Multiple A-type cyclins express sequentially during the cell cycle in Nicotiana tabacum BY2 cells

H2 supplied via ammonia borane stimulates lateral root branching via phytomelatonin signaling

A reliable and stable hydrogen gas (H2) supply will benefit agricultural laboratory and field trials. Here, we assessed ammonia borane (AB), an efficient hydrogen storage material used in the energy industry, and determined its effect on plant physiology and the corresponding mechanism. Through hydroponics and pot experiments, we discovered that AB increases tomato (Solanum lycopersicum) lateral root (LR) branching and this function depended on the increased endogenous H2 level caused by the sustainable H2 supply. In particular, AB might trigger LR primordia initiation. Transgenic tomato and Arabidopsis (Arabidopsis thaliana) expressing hydrogenase1 (CrHYD1) from Chlamydomonas reinhardtii not only accumulated higher endogenous H2 and phytomelatonin levels but also displayed pronounced LR branching. These endogenous H2 responses achieved by AB or genetic manipulation were sensitive to the pharmacological removal of phytomelatonin, indicating the downstream role of phytomelatonin in endogenous H2 control of LR formation. Consistently, extra H2 supply failed to influence the LR defective phenotypes in phytomelatonin synthetic mutants. Molecular evidence showed that the phytomelatonin-regulated auxin signaling network and cell-cycle regulation were associated with the AB/H2 control of LR branching. Also, AB and melatonin had little effect on LR branching in the presence of auxin synthetic inhibitors. Collectively, our integrated approaches show that supplying H2 via AB increases LR branching via phytomelatonin signaling. This finding might open the way for applying hydrogen storage materials to horticultural production.

Conservation and divergence of flg22, pep1 and nlp20 in activation of immune response and inhibition of root development

Many pattern-recognition receptors (PRRs) and their corresponding ligands have been identified. However, it is largely unknown how similar and different these ligands are in inducing plant innate immunity and affecting plant development. In this study, we examined three well characterized ligands in Arabidopsis thaliana, namely flagellin 22 (flg22), plant elicitor peptide 1 (pep1) and a conserved 20-amino-acid fragment found in most necrosis and ethylene-inducing peptide 1-like proteins (nlp20). Our quantitative analyses detected the differences in amplitude in the early immune responses of these ligands, with nlp20-induced responses typically being slower than those mediated by flg22 and pep1. RNA sequencing showed the shared differentially expressed genes (DEGs) was mostly enriched in defense response, whereas nlp20-regulated genes represent only a fraction of those genes differentially regulated by flg22 and pep1. The three elicitors all inhibited primary root growth, especially pep1, which inhibited both auxin transport and signaling pathway. In addition, pep1 significantly inhibited the cell division and genes involved in cell cycle. Compared with flg22 and nlp20, pep1 induced much stronger expression of its receptor in roots, suggesting a potential positive feedback regulation in the activation of immune response. Despite PRRs and their co-receptor BAK1 were necessary for both PAMP induced immune response and root growth inhibition, bik1 mutant only showed impaired defense response but relatively normal root growth inhibition, suggesting BIK1 acts differently in these two biological processes.

Genome-Wide Identification, Characterization, and Transcriptomic Analysis of the Cyclin Gene Family in Brassica rapa

Cyclins are involved in cell division and proliferation by activating enzymes required for the cell cycle progression. Our genome-wide analysis identified 76 cyclin genes in Brassica rapa, which were divided into nine different types (A-, B-, C-, D-, H-, L-, P-, T-, and SDS-type). Cyclin genes were unevenly scattered on all chromosomes, with a maximum of 10 on A08 and a minimum of 2 on A04. The gene structure and conserved motif analysis showed that the cyclins which belonged to the same type or subgroup have a comparable intron/exon pattern or motif. A total of 14 collinear gene pairs suggested that the B. rapa cyclin genes experienced a mass of segmental duplication. The Ka/Ks analysis revealed that the Brcyclin gene family has undergone an extensive purifying pressure. By analyzing the cis-elements in the promoters, we identified 11 cis-elements and five of them are related to the hormone response. We observed 48 potential miRNAs targeting 44 Brcyclin genes, which highlighted the involvement of miRNAs in the regulation of cyclin genes. An association analysis between the leaf size and SNPs in mutants and a transcriptome analysis of two Chinese cabbage-cabbage translocation lines also showed that the Brcyclin gene family was involved in the development of the leaves. The functional characterization of the B. rapa cyclin gene family will provide the foundation for future physiological and genetic studies in the regulation of leaf growth.

OsARP6 Is Involved in Internode Elongation by Regulating Cell-Cycle-Related Genes

The SWR1 complex (SWR1-C) is important for the deposition of histone variant H2A.Z into chromatin to regulate gene expression. Characterization of SWR1-C subunits in Arabidopsis thaliana has revealed their role in variety of developmental processes. Oryza sativa actin related protein 6 (OsARP6) is a subunit of rice SWR1-C. Its role in rice plant development is unknown. Here, we examined the subcellular localization, expression patterns, and loss of function phenotypes for this protein and found that OsARP6 is a nuclear localized protein, and is broadly expressed. OsARP6 interacted with OsPIE1, a central ATPase subunit of rice SWR1-C. The osarp6 knockout mutants displayed pleiotropic phenotypic alterations in vegetative and reproductive traits, including semi-dwarf phenotype, lower tillers number, short leaf length, changes in spikelet morphology, and seed abortion. Microscopic thin sectioning of the top internode revealed that the dwarf phenotype of osarp6 was due to reduced number of cells rather than reduced cell length. The altered transcript level of genes involved in cell division suggested that OsARP6 affects cell cycle regulation. In addition, H2A.Z levels were reduced at the promoters and transcription start sites (TSS) of the regulated genes in osarp6 plants. Together, these results suggest that OsARP6 is involved in rice plant development, and H2A.Z deposition.

Overexpression Populus d-Type Cyclin Gene PsnCYCD1;1 Influences Cell Division and Produces Curved Leaf in Arabidopsis thaliana

d-type cyclins (CYCDs) are a special class of cyclins and play extremely important roles in plant growth and development. In the plant kingdom, most of the existing studies on CYCDs have been done on herbaceous plants, with few on perennial woody plants. Here, we identified a Populus d-type cyclin gene, PsnCYCD1;1, which is mainly transcribed in leaf buds and stems. The promoter of PsnCYCD1;1 activated GUS gene expression and transgenic Arabidopsis lines were strongly GUS stained in whole seedlings and mature anthers. Moreover, subcellular localization analysis showed the fluorescence signal of PsnCYCD1;1-GFP fusion protein is present in the nucleus. Furthermore, overexpression of the PsnCYCD1;1 gene in Arabidopsis can promote cell division and lead to small cell generation and cytokinin response, resulting in curved leaves and twisted inflorescence stems. Moreover, the transcriptional levels of endogenous genes, such as ASs, KNATs, EXP10, and PHB, were upregulated by PsnCYCD1;1. Together, our results indicated that PsnCYCD1;1 participates in cell division by cytokinin response, providing new information on controlling plant architecture in woody plants.

Genome-Wide Analysis of the Cyclin Gene Family and Their Expression Profile in Medicago truncatula

Cyclins, together with highly conserved cyclin-dependent kinases (CDKs), play an important role in the process of cell cycle in plants, but less is known about the functions of cyclins in legume plants, especially Medicago truncatula. Our genome-wide analysis identified 58, 103, and 51 cyclin members in the M. truncatula, Glycine max, and Phaseolus vulgaris genomes. Phylogenetic analysis suggested that these cyclins could be classified into 10 types, and the CycB-like types (CycBL1-BL8) were the specific subgroups in M. truncatula, which was one reason for the expansion of the B-type in M. truncatula. All putative cyclin genes were mapped onto their own chromosomes of each genome, and 9 segmental duplication gene pairs involving 20 genes were identified in M. truncatula cyclins. Determined by quantitative real-time PCR, the expression profiling suggested that 57 cyclins in M. truncatula were differentially expressed in 9 different tissues, while a few genes were expressed in some specific tissues. Using the publicly available RNAseq data, the expression of Mtcyclins in the wild-type strain A17 and three nodule mutants during rhizobial infection showed that 23 cyclins were highly upregulated in the nodulation (Nod) factor-hypersensitive mutant sickle (skl) mutant after 12 h of rhizobium inoculation. Among these cyclins, six cyclin genes were also specifically expressed in roots and nodules, which might play specific roles in the various phases of Nod factor-mediated cell cycle activation and nodule development. Our results provide information about the cyclin gene family in legume plants, serving as a guide for further functional research on plant cyclins.

Genome-Wide Analysis of the D-type Cyclin Gene Family Reveals Differential Expression Patterns and Stem Development in the Woody Plant Prunus mume

Cyclins, a prominent class of cell division regulators, play an extremely important role in plant growth and development. D-type cyclins (CYCDs) are the rate-limiting components of the G1 phase. In plants, studies of CYCDs are mainly concerned with herbaceous plants, yet little information is available about these genes in perennial woody plants, especially ornamental plants. Here, twelve Prunus mume CYCD (PmCYCDs) genes are identified and characterized. The PmCYCDs were named on the basis of orthologues in Arabidopsis thaliana and Oryza sativa. Gene structure and conserved domains of each subgroup CYCDs was similar to that of their orthologues in A. thaliana and O. sativa. However, PmCYCDs exhibited different tissue-specific expression patterns in root, stem, leaf, bud, and fruit organs. The results of qRT-PCR showed that all PmCYCDs, except PmCYCD5;2 and PmCYCD7;1, were primarily highly expressed in leaf buds, shoots, and stems. In addition, the transcript levels of PmCYCD genes were analyzed in roots under different treatments, including exogenous applications of NAA, 6-BA, GA3, ABA, and sucrose. Interestingly, although PmCYCDs were induced by sucrose, the extent of gene induction among PmCYCD subgroups varied. The induction of PmCYCD1;2 by hormones depended on the presence of sucrose. PmCYCD3;1 was stimulated by NAA, and induction was strengthened when sugar and hormones were applied together. Taken together, our study demonstrates that PmCYCDs are functional in plant stem development and provides a basis for selecting members of the cyclin gene family as candidate genes for ornamental plant breeding.

Cell division and turgor mediate enhanced plant growth in Arabidopsis plants treated with the bacterial signalling molecule lumichrome

Transcriptomic analysis indicates that the bacterial signalling molecule lumichrome enhances plant growth through a combination of enhanced cell division and cell enlargement, and possibly enhances photosynthesis. Lumichrome (7,8 dimethylalloxazine), a novel multitrophic signal molecule produced by Sinorhizobium meliloti bacteria, has previously been shown to elicit growth promotion in different plant species (Phillips et al. in Proc Natl Acad Sci USA 96:12275-12280, https://doi.org/10.1073/pnas.96.22.12275 , 1999). However, the molecular mechanisms that underlie this plant growth promotion remain obscure. Global transcript profiling using RNA-seq suggests that lumichrome enhances growth by inducing genes impacting on turgor driven growth and mitotic cell cycle that ensures the integration of cell division and expansion of developing leaves. The abundance of XTH9 and XPA4 transcripts was attributed to improved mediation of cell-wall loosening to allow turgor-driven cell enlargement. Mitotic CYCD3.3, CYCA1.1, SP1L3, RSW7 and PDF1 transcripts were increased in lumichrome-treated Arabidopsis thaliana plants, suggesting enhanced growth was underpinned by increased cell differentiation and expansion with a consequential increase in biomass. Synergistic ethylene-auxin cross-talk was also observed through reciprocal over-expression of ACO1 and SAUR54, in which ethylene activates the auxin signalling pathway and regulates Arabidopsis growth by both stimulating auxin biosynthesis and modulating the auxin transport machinery to the leaves. Decreased transcription of jasmonate biosynthesis and responsive-related transcripts (LOX2; LOX3; LOX6; JAL34; JR1) might contribute towards suppression of the negative effects of methyl jasmonate (MeJa) such as chlorophyll loss and decreases in RuBisCO and photosynthesis. This work contributes towards a deeper understanding of how lumichrome enhances plant growth and development.

Suppression of microRNA159 impacts multiple agronomic traits in rice (Oryza sativa L.)

BACKGROUND

microRNAs (miRNAs) are important regulators in plant growth and development. miR159 is a conserved miRNA among different plant species and has various functions in plants. Studies on miR159 are mostly done on model plant, Arabidopsis thaliana. In rice, studies on miR159 were either based upon genome-wide expression analyses focused upon responses to different nitrogen forms and abiotic stress or upon phenotypic studies of transgenic plants overexpressing its precursor. STTM (Short Tandem Target Mimic) is an effective tool to block the activity of endogenous mature miRNA activity in plant. Therefore, specific roles of miR159 in rice could be explored by down regulating miR159 through STTM.

RESULTS

In this study, expression of mature miR159 was successfully suppressed by STTM which resulted in the increased expressions of its two targets genes, OsGAMYB and OsGAMYBL1 (GAMYB-LIKE 1). Overall, STTM159 plants exhibited short stature along with smaller organ size and reduction in stem diameter, length of flag leaf, main panicle, spikelet hulls and grain size. Histological analysis of stem, leaf and mature spikelet hull showed the reduced number of small vascular bundles (SVB), less number of small veins (SV) between two big veins (LV) and less cell number in outer parenchyma. Gene Ontology (GO) enrichment analysis of differentially expressed genes between wild type plants and STTM159 transgenic plants showed that genes involved in cell division, auxin, cytokinin (CK) and brassinosteroids (BRs) biosynthesis and signaling are significantly down-regulated in STTM159 plants.

CONCLUSION

Our data suggests that in rice, miR159 positively regulates organ size, including stem, leaf, and grain size due to the promotion of cell division. Further analysis from the RNA-seq data showed that the decreased cell divisions in STTM159 transgenic plants may result, at least partly from the lower expression of the genes involved in cell cycle and hormone homeostasis, which provides new insights of rice miR159-specific functions.

Timing of the G1/S transition in tobacco pollen vegetative cells as a primary step towards androgenesis in vitro

KEY MESSAGE

Mid-bicellular pollen vegetative cells in tobacco escape from G1 arrest and proceed to the G1/S transition towards androgenesis within 1 day under glutamine starvation conditions in vitro. In the Nicotiana tabacum pollen culture system, immature pollen grains at the mid-bicellular stage can mature in the presence of glutamine; however, if glutamine is absent, they deviate from their native cell fate in a few days. The glutamine-starved pollen grains cannot undergo maturation, even when supplied with glutamine later. Instead, they undergo cell division towards androgenesis slowly within 10 days in a medium containing appropriate nutrients. During the culture period, they ought to escape from G1 arrest to proceed into S phase as the primary step towards androgenesis. However, this event has not been experimentally confirmed. Here, we demonstrated that the pollen vegetative cells proceeded to the G1/S transition within approximately 15-36 h after the start of culture. These results were obtained by analyzing transgenic pollen possessing a fusion gene encoding nuclear-localizing GFP under the control of an E2F motif-containing promoter isolated from a gene encoding one of DNA replication licensing factors. Observations using a 5-ethynyl-2'-deoxyuridine DNA labeling and detection technique uncovered that the G1/S transition was soon followed by S phase. These hallmarks of vegetative cells undergoing dedifferentiation give us new insights into upstream events causing the G1/S transition and also provide a novel strategy to increase the frequency of the androgenic response in tobacco and other species, including recalcitrants.

The role for CYCLIN A1;2/TARDY ASYNCHRONOUS MEIOSIS in differentiated cells in Arabidopsis

The Arabidopsis A1-type cyclin, CYCA1;2, also named TARDY ASYNCHRONOUS MEIOSIS (TAM), is known for its positive role in meiotic cell cycle progression, but its function in other cells has not been characterized. This paper reports the role of CYCA1;2/TAM in differentiated cells in vegetative organs. The pattern of CYCA1;2/TAM expression was investigated by promoter and protein fusions using the β-glucuronidase and the green fluorescent protein, respectively. The relevance of the promoter region used in these gene fusion constructs was verified by the effective complementation of the phenotype of the diploid null allele, tam-2 2C by a genomic fragment containing the wild-type coding region of CYCA1;2/TAM and the promoter region. CYCA1;2/TAM expression was found primarily in non-proliferating cells such as guard cells, trichomes, and mesophyll cells, and in vascular tissue. In two types of overexpression lines, one containing the CYCA1;2/TAM transgene driven by the ARABIDOPSIS SKP1-LIKE1 (ASK1) promoter and the other CYCA1;2/TAM-GFP driven by the cauliflower mosaic virus 35S promoter, the largest differences between the transgene transcript levels were approximately 72- and 45-folds, respectively, but the TAM-GFP signal levels in the mesophyll and stomata in the 35S:TAM-GFP lines only differ slightly. Furthermore, the GFP signals in the mesophyll and stomata in the TAM:TAM-GFP and 35S:TAM-GFP lines were all at similarly low levels. These results indicate that the CYCA1;2/TAM protein is likely maintained at low levels in these cells through post-transcriptional regulation. Loss of function in CYCA1;2/TAM resulted in increases in the nuclear size in both trichomes and guard cells. Surprisingly, overexpression of CYCA1;2/TAM led to similar increases. The large increases in trichome nuclear size likely reflected ploidy increases while the moderate increases in guard cell nuclear size did not justify for a ploidy increase. These nuclear size increases were not clearly correlated with trichome branch number increases and guard cell size increases, respectively. These results suggest that cellular homeostasis of the CYCA1;2/TAM protein is linked to the control of nuclear sizes in trichomes and guard cells.

Over-expression of SlCycA3 gene in Arabidopsis accelerated the cell cycle transition

We characterised an A-type cyclin SlCycA3 (AJ243453) from tomato (Solanum lycopersicum L.). Phylogenetic analysis based on the deduced amino acid sequence revealed that SlCycA3 was 71% identical to A3-type cyclin in Nicotiana tabacum L. (CAA63540), 48% identical to its homologue found in Arabidopsis thaliana (NP_199122), and 48% identical to its homologue in Pisum sativum L. (CAB77269). SlCycA3 gene was transformed into Arabidopsis plants in order to study its function. The hypocotyl length of transgenic plants was approximately half the length of wild-type plants, and the cell size in the transgenic lines was also smaller. The transgenic plants had longer roots than the wild type. Overexpression of SlCycA3 gene accelerated the cell cycle from G1/S transition to early M-phase, thereby accelerating the cell division. When the plants were treated with IAA and 3-indolebutyric acid (IBA) for 2 days, the transgenic plants produced more lateral roots than wild type. Treatment with IBA significantly increased the cell number in the G2-phase in transgenic plants compared with wild type after treatment for 10 days, whereas the proportion of cells in the S-phase was strongly increased by IAA treatment both in wild-type and transgenic plants. These results suggest a possible key role for cyclin in regulating root growth and development and provide some evidence of cell division underlying hormone treatment in plants.

Cyclin-dependent kinase complexes in developing maize endosperm: evidence for differential expression and functional specialization

Endosperm development in maize (Zea mays L.) and related cereals comprises a cell proliferation stage followed by a period of rapid growth coupled to endoreduplication. Regulation of the cell cycle in developing endosperm is poorly understood. We have characterized various subunits of cyclin-dependent kinase (CDK) complexes, master cell cycle regulators in all eukaryotes. A-, B-, and D-type cyclins as well as A- and B-type cyclin-dependent kinases were characterized with respect to their RNA and protein expression profiles. Two main patterns were identified: one showing expression throughout endosperm development, and another characterized by a sharp down-regulation with the onset of endoreduplication. Cyclin CYCB1;3 and CYCD2;1 proteins were distributed in the cytoplasm and nucleus of cells throughout the endosperm, while cyclin CYCD5 protein was localized in the cytoplasm of peripheral cells. CDKB1;1 expression was strongly associated with cell proliferation. Expression and cyclin-binding patterns suggested that CDKA;1 and CDKA;3 are at least partially redundant. The kinase activity associated with the cyclin CYCA1 was highest during the mitotic stage of development, while that associated with CYCB1;3, CYCD2;1 and CYCD5 peaked at the mitosis-to-endoreduplication transition. A-, B- and D-type cyclins were more resistant to proteasome-dependent degradation in endoreduplicating than in mitotic endosperm extracts. These results indicated that endosperm development is characterized by differential expression and activity of specific cyclins and CDKs, and suggested that endoreduplication is associated with reduced cyclin proteolysis via the ubiquitin-proteasome pathway.

Overexpression of the anaphase-promoting complex (APC) genes in Nicotiana tabacum promotes increasing biomass accumulation

The anaphase-promoting complex (APC) plays pivotal roles in cell cycle pathways related to plant development. In this study, we present evidence that overproduction of APC10 from Arabidopsis thaliana in tobacco (Nicotiana tabacum) plants promotes significant increases in biomass. Analyzes of plant's fresh and dried weight, root length, number of days to flower and number of seeds of plants overexpressing AtAPC10 verified an improved agronomic performance of the transgenic plants. Detailed analyzes of the leaf growth at the cellular level, and measurements of leaf cell number, showed that AtAPC10 also produce more cells, showing an enhancement of proliferation in these plants. In addition, crossing of plants overexpressing AtAPC10 and AtCDC27a resulted in a synergistic accumulation of biomass and these transgenic plants exhibited superior characteristics compared to the parental lines. The results of the present study suggest that transgenic plants expressing AtAPC10 and AtAPC10/AtCDC27a concomitantly are promising leads to develop plants with higher biomass.

Concentration-dependent effects of narciclasine on cell cycle progression in Arabidopsis root tips

BACKGROUND

Narciclasine (NCS) is an Amaryllidaceae alkaloid isolated from Narcissus tazetta bulbs. NCS has inhibitory effects on a broad range of biological activities and thus has various potential practical applications. Here we examine how NCS represses plant root growth.

RESULTS

Results showed that the inhibition of NCS on cell division in Arabidopsis root tips and its effects on cell differentiation are concentration-dependent; at low concentrations (0.5 and 1.0 μM) NCS preferentially targets mitotic cell cycle specific/cyclin complexes, whereas at high concentration (5.0 μM) the NCS-stimulated accumulation of Kip-related proteins (KRP1 and RP2) affects the CDK complexes with a role at both G1/S and G2/M phases.

CONCLUSIONS

Our findings suggest that NCS modulates the coordination between cell division and differentiation in Arabidopsis root tips and hence affects the postembryonic development of Arabidopsis seedlings.

Developmental regulation of CYCA2s contributes to tissue-specific proliferation in Arabidopsis

In multicellular organisms, morphogenesis relies on a strict coordination in time and space of cell proliferation and differentiation. In contrast to animals, plant development displays continuous organ formation and adaptive growth responses during their lifespan relying on a tight coordination of cell proliferation. How developmental signals interact with the plant cell-cycle machinery is largely unknown. Here, we characterize plant A2-type cyclins, a small gene family of mitotic cyclins, and show how they contribute to the fine-tuning of local proliferation during plant development. Moreover, the timely repression of CYCA2;3 expression in newly formed guard cells is shown to require the stomatal transcription factors FOUR LIPS/MYB124 and MYB88, providing a direct link between developmental programming and cell-cycle exit in plants. Thus, transcriptional downregulation of CYCA2s represents a critical mechanism to coordinate proliferation during plant development.

Characterization of five polyamine oxidase isoforms in Arabidopsis thaliana

The genome of Arabidopsis thaliana contains five genes (AtPAO1 to AtPAO5) encoding polyamine oxidase (PAO) which is an enzyme responsible for polyamine catabolism. To understand the individual roles of the five AtPAOs, here we characterized their tissue-specific and space-temporal expression. AtPAO1 seems to have a specific function in flower organ. AtPAO2 was expressed in shoot meristem and root tip of seedlings, and to a higher extent in the later growth stage within restricted parts of the organs, such as shoot meristem, leaf petiole and also in anther. The expression of AtPAO3 was constitutive, but highest in flower organ. AtPAO3 promoter activity was detected in cotyledon, distal portion of root, boundary region of mature rosette leaf and in filaments of flower. AtPAO4 was expressed at higher level all over young seedlings including roots, and in the mature stage its expression was ubiquitous with rather lower level in stem. AtPAO5 expression was observed in the whole plant body throughout various growth stages. Its highest expression was in flowers, particularly in sepals, but not in petals. Furthermore, we determined the substrate specificity of AtPAO1 to AtPAO4. None of the AtPAO enzymes recognized putrescine (Put). AtPAO2 and AtPAO3 showed almost similar substrate recognition patterns in which the most preferable substrate is spermidine (Spd) followed by less specificity to other tetraamines tested. AtPAO4 seemed to be spermine (Spm)-specific. More interestingly, AtPAO1 preferred thermospermine (T-Spm) and norspermine (NorSpm) to Spm, but did not recognize Spd. Based on the results, the individual function of AtPAOs is discussed.

The expression of Orysa;CycB1;1 is essential for endosperm formation and causes embryo enlargement in rice

The cell cycle is an important process during seed development in plants and its progression is driven by a number of core regulators such as the cyclins. Currently, however, little is known regarding the role of the cyclins in embryo and endosperm development in cereals. In our current study, we show that the knockdown of Orysa;CycB1;1 in rice results in the production of abnormal seeds, which at maturity contain only an enlarged embryo. It was further found that a delayed and abnormal cellularization occurred in the endosperm in these knockdown seeds which eventually became abortive. Moreover, the observed development of the enlarged embryo was also morphologically abnormal and found to be caused by an enlarged cell size rather than an increased cell number. Expression analysis showed that Orysa;CycB1;1 transcripts were localized in the endosperm and embryo. Genome-wide transcriptional profiling further indicated that a large number of genes are responsible for the phenotype of the enlarged embryo. The results of the knockdown of Orysa;CycB1;1 via an endosperm or an embryo-specific promoter also suggest that the enlarged embryo may be correlated to the abortive endosperm. Our results suggest that Orysa;CycB1;1 expression is critical for endosperm formation via the regulation of mitotic division, and that the endosperm plays an important role in maintenance of embryo development in rice.

Rhodococcus fascians infection accelerates progression of tobacco BY-2 cells into mitosis through rapid changes in plant gene expression

* To characterize plant cell cycle activation following Rhodococcus fascians infection, bacterial impact on cell cycle progression of tobacco BY-2 cells was investigated. * S-phase-synchronized BY-2 cells were cocultivated with R. fascians and cell cycle progression was monitored by measuring mitotic index, cell cycle gene expression and flow cytometry parameters. Cell cycle alteration was further investigated by cDNA-AFLP (amplified fragment length polymorphism). * It was shown that cell cycle progression of BY-2 cells was accelerated only upon infection with bacteria whose virulence gene expression was induced by a leafy gall extract. Thirty-eight BY-2 genes showed a differential expression within 6 h post-infection. Among these, seven were previously associated with specific plant cell cycle phases (in particular S and G2/M phases). Several genes also showed a differential expression during leafy gall formation. * R. fascians-infected BY-2 cells provide a simple model to identify plant genes related to leafy gall development. R. fascians can also be regarded as a useful biotic agent to alter cell cycle progression and, thereby, gain a better understanding of cell cycle regulation in plants.

Cell cycle regulated D3-type cyclins form active complexes with plant-specific B-type cyclin-dependent kinase in vitro

Tobacco (Nicotiana tabacum L.) cv Bright Yellow-2 (BY-2) cells are the most highly synchronizable plant cell culture, and previously we used them to analyze cell cycle regulation of cyclin-dependent kinases (CDKs) containing the cyclin binding motifs PSTAIRE (CDKA) and PPTA/TLRE (CDKB). Here we describe the analysis of tobacco CycD3 cyclins whose transcripts predominantly accumulate during G2 to M phase, which represents a unique feature of this type of cyclin D in plants. Although protein levels of CycD3s fluctuate with different patterns during the cell cycle, kinase assays revealed that the CycD3-associated kinases phosphorylate histone H1 and the tobacco retinoblastoma related protein (NtRBR1) with two peaks at the G1/S and G2/M boundaries. In vitro pull-down assays revealed that cell cycle-regulated CycD3s bind to CDKA, but more weakly than does CycD3;3, and that they also bind to CDKB and the CDK inhibitor NtKIS1a. Mutations in the cyclin box of the CycD3s showed that two amino acids are required for binding with CDKA and NtKIS1a, but no diminished interaction was observed with CDKB. A reconstituted kinase assay was adapted for use with bacterially produced GST-CycD3s, and kinase activity could be activated by incubation of extracts from exponentially growing BY-2 cells. Such activated complexes contained CDKA and CDKB, and the reconstituted GST-CycD3 mutants, retaining binding ability to CDKB, showed kinase activity, suggesting that these cell cycle-regulated CycD3s form active complexes with both A- and B-type CDKs in vitro.

Crosstalk between auxin, cytokinins, and sugars in the plant cell cycle

Plant meristems are utilization sinks, in which cell division activity governs sink strength. However, the molecular mechanisms by which cell division activity and sink strength are adjusted to a plant's developmental program in its environmental setting are not well understood. Mitogenic hormonal as well as metabolic signals drive and modulate the cell cycle, but a coherent idea of how this is accomplished, is still missing. Auxin and cytokinins are known as endogenous mitogens whose concentrations and timing, however, can be externally affected. Although the sites and mechanisms of signal interaction in cell cycle control have not yet been unravelled, crosstalk of sugar and phytohormone signals could be localized to several biochemical levels. At the expression level of cell cycle control genes, like cyclins, Cdks, and others, synergistic but also antagonistic interactions could be demonstrated. Another level of crosstalk is that of signal generation or modulation. Cytokinins affect the activity of extracellular invertases and hexose-uptake carriers and thus impinge on an intracellular sugar signal. With tobacco BY-2 cells, a coordinated control of cell cycle activity at both regulatory levels could be shown. Comparison of the results obtained with the root cell-representing BY-2 cells with literature data from shoot tissues or green cell cultures of Arabidopsis and Chenopodium suggests opposed and tissue-specific regulatory patterns of mitogenic signals and signal crosstalk in root and shoot meristems.

Nitric oxide modulates the expression of cell cycle regulatory genes during lateral root formation in tomato

Nitric oxide (NO) is a bioactive molecule involved in diverse physiological functions in plants. It has previously been reported that the NO donor sodium nitroprusside (SNP) applied to germinated tomato seeds was able to induce lateral root (LR) formation in the same way that auxin treatment does. In this paper, it is shown that NO modulates the expression of cell cycle regulatory genes in tomato pericycle cells and leads, in turn, to induced LR formation. The addition of the NO scavenger CPTIO at different time points during auxin-mediated LR development indicates that NO is required for LR primordia formation and not for LR emergence. The SNP-mediated LR promotion could be prevented by the cell cycle inhibitor olomoucine, suggesting that NO is involved in cell cycle regulation. A system was developed in which the formation of LRs was synchronized. It was based on the control of NO availability in roots by treatment with the NO scavenger CPTIO. The expression of the cell cycle regulatory genes encoding CYCA2;1, CYCA3;1, CYCD3;1, CDKA1, and the Kip-Related Protein KRP2 was studied using RT-PCR analysis in roots with synchronized and non-synchronized LR formation. NO mediates the induction of the CYCD3;1 gene and the repression of the CDK inhibitor KRP2 gene at the beginning of LR primordia formation. In addition, auxin-dependent cell cycle gene regulation was dependent on NO.

The genes encoding Arabidopsis ORC subunits are E2F targets and the two ORC1 genes are differently expressed in proliferating and endoreplicating cells

Initiation of eukaryotic DNA replication depends on the function of pre-replication complexes (pre-RC), one of its key component being the six subunits origin recognition complex (ORC). In spite of a significant degree of conservation among ORC proteins from different eukaryotic sources, the regulation of their availability varies considerably in different model systems and cell types. Here, we show that the six ORC genes of Arabidopsis thaliana are regulated at the transcriptional level during cell cycle and development. We found that Arabidopsis ORC genes, except AtORC5, contain binding sites for the E2F family of transcription factors. Expression of AtORC genes containing E2F binding sites peaks at the G1/S-phase. Analysis of AtORC gene expression in plants with reduced E2F activity, obtained by expressing a dominant negative version of DP, the E2F heterodimerization partner, and with increased E2F activity, obtained by inactivation of the retinoblastoma protein, led us to conclude that all AtORC genes, except AtORC5 are E2F targets. Interestingly, Arabidopsis contains two AtORC1 (a and b) genes, highly conserved at the amino acid level but with unrelated promoter sequences. AtORC1b expression is restricted to proliferating cells. However, AtORC1a is preferentially expressed in endoreplicating cells based on our analysis in endoreplicating tissues and in a mutant with altered endocycle pattern. This suggests a differential expression of the two ORC1 genes in Arabidopsis.

Dedifferentiation of tobacco cells is associated with ribosomal RNA gene hypomethylation, increased transcription, and chromatin alterations

Epigenetic changes accompanying plant cell dedifferentiation and differentiation are reported in 35S ribosomal DNA (rDNA) of tobacco (Nicotiana tabacum). There was a reduction of CG and CNG methylation in both intergenic and genic regions of the rDNA cistron in fully dedifferentiated callus and root compared to leaf. The rDNA hypomethylation was not random, but targeted to particular rDNA gene families at units that are clustered within the tandem array. The process of hypomethylation was initiated as early as 2 weeks after the callus induction and established epigenetic patterns were stably maintained throughout prolonged culture. However, regenerated plants and their progeny showed partial and complete remethylation of units, respectively. Nuclear run-on assays revealed a 2-fold increase of primary (unprocessed) ribosomal RNA transcripts in callus compared to leaf tissue. However, the abundance of mature transcripts in callus was elevated by only about 25%. Fluorescence in situ hybridization analysis of interphase nuclei showed high levels of rDNA chromatin condensation in both callus and leaf, with substantially less decondensed rDNA than is observed in meristematic root-tip cells. It is likely that the regions of the rDNA locus showing decondensation correspond to the clusters of hypomethylated units that occur in the tandem array at each locus. The data together indicate that the establishment of pluripotency and cell proliferation occurring with callus induction is associated with enhanced ribosomal RNA gene expression and overall rDNA hypomethylation, but is not associated with material-enhanced relaxation of chromatin structure (decondensation) at rDNA loci.

Cyclin-dependent kinase inhibitors in maize endosperm and their potential role in endoreduplication

Two maize (Zea mays) cyclin-dependent kinase (CDK) inhibitors, Zeama;KRP;1 and Zeama;KRP;2, were characterized and shown to be expressed in developing endosperm. Similar to the CDK inhibitors in Arabidopsis (Arabidopsis thaliana) and tobacco (Nicotiana tabacum), the maize proteins contain a carboxy-terminal region related to the inhibitory domain of the mammalian Cip/Kip inhibitors. Zeama;KRP;1 is present in the endosperm between 7 and 21 d after pollination, a period that encompasses the onset of endoreduplication, while the Zeama;KRP;2 protein declines during this time. Nevertheless, Zeama;KRP;1 accounts for only part of the CDK inhibitory activity that peaks coincident with the endoreduplication phase of endosperm development. In vitro assays showed that Zeama;KRP;1 and Zeama;KRP;2 are able to inhibit endosperm Cdc2-related CKD activity that associates with p13(Suc1). They were also shown to specifically inhibit cyclin A1;3- and cyclin D5;1-associated CDK activities, but not cyclin B1;3/CDK. Overexpression of Zeama;KRP;1 in maize embryonic calli that ectopically expressed the wheat dwarf virus RepA protein, which counteracts retinoblastoma-related protein function, led to an additional round of DNA replication without nuclear division.

Global analysis of the core cell cycle regulators of Arabidopsis identifies novel genes, reveals multiple and highly specific profiles of expression and provides a coherent model for plant cell cycle control

Arabidopsis has over 80 genes encoding conserved and plant-specific core cell cycle regulators, but in most cases neither their timing of expression in the cell cycle is known nor whether they represent redundant and/or tissue-specific functions. Here we identify novel cell cycle regulators, including new cyclin-dependent kinases related to the mammalian galactosyltransferase-associated protein kinase p58, and new classes of cyclin-like and CDK-like proteins showing strong tissue specificity of expression. We analyse expression of all cell cycle regulators in synchronized Arabidopsis cell cultures using multiple approaches including Affymetrix microarrays, massively parallel signature sequencing and real-time reverse transcriptase polymerase chain reaction, and in plant material using the results of over 320 microarray experiments. These global analyses reveal that most core cell cycle regulators are expressed across almost all tissues and more than 85% are expressed at detectable levels in the cell suspension culture, allowing us to present a unified model of transcriptional regulation of the plant cell cycle. Characteristic patterns of D-cyclin expression in early and late G1 phase, either limited to the re-entry cycle or continuously oscillating, suggest that several CYCD genes with strong oscillatory regulation in late G1 may play the role of cyclin E in plants. Alone amongst the six groups of A and B type cyclins, members of CYCA3 peak in S-phase suggest it is a major component of S-phase kinases, whereas others show a peak in G2/M. 82 genes share this G2/M regulatory pattern, about half being new candidate mitotic genes of previously unknown function.

Endogenous cytokinin oscillations control cell cycle progression of tobacco BY-2 cells

The significance of cytokinins for the progression of the cell cycle is well known. Cytokinins contribute to the control of the expression of D-cyclins and other cell cycle genes, but knowledge as to how they affect the progression of the cell cycle is still limited. Highly synchronized tobacco BY-2 cells with clearly defined cell cycle stages were employed to determine cytokinin patterns in detail throughout the entire cycle. Concentrations of trans-zeatin, and of some other cytokinins, oscillated during the course of the cell cycle, increasing substantially at all four phase transitions and decreasing again to a minimum value during the course of each subsequent phase. Addition of exogenous cytokinins or inhibition of cytokinin biosynthesis promoted the progression of the cell cycle when the effects of these manipulations intensified the endogenous fluctuations, whereas the progression of the cycle was retarded when the amplitude of the fluctuations was decreased. The results show that the attainment of low concentrations of cytokinins is as important as the transient increases in concentration for a controlled progression from one phase of the cell cycle to the next. Cytokinin oxidase/dehydrogenase activity also showed fluctuations during the course of the cell cycle, the timing of which could at least partly explain oscillations of cytokinin levels. The activities of the enzyme were sufficient to account for the rates of cytokinin disappearance observed subsequent to a phase transition.

Molecular characterization of the tobacco SET domain protein NtSET1 unravels its role in histone methylation, chromatin binding, and segregation

Plants contain a great number of genes encoding a distinctive class of SET domain proteins which harbor a plant-specific N-terminal part together with a C-terminal part showing highest sequence similarity to the catalytic domain of the yeast CLR4, the human SUV39H1 and G9a histone-methyltransferases (HMTases). Here we show that NtSET1, a representative member of this class from tobacco, methylated both K9 and K27 of histone H3 in vitro. Ectopic expression of NtSET1, by an inducible promoter, increased the amount of dimethylated H3K9 and induced chromosome-segregation defects in tobacco BY2 cells. Deletion analyses show that the HMTase activity, the association with specific chromatin regions and with condensed chromosomes, and the cellular effects largely depended on the C-terminal region including the SET domain of the protein. Nevertheless, the N-terminal part of NtSET1 was capable of targeting the green fluorescent protein to interphase chromatin. Finally, we show that NtSET1 bound LHP1, the Arabidopsis homolog of animal heterochromatin protein 1 (HP1), and that LHP1 co-localized with heterochromatin containing high amounts of dimethylated H3K9, suggesting a role for NtSET1 in heterochromatic function. Taken together, our results provide new insights into the molecular and global chromatin-binding activities of this particular class member of plant SET domain proteins.

Sex-Specific Cell Division during Development of Unisexual Flowers in the Dioecious Plant Silene latifolia

We analyzed cell division patterns during the differentiation of unisexual flowers of the dioecious plant Silene latifolia using in situ hybridization with histone H4 and cyclin A1 genes. The gene expression patterns indicated that the activation of cell divisions in whorls 3 and 4 was reversed in young male and female flower buds. During maturation of flower buds, a remarkable reduction in cell division activity occurred in the male gynoecium primordium and female stamen primordia. Our analyses showed that differential activation and reduction of cell division strongly correlated with sex-specific promotion and cessation in the sex differentiation of unisexual flowers.

The tobacco A-type cyclin, Nicta;CYCA3;2, at the nexus of cell division and differentiation

Although most of the components of the cell cycle machinery are conserved in all eukaryotes, plants differ strikingly from animals by the absence of a homolog of E-type cyclin, an important regulator involved in G1/S-checkpoint control in animals. By contrast, plants contain a complex range of A-type cyclins, with no fewer than 10 members in Arabidopsis. We previously identified the tobacco A-type cyclin Nicta;CYCA3;2 as an early G1/S-activated gene. Here, we show that antisense expression of Nicta;CYCA3;2 in tobacco plants induces defects in embryo formation and impairs callus formation from leaf explants. The green fluorescent protein (GFP)-Nicta;CYCA3;2 fusion protein was localized in the nucleoplasm. Transgenic tobacco plants overproducing GFP-Nicta;CYCA3;2 could not be regenerated from leaf disc transformation, whereas some transgenic Arabidopsis plants were obtained by the floral-dip transformation method. Arabidopsis plants that overproduce GFP-Nicta;CYCA3;2 showed reduced cell differentiation and endoreplication and a dramatically modified morphology. Calli regenerated from leaf explants of these transgenic Arabidopsis plants were defective in shoot and root regeneration. We propose that Nicta;CYCA3;2 has important functions, analogous to those of cyclin E in animals, in the control of plant cell division and differentiation.

Interaction of NtCDPK1 calcium-dependent protein kinase with NtRpn3 regulatory subunit of the 26S proteasome in Nicotiana tabacum

Using a yeast two-hybrid system, we identified NtRpn3, a regulatory subunit of 26S proteasome, as an interacting protein of NtCDPK1 calcium-dependent protein kinase in Nicotiana tabacum. Rpn3 in yeast is an essential protein involved in proteolysis of cell cycle regulatory proteins, and the carrot homolog of Rpn3 was previously isolated as a nuclear antigen that is mainly expressed in the meristem. NtCDPK1 physically interacts with NtRpn3 in vitro in a Ca2+-independent manner and phosphorylates NtRpn3 in a Ca2+-dependent manner with Mg2+ as a cofactor. NtCDPK1 and NtRpn3 are co-localized in the nucleus, nuclear periphery, and around plasma membrane in vivo. Both NtCDPK1 and AtRpn3, an NtRpn3 homolog of Arabidopsis, are mainly expressed in the rapidly proliferating tissues including shoot and root meristems, and developing floral buds. Virus-induced gene silencing of either NtRpn3 or NtCDPK1 resulted in the phenotypes of abnormal cell morphology and premature cell death in newly emerged leaves. Finally, NtCDPK1 interacts with NtRpn3 in vivo as shown by co-immunoprecipitation. Based on these results, we propose that NtCDPK1 and NtRpn3 are interacting in a common signal transduction pathway possibly for regulation of cell division, differentiation, and cell death in tobacco.

A plant cyclin B2 is degraded early in mitosis and its ectopic expression shortens G2-phase and alleviates the DNA-damage checkpoint

Mitotic progression is timely regulated by the accumulation and degradation of A- and B-type cyclins. In plants, there are three classes of A-, and two classes of B-type cyclins, but their specific roles are not known. We have generated transgenic tobacco plants in which the ectopic expression of a plant cyclin B2 gene is under the control of a tetracycline-inducible promoter. We show that the induction of cyclin B2 expression in cultured cells during G2 phase accelerates the entry into mitosis and allows cells to override the replication checkpoint induced by hydroxyurea in the simultaneous presence of caffeine or okadaic acid, drugs that are known to alleviate checkpoint control. These results indicate that in plants, a B2-type cyclin is a rate-limiting regulator for the entry into mitosis and a cyclin B2-CDK complex might be a target for checkpoint control pathways. The cyclin B2 localization and the timing of its degradation during mitosis corroborate these conclusions: cyclin B2 protein is confined to the nucleus and during mitosis it is only present during a short time window until mid prophase, but it is effectively degraded from this timepoint onwards. Although cyclin B2 is not present in cells arrested by the spindle checkpoint in metaphase, cyclin B1 is accumulating in these cells. Ectopic expression of cyclin B2 in developing plants interferes with differentiation events and specifically blocks root regeneration, indicating the importance of control mechanisms at the G2- to M-phase transition during plant developmental processes.

The plant cell cycle

Cell division in plants is controlled by the activity of cyclin-dependent kinase (CDK) complexes. Although this basic mechanism is conserved with all other eukaryotes, plants show novel features of cell-cycle control in the molecules involved and their regulation, including novel CDKs showing strong transcriptional regulation in mitosis. Plant development is characterized by indeterminate growth and reiteration of organogenesis and is therefore intimately associated with cell division. This may explain why plants have a large number of cell-cycle regulators that appear to have overlapping and distinct functions. Here we review the recent considerable progress in understanding how core cell-cycle regulators are involved in integrating and coordinating cell division at the molecular level.

Transcriptome analysis during cell division in plants

Using synchronized tobacco Bright Yellow-2 cells and cDNA-amplified fragment length polymorphism-based genomewide expression analysis, we built a comprehensive collection of plant cell cycle-modulated genes. Approximately 1,340 periodically expressed genes were identified, including known cell cycle control genes as well as numerous unique candidate regulatory genes. A number of plant-specific genes were found to be cell cycle modulated. Other transcript tags were derived from unknown plant genes showing homology to cell cycle-regulatory genes of other organisms. Many of the genes encode novel or uncharacterized proteins, indicating that several processes underlying cell division are still largely unknown.

Cell cycle-regulated gene expression in Arabidopsis

Regulated gene expression is an important mechanism for controlling cell cycle progression in yeast and mammals, and genes involved in cell division-related processes often show transcriptional regulation dependent on cell cycle position. Analysis of cell cycle processes in plants has been hampered by the lack of synchronizable cell suspensions for Arabidopsis, and few cell cycle-regulated genes are known. Using a recently described synchrony system, we have analyzed RNA from sequential samples of Arabidopsis cells progressing through the cell cycle using Affymetrix Genearrays. We identify nearly 500 genes that robustly display significant fluctuation in expression, representing the first genomic analysis of cell cycle-regulated gene expression in any plant. In addition to the limited number of genes previously identified as cell cycle-regulated in plants, we also find specific patterns of regulation for genes known or suspected to be involved in signal transduction, transcriptional regulation, and hormonal regulation, including key genes of cytokinin response. Genes identified represent pathways that are cell cycle-regulated in other organisms and those involved in plant-specific processes. The range and number of cell cycle-regulated genes show the close integration of the plant cell cycle into a variety of cellular control and response pathways.

Forkhead-associated domains of the tobacco NtFHA1 transcription activator and the yeast Fhl1 forkhead transcription factor are functionally conserved

NtFHA1 encodes a novel protein containing the forkhead-associated (FHA) domain and the acidic domain in Nicotiana tabacum. NtFHA1 functions as a transactivator and is targeted to the nucleus. The sequence of the FHA domain of NtFHA1 is significantly homologous to that of the Fhl1 forkhead transcription factor of yeast. FHL1 was previously identified as a suppressor of RNA polymerase III mutations, and the fhl1 deletion mutant exhibited severe growth defects and impaired rRNA processing. Ectopic expression of the FHA domain of NtFHA1 (but not its mutant form) resulted in severe growth retardation in yeast. Similarly, expression of Fhl1, its FHA domain, or chimeric Fhl1 containing the NtFHA1 FHA domain also inhibited yeast growth. Yeast cells overexpressing the FHA domains of NtFHA1 and Fhl1 contained lower levels of mature rRNAs and exhibited rRNA-processing defects, similar to the fhl1 null mutant. Chimeric Fhl1 (but not the mutant form with a small deletion in its FHA domain) fully complemented the growth and rRNA-processing defects of the fhl1 null mutant, demonstrating that the FHA domain of NtFHA1 can functionally substitute for the FHA domain of Fhl1. These results demonstrate that the FHA domains of NtFHA1 and Fhl1 are conserved in their structure and function and that the FHA domain of Fhl1 is critically involved in regulation of rRNA processing in yeast. NtFHA1 function in plants may be analogous to Fhl1 function in yeast.

Plant D-type cyclins and the control of G1 progression

The basic pattern of controls that operate during the G1 phase of the plant cell cycle shows much closer similarity to animals than to the yeasts and other fungi. The activity of D-type cyclin (CycD) kinases is induced in response to stimulatory signals, and these phosphorylate the plant homologue of the retinoblastoma tumour susceptibility (Rb) protein. It is likely that Rb phosphorylation results in the activation of genes under the control of E2F transcription factors, including those required for S phase entry. As the initial triggers of the cascade, attention has focused on the CycDs, and a family of 10 genes is present in Arabidopsis, divided into three major and three minor groups. Analysis to date suggests that these groups are functionally distinct.

Manipulation of leaf shape by modulation of cell division

The role of cell division as a causal element in plant morphogenesis is debatable, with accumulating evidence supporting the action of cell division-independent mechanisms. To directly test the morphogenic function of cell division, we have utilised a microinduction technique to locally and transiently manipulate the expression in transgenic plants of two genes encoding putative effectors of the cell cycle, a tobacco A-type cyclin and a yeast cdc25. The results show that local expression of these genes leads to modulation of cell division patterns. Moreover, whereas altered cell division in the apical meristem had no influence on organogenesis, local induction of cell proliferation on the flanks of young leaf primordia led to a dramatic change in lamina development and, thus, leaf shape. These data indicate that the role of cell division in plant morphogenesis is context dependent and identify cell division in the leaf primordium as a potential target for factors regulating leaf shape.

Sub-cellular localisation of GFP-tagged tobacco mitotic cyclins during the cell cycle and after spindle checkpoint activation

We have previously shown that the tobacco cyclin B1;1 protein accumulates during the G2 phase of the cell cycle and is subsequently destroyed during mitosis. Here, we investigated the sub-cellular localisation of two different B1-types and one A3-type cyclin during the cell cycle by using confocal imaging and differential interference contrast (DIC) microscopy. The cyclins were visualised as GFP-tagged fusion proteins in living tobacco cells. Both B1-type cyclins were found in the cytoplasm and in the nucleus during G2 but when cells entered into prophase, both cyclins became associated with condensing chromatin and remained on chromosomes until metaphase. As cells exited metaphase, the B1-type cyclins became degraded, as shown by time-lapse images. A stable variant of cyclin B1;1-GFP fusion protein, in which the destruction box had been mutated, maintained its association with the nuclear material at later phases of mitosis such as anaphase and telophase. Furthermore, we demonstrated that cyclin B1;1 protein is stabilised in metaphase-arrested cells after microtubule destabilising drug treatments. In contrast to the B1-type cyclins, the cyclin A3;1 was found exclusively in the nucleus in interphase cells and disappeared earlier than the cyclin B1 proteins during mitosis.

NtSET1, a member of a newly identified subgroup of plant SET-domain-containing proteins, is chromatin-associated and its ectopic overexpression inhibits tobacco plant growth

The SET- and chromo-domains are recognized as signature motifs for proteins that contribute to epigenetic control of gene expression through effects on the regional organization of chromatin structure. This paper reports the identification of a novel subgroup of SET-domain-containing proteins in tobacco and Arabidopsis, which show highest homologies with the Drosophila position-effect-variegation repressor protein SU(VAR)3-9 and the yeast centromer silencing protein CLR4. The tobacco SET-domain-containing protein (NtSET1) was fused to the green fluorescence protein (GFP) that serves as a visual marker for localization of the recombinant protein in living cells. Whereas control GFP protein alone was uniformly dispersed within the nucleus and cytoplasm, the NtSET1-GFP fusion protein showed a non-uniform localization to multiple nuclear regions in interphase tobacco TBY2 cells. During mitosis, the NtSET1-GFP associated with condensed chromosomes with a non-random distribution. The NtSET1 thus appears to have distinct target regions in the plant chromatin. Overexpression of the NtSET1-GFP in transgenic tobacco inhibited plant growth, implicating the possible involvement of the NtSET1 in transcriptional repression of growth control genes through the formation of higher-order chromatin domains.

Cell cycle controls and the development of plant form

The relationship between cell division and plant form has long been a battleground for the debate between those proclaiming and disclaiming an important role for cell division in morphogenetic and developmental processes. Recent evidence suggests that cell division and morphogenesis are intimately interconnected, and whereas overall architecture is determined by patterning genes, the elaboration and execution of developmental programmes require proper control of the cell-division cycle.

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.

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.

Plant A-type cyclins

Although the basic mechanisms which control the progression through the cell cycle appear to be conserved in all higher eukaryotes, the unique features of the plant developmental programme must be somehow reflected in a plant-specific regulation of the factors which control cell division. In the past few years, considerable progress has been achieved in identifying the major components of the cell cycle machinery in plants, especially the cyclin-dependent kinases (CDKs) and their regulatory subunits, the cyclins. The question of how these components direct expression of specific genes at specific stages of the cell cycle, and how they are themselves regulated, constitutes a challenge for the present and for the years to come. This review summarizes our current knowledge of a particular class of plant cyclins, the A-type cyclins, which can be further subdivided into three structural groups. The putative functions of these A-type cyclins are discussed in relation to the presence of remarkable motifs in their amino acid sequences, and to their specific transcriptional regulation, protein amount and subcellular localization.

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.

Cell cycle function of a Medicago sativa A2-type cyclin interacting with a PSTAIRE-type cyclin-dependent kinase and a retinoblastoma protein

In plants multiple A-type cyclins with distinct expression patterns have been isolated and classified into three subgroups (A1-A3), while in animal somatic cells a single type of cyclin A is required for cell-cycle regulation from the S to M phases. We studied the function of an A2-type cyclin from Medicago sativa (Medsa;cycA2) which, in contrast to animal and most plant A-type cyclins, was expressed in all phases of the cell cycle. Using synchronized alfalfa cell cultures and anti-Medsa;CycA2 polyclonal antibodies, we showed that while the mRNA level increased steadily from the late G1 to the G2-M phase, the protein level after a rapid increase in S-phase reached a plateau during the G2 phase. In the yeast two-hybrid system, the Medsa;CycA2 protein interacted with the PSTAIRE-motif-containing cyclin-dependent kinase Cdc2MsA and with the maize retinoblastoma protein. Unexpectedly, the CycA2-associated kinase activity was biphasic: a first activity peak occurred in the S phase while the major one occurred during the G2/M transition, with no apparent dependence upon the actual levels of the Medsa;CycA2 and Cdc2MsA proteins. Immunohistological localization of the cyclin A2 protein by immunofluorescence and immunogold labelling revealed the presence of Medsa;CycA2 in the nucleus of the interphase and prophase cells, while it was undetectable thereafter during mitosis. Together these data suggest that Medsa;CycA2 plays a role both in the S phase and at the G2/M transition.

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.