Plant Molecular Biology 43: 621–633, 2000. Dirk Inzé (Ed.), The Plant Cell Cycle. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 621 The role and regulation of D-type cyclins in the plant cell cycle Marcel Meijer and James A. H. Murray * Institute of Biotechnology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QT, UK ( * author for correspondence; e-mail: j.murray@biotech.cam.ac.uk) Key words: D-type cyclins, differentiation, G 1 /S control, plant cell cycle, proliferation, retinoblastoma protein Abstract The G 1 phase of the cell cycle represents a period of commitment to cell division, both for cells stimulated to resume division from a resting or quiescent state, and for cells involved in repeated cell cycles. During this period, various signals that affect the cells’ ability to divide must be assessed and integrated. G 1 culminates in the entry of cells into S phase, when DNA replication occurs. In addition, it is likely that several types of differentiation decision may be taken by cells in the G 1 phase. In both animals and plants, it appears that D-type cyclins play an important role in the cell cycle responses to external signals, by forming the regulatory subunit of cyclin-dependent kinase complexes. The phosphorylation targets of D-cyclin kinases in mammalian cells are the retinoblastoma (Rb) protein and close relatives. Unphosphorylated Rb can associate with E2F transcription factors, preventing transcription of genes under E2F control until the G 1 /S boundary is reached. The conservation of Rb and E2F proteins in plants suggests that this pathway is therefore conserved in all higher eukaryotes, although it is absent in fungi and yeasts. Here we review the current understanding of the roles and regulations of D-type (CycD) cyclins in plants. Introduction The co-ordination of cell division with cell growth and differentiation is necessary to create complex multicel- lular organisms, and is achieved within the framework of a specific developmental plan that defines the char- acteristics of the particular organism (White-Cooper and Glover, 1995; Meyerowitz, 1997). Plants also need to modulate this primary pattern of growth and development in order to respond flexibly to changes in their environment, since they are unable to physically move to optimal locations (De Veylder, 1998; Fran- cis, 1998). The control of cell division in plants must therefore sense and interact with external signals. Almost half a century ago, Howard and Pelc (1953) introduced the terminology of the eukaryotic cell divi- sion cycle, recognising the fundamental importance of the separation of DNA replication (S phase) from cell division (M phase) by the two gap phases (G 1 and G 2 ) in the sequence G 1 -S-G 2 -M. This arrangement allows for the precise control of DNA replication and mito- sis, and it is not surprising that an ordered series of molecular and cellular processes define the order and control of the cycle. Cells that temporarily or perma- nently lose the capacity to divide normally stop in the G 1 phase with 2C DNA content, although in plants there appears to be much more flexibility, with both G 1 and G 2 being important cell cycle exit points. It should be noted however that the frequent occurrence of endoreduplication (chromosome replication with- out subsequent mitosis) can create G 1 cells with 4C or higher DNA content (see below). Non-cycling or quiescent cells are often said to be in G 0 to distinguish them from actively cycling G 1 cells, although there is no molecular definition of this state in plants (see Loeffler and Potten, 1996). Progression through the eukaryotic cell cycle is largely regulated at two principal control points, one late in G 1 phase and the other at the G 2 /M boundary. A further important control exists at the metaphase- anaphase transition, and no doubt further subsidiary controls also exist. Transit through these control [ 77 ]