THE CELL-CYCLE IN PLANT DEVELOPMENT

The aim of this review is to discuss the molecular controls of the cell cycle in relation to higher plant development. An analysis is made of the current models of the cell cycle based on the biochemistry and genetics of the budding yeast, Saccharomyces cerevisiae and the fission yeast, Schizosaccharomyces pombe. What emerges are universal mechanisms observed in a wide range of taxonomic groups involving a group of protein kinases which regulate the transition from both post-synthetic interphase (G2) to mitosis and from pre-synthetic interphase (G1) to DNA synthetic-(S) phase. The data are consistent in showing the activity of protein kinase complexes operating in conjunction with at least one dephosphorylating enzyme. The natural substrate(s) for the kev cell division cycle gene product, p34cdc2, has vet to be resolved although the nuclear lamins and microtubular apparatus are strong candidates. These models serve as a basis for assessing the cell cycle in higher plants. Mitosis and various stages of nuclear DNA replication are considered in relation to the presumed initiation and termination factors that regulate these events. In order to make a link between the cell cycle and plant development special consideration has been given to plant meristems. In particular, the activity of the cell cycle in cells that have the capacity to regenerate whole tissue systems (`stem cells') within the meristem is discussed. In the root meristem, the quiescent centre cells conform to a stem cell population; a non-cycling stem cell may be immune to the morphogenetic signals that cause cycling cells to arrest and differentiate. The pericycle may act as a vestigial stem cell population. The shoot apex is also discussed in relation to both vegetative and floral growth. Although gradients of cell division exist in shoot meristems it is far less obvious where `stem', or founder, cells reside in the apex. The way in which the cell cycle shortens on transition to floral growth is considered critical for identifying when the meristem becomes florally determined. Temperature and toxic metals are given special attention where it is emphasized that G1 phase becomes protracted when plants are stressed. Species that can tolerate stressful environments may have meristems in which a greater number of cells are competent for division. Finally, the cell cycle in vitro is discussed in relation to rapid changes in gene expression which are linked to the transition from G1 to S phase. The latter emerges as a kev cell cycle transition for plant meristems both in vivo and in vitro.