Developmental control of the cell cycle.

Abstract

Whereas animal development has a defined and early embryonic phase during which all of the major organ systems are put in place, plant growth continues throughout the life of the organism. Moreover, most of the above ground plant growth that we see is modular and the modules, known as phytomers, are formed by the activity of shoot apical meristem. This results in at least two distinct types of growth: indeterminate growth as shown by the meristem, producing tissues of undefined maximum size; and determinate growth as shown by leaves and other lateral organs, producing organs of limited maximum size. We are interested in how plant cell proliferation is regulated within the context of development. Mutations in developmental genes can affect the pattern and the extent of cell growth or proliferation. Thus it can be argued that such genes regulate cell growth division. The molecular mechanisms by which developmental genes influence cell division and proliferation have remained largely obscure. Developmental patterning and cell cycle control appear to be co-ordinated throughout plant growth and this is reflected by temporal and spatial regulation of regulatory genes such as cyclins and cyclin dependent protein kinases. Using an approach based on RNA in situ hybridisation, we have characterised the expression patterns of a number of cell cycle genes (Fobert et al., 1994; Fobert et al., 1996; Gaudin et al., 2000). Based on this survey, many cell cycle control genes are expressed throughout the plant in a cell cycle-stage specific manner, and produce a spotty pattern (Fig. 1). Double label in situ methods (Fobert et al., 1994) and synchronised cell (Sorrell et al., 2001) studies permit the further resolution of expression periods (Fig. 1). Many of the genes, such as B-cyclins, kinesins and ICKs expressed during the G2/M phase, contain characteristic motifs, called MSA elements, within their 5# non-coding regions. Molecular analysis of these MSA elements indicates that they are necessary and sufficient to direct G2/M gene expression and that they bind to 3 repeat Myb proteins (Ito et al., 2001). The plant 3 repeat Myb proteins are similar to the mammalian cMybs that are implicated in G1/S progression. Therefore, their role has been conserved in that both animal and plant genes regulate cell cycle phase transitions, but the phase affected is different. Plant 3 repeat Myb genes form two classes, one of which appears to act positively and is up regulated during G2/M; the other acts as an inhibitor and is expressed throughout the cell cycle. The transcripts of other cell cycle related genes do not change markedly during the cell cycle. These include the PSTAIR cdk genes and cyclin A and D genes. These functions may be regulated post-transcriptionally. Gainand loss-of-function transgenic and mutant approaches are being used to dissect the function of such genes. The alcA/alcR gene switch is a two component chemically inducible system for regulating gene expression in a range of organisms. Briefly, this system involves two components, originally from the filamentous fungus, Aspergillus nidulans. The transactivator, ALCR, is activated by a treatment of the plant with a low level of ethanol and a chimeric promoter, pAlcA/35S that contains a minimal 35S promoter and the upstream regulatory sequences of the Aspergillus alcA gene containing the binding sites of ALCR. The ethanol switch is functional in tobacco and has been transferred to Arabidopsis (Roslan et al., 2001) where we are using it to up and down regulate the expression of key developmental and cell cycle regulatory genes. We have created a range of transactivator lines that express the alcR transactivator in specific domains of the meristem or leaves. These lines should be generally useful for manipulating meristem growth or development. * Corresponding author. Tel.: +44-1603-450288; fax: +44-1603-450022. E-mail address: john.doonan@bbsrc.ac.uk (J.H. Doonan). Cell Biology International 27 (2003) 283–285 Cell Biology International