and cytokinesis. This network of regulatory proteins involved in monitoring cell cycle progression is also well suited to coupling cell cycle stages with the environmental conditions and facilitating quality control.
      This chapter discusses the basic mechanisms of cell division, as well as some important aspects of growth control and DNA replication in plants. The discussion of cell division control mechanisms will initially focus on the molecules fundamentally responsible for cell cycle control in all eukaryotes. Although much progress has recently been made in dissecting the molecular events that control growth and cell division in plants, our understanding of these processes remains far better in other model systems. In particular, much of the experimental evidence discussed in this chapter is the result of research in budding yeast (Saccharomyces cerevisiae), fission yeast (Schizosaccharomyces pombe), and animal cell cultures, as well as in genetic systems such as the fruitfly Drosophila melanogaster. The molecular mechanisms elucidated in these model organisms are often also used in plants to regulate growth throughout plant development. To facilitate comparison of information from the various models discussed later in the chapter, we will compare and contrast cell division in plants and animals.

Figure 11.1
Cell growth and cell division are coupled to maintain an optimal cell volume. In most cases, if division proceeds without intervening periods of cell growth, cells become progressively smaller and die. (Exceptions are frequently observed in fertilized eggs, which are very large cells and may initially subdivide without growing.) Genotype and environmental factors (e.g., nutrient availability) can also influence cell size. The division-without-growth phenotype is observed in the fission yeast (Schizosaccharomyces pombe) mutant for the Wee1 protein kinase, which ordinarily prevents premature mitosis (see Section 11.5.4). The left panel shows cells in which wee1 is disrupted by another gene (ura4). With adequate nutrients available, a specific maximum cell volume is set and cells oscillate between this unit volume (immediately before division) and half of this volume (immediately after division). This homeostasis of cell volume is partly controlled by the wee1⁺ gene (middle panel). In above-optimal conditions, the set maximum cell volume increases. This situation can be mimicked by increasing the amount of the wee1 protein kinase (right panel).