Cell cycle control and plant morphogenesis: is there an essential link? Adriana S. Hemerly, 1 Paulo C.G. Ferreira, 1 Marc Van Montagu, 2* and Dirk Inze ´ 2,3 Summary Plant morphogenesis has some interesting features that may have consequences for the regulation of cell division. In particular, the immobility of plant cells implies the necessity for highly accurate controls, in contrast with the flexibility of many developmental processes in animals. An important question in plant development concerns the status of the relationship between plant morphogenesis and cell division. In this review, we discuss the current knowledge of the molecular mechanisms controlling the plant cell cycle and how this could be differentially regulated during plant morphogenesis. The plant genes involved are homologous to those of other higher eukaryotes, suggesting a similar cell cycle machinery. A variety of mechanisms control these genes, reflecting the complexity of internal and environmental signals to which plants should respond. This intricate network requires an upstream control mechanism to function as a failsafe system and to govern cell division and growth to produce the correct plant shape. BioEssays 21:29–37, 1999.  1999 John Wiley & Sons, Inc. Introduction Studies of the various morphogenetic processes in plant development often involve the determination of patterns and frequencies of cell divisions. But are cell divisions essential to define plant shape? The plant body is formed through the processes of growth, morphogenesis, and cellular differentiation. Cell division is primarily responsible for generating plant growth and results in organs with particular shapes (together with cell expan- sion) and various cellular types (cellular differentiation). These three overlapping processes require a fundamentally inte- grated control of cell cycle progression and developmental signals to drive plant growth. Such a mechanism requires that cell divisions be differentially activated and suppressed to permit the formation of high-order structures. Hence, when considering the controls of cell division in plants, mechanisms acting at two distinct levels can be distinguished: first, the basic molecular control of the cell cycle and the developmen- tal program, which governs particular rates and orientations of cell division; and second, these regulatory mechanisms must be interconnected and linked to an adaptable cell cycle. A better understanding of the integration of cell cycle controls and the differential regulation of cell division in plants will help establish the status of the precise relationships between cell division and plant morphogenesis. Considerable effort has been made to unravel the mecha- nisms controlling cell division in plants. Here, some relevant findings are reviewed. On the basis of yeast and animal data, we discuss the possible controls governing the basic cell cycle progression in plants and how it could be differentially regulated during development. We also address the impor- 1 Departamento de Bioquı ´mica Me ´ dica, UFRJ, Rio de Janeiro, RJ, Brazil. 2 Laboratorium voor Genetica, Departement Genetica, Vlaams Interuni- versitair Instituut voor Biotechnologie (VIB), Universiteit Gent, Gent, Belgium. 3 Laboratoire Associe ´ de l’Institut National de la Recherche Agronomi- que (France), Universiteit Gent, Gent, Belgium. Funding agencies: Belgian Programme on Interuniversity Poles of Attraction, Prime Minister’s Office, Science Policy Programming; Grant number: P4/15; and European Union BIOTECH program; Grant number: BIO4-CT96–0217. *Correspondence to: M. Van Montagu, Laboratorium voor Genetica Universiteit Gent, K.L. Ledeganckstraat 35, B-9000 Gent, Belgium. E-mail mamon@gengenp.rug.ac.be Review articles BioEssays 21:29–37,  1999 John Wiley & Sons, Inc. BioEssays 21.1 29