A family of cyclin D homologs from plants differentially controlled by growth regulators and containing the conserved retinoblastoma protein interaction motif

The shoot apical meristem: the dynamics of a stable structure

The shoot apical meristem (SAM) is a group of proliferating, embryonic-type cells that generates the aerial parts of the plant. SAMs are highly organized and stable structures that can function for years or even centuries. This is in apparent contradiction to the behaviour of their constituent cells, which continuously proliferate and differentiate. To reconcile the dynamic nature of the cells with the stability of the overall system the existence of elaborate signalling networks has been proposed. This is supported by recent work suggesting that the exchange of signals between cells, rather than a rigidly predetermined genetic program, is required for the establishment and functioning of an organized meristem. Together these interactions form a stable network, set up during embryogenesis, that assures the coordination of cell behaviour throughout development. Besides meristem-specific signalling cascades such as the CLAVATA receptor kinase pathway, which controls meristem size, these interactions involve plant hormones. In particular, cytokinins and auxins are implicated in the maintenance of meristem identity and phyllotaxis, respectively.

Auxin cross-talk: integration of signalling pathways to control plant development

Plants sense and respond to endogenous signals and environmental cues to ensure optimal growth and development. Plant cells must integrate the myriad transduction events into a comprehensive network of signalling pathways and responses. The phytohormone auxin occupies a central place within this transduction network, frequently acting in conjunction with other signals, to co-ordinately regulate cellular processes such as division, elongation and differentiation. As a non-cell autonomous signal, auxin also interacts with other signalling pathways to regulate inter-cellular developmental processes. As part of this especially themed edition of Plant Molecular Biology, we will review examples of 'cross-talk' between auxin and other signalling pathways. Given the current state of knowledge, we have deliberately focused our efforts reviewing auxin interactions with other phytohormone and light signalling pathways. We conclude by discussing how new genomic approaches and the Arabidopsis genome sequence are likely to impact this area of research in the future.

Multiple routes communicating nitrogen availability from roots to shoots: a signal transduction pathway mediated by cytokinin

In higher plants, inorganic nitrogen has crucial effects on growth and development, providing cellular components and modulating gene expression. To date, not only nitrogen assimilatory genes but also a substantial number of genes with other functions have been shown to be selectively regulated by the availability of nitrogen. In terms of the communicating substance(s) between root and shoot, accumulating evidence suggests that nitrate itself is the primary signal molecule triggering the activation of transcription of nitrate assimilation and related genes. On the other hand, some of the genes involved in photosynthesis, cell cycling and translation machinery are also regulated, at least in part, by nitrate and other nitrogen sources and, in some cases, the effect can be mimicked by cytokinin treatment. Spatial and temporal studies on the accumulation levels and the translocation of cytokinin in response to nitrate replenishment in maize showed subsequent accumulation of various cytokinin species in the roots, xylem sap and leaves. In Arabidopsis thaliana, trans-zeatin riboside-5'-monophosphate and/or trans-zeatin riboside also accumulated in the roots in response to nitrate resupply. These studies suggest that cytokinin metabolism and translocation could be commonly modulated by nitrogen availability in higher plants. Thus, in addition to nitrate, cytokinin could be another root-to-shoot signal communicating nitrogen availability.

Endoreduplication is not inhibited but induced by aphidicolin in cultured cells of tobacco

Endoreduplication is a common process in plants that allows cells to increase their DNA content. In the tobacco cell cultures studied in this work it can be induced by simple hormone deprivation. Mesophyll protoplast-derived cells cultured in the presence of NAA (auxin) and BAP (cytokinin) keep on dividing, while elongation and concomitant DNA endoreduplication are induced and maintained in a medium containing only NAA. If aphidicolin is given to the two types of culture, no effect is observed on elongating, endoreduplicating cells. However, the cells programmed for division switch to elongation and DNA endoreduplication. Thus aphidicolin, an inhibitor of the replicative DNA polymerases, alpha and delta, does not inhibit endoreduplication, and furthermore actually induces it when the mitotic cell cycle is blocked. DNA duplication and cell growth can only be completely blocked if ddTTP, an inhibitor of DNA polymerase-beta, is given together with aphidicolin. This result implies that an aphidicolin-resistant DNA polymerase, such as the repair-associated DNA polymerase-beta, can mediate DNA synthesis during endoreduplication and can substitute for polymerases-alpha and -delta when the latter are inhibited. Similar results are obtained in cultures of the BY-2 cell line by withdrawing auxins from the culture medium. In this cell line endoreduplication is induced only in a small proportion of the cells. A greater proportion of the cells are blocked in the G(2) phase of the cell cycle.

Genome-wide analysis of core cell cycle genes in Arabidopsis

Cyclin-dependent kinases and cyclins regulate with the help of different interacting proteins the progression through the eukaryotic cell cycle. A high-quality, homology-based annotation protocol was applied to determine the core cell cycle genes in the recently completed Arabidopsis genome sequence. In total, 61 genes were identified belonging to seven selected families of cell cycle regulators, for which 30 are new or corrections of the existing annotation. A new class of putative cell cycle regulators was found that probably are competitors of E2F/DP transcription factors, which mediate the G1-to-S progression. In addition, the existing nomenclature for cell cycle genes of Arabidopsis was updated, and the physical positions of all genes were compared with segmentally duplicated blocks in the genome, showing that 22 core cell cycle genes emerged through block duplications. This genome-wide analysis illustrates the complexity of the plant cell cycle machinery and provides a tool for elucidating the function of new family members in the future.

Molecular analysis of signals controlling dormancy and growth in underground adventitious buds of leafy spurge

Dormancy and subsequent regrowth of adventitious buds is a critical physiological process for many perennial plants. We have used the expression of hormone and cell cycle-responsive genes as markers to follow this process in leafy spurge (Euphorbia esula). In conjunction with earlier studies, we show that loss of mature leaves results in decreased sugar levels and increased gibberellin perception in underground adventitious buds. Gibberellin is sufficient for induction of S phase-specific but not M phase-specific gene expression. Loss of both apical and axillary buds or inhibition of polar auxin transport did not result in induction of S phase- or M phase-specific gene expression. Loss of polar auxin transport was necessary for continuation of the cell cycle and further bud development if the S phase was previously initiated.

Synchronous Arabidopsis suspension cultures for analysis of cell-cycle gene activity

Synchronized suspension cultures are powerful tools in plant cell-cycle studies. However, few Arabidopsis cell cultures are available, and synchrony extending over several sequential phases of the cell cycle has not been reported. Here we describe the first useful synchrony in Arabidopsis, achieved by selecting the rapidly dividing Arabidopsis cell suspensions MM1 and MM2d. Synchrony may be achieved either by removing and re-supplying sucrose to the growth media or by applying an aphidicolin block/release. Synchronization with aphidicolin produced up to 80% S-phase cells and up to 92% G2 cells, together with clear separation of different cell-cycle phases. These synchronization procedures can be used for analysis of gene expression and protein activity. We show that representatives of three CDK gene classes of Arabidopsis (CDKA, CDKB1 and CDKB2) show differential expression timing, and that three CDK inhibitor genes show strikingly different expression patterns during cell-cycle re-entry. We propose that ICK2 (KRP2) may have a specific role in this process.

ABA and sugar interactions regulating development: cross-talk or voices in a crowd?

Plant growth and development are controlled by the concerted action of many signaling pathways that integrate information from environmental signals with that from developmental and metabolic cues. Physiological studies have demonstrated that abscisic acid and sugars have both similar and antagonistic effects on diverse processes, including seed development, germination, and seedling growth. Recent genetic studies have identified several loci that are involved in both sugar and hormonal responses. It is rarely clear whether these apparent linkages reflect direct or indirect interactions between sugar and hormone signaling pathways, but the identification of gene products that are encoded at these loci is allowing these possibilities to be tested.

Spatial analysis of plant metabolism: sucrose imaging within Vicia faba cotyledons reveals specific developmental patterns

During legume embryogenesis the differentiation of the cotyledons proceeds gradually in a wave-like manner. The process is metabolically and genetically controlled and regulated by sugars. In order to perform a spatial and temporal analysis of the sugar distribution pattern a new method was developed to specifically measure sucrose directly in tissues via bioluminescence and single photon counting. This enabled a quantitative sucrose imaging with a resolution close to the single cell level. The procedure was applied on sections of Vicia faba cotyledons covering the main stages of histodifferentiation. Young embryos before the storage phase contained moderate levels of sucrose, which were evenly distributed. At the onset of maturation high concentrations were present within a tissue layer covering the outward half of the coytledons. This layer was directly underneath the epidermis expressing a sucrose transporter gene indicating that epidermal transporters caused the high sucrose accumulation in the underlying tissue. At that stage the sucrose gradient was inversely oriented compared with cell size and the starch content. Cells within the interior were larger, contained starch but low sucrose. Thus, the sucrose pattern is controlled by uptake activity and permeability within the parenchyma. However, during the main storage phase actively elongating and starch accumulating cells contain highest sucrose concentrations indicating that differences in growth and starch accumulation also affect intracotyledonary sugar distribution. High sucrose concentrations were correlated with transcript levels of sucrose synthase and ADP-Glc pyrophosphorylase indicating a signaling function for sucrose to induce starch biosynthesis on the gene expression level. Carbon flux through the sucrose synthase pathway towards starch increased when hexoses levels decreased.

Cellular basis of shoot apical meristem development

Shoot apical meristems are composed of proliferating, embryonic type cells, that generate tissues and organs throughout the life of the plant. This review covers the cell biology of the higher plant shoot apical meristem (SAM). The first section describes the molecular basis of plant cell growth and division. The genetic mechanisms, that operate in meristem function and the identification of several key regulators of meristem behavior are described in the second section, and intercellular communication and coordination of cellular behavior in the third part. Finally, we discuss some recent results that indicate interaction between the cellular regulators, such as the cell cycle control genes and developmental regulators.

A matter of size: developmental control of organ size in plants

The intrinsic size of plant organs is determined by developmental signals, yet the molecular and genetic mechanisms that control organ size are largely unknown. Ongoing functional analysis of Arabidopsis genes is defining important regulators involved in these mechanisms. Key features of this control are the coordinated activation of growth and cell division by growth regulators and the maintenance of meristematic competence by the ANT gene, which acts as an organ-size checkpoint. Alterations of genome size by polyploidization and endoreduplication can reset this checkpoint by ploidy-dependent, epigenetically regulated differential gene expression. In addition, the regulation of polarized growth and phytohormone signaling also affect final organ size. These findings reveal unique aspects of plant organ-size control that are distinct from animal organ-size control.

A cell-cycle-regulated kinase activity phosphorylates plant retinoblastoma protein and contains, in Arabidopsis, a CDKA/cyclin D complex

The activity of cyclin-dependent kinases (CDK) is crucial for cell-cycle transitions. Here, we report the identification of a CDK activity that phosphorylates the retinoblastoma-related (RBR) protein. A CDK/cyclin complex that binds to and phosphorylates RBR may be isolated from various plant sources, e.g. wheat, maize, Arabidopsis thaliana and tobacco, and from cells growing under various conditions. The presence of an RBR-associated CDK activity correlates with the proliferative activity, suggesting that phosphorylation of RBR is a major event in actively proliferating tissues. In A. thaliana, this activity comprises a PSTAIRE CDKA and at least cyclin D2. Furthermore, this CDK activity is cell-cycle-regulated, as revealed by studies with highly synchronized tobacco BY-2 cells where it is maximal in late G1 and early S phase cells and progressively decreases until G2 phase. Aphidicolin-arrested but not roscovitine-arrested cells contain a PSTAIRE-type CDK that binds to and phosphorylates RBR. Thus, association with a D-type cyclin is a likely mechanism leading to CDK activation late in G1. Our studies constitute the first report measuring the activity of CDK/cyclin complexes formed in vivo on RBR, an activity that fluctuates in a cell-cycle-dependent manner. This work provides the basis for further studies on the impact of phosphorylation of RBR on its function during the cell cycle and development.

Arabidopsis E2F1 binds a sequence present in the promoter of S-phase-regulated gene AtCDC6 and is a member of a multigene family with differential activities

Mammalian E2F transcription factors are composed of E2F and DP subunits, and with their negative regulators, Rb-related proteins, govern expression of cell-division-related genes. Six E2Fs and two DPs are present in mammals, but only single E2F genes are known from wheat, tobacco and carrot. Here we show that E2Fs are a multigene family in Arabidopsis, and report isolation of three E2F-like clones AtE2F1-3, with 45-62% identity to wheat, tobacco and carrot E2Fs. Sequence analysis reveals that AtE2F1 and AtE2F3 are closely related to previously identified plant E2Fs, whereas AtE2F2 is related to human E2F6 and Drosophila dE2F2 which are unusual in lacking transcriptional activation potential. Consistent with this, we show that AtE2FI and AtE2F3 activate transcription in yeast cells and bind a plant Rb protein, but AtE2F2 cannot activate transcription or bind Rb. Consensus E2F-binding sites were identified in promoters of several cell cycle related genes, including the D-type cyclin CycD3 and the Arabidopsis homologue of the replication origin protein CDC6. Accumulation of AtE2F1-3 was observed in partially synchronised Arabidopsis cells re-entering the cell cycle, before induction of CycD3 and CDC6 expression in late G1. AtE2F1 complexes bound to consensus E2F sequences and to the AtCDC6 promoter in vitro. We conclude that Arabidopsis contains a family of functionally distinct E2F genes, most probably involved in the G1-to-S phase progression.

The Arabidopsis AMP1 gene encodes a putative glutamate carboxypeptidase

Arabidopsis amp1 mutants show pleiotropic phenotypes, including altered shoot apical meristems, increased cell proliferation, polycotyly, constitutive photomorphogenesis, early flowering time, increased levels of endogenous cytokinin, and increased cyclin cycD3 expression. We have isolated the AMP1 gene by map-based cloning. The AMP1 cDNA encodes a 706;-amino acid polypeptide with significant similarity to glutamate carboxypeptidases. The AMP1 mRNA was expressed in all tissues examined, with higher expression in roots, stems, inflorescences, and siliques. Microarray analysis identified four mRNA species with altered expression in two alleles of amp1, including upregulation of CYP78A5, which has been shown to mark the shoot apical meristem boundary. The similarity of the AMP1 protein to glutamate carboxypeptidases, and in particular to N-acetyl alpha-linked acidic dipeptidases, suggests that the AMP1 gene product modulates the level of a small signaling molecule that acts to regulate a number of aspects of plant development, in particular the size of the apical meristem.

The Arabidopsis AMP1 gene encodes a putative glutamate carboxypeptidase

Arabidopsis amp1 mutants show pleiotropic phenotypes, including altered shoot apical meristems, increased cell proliferation, polycotyly, constitutive photomorphogenesis, early flowering time, increased levels of endogenous cytokinin, and increased cyclin cycD3 expression. We have isolated the AMP1 gene by map-based cloning. The AMP1 cDNA encodes a 706;-amino acid polypeptide with significant similarity to glutamate carboxypeptidases. The AMP1 mRNA was expressed in all tissues examined, with higher expression in roots, stems, inflorescences, and siliques. Microarray analysis identified four mRNA species with altered expression in two alleles of amp1, including upregulation of CYP78A5, which has been shown to mark the shoot apical meristem boundary. The similarity of the AMP1 protein to glutamate carboxypeptidases, and in particular to N-acetyl alpha-linked acidic dipeptidases, suggests that the AMP1 gene product modulates the level of a small signaling molecule that acts to regulate a number of aspects of plant development, in particular the size of the apical meristem.

CYTOKININ METABOLISM AND ACTION

Cytokinins are structurally diverse and biologically versatile. The chemistry and physiology of cytokinin have been studied extensively, but the regulation of cytokinin biosynthesis, metabolism, and signal transduction is still largely undefined. Recent advances in cloning metabolic genes and identifying putative receptors portend more rapid progress based on molecular techniques. This review centers on cytokinin metabolism with connecting discussions on biosynthesis and signal transduction. Important findings are summarized with emphasis on metabolic enzymes and genes. Based on the information generated to date, implications and future research directions are presented.

A novel and highly divergent Arabidopsis cyclin isolated by complementation in budding yeast

A novel cyclin, CycJ18, was isolated by complementation of G1 cyclin-deficient budding yeast with an Arabidopsis cDNA library. CycJ18 shares only 20% identity in its conserved cyclin box domain with other cyclins, and is predominantly expressed in young seedlings. CycJ18 is a member of a potential new plant cyclin class.

The fas locus of the phytopathogen Rhodococcus fascians affects mitosis of tobacco BY-2 cells

The effect of Rhodococcus fascians, the causal agent of leafy gall disease, on the mitotic behavior of synchronized tobacco Bright Yellow-2 (BY-2) cells was investigated. Incubation of aphidicolin-synchronized BY-2 cells with R. fascians cells specifically resulted in a broader mitotic index peak, an effect that was linked to an intact and expressed fas virulence locus. The obtained results pointed towards an effect of R. fascians on the prophase of mitosis. The relevance of these results to the virulence of the bacterium is discussed.

The regulation of ornithine decarboxylase gene expression by sucrose and small upstream open reading frame in tomato (Lycopersicon esculentum Mill)

We identified a near-full-length cDNA clone encoding ornithine decarboxylase (ODC) from tomato (Lycopersicon esculentum Mill). It contained a small upstream open reading frame (uORF) within its 5' untranslated region. An in vitro translation assay demonstrated that the uORF repressed expression of downstream ORF. Neither nucleotide nor predicted peptide sequence of the uORF was responsible for the repression. The presence of upstream AUG codon was shown to be responsible. ODC expression appeared to be organ specific. The ODC gene was expressed in roots, hypocotyls and sink leaves but not in source leaves. ODC transcripts were observed in apical meristem of primary roots, and were distributed in cells of cortex layer preferentially. ODC expression responded immediately to sucrose availability via the sucrose-specific pathway independent of hexokinase. Sucrose induction of ODC gene was seen in roots, hypocotyls and flowers but not in mature leaves. Moreover, only the root apical meristem responded to sucrose availability. These observations indicate that the spatial pattern of ODC expression is closely associated with cell proliferation and that sucrose sensing plays a major role in the spatial pattern of ODC expression. Also, the differential regulation of ODC and arginine decarboxylase gene expression by factors modulating plant growth suggests that they would have different physiological roles in plant development.

De novo cortical cell division triggered by the phytopathogen Rhodococcus fascians in tobacco

Plant growth, development, and morphology can be affected by several environmental stimuli and by specific interactions with phytopathogens. In many cases, plants respond to pathogenic stimuli by adapting their hormone levels. Here, the interaction between the phytopathogen Rhodococcus fascians and one of its host plants, tobacco, was analyzed phenotypically and molecularly. To elucidate the basis of the cell division modulation and shoot primordia initiation caused by R. fascians, tobacco plants were infected at leaf axils and shoot apices. Adventitious meristems that gave rise to multiple-shoot primordia (leafy galls) were formed. The use of a transgenic line carrying the mitotic CycB1 promoter fused to the reporter gene coding for beta-glucuronidase from Escherichia coli (uidA), revealed that stem cortical cells were stimulated to divide in an initial phase of the leafy gall ontogenesis. Local cytokinin and auxin levels throughout the infection process as well as modulation of expression of the cell cycle regulator gene Nicta;CycD3;2 are discussed.

Investigating the hows and whys of DNA endoreduplication

Endoreduplication is a form of nuclear polyploidization that results in multiple, uniform copies of chromosomes. This process is common in plants and animals, especially in tissues with high metabolic activity, and it generally occurs in cells that are terminally differentiated. In plants, endoreduplication is well documented in the endosperm and cotyledons of developing seeds, but it also occurs in many tissues throughout the plant. It is thought that endoreduplication provides a mechanism to increase the level of gene expression, but the function of this process has not been thoroughly investigated. Numerous observations have been made of endoreduplication, or at least extra cycles of S-phase, as a consequence of mutations in genes controlling several aspects of cell cycle regulation. However, until recently there were few studies directed at the molecular mechanisms responsible for this specialized cell cycle. It is suggested that endoreduplication requires nothing more elaborate than a loss of M-phase cyclin-dependent kinase activity and oscillations in the activity of S-phase cyclin-dependent kinase.

Cyclin/Cdk complexes: their involvement in cell cycle progression and mitotic division

DNA replication and mitosis are dependent on the activity of cyclin-dependent protein kinase (CDK) enzymes, which are heterodimers of a catalytic subunit with a cyclin subunit. Cyclin binding to specific individual proteins is thought to provide potential substrates to Cdk. Protein binding by cyclins is assessed in terms of its mechanisms and biological significance, using evidence from diverse organisms including substrate specificity in animal Cdk enzymes containing D-, A-, and B-type cyclins and extensive cyclin gene manipulations in yeasts. Assembly of protein complexes with cyclin/Cdk is noted and the capacity of the cyclin-dependent kinase subunit Cks, in such complex, to extend the range of Cdk substrates is documented and discussed in terms of cell cycle regulation. Cell cycle progression involves changing abundance of individual cyclins, due to changing rates of their transcription or proteolysis, with consequent changes in the substrates of CDK through the cell cycle. Some overlap of the functions of individual cyclins in vivo has been identified by cyclin deletions and is suggested to follow a pattern in which cyclins can commonly complete functions initiated by the preceding cyclins well enough to preserve viability as groups of cyclins are removed by proteolysis. Cyclin accumulation is particularly important in terminating the G1 phase, when it raises CDK activity and starts events leading to DNA replication. It is suggested that plants share this mechanism. The distribution of cyclins and Cdk in maize root tip cells during mitosis and cytokinesis indicates the presence of Cdk1 (Cdc2a) and cyclin CycB1zm;2 at the mature and disassembling preprophase band and the presence of CycB1zm;2 at condensing and condensed chromosomes. Both observations correlate with the earlier-reported capacity of injected metaphase cyclin/CDK to accelerate preprophase band disassembly and chromosome condensation and with observations of the location of Cdk and cyclins in other laboratories. Additionally CycB1zm;2 is seen at the nuclear envelope during its breakdown, which correlates with an acceleration of the process by injected metaphase cyclin B/CDK. A phenomenon possibly unique to the plant kingdom is the persistence of mitotic cyclins after anaphase. Participation of cyclins in cytokinesis is indicated by the concentration of the mitotic cyclin CycA1;zm;1 at the phragmoplast. It is suggested that cyclins have a general function of spatially focusing Cdk activity and that in the plant cell the concentrations of cyclins are important mediators of CDK activity at the cytoskeleton, chromosomes, spindle, nuclear envelope, and phragmoplast.

Characterization of two distinct DP-related genes from Arabidopsis thaliana

E2F/DP complexes play a pivotal role in the regulation of the G1/S transition in animals. Recently, plant E2F homologs have been cloned, but DP-related sequences have not been identified so far. Here we report that Arabidopsis thaliana contains at least two different DP-related genes, AtDPa and AtDPb. They exhibit an overall domain organization similar to that of their animal counterparts, although phylogenetic analysis demonstrated that they form a separate subgroup. AtDPs efficiently heterodimerize in vitro with the Arabidopsis E2F-related proteins, AtE2Fa and AtE2Fb through their dimerization domains. AtDPa and AtE2Fa are predominantly produced in actively dividing cells with highest transcript levels in early S phase cells.

Promotive effect of brassinosteroids on cell division involves a distinct CycD3-induction pathway in Arabidopsis

Brassinosteroids (BRs) are steroid hormones that play an essential role in plant growth and development. However, the contradictory results of previous studies make their role in cell division unclear. Using a cDNA array, we identified genes that respond to BR in the det2 suspension culture of Arabidopsis, and found that epi-brassinolide upregulated transcription of the CycD3, a D-type plant cyclin gene through which cytokinin activates cell division. RNA gel-blot analysis and cell culturing showed that epi-brassinolide may promote cell division through CycD3, and can substitute cytokinin in culturing of Arabidopsis callus and suspension cells. The CycD3 induction by epi-brassinolide was further shown to involve de novo protein synthesis, but no protein phosphorylation or dephosphorylation. Induction was also found to occur in cells of a BR-insensitive mutant, bri1, suggesting that BR induces CycD3 transcription through a previously unknown signal pathway in plants.

An enhancer trap line associated with a D-class cyclin gene in Arabidopsis

In yeast and animals, cyclins have been demonstrated to be important regulators of cell cycle progression. In recent years, a large number of A-, B-, and D-class cyclins have been isolated from a variety of plant species. One class of cyclins, the D-class cyclins, is important for progression through G1 phase of the cell cycle. In Arabidopsis, four D-class cyclins have been isolated and characterized (CYCLIN-D1;1, CYCLIN-D2;1, CYCLIN-D3;1, and CYCLIN-D4;1). In this report we describe the characterization of a fifth D-class cyclin gene, CYCLIN-D3;2 (CYCD3;2), from Arabidopsis. An enhancer trap line, line 5580, contains a T-DNA insertion in CYCD3;2. Enhancer trap line 5580 exhibits expression in young vegetative and floral primordia. In line 5580, T-DNA is inserted in the first exon of the CYCD3;2 gene; in homozygous 5580 plants CYCD3;2 RNA is not detectable. Even though CYCD3;2 gene function is eliminated, homozygous 5580 plants do not exhibit an obvious growth or developmental phenotype. Via in situ hybridization we demonstrate that CYCD3;2 RNA is expressed in developing vegetative and floral primordia. In addition, CYCD3;2 is also capable of rescuing a yeast strain that is deficient in G1 cyclin activity.

Cell cycle activation by plant parasitic nematodes

Sedentary nematodes are important pests of crop plants. They are biotrophic parasites that can induce the (re)differentiation of either differentiated or undifferentiated plant cells into specialized feeding cells. This (re)differentiation includes the reactivation of the cell cycle in specific plant cells finally resulting in a transfer cell-like feeding site. For growth and development the nematodes fully depend on these cells. The mechanisms underlying the ability of these nematodes to manipulate a plant for its own benefit are unknown. Nematode secretions are thought to play a key role both in plant penetration and feeding cell induction. Research on plant-nematode interactions is hampered by the minute size of cyst and root knot nematodes, their obligatory biotrophic nature and their relatively long life cycle. Recently, insights into cell cycle control in Arabidopsis thaliana in combination with reporter gene technologies showed the differential activation of cell cycle gene promoters upon infection with cyst or root knot nematodes. In this review, we integrate the current views of plant cell fate manipulation by these sedentary nematodes and made an inventory of possible links between cell cycle activation and local, nematode-induced changes in auxin levels.

Cell cycle regulation in the course of nodule organogenesis in Medicago

The molecular mechanisms of de novo meristem formation, cell differentiation and the integration of the cell cycle machinery into appropriate stages of the developmental programmes are still largely unknown in plants. Legume root nodules, which house nitrogen-fixing rhizobia, are unique plant organs and their development may serve as a model for organogenetic processes in plants. Nodules form and are essential for the plant only under limitation of combined nitrogen in the soil. Moreover, their development is triggered by external mitogenic signals produced by their symbiotic partners, the rhizobia. These signals, the lipochitooligosaccharide Nod factors, act as host-specific morphogens and induce the re-entry of root cortical cells into mitotic cycles. Maintenance of cell division activity leads to the formation of a persistent nodule meristem from which cells exit continuously and enter the nodule differentiation programme, involving multiple cycles of endoreduplication and enlargement of nuclear and cell volumes. While the small diploid 2C cells remain uninfected, the large polyploid cells can be invaded and, after completing the differentiation programme, host the nitrogen-fixing bacteroids. This review summarizes the present knowledge on cell cycle reactivation and meristem formation in response to Nod factors and reports on a novel plant cell cycle regulator that can switch mitotic cycles to differentiation programmes.

Developmental control of cell division patterns in the shoot apex

The shoot apical meristem is a group of rapidly dividing cells that generate all aerial parts of the plant. It is a highly organised structure, which can be divided into functionally distinct domains, characterised by specific proliferation rates of the individual cells. Genetic studies have enabled the identification of regulators of meristem function. These factors are involved in the formation and maintenance of the meristem, as well as in the formation of the primordia. Somehow, they must also govern cell proliferation rates within the shoot apex. Possible links between meristem regulators and the cell cycle machinery will be discussed. In order to analyse the role of cell proliferation in development, cell cycle gene expression has been perturbed using transgenic approaches and mutation. The effect of these alterations on growth and development at the shoot apex will be presented. Together, these studies give a first insight into the regulatory networks controlling the cell cycle and into the significance of cell proliferation in plant development.

Regulation of cyclin-dependent kinases in Arabidopsis thaliana

In plants, different families of cyclin-dependent kinases (CDKs) and cyclins have been identified, indicating that also in plants the progression through the cell cycle is regulated by CDKs. In all eukaryotes, CDKs exert their activity through well-controlled phosphorylations of specific substrates on serine/threonine residues. Such post-translational modifications are universal mechanisms in signal transduction pathways. They allow the organism to differentiate, regulate growth and/or adapt to environmental changes, the latter being crucial for plants because of their sedentary life-style. This adaptation might explain the occurrence of a special CDK type with plant-specific features. This review focuses on the involvement of plant CDKs in different phases of the cell cycle in Arabidopsis thaliana and outlines their regulation by binding to other proteins, and by phosphorylation and dephosphorylation.

CDK-related protein kinases in plants

Cyclin-dependent kinases (CDK) form a conserved superfamily of eukaryotic serine-threonine protein kinases, which require binding to a cyclin protein for activity. CDK are involved in different aspects of cell biology and notably in cell cycle regulation. The comparison of nearly 50 plant CDK-related cDNAs with a selected set of their animal and yeast counterparts reveals five classes of these genes in plants. These are described here with respect to their phylogenetic, structural and functional properties. A plant-wide nomenclature of CDK-related genes is proposed, using a system similar to that of the plant cyclin genes. The most numerous class, CDKA, includes genes coding for CDK with the PSTAIRE canonical motif. CDKB makes up a class of plant-specific CDK divided into two groups: CDKB1 and CDKB2. CDKC, CDKD and CDKE form less numerous classes. The CDKD class includes the plant orthologues of metazoan CDK7, which correspond to the CDK-activating kinase (CAK). At present, no functional information is available in plants for CDKC and CDKE.

The role and regulation of D-type cyclins in the plant cell cycle

The G1 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. G1 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 G1 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 G1/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.

Factors controlling cyclin B expression

Cyclins control the transition between the phases of the eukaryotic cell cycle as regulatory subunits of the cyclin-dependent kinases (CDKs). Phase-specific activation of the CDK is in part regulated by phase-specific expression of their cyclin component. In most eukaryotic cells including higher plant, B-type cyclin genes are expressed specifically at G2/M phase during the cell cycle. Promoters from yeast, plant and animal B-type cyclin genes are all activated in a cell cycle-regulated manner. In yeast, a transcription factor, Mcm1, in cooperation with an uncloned factor SFF, regulates the cell cycle-dependent promoter activation of mitotic B-type cyclin genes, CLB1 and CLB2. Activity of the human cyclin B1 promoter is regulated by a complex mechanism involving multiple cis-acting elements, none of which are sufficient for G2/M-specific promoter activation. In contrast, plants employ a simple mechanism for cell cycle-regulated promoter activation of B-type cyclin genes. Plant B-type cyclin gene promoters contain a common cis-acting element, called the MSA element, which is necessary and sufficient for the phase-specific promoter activation. MSA-like sequences are also found in the promoters of G2/M-specific genes encoding kinesin-like proteins, suggesting that a defined set of G2/M-specific genes are co-regulated by a common MSA-mediated mechanism in plants. Thus, the molecular mechanisms regulating B-type cyclin gene expression are evolutionarily divergent, and the MSA-mediated mechanism seems to be specific to plants. The consensus sequence of the MSA element resembles the binding sites of animal Myb transcription factors. A set of our data suggest the possibility that plant Myb may have unexpected roles in G2/M by inducing B-type cyclin genes, together with other cell cycle-related genes in plants.

A geminivirus replication protein interacts with the retinoblastoma protein through a novel domain to determine symptoms and tissue specificity of infection in plants

Geminiviruses replicate in nuclei of mature plant cells after inducing the accumulation of host DNA replication machinery. Earlier studies showed that the viral replication factor, AL1, is sufficient for host induction and interacts with the cell cycle regulator, retinoblastoma (pRb). Unlike other DNA virus proteins, AL1 does not contain the pRb binding consensus, LXCXE, and interacts with plant pRb homo logues (pRBR) through a novel amino acid sequence. We mapped the pRBR binding domain of AL1 between amino acids 101 and 180 and identified two mutants that are differentially impacted for AL1-pRBR interactions. Plants infected with the E-N140 mutant, which is wild-type for pRBR binding, developed wild-type symptoms and accumulated viral DNA and AL1 protein in epidermal, mesophyll and vascular cells of mature leaves. Plants inoculated with the KEE146 mutant, which retains 16% pRBR binding activity, only developed chlorosis along the veins, and viral DNA, AL1 protein and the host DNA synthesis factor, proliferating cell nuclear antigen, were localized to vascular tissue. These results established the importance of AL1-pRBR interactions during geminivirus infection of plants.

Sugar control of the plant cell cycle: differential regulation of Arabidopsis D-type cyclin gene expression

In most plants, sucrose is the major transported carbon source. Carbon source availability in the form of sucrose is likely to be a major determinant of cell division, and mechanisms must exist for sensing sugar levels and mediating appropriate control of the cell cycle. We show that sugar availability plays a major role during the G(1) phase by controlling the expression of CycD cyclins in Arabidopsis. CycD2 mRNA levels increase within 30 min of the addition of sucrose; CycD3 is induced after 4 h. This corresponds to induction of CycD2 expression early in G(1) and CycD3 expression in late G(1) near the S-phase boundary. CycD2 and CycD3 induction is independent both of progression to a specific point in the cell cycle and of protein synthesis. Protein kinase activity of CycD2- and CycD3-containing cyclin-dependent kinases is consistent with the observed regulation of their mRNA levels. CycD2 and CycD3 therefore act as direct mediators of the presence of sugar in cell cycle commitment. CycD3, but not CycD2, expression responds to hormones, for which we show that the presence of sugars is required. Finally, protein phosphatases are shown to be involved in regulating CycD2 and CycD3 induction. We propose that control of CycD2 and CycD3 by sucrose forms part of cell cycle control in response to cellular carbohydrate status.

SUGAR-INDUCED SIGNAL TRANSDUCTION IN PLANTS

Sugars have important signaling functions throughout all stages of the plant's life cycle. This review presents our current understanding of the different mechanisms of sugar sensing and sugar-induced signal transduction, including the experimental approaches used. In plants separate sensing systems are present for hexose and sucrose. Hexokinase-dependent and -independent hexose sensing systems can further be distinguished. There has been progress in understanding the signal transduction cascade by analyzing the function of the SNF1 kinase complex and the regulatory PRL1 protein. The role of sugar signaling in seed development and in seed germination is discussed, especially with respect to the various mechanisms by which sugar signaling controls gene expression. Finally, recent literature on interacting signal transduction cascades is discussed, with particular emphasis on the ethylene and ABA signal transduction pathways.

Cyclin D control of growth rate in plants

The mechanisms by which plants modulate their growth rate in response to environmental and developmental conditions are unknown, but are presumed to involve specialized regions called meristems where cell division is concentrated. The possible role of cell division in influencing meristem activity and overall plant growth rate is controversial, with a prevailing view that cell division is secondary to higher order meristem controls. Here we show that a reduction in the length of the cell-cycle G1 phase and faster cell cycling occur when the rate of cell division in transgenic tobacco plants is increased by the plant D-type cyclin CycD2 (ref. 8). The plants have normal cell and meristem sizes, but elevated overall growth rates, an increased rate of leaf initiation and accelerated development in all stages from seedling to maturity. We conclude that cell division is a principal determinant of meristem activity and overall growth rate, and propose that modulation of plant growth rate is achieved through regulation of G1.

Regulation of sugar, amino acid and peptide plant membrane transporters

During the past few years, various cDNAs encoding the proton cotransporters which mediate the uptake of sucrose, hexoses, amino acids and peptides across the plant plasma membrane have been cloned. This has made possible some preliminary insight into the regulation of the activity of these transporters at various levels. The paper summarises the present status of knowledge and gaps relative to their transcriptional control (organ, tissue and cell specificity, response to the environment) and post-transcriptional control (targeting and turnover, kinetic and thermodynamic control, lipidic environment, phosphorylation). This outline and the description of a few cases (the sink/source transition of the leaf, the pollen grain, the legume seed) serve as a basis for suggesting some directions for future research.

Control of plant growth and development through manipulation of cell-cycle genes

The plant embryo is a relatively simple structure consisting of a primordial shoot and root, whose development is frozen in the form of a seed. Most development of the mature plant takes place post-embryonically, and is the consequence of cell division and organogenesis in small regions known as meristems, which originate in the embryonic shoot and root apices. Significant recent progress has been made in understanding the mechanisms that control the plant cell cycle at a molecular level, and the first attempts have been made to control plant growth through modulation of cell-cycle genes. These results suggest that there is significant potential to control plant growth and architecture through manipulation of cell division rates. However, a full realisation of the promise of such strategies will probably require a much greater understanding of cell division control and how its upstream regulation is co-ordinated by spatial relationships between cells and by environmental signals.

The expression of D-cyclin genes defines distinct developmental zones in snapdragon apical meristems and is locally regulated by the Cycloidea gene

Three D-cyclin genes are expressed in the apical meristems of snapdragon (Antirrhinum majus). The cyclin D1 and D3b genes are expressed throughout meristems, whereas cyclin D3a is restricted to the peripheral region of the meristem, especially the organ primordia. During floral development, cyclin D3b expression is: (a) locally modulated in the cells immediately surrounding the base of organ primordia, defining a zone between lateral organs that may act as a developmental boundary; (b) locally modulated in the ventral petals during petal folding; and (c) is specifically repressed in the dorsal stamen by the cycloidea gene. Expression of both cyclin D3 genes is reduced prior to the cessation of cell cycle activity, as judged by histone H4 expression. Expression of all three D-cyclin genes is modulated by factors that regulate plant growth, particularly sucrose and cytokinin. These observations may provide a molecular basis for understanding the local regulation of cell proliferation during plant growth and development.

DNA replication and cell cycle in plants: learning from geminiviruses

Plant cell growth and development depend on continuous cell proliferation which is restricted to small regions of the plant called meristems. Infection by geminiviruses, small DNA viruses whose replicative cycle relies on host cell factors, is excluded from those proliferating areas. Since most of the replicative factors are present, almost exclusively, in proliferating cells, geminivirus infection is believed to induce a cellular state permissive for viral DNA replication, e.g. S-phase or, at least, some specific S-phase functions. The molecular basis for this effect seems to be the interference that certain geminivirus proteins exert on the retinoblastoma-related (RBR) pathway, which analogously to that of animal cells, regulates plant cell cycle activation and G(1)-S transition. In some cases, geminiviruses induce cell proliferation and abnormal growth. Mechanisms other than sequestering plant RBR probably contribute to the multiple effects of geminivirus proteins on cellular gene expression, cell growth control and cellular DNA replication. Current efforts to understand the coupling of geminivirus DNA replication to cell cycle and growth control as well as the directions in which future research is aiming are reviewed.

Rapid effects of nitrogen form on leaf morphogenesis in tobacco

Ammonium (NH4+) instead of nitrate (NO3-) as the nitrogen (N) source for tobacco (Nicotiana tabacum L.) cultivated in a pH-buffered nutrient solution resulted in decreased shoot and root biomass. Reduction of shoot fresh weight was mainly related to inhibition of leaf growth, which was already detectable after short-term NH4+ treatments of 24 h, and even at a moderate concentration level of 2 mM. Microscopic analysis of the epidermis of fully expanded leaves revealed a decrease in cell number (50%) and in cell size (30%) indicating that both cell division and cell elongation were affected by NH4+ application. Changes in various physiological parameters known to be associated with NH4(+)-induced growth depression were examined both in long-term and short-term experiments: the concentrations of total N, soluble sugars and starch as well as the osmotic potential, the apparent hydraulic conductivity and the rate of water uptake were not reduced by NH4+ treatments (duration 1-12 d), suggesting that leaf growth was neither limited by the availability of N and carbohydrates, nor by a lack of osmotica or water supply. Although the concentration of K+ in leaf press sap declined in expanding leaves by approximately 15% in response to NH4+ nutrition, limitation of mineral nutrients seems to be unlikely in view of the fast response of leaf growth at 24 h after the start of the NH4+ treatment. No inhibitory effects were observed when NH4+ and NO3- were applied simultaneously (each 1 mM) resulting in a NO3-/NH4+ net uptake ratio of 6:4. These findings suggest that the rapid inhibition of leaf growth was not primarily related to NH4+ toxicity, but to the lack of NO3(-)-supply. Growth inhibition of plants fed solely with NH4+ was associated with a 60% reduction of the zeatine + zeatine riboside (Z + ZR) cytokinin fraction in the xylem sap after 24 h. Furthermore Z + ZR levels declined to almost zero within the next 4 d after start of the NH4+ treatment. In contrast, the concentrations of the putative Z + ZR precursors isopentenyl-adenine and isopentenyl-adenosine (i-Ade + i-Ado) were not affected by NH4+ application. Since cytokinins are involved in the regulation of both cell division and cell elongation, it seems likely that the presence of NO3- is required to maintain biosynthesis and/or root to shoot transfer of cytokinins at a level that is sufficient to mediate normal leaf morphogenesis.

Plant organ size control: AINTEGUMENTA regulates growth and cell numbers during organogenesis

The control of cell proliferation during organogenesis plays an important role in initiation, growth, and acquisition of the intrinsic size of organs in higher plants. To understand the developmental mechanism that controls intrinsic organ size by regulating the number and extent of cell division during organogenesis, we examined the function of the Arabidopsis regulatory gene AINTEGUMENATA (ANT). Previous observations revealed that ANT regulates cell division in integuments during ovule development and is necessary for floral organ growth. Here we show that ANT controls plant organ cell number and organ size throughout shoot development. Loss of ANT function reduces the size of all lateral shoot organs by decreasing cell number. Conversely, gain of ANT function, via ectopic expression of a 35S::ANT transgene, enlarges embryonic and all shoot organs without altering superficial morphology by increasing cell number in both Arabidopsis and tobacco plants. This hyperplasia results from an extended period of cell proliferation and organ growth. Furthermore, cells ectopically expressing ANT in fully differentiated organs exhibit neoplastic activity by producing calli and adventitious roots and shoots. Based on these results, we propose that ANT regulates cell proliferation and organ growth by maintaining the meristematic competence of cells during organogenesis.

Plant organ size control: AINTEGUMENTA regulates growth and cell numbers during organogenesis

The control of cell proliferation during organogenesis plays an important role in initiation, growth, and acquisition of the intrinsic size of organs in higher plants. To understand the developmental mechanism that controls intrinsic organ size by regulating the number and extent of cell division during organogenesis, we examined the function of the Arabidopsis regulatory gene AINTEGUMENATA (ANT). Previous observations revealed that ANT regulates cell division in integuments during ovule development and is necessary for floral organ growth. Here we show that ANT controls plant organ cell number and organ size throughout shoot development. Loss of ANT function reduces the size of all lateral shoot organs by decreasing cell number. Conversely, gain of ANT function, via ectopic expression of a 35S::ANT transgene, enlarges embryonic and all shoot organs without altering superficial morphology by increasing cell number in both Arabidopsis and tobacco plants. This hyperplasia results from an extended period of cell proliferation and organ growth. Furthermore, cells ectopically expressing ANT in fully differentiated organs exhibit neoplastic activity by producing calli and adventitious roots and shoots. Based on these results, we propose that ANT regulates cell proliferation and organ growth by maintaining the meristematic competence of cells during organogenesis.

The plant cell cycle

Molecular controls of the plant cell cycle must integrate environmental signals within developmental contexts. Recent advances highlight the fundamental conservation of underlying cell cycle mechanisms between animals and plants, overlaid by a rich molecular and regulatory diversity that is specific to plant systems. Here we review plant cell cycle regulators and their control.

His-Asp phosphotransfer possibly involved in the nitrogen signal transduction mediated by cytokinin in maize: molecular cloning of cDNAs for two-component regulatory factors and demonstration of phosphotransfer activity in vitro

Implication of His-to-Asp and/or Asp-to-His (His-Asp) phosphorelay has been recently reported in signal transduction pathways initiated by ethylene and cytokinin. These signaling systems are generally composed of sensor His-protein kinases, His-containing phosphotransfer (HPt) domains, and response regulator domains. In this study, we isolated maize cDNAs, designated as ZmRR2 and ZmHP2, which encode a response regulator domain and HPt domain, respectively, and we identified their His-to-Asp phosphotransfer activity in vitro. The putative translated product of ZmRR2 was highly similar to that of ZmRR1 (78% identity), a maize response regulator homologue. The putative translated product of ZmHP2 showed similarity to that of HPt domains from Arabidopsis thaliana (AHP1-AHP3: 44 to 47% identity) and Saccharomyces cerevisiae (Ypdlp: 24% identity). In vitro experiments demonstrated that the putative signaling factors can transfer the phosphoryl group from His-80 of ZmHP2 to Asp-90 of ZmRRs. Treating detached leaves with t-zeatin or supplying inorganic nitrogen to the whole plant induced the accumulation of ZmRR1 and ZmRR2 transcripts. On the other hand, the steady-state transcript level of ZmHP2 was not affected by cytokinin or inorganic nitrogen sources. These results indicate that His-Asp phosphotransfer may be involved in the transduction of nitrogen signals mediated by cytokinin, and that multiple response regulators participate in the signaling pathways.

Molecular and biochemical characterization of the involvement of cyclin-dependent kinase A during the early development of tomato fruit

Following fruit set, the early development of tomato (Lycopersicon esculentum Mill.) fruit comprises two distinct phases: a cell division phase and a consecutive phase of cell expansion until the onset of ripening. In this study, we analyzed cytological and molecular changes characterizing these early phases of tomato fruit development. First we investigated the spatial and temporal regulation of the mitotic activity during fruit development. The DNA content of isolated nuclei from the different fruit tissues was determined by flow cytometry analysis. The results confirm the data of mitotic activity measurements and show that cell differentiation, leading to expanded cells, is characterized by endoreduplication. Second, we isolated two cDNAs, named Lyces;CDKA1 (accession no. Y17225) and Lyces;CDKA2 (accession no. Y17226), encoding tomato homologs of the cyclin-dependent kinase (CDK) p34(cdc2). Tomato CDKA gene expression was followed at both the transcriptional and translational levels during fruit development. The transcripts for Lyces;CDKA1 and Lyces;CDKA2 and the corresponding CDKA proteins are predominantly accumulated during the phase of cell division between anthesis and 5 d post anthesis (DPA). In whole fruits, the maximum CDK activity was obtained between 5 and 10 DPA. The determination of the kinase activity using protein extracts from the different fruit tissues was in agreement with mitotic activity analysis. It showed the particular disappearance of the activity in the gel tissue as early as 15 DPA. The overall data of CDK activity measurements suggest a strong post-translational regulation of CDK at the temporal and spatial levels during early tomato fruit development.

Isolation and characterization of the E2F-like gene in plants

The transcription factor E2F regulates the expression of genes involved in the progression of G1/S transition and DNA replication in mammalian cells. We cloned and characterized a cDNA (NtE2F) corresponding to a E2F homolog of tobacco (Nicotiana tabacum). The transcription of NtE2F was induced as cells progressed from G1 to the S phase and expressed much earlier than that of the proliferating cell nuclear antigen (PCNA) gene. We demonstrated that NtE2F can interact with the tobacco retinoblastoma (Rb)-related protein in a yeast two-hybrid assay. To further characterize NtE2F, the trans-activation activity of NtE2F was examined by using a transient assay in the tobacco Bright Yellow-2 (BY-2) cells with NtE2F fused to the DNA-binding domain of the veast transcriptional activator GAL4. NtE2F activated the transcription of the beta-glucuronidase (GUS) reporter gene driven by a cauliflower mosaic virus (CaMV) 35S core promoter containing the GAL4-binding sequence. This is the first report of the identification of a functionally equivalent E2F-like gene in plants.