Summary Plant growth responds rapidly to developmental and environmental signals, but the underlying changes in cell division activity are poorly understood. A labile cyclin‐GUS reporter was developed to facilitate the spatio‐temporal analysis of cell division patterns. The chimeric reporter protein is turned over every cell cycle and hence its histochemical activity accurately reports individual mitotic cells. Using Arabidopsis plants transformed with cyclin‐GUS, we visualized patterns of mitotic activity in wounded leaves which suggest a role for cell division in structural reinforcement.

During postembryonic plant development, cell division is coupled to cell growth. There is a stringent requirement to couple these processes in shoot and root meristems. As cells pass through meristems, they transit through zones with high rates of cell growth and proliferation during organogenesis. This transition implies a need for coordinate regulation of genes underpinning these two fundamental cell functions. Here, we report a mechanism for coregulation of cell division control genes and cell growth effectors. We identified a GCCCR motif necessary and sufficient for high-level cyclin CYCB1;1 expression at G 2/M. This motif is overrepresented in many ribosomal protein gene promoters and is required for high-level expression of the S27 and L24 ribosomal subunit genes we examined. p33 TCP20 , encoded by the Arabidopsis TCP20 gene, binds to the GCCCR element in the promoters of cyclin CYCB1;1 and ribosomal protein genes in vitro and in vivo. We propose a model in which organ growth rates, and possibly shape in aerial organs, are regulated by the balance of positively and negatively acting teosinte-branched, cycloidea, PCNA factor (TCP) genes in the distal meristem boundary zone where cells become mitotically quiescent before expansion and differentiation.

During early seed development, nuclear divisions in the endosperm are not followed by cell division, leading to the development of a syncytium. The simple organization of the Arabidopsis endosperm provides a model in which to study the regulation of the cell cycle in relation to development. To monitor nuclear divisions, we constructed a HISTONE 2B::YELLOW FLUORESCENT PROTEIN gene fusion ( H2B::YFP ) . To validate its use as a vital marker for chromatin in plants, H2B::YFP was expressed constitutively in Arabidopsis. This enabled the observation of mitoses in living root meristems. H2B::YFP was expressed specifically in Arabidopsis syncytial endosperm by using GAL4 transactivation. Monitoring mitotic activity in living syncytial endosperm showed that the syncytium was organized into three domains in which nuclei divide simultaneously with a specific time course. Each mitotic domain has a distinct spatiotemporal pattern of mitotic CYCLIN B1;1 accumulation. The polar spatial organization of the three mitotic domains suggests interactions between developmental mechanisms and the regulation of the cell cycle. INTRODUCTIONRegulation of the cell cycle in plants and animals involves a remarkable number of conserved genes and mechanisms (Huntley and Murray, 1999;Mironov et al., 1999). In animals, regulation of the entry into G1 has proven to be critical for some developmental steps, such as Drosophila wing patterning (Johnston and Edgar, 1998) and the integration between trophic factors and proliferation (Conlon and Raff, 1999;Galloni and Edgar, 1999). Recently, an essential block of entry into mitosis has been characterized for the coordination between proliferation and gastrulation in Drosophila (Grosshans and Wieschaus, 2000;Mata et al., 2000;Seher and Leptin, 2000). It has been shown in plants that endogenous cell cycle regulation is very precise in meristems, where cells are produced in roots (Berger et al., 1998a), and in the shoot apex (Meyerowitz, 1997;Laufs et al., 1998). However, little is known about the general mechanisms that govern cell proliferation in relation to patterning in plants. To undertake such an analysis, we propose to use a simple developmental system, the syncytial endosperm in Arabidopsis.Several developmental programs in multicellular organisms are characterized by multiple nucleate structures called syncytia or coenocytes. These structures originate either from the fusion of multiple cells, as in osteoclasts (Jotereau and Le Douarin, 1978), or from a single cell in which nuclear divisions proceed without cytokinesis. The best-studied examples of the latter type are the Drosophila oocyte (Foe et al., 1993) and the hyphae of the fungus Aspergillus nidulans (Doonan, 1992). In these systems, nuclei divide synchronously and thus provide simple models in which to study the developmental control of the cell cycle (Edgar and Lehner, 1996). In plants, the development of syncytia is typical of the endosperm (Vijayaraghavan and Prabhakar, 1984;Lopes and Larkins, 1993; Berger, 1999). Endosperm is inc...

Plant species is considered to be one of the most important factors in shaping rhizobacterial communities, but specific plant–microbe interactions in the rhizosphere are still not fully understood. Arabidopsis thaliana, for which a large number of naturally occurring ecotype accessions exist, lacks mycorrhizal associations and is hence an ideal model for rhizobacterial studies. Eight Arabidopsis accessions were found to exert a marked selective influence on bacteria associated with their roots, as determined by terminal-restriction fragment length polymorphism (T-RFLP) and ribosomal intergenic spacer analysis (RISA). Community differences in species composition and relative abundance were both significant (P <0.001). The eight distinct and reproducible accession-dependent community profiles also differed from control bulk soil. Root exudates of these variants were analysed by high performance liquid chromatography (HPLC) to try to establish whether the unique rhizobacterial assemblages among accessions could be attributed to plant-regulated chemical changes in the rhizosphere. Natural variation in root exudation patterns was clearly exhibited, suggesting that differences in exudation patterns among accessions could be influencing bacterial assemblages. Other factors such as root system architecture are also probably involved. Finally, to investigate the Arabidopsis rhizosphere further, the phylogenetic diversity of rhizobacteria from accession Cvi-0 is described.

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