Distinct cyclin D genes show mitotic accumulation or constant levels of transcripts in tobacco bright yellow-2 cells

Functional characterization of D-type cyclins involved in cell division in rice

BACKGROUND

D-type cyclins (CYCD) regulate the cell cycle G1/S transition and are thus closely involved in cell cycle progression. However, little is known about their functions in rice.

RESULTS

We identified 14 CYCD genes in the rice genome and confirmed the presence of characteristic cyclin domains in each. The expression of the OsCYCD genes in different tissues was investigated. Most OsCYCD genes were expressed at least in one of the analyzed tissues, with varying degrees of expression. Ten OsCYCD proteins could interact with both retinoblastoma-related protein (RBR) and A-type cyclin-dependent kinases (CDKA) forming holistic complexes, while OsCYCD3;1, OsCYCD6;1, and OsCYCD7;1 bound only one component, and OsCYCD4;2 bound to neither protein. Interestingly, all OsCYCD genes except OsCYCD7;1, were able to induce tobacco pavement cells to re-enter mitosis with different efficiencies. Transgenic rice plants overexpressing OsCYCD2;2, OsCYCD6;1, and OsCYCD7;1 (which induced cell division in tobacco with high-, low-, and zero-efficiency, respectively) were created. Higher levels of cell division were observed in both the stomatal lineage and epidermal cells of the OsCYCD2;2- and OsCYCD6;1-overexpressing plants, with lower levels seen in OsCYCD7;1-overexpressing plants.

CONCLUSIONS

The distinct expression patterns and varying effects on the cell cycle suggest different functions for the various OsCYCD proteins. Our findings will enhance understanding of the CYCD family in rice and provide a preliminary foundation for the future functional verification of these genes.

GmMs1 encodes a kinesin-like protein essential for male fertility in soybean (Glycine max L.)

The application of heterosis is a promising approach for greatly increasing yield in soybean (Glycine max L.). Nuclear male sterility is essential for hybrid seed production and the utilization of heterosis. Here we report the cloning of the gene underlying the soybean male-sterile mutant ms-1, which has been widely used for recurrent selection in soybean breeding programs. We initially delimited the ms1 locus to a 16.15 kb region on chromosome 13, based on SLAF_BSA sequencing followed by genotyping of an F2 population segregating for the locus. Compared with the same region in fertile plants, the mutant region lacks a sequence of approximately 38.7 kb containing five protein-coding genes, including an ortholog of the kinesin-like protein gene NACK2, named GmMs1. The GmMs1 knockout plants generated via CRISPR/Cas-mediated gene editing displayed a complete male-sterile phenotype. Metabolic profiling showed that fertile anthers accumulated starch and sucrose normally, whereas sterile anthers had higher anthocyanin levels and lower flavonoid levels and lower antioxidant enzyme activities. These results provide insights into the molecular mechanisms governing male sterility and demonstrate that GmMs1 could be used to create male-sterile lines through targeted mutagenesis. These findings pave the way for designing seed production technology and an intelligent male-sterile line system to utilize heterosis in soybean.

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.

Visualization of Protein Coding, Long Noncoding, and Nuclear RNAs by Fluorescence in Situ Hybridization in Sections of Shoot Apical Meristems and Developing Flowers

In addition to transcriptional regulation, gene expression is further modulated through mRNA spatiotemporal distribution, by RNA movement between cells, and by RNA localization within cells. Here, we have adapted RNA fluorescence in situ hybridization (FISH) to explore RNA localization in Arabidopsis (Arabidopsis thaliana). We show that RNA FISH on sectioned material can be applied to investigate the tissue and subcellular localization of meristem and flower development genes, cell cycle transcripts, and plant long noncoding RNAs. We also developed double RNA FISH to dissect the coexpression of different mRNAs at the shoot apex and nuclear-cytoplasmic separation of cell cycle gene transcripts in dividing cells. By coupling RNA FISH with fluorescence immunocytochemistry, we further demonstrate that a gene's mRNA and protein may be simultaneously detected, for example revealing uniform distribution of PIN-FORMED1 (PIN1) mRNA and polar localization of PIN1 protein in the same cells. Therefore, our method enables the visualization of gene expression at both transcriptional and translational levels with subcellular spatial resolution, opening up the possibility of systematically tracking the dynamics of RNA molecules and their cognate proteins in plant cells.

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.

Tomato leaf curl Yunnan virus-encoded C4 induces cell division through enhancing stability of Cyclin D 1.1 via impairing NbSKη -mediated phosphorylation in Nicotiana benthamiana

The whitefly-transmitted geminiviruses induce severe developmental abnormalities in plants. Geminivirus-encoded C4 protein functions as one of viral symptom determinants that could induce abnormal cell division. However, the molecular mechanism by which C4 contributes to cell division induction remains unclear. Here we report that tomato leaf curl Yunnan virus (TLCYnV) C4 interacts with a glycogen synthase kinase 3 (GSK3)/SHAGGY-like kinase, designed NbSKη, in Nicotiana benthamiana. Pro32, Asn34 and Thr35 of TLCYnV C4 are critical for its interaction with NbSKη and required for C4-induced typical symptoms. Interestingly, TLCYnV C4 directs NbSKη to the membrane and reduces the nuclear-accumulation of NbSKη. The relocalization of NbSKη impairs phosphorylation dependent degradation on its substrate-Cyclin D1.1 (NbCycD1;1), thereby increasing the accumulation level of NbCycD1;1 and inducing the cell division. Moreover, NbSKη-RNAi, 35S::NbCycD1;1 transgenic N. benthamiana plants have the similar phenotype as 35S::C4 transgenic N. benthamiana plants on callus-like tissue formation resulted from abnormal cell division induction. Thus, this study provides new insights into mechanism of how a viral protein hijacks NbSKη to induce abnormal cell division in plants.

Modulation of CycD3;1-CDK complexes by phytohormones and sucrose during maize germination

Maize CycD3;1 associates to CDKA or CDKB1;1 proteins during germination and the complexes formed develop kinase activity. These complexes appear to vary in size as germination proceeds, suggesting association to different sets of proteins. CycD3;1 and associated CDK proteins respond to phytohormones and sucrose. Results revealed a reduction in the CycD3;1 protein amount along germination in the presence of indoleacetic acid (IAA) or abscisic acid (ABA), although in the latter protein levels recover at the end of germination. While the levels of CDKA increase with IAA, they decrease with ABA. Both phytohormones, IAA and ABA, increase levels of CDKB1;1 only during the early germination times. CycD3;1 associated kinase activity is only reduced by both phytohormones towards the end of the germination period. On the other hand, lack of sucrose in the imbibition medium strongly reduces CycD3;1 protein levels without affecting the levels of neither CDKA nor CDKB1;1. The corresponding CycD3;1 associated kinase activity is also severely decreased. The presence of sucrose in the medium appears to stabilize the CycD3;1 protein levels.

Superoxide serves as a putative signal molecule for plant cell division: overexpression of CaRLK1 promotes the plant cell cycle via accumulation of O2- and decrease in H2 O2

Reactive oxygen species (ROS) exert both positive and negative effects on plant growth and development and therefore receive a great deal of attention in current research. A hot pepper, Capsicum annuum receptor-like kinase 1 (CaRLK1) was ectopically expressed in Nicotiana tabacum BY-2 cell and Nicotiana benthamiana plants. This ectopic expression of CaRLK1 enhanced cell division and proliferation in both heterologous systems. Apparently, CaRLK1 is involved in controlling the cell cycle, possibly by inducing expressions of cyclin B1, cyclin D3, cyclin-dependent protein kinase 3, condensin complex subunit 2 and anaphase-promoting complex subunit 11 genes. CaRLK1 overexpression also increased transcript accumulation of NADPH oxidase genes, generation of O2- and catalase (CAT) activity/protein levels. In parallel, it decreased cellular H2 O2 levels and cell size. Treatment with Tiron or diphenyleneiodonium (DPI) both decreased the cell division rate and O2- concentrations, but increased cellular H2 O2 levels. Tobacco BY-2 cells overexpressing CaRLK1 were more sensitive to amino-1,2,4-triazole (3-AT), a CAT inhibitor, than control cells, suggesting that the increased H2 O2 levels may not function as a signal for cell division and proliferation. Overexpression of CaRLK1 stimulated progression of the cell cycle from G0 /G1 phase into the S phase. It is concluded that the CaRLK1 protein plays a pivotal role in controlling the level of O2- as signaling molecule which promotes cell division, concomitant with a reduction in H2 O2 by the induction of CAT activity/protein.

Cell Cycle-independent Role of Cyclin D3 in Host Restriction of Influenza Virus Infection

To identify new host factors that modulate the replication of influenza A virus, we performed a yeast two-hybrid screen using the cytoplasmic tail of matrix protein 2 from the highly pathogenic H5N1 strain. The screen revealed a high-score interaction with cyclin D3, a key regulator of cell cycle early G₁ phase. M2-cyclin D3 interaction was validated through GST pull-down and recapitulated in influenza A/WSN/33-infected cells. Knockdown of Ccnd3 by small interfering RNA significantly enhanced virus progeny titers in cell culture supernatants. Interestingly, the increase in virus production was due to cyclin D3 deficiency per se and not merely a consequence of cell cycle deregulation. A combined knockdown of Ccnd3 and Rb1, which rescued cell cycle progression into S phase, failed to normalize virus production. Infection by influenza A virus triggered redistribution of cyclin D3 from the nucleus to the cytoplasm, followed by its proteasomal degradation. When overexpressed in HEK 293T cells, cyclin D3 impaired binding of M2 with M1, which is essential for proper assembly of progeny virions, lending further support to its role as a putative restriction factor. Our study describes the identification and characterization of cyclin D3 as a novel interactor of influenza A virus M2 protein. We hypothesize that competitive inhibition of M1-M2 interaction by cyclin D3 impairs infectious virion formation and results in attenuated virus production. In addition, we provide mechanistic insights into the dynamic interplay of influenza virus with the host cell cycle machinery during infection.

Involvement of DNA methylation in the control of cell growth during heat stress in tobacco BY-2 cells

The alteration of growth patterns, through the adjustment of cell division and expansion, is a characteristic response of plants to environmental stress. In order to study this response in more depth, the effect of heat stress on growth was investigated in tobacco BY-2 cells. The results indicate that heat stress inhibited cell division, by slowing cell cycle progression. Cells were stopped in the pre-mitotic phases, as shown by the increased expression of CycD3-1 and by the decrease in the NtCycA13, NtCyc29 and CDKB1-1 transcripts. The decrease in cell length and the reduced expression of Nt-EXPA5 indicated that cell expansion was also inhibited. Since DNA methylation plays a key role in controlling gene expression, the possibility that the altered expression of genes involved in the control of cell growth, observed during heat stress, could be due to changes in the methylation state of their promoters was investigated. The results show that the altered expression of CycD3-1 and Nt-EXPA5 was consistent with changes in the methylation state of the upstream region of these genes. These results suggest that DNA methylation, controlling the expression of genes involved in plant development, contributes to growth alteration occurring in response to environmental changes.

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.

CbCBF from Capsella bursa-pastoris enhances cold tolerance and restrains growth in Nicotiana tabacum by antagonizing with gibberellin and affecting cell cycle signaling

Plant cells respond to cold stress via a regulatory mechanism leading to enhanced cold acclimation accompanied by growth retardation. The C-repeat binding factor (CBF) signaling pathway is essential for cold response of flowering plants. Our previously study documented a novel CBF-like gene from the cold-tolerant Capsella bursa-pastoris named CbCBF, which was responsive to chilling temperatures. Here, we show that CbCBF expression is obviously responsive to chilling, freezing, abscisic acid, gibberellic acid (GA), indoleacetic acid or methyl jasmonate treatments and that the CbCBF:GFP fusion protein was localized to the nucleus. In addition, CbCBF overexpression conferred to the cold-sensitive tobacco plants enhanced tolerance to chilling and freezing, as well as dwarfism and delayed flowering. The leaf cells of CbCBF overexpression tobacco lines attained smaller sizes and underwent delayed cell division with reduced expression of cyclin D genes. The dwarfism of CbCBF transformants can be partially restored by GA application. Consistently, CbCBF overexpression reduced the bioactive gibberellin contents and disturbed the expression of gibberellin metabolic genes in tobacco. Meanwhile, cold induced CbCBF expression and cold tolerance in C. bursa-pastoris are reduced by GA. We conclude that CbCBF confers cold resistance and growth inhibition to tobacco cells by interacting with gibberellin and cell cycle pathways, likely through activation of downstream target genes.

Stretched cell cycle model for proliferating lymphocytes

Stochastic variation in cell cycle time is a consistent feature of otherwise similar cells within a growing population. Classic studies concluded that the bulk of the variation occurs in the G1 phase, and many mathematical models assume a constant time for traversing the S/G2/M phases. By direct observation of transgenic fluorescent fusion proteins that report the onset of S phase, we establish that dividing B and T lymphocytes spend a near-fixed proportion of total division time in S/G2/M phases, and this proportion is correlated between sibling cells. This result is inconsistent with models that assume independent times for consecutive phases. Instead, we propose a stretching model for dividing lymphocytes where all parts of the cell cycle are proportional to total division time. Data fitting based on a stretched cell cycle model can significantly improve estimates of cell cycle parameters drawn from DNA labeling data used to monitor immune cell dynamics.

Evolution of networks and sequences in eukaryotic cell cycle control

The molecular networks regulating the G1-S transition in budding yeast and mammals are strikingly similar in network structure. However, many of the individual proteins performing similar network roles appear to have unrelated amino acid sequences, suggesting either extremely rapid sequence evolution, or true polyphyly of proteins carrying out identical network roles. A yeast/mammal comparison suggests that network topology, and its associated dynamic properties, rather than regulatory proteins themselves may be the most important elements conserved through evolution. However, recent deep phylogenetic studies show that fungal and animal lineages are relatively closely related in the opisthokont branch of eukaryotes. The presence in plants of cell cycle regulators such as Rb, E2F and cyclins A and D, that appear lost in yeast, suggests cell cycle control in the last common ancestor of the eukaryotes was implemented with this set of regulatory proteins. Forward genetics in non-opisthokonts, such as plants or their green algal relatives, will provide direct information on cell cycle control in these organisms, and may elucidate the potentially more complex cell cycle control network of the last common eukaryotic ancestor.

Regulatory dephosphorylation of CDK at G₂/M in plants: yeast mitotic phosphatase cdc25 induces cytokinin-like effects in transgenic tobacco morphogenesis

BACKGROUND

During the last three decades, the cell cycle and its control by cyclin-dependent kinases (CDKs) have been extensively studied in eukaryotes. This endeavour has produced an overall picture that basic mechanisms seem to be largely conserved among all eukaryotes. The intricate regulation of CDK activities includes, among others, CDK activation by CDC25 phosphatase at G₂/M. In plants, however, studies of this regulation have lagged behind as a plant Cdc25 homologue or other unrelated phosphatase active at G₂/M have not yet been identified.

SCOPE

Failure to identify a plant mitotic CDK activatory phosphatase led to characterization of the effects of alien cdc25 gene expression in plants. Tobacco, expressing the Schizosaccharomyces pombe mitotic activator gene, Spcdc25, exhibited morphological, developmental and biochemical changes when compared with wild type (WT) and, importantly, increased CDK dephosphorylation at G₂/M. Besides changes in leaf shape, internode length and root development, in day-neutral tobacco there was dramatically earlier onset of flowering with a disturbed acropetal floral capacity gradient typical of WT. In vitro, de novo organ formation revealed substantially earlier and more abundant formation of shoot primordia on Spcdc25 tobacco stem segments grown on shoot-inducing media when compared with WT. Moreover, in contrast to WT, stem segments from transgenic plants formed shoots even without application of exogenous growth regulator. Spcdc25-expressing BY-2 cells exhibited a reduced mitotic cell size due to a shortening of the G₂ phase together with high activity of cyclin-dependent kinase, NtCDKB1, in early S-phase, S/G₂ and early M-phase. Spcdc25-expressing tobacco ('Samsun') cell suspension cultures showed a clustered, more circular, cell phenotype compared with chains of elongated WT cells, and increased content of starch and soluble sugars. Taken together, Spcdc25 expression had cytokinin-like effects on the characteristics studied, although determination of endogenous cytokinin levels revealed a dramatic decrease in Spcdc25 transgenics.

CONCLUSIONS

The data gained using the plants expressing yeast mitotic activator, Spcdc25, clearly argue for the existence and importance of activatory dephosphorylation at G₂/M transition and its interaction with cytokinin signalling in plants. The observed cytokinin-like effects of Spcdc25 expression are consistent with the concept of interaction between cell cycle regulators and phytohormones during plant development. The G₂/M control of the plant cell cycle, however, remains an elusive issue as doubts persist about the mode of activatory dephosphorylation, which in other eukaryotes is provided by Cdc25 phosphatase serving as a final all-or-nothing mitosis regulator.

The D-type cyclin gene (Nicta;CycD3;4) controls cell cycle progression in response to sugar availability in tobacco

D-type cyclins play key roles in the G1-to-S phase transition that occurs in response to nutrient and hormonal signals. In higher plants, sucrose is the major transported carbon source, and is likely to be a major determinant of cell division. To elucidate how sugar affects on the regulation of cell cycle machinery and plant development, we examined the role of carbon sources on the expression of cell-cycle-related genes in transgenic tobacco plants overexpressing Nicta;CycD3;4. The Nicta;CycD3;4 overexpressed transgenic plants showed accelerated growth and remarkable increase in the number of cells in the S and G2 phases in response to sucrose concentrations. Increased expressions level of Nicta;CycD3;4 gene was observed in transgenic tobacco plants grown on 1/2 strength MS medium supplemented with a high concentration of sugar. Moreover, the expression of sugar-sensing-related gene, invertase, was also maintained at a high level in transgenic tobacco plants with elevated sugar availability. These findings indicate that sugar availability plays a role during the G1 phase and the transition of the G1-to-S phase of cell cycle by controlling the expression of Nicta;CycD3;4.

Spatio-temporal changes in cell division, endoreduplication and expression of cell cycle-related genes in pollinated and plant growth substances-treated ovaries of cucumber

We investigated the temporal and spatial changes in cell division, endoreduplication and expression of cell cycle-related genes in developing cucumber fruits at 0-20 days after anthesis (DAA). Cell division was intense at 0-4 DAA and then decreased until to 8 DAA. Meanwhile, endoreduplication started at 4 DAA and increased gradually to 20 DAA, accompanied by an increase in fruit weight. Cell division was mainly observed in the exocarp, while endoreduplication occurred mostly in the endocarp and pulp. Among the six cell cycle-related genes examined, two mitotic cyclin genes (CycA and CycB) and CDKB had the highest transcript levels within 2 DAA, while transcripts of two CycD3 genes and CDKA peaked at 4 DAA and 20 DAA, respectively. Naphthaleneacetic acid (NAA), N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU) and 24-epibrassinolide (EBR) all induced parthenocarpic growth as well as active cell division, and enhanced transcripts of cell cycle-related genes. In comparison, gibberellic acid (GA(3)) had little effect on the induction of parthenocarpy and transcripts of cell cycle-related genes. These results provide evidence for the important roles of cell division and endoreduplication during cucumber fruit development, and suggest the essential roles of cell cycle-related genes and plant growth substances in fruit development.

Expressions of ECE-CYC2 clade genes relating to abortion of both dorsal and ventral stamens in Opithandra (Gesneriaceae)

BACKGROUND

ECE-CYC2 clade genes known in patterning floral dorsoventral asymmetry (zygomorphy) in Antirrhinum majus are conserved in the dorsal identity function including arresting the dorsal stamen. However, it remains uncertain whether the same mechanism underlies abortion of the ventral stamens, an important morphological trait related to evolution and diversification of zygomorphy in Lamiales sensu lato, a major clade of predominantly zygomorphically flowered angiosperms. Opithandra (Gesneriaceae) is of particular interests in addressing this question as it is in the base of Lamiales s.l., an early representative of this type zygomorphy.

RESULTS

We investigated the expression patterns of four ECE-CYC2 clade genes and two putative target cyclinD3 genes in Opithandra using RNA in situ hybridization and RT-PCR. OpdCYC gene expressions were correlated with abortion of both dorsal and ventral stamens in Opithandra, strengthened by the negatively correlated expression of their putative target OpdcyclinD3 genes. The complement of OpdcyclinD3 to OpdCYC expressions further indicated that OpdCYC expressions were related to the dorsal and ventral stamen abortion through negative effects on OpdcyclinD3 genes.

CONCLUSION

These results suggest that ECE-CYC2 clade TCP genes are not only functionally conserved in the dorsal stamen repression, but also involved in arresting ventral stamens, a genetic mechanism underlying the establishment of zygomorphy with abortion of both the dorsal and ventral stamens evolved in angiosperms, especially within Lamiales s.l.

Serpin genes AtSRP2 and AtSRP3 are required for normal growth sensitivity to a DNA alkylating agent in Arabidopsis

BACKGROUND

The complex responses of plants to DNA damage are incompletely understood and the role of members of the serpin protein family has not been investigated. Serpins are functionally diverse but structurally conserved proteins found in all three domains of life. In animals, most serpins have regulatory functions through potent, irreversible inhibition of specific serine or cysteine proteinases via a unique suicide-substrate mechanism. Plant serpins are also potent proteinase inhibitors, but their physiological roles are largely unknown.

RESULTS

Six Arabidopsis genes encoding full-length serpins were differentially expressed in developing seedlings and mature tissues. Basal levels of AtSRP2 (At2g14540) and AtSRP3 (At1g64030) transcripts were highest in reproductive tissues. AtSRP2 was induced 5-fold and AtSRP3 100-fold after exposure of seedlings to low concentrations of methyl methanesulfonate (MMS), a model alkylating reagent that causes DNA damage. Homozygous T-DNA insertion mutants atsrp2 and atsrp3 exhibited no differential growth when mutant and wild-type plants were left untreated or exposed to gamma-radiation or ultraviolet light. In contrast, atsrp2 and atsrp3 plants exhibited greater root length, leaf number and overall size than wild-type plants when exposed to MMS. Neither of the two serpins was required for meiosis. GFP-AtSRP2 was localized to the nucleus, whereas GFP-AtSRP3 was cytosolic, suggesting that they target different proteinases. Induction of cell cycle- and DNA damage-related genes AtBRCA1, AtBARD1, AtRAD51, AtCYCB1;1 and AtCYCD1;1, but not AtATM, was reduced relative to wild-type in atsrp2 and atsrp3 mutants exposed to MMS.

CONCLUSION

Expression of specific serpin genes (AtSRP2 and AtSRP3 in Arabidopsis) is required for normal responses of plants following exposure to alkylating genotoxins such as MMS.

Eternal youth, the fate of developing Arabidopsis leaves upon Rhodococcus fascians infection

The phytopathogenic actinomycete Rhodococcus fascians induces neoplastic shooty outgrowths on infected hosts. Upon R. fascians infection of Arabidopsis (Arabidopsis thaliana), leaves are formed with small narrow lamina and serrated margins. These symptomatic leaves exhibit reduced tissue differentiation, display more but smaller cells that do not endoreduplicate, and accumulate in the G1 phase of the cell cycle. Together, these features imply that leaf growth occurs primarily through mitotic cell division and not via cell expansion. Molecular analysis revealed that cell cycle gene expression is activated continuously throughout symptomatic leaf development, ensuring persistent mitotic cycling and inhibition of cell cycle exit. The transition at the two major cell cycle checkpoints is stimulated as a direct consequence of the R. fascians signals. The extremely reduced phenotypical response of a cyclind3;1-3 triple knockout mutant indicates that the D-type cyclin/retinoblastoma/E2F transcription factor pathway, as a major mediator of cell growth and cell cycle progression, plays a key role in symptom development and is instrumental for the sustained G1-to-S and G2-to-M transitions during symptomatic leaf growth.

Functions, regulation and cellular localization of plant cyclin-dependent kinase inhibitors

The cell cycle is regulated by the cyclin-dependent kinase (CDK), and CDK inhibitors can bind to CDKs and inhibit their activities. This review examines plant CDK inhibitors, with particular emphasis on their molecular and cellular functions, regulation and cellular localization. In plants, a family of ICK/KRP CDK inhibitors represented by ICK1 is known and another type of CDK inhibitor represented by the SIMESE (SIM) has recently been reported. Considerable understanding has been gained with the ICK/KRP CDK inhibitors. These plant CDK inhibitors share only limited sequence similarity in the C-terminal region with the KIP/CIP family of mammalian CDK inhibitors. The ICK/KRP CDK inhibitors thus provide good tools to understand the basic machinery as well as the unique aspects of the plant cell cycle. The ICK/KRP CDK inhibitors interact with D-type cyclins or A-type CDKs or both. Several functional regions and motifs have been identified in ICK1 for CDK inhibition, nuclear localization and protein instability. Clear evidence shows that ICK/KRP proteins are important for the cell cycle and endoreduplication. Preliminary evidence suggests that they may also be involved in cell differentiation and cell death. Results so far show that plant CDK inhibitors are exclusively localized in the nucleus. The molecular sequences regulating the localization and functional significance will be discussed.

Transgenic tobacco plants overexpressing the Nicta; CycD3; 4 gene demonstrate accelerated growth rates

D-type cyclins control the onset of cell division and the response to extracellular signals during the G1 phase. In this study, we transformed a D-type cyclin gene, Nicta;CycD3;4, from Nicotiana tabacum using an Agrobacterium-mediated method. A predicted 1.1 kb cyclin gene was present in all of the transgenic plants, but not in wild-type. Northern analyses showed that the expression level of the Nicta;CycD3;4 gene in all of the transgenic plants was strong when compared to the wild-type plants, suggesting that Nicta;CycD3;4 gene driven by the CaMV 35S promoter was being overexpressed. Our results revealed that transgenic plants overexpressing Nicta;CycD3;4 had an accelerated growth rate when compared to wild-type plants, and that the transgenic plants exhibited a smaller cell size and a decreased cell population in young leaves when compared to wild-type plants.

Tobacco cells transformed with the fission yeast Spcdc25 mitotic inducer display growth and morphological characteristics as well as starch and sugar status evocable by cytokinin application

In plants, the G2/M control of cell cycle remains an elusive issue as doubts persist about activatory dephosphorylation--in other eukaryotes provided by CDC25 phosphatase and serving as a final all-or-nothing mitosis regulator. We report on the effects of tobacco (Nicotiana tabacum L., cv. Samsun) transformation with fission yeast (Schizosaccharomyces pombe) cdc25 (Spcdc25) on cell characteristics. Transformed cell suspension cultures showed higher dry mass accumulation during the exponential phase and clustered more circular cell phenotypes compared to chains of elongated WT cells. Similar cell parameters, as in the transformants, can be induced in WT by cytokinins. Spcdc25 cells, after cytokinin treatment, showed giant cell clusters and growth inhibition. In addition, Spcdc25 expression led to altered carbohydrate status: increased starch and soluble sugars with higher sucrose:hexoses ratio, inducible in WT by cytokinin treatment. Taken together, the Spcdc25 transformation had a cytokinin-like effect on studied characteristics. However, endogenous cytokinin determination revealed markedly lower cytokinin levels in Spcdc25 transformants. This indicates that the cells sense Spcdc25 expression as an increased cytokinin availability, manifested by changed cell morphology, and in consequence decrease endogenous cytokinin levels. Clearly, the results on cell growth and morphology are consistent with the model of G2/M control including cytokinin-regulated activatory dephosphorylation. Nevertheless, no clear link is obvious between Spcdc25 transformation and carbohydrate status and thus the observed cytokinin-like effect on carbohydrate levels poses a problem. Hence, we propose that Spcdc25-induced higher CDK(s) activity at G2/M generates a signal-modifying carbohydrate metabolism to meet high energy and C demands of forthcoming cell division.

A role of brassinosteroids in early fruit development in cucumber

Brassinosteroids (BRs) are essential for many biological processes in plants, however, little is known about their roles in early fruit development. To address this, BR levels were manipulated through the application of exogenous BRs (24-epibrassinolide, EBR) or a BR biosynthesis inhibitor (brassinazole, Brz) and their effects on early fruit development, cell division, and expression of cyclin and cyclin-dependent kinases (CDKs) genes were examined in two cucumber cultivars that differ in parthenocarpic capacity. The application of EBR induced parthenocarpic growth accompanied by active cell division in Jinchun No. 4, a cultivar without parthenocarpic capacity, whereas Brz treatment inhibited fruit set and, subsequently, fruit growth in Jinchun No. 2, a cultivar with natural parthenocarpic capacity, and this inhibitory effect could be rescued by the application of EBR. RT-PCR analysis showed both pollination and EBR induced expression of cell cycle-related genes (CycA, CycB, CycD3;1, CycD3;2, and CDKB) after anthesis. cDNA sequences for CsCycD3;1 and CsCycD3;2 were isolated through PCR amplification. Both CsCycD3;1 and CsCycD3;2 transcripts were up-regulated by EBR treatment and pollination but strongly repressed by Brz treatment. Meanwhile, BR6ox1 and SMT transcripts, two genes involved in BR synthesis, exhibited feedback regulation. These results strongly suggest that BRs play an important role during early fruit development in cucumber.

Maize D4;1 and D5 cyclin proteins in germinating maize. Associated kinase activity and regulation by phytohormones

We have previously reported the expression of four different maize D cyclins during seed germination and showed that cytokinins and auxins stimulate the expression of every cyclin in a differential way. In this paper we characterize the behavior at the protein level of two of these cyclins, CycD5 and CycD4;1. Antibodies were raised against CycD5;2 (which very likely also recognizes D5;1) and CycD4;1 and Western blot studies demonstrated that neither BA nor indol-3 acetic acid (IAA) stimulate cyclin accumulation during germination, compared with control levels. However, phytohormones, particularly IAA, modify the kinase activity associated to D cyclins preferentially at early hours of germination. The associated kinase moiety to D cyclins appears to be of a Cdk-A type because this protein immunoprecipitates with D cyclins and because kinase activity is strongly inhibited by both olomoucine and also by a peptide corresponding to the carboxy end of a maize kip related protein (KRP) protein. There is thus no correlation between mRNA and protein expression for these maize D cyclins during seed germination, although phytohormones may stimulate a signaling cascade that stimulates activation of protein kinase activity in cyclin-Cdk complexes.

Auxin is required for the assembly of A-type cyclin-dependent kinase complexes in tobacco cell suspension culture

Although activation of A-type cyclin-dependent kinase (CDKA) is required for plant cell division, little is known about how CDKA is activated before commitment to cell division. Here, we show that auxin is required for the formation of active CDKA-associated complexes, promoting assembly of the complex in tobacco suspension culture Bright Yellow-2 (BY-2) cells. Protein gel blot analysis revealed that CDKA levels increased greatly after stationary-phase BY-2 cells were subcultured into fresh medium to re-enter the cell cycle. However, these increasing levels subsided when cells were subcultured into auxin-deprived medium, and a subtle increase was observed after subculturing into sucrose-deprived medium. Additionally, p13(suc1)-associated kinase activity did not increase significantly after subculturing into either auxin- or sucrose-deprived medium, but increased strongly after subculturing into medium containing both auxin and sucrose. Using gel filtration, we found that p13(suc1)-associated kinase activity against tobacco retinoblastoma-related protein was maximal in fractions corresponding to the molecular mass of the cyclin/CDKA complex. Interestingly, this peak distribution of high molecular-mass fractions of CDKA disappeared after cells were subcultured into auxin-deprived medium. These findings suggest an important role for auxin in the assembly of active CDKA-associated complexes.

The plant cell cycle--15 years on

The basic components of the plant cell cycle are G1 (postmitotic interphase), S-phase (DNA synthesis phase), G2 (premitotic interphase) and mitosis/cytokinesis. Proliferating cells are phosphoregulated by cyclin-dependent protein kinases (CDKs). Plant D-type cyclins are sensors of the G0 to G1 transition, and are also important for G2/M. At G1/S, the S-phase transcription factor, E2F, is released from inhibitory retinoblastoma protein. Negative regulation of G1 events is through KRPs (Kip-related proteins). Plant S-phase genes are similar to animal ones, but timing of expression can be different (e.g. CDC6 at the start of S-phase) and functional evidence is limited. At G2/M, A-type and the unique B-type CDKs when bound to A, B and D cyclins, drive cells into division; they are negatively regulated by ICK1/2 and perhaps also by WEE1 kinase. In Arabidopsis, a putative CDC25 lacks a regulatory domain. Mitosis depends on correct temporal activity of CDKs, Aurora kinases and anaphase promotion complex; CDK-cyclin B activity beyond metaphase is catastrophic. Endoreduplication (re-replication of DNA in the absence of mitosis) is characterized by E2F expression and down-regulation of mitotic cyclins. Some cell size data support, whilst others negate, the idea of cell size having an impact on development.

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 Regulation in Plant Development

Cell cycle regulation is of pivotal importance for plant growth and development. Although plant cell division shares basic mechanisms with all eukaryotes, plants have evolved novel molecules orchestrating the cell cycle. Some regulatory proteins, such as cyclins and inhibitors of cyclin-dependent kinases, are particularly numerous in plants, possibly reflecting the remarkable ability of plants to modulate their postembryonic development. Many plant cells also can continue DNA replication in the absence of mitosis, a process known as endoreduplication, causing polyploidy. Here, we review the molecular mechanisms that regulate cell division and endoreduplication and we discuss our understanding, albeit very limited, on how the cell cycle is integrated with plant development.

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.

The emerging importance of cyclin-dependent kinase inhibitors in the regulation of the plant cell cycle and related processesThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

The cell division cycle in plants as in other eukaryotes is controlled by the cyclin-dependent kinase (CDK). This CDK paradigm determines that developmental cues and environmental signals need to impinge on the CDK complex to affect the cell cycle. An important part of understanding cell cycle regulation is to understand how CDK is regulated by various factors. In addition, there are features that set the cell cycle regulation in plants apart from that in other eukaryotes such as animals. Our knowledge of the molecular mechanisms that underlie the differences is poor. A family of plant CDK inhibitor proteins has been identified. The plant CDK inhibitors share similarity with a family of animal CDK inhibitors in a small region, while most of the sequence and the structural layout of the plant CDK inhibitors are different from the animal counterparts. Studies of plant CDK inhibitors have been performed mostly with the CDK inhibitors from Arabidopsis called ICKs (also referred to as KRPs). ICKs interact with D-type cyclins and A-type CDK. Overexpression of ICKs has been shown to affect cell division, plant growth, and morphogenesis. Studies of ICKs have also provided insightful information on the control of endoreduplication in plants. These aspects as well as cellular localization and protein regulation of ICKs are reviewed.

Elicitor-induced down-regulation of cell cycle-related genes in tobacco cells

The fungal elicitors, a xylanase from Trichoderma viride and an extract from the cell wall of Phytophthora infestans, are shown to cause a rapid reduction of the mRNA levels of various cell cycle-related genes, including MAP kinase genes and cyclin genes, in cultured tobacco cells (Nicotiana tabacum cv. Xanthi, line XD6S). Pharmacological analyses suggest that the elicitor-induced decrease in Bi-type cyclin (Nicta;CycB1;3) and A1-type cyclin (Nicta;CycA1;1) mRNAs may be due to transcriptional repression, and that in D3-type cyclin (Nicta;CycD3;2) mRNA due to destabilization of the mRNA molecule itself. The activity of protein kinases is required for both the activation of defence genes and the repression of cyclin genes. The transcriptional activity of the promoter of the B1-class cyclin gene decreases upon elicitor treatment. The transactivation activity of NtmybA2, a tobacco Myb transcription activator for the M phase-specific cis-acting elements in the promoter of the B-type cyclin gene, is inhibited by elicitor treatment. In addition, the mRNA levels of NtmybA2 and two other related genes, NtmybA1 and NtmybB, decrease in response to the elicitor. Finally, we discuss a negative cross-talk between signal transduction pathways for growth and defence responses, which might be important for adaptation to environmental stress by potential pathogens.

Cell cycle synchronization of tobacco BY-2 cells

Synchronization is a powerful technique for understanding cell cycle events. Here, we describe the procedure for synchronizing tobacco bright yellow 2 (BY-2) cell line, with which an exceptionally high level of synchrony can be achieved. It basically relies on an "arrest-and-release" strategy using aphidicolin, an inhibitor of DNA replication, and propyzamide, a plant-microtubule disruptant. In a single-step process using aphidicolin alone, a cell population with about 70% of the cells at mitosis can be achieved, whereas by a two-step method using the two inhibitors sequentially, the level of synchrony can reach over 90%. The method of choice depends not only on the peak mitotic cell proportion but also on the cell cycle stage that is targeted for analysis. Both procedures take about 1.5 days, and cell cycle progression can be observed from the S phase to the next G1 phase at about 12 h after a 24 h-period treatment with aphidicolin.

Systematic spatial analysis of gene expression during wheat caryopsis development

The cereal caryopsis is a complex tissue in which maternal and endosperm tissues follow distinct but coordinated developmental programs. Because of the hexaploid genome in wheat (Triticum aestivum), the identification of genes involved in key developmental processes by genetic approaches has been difficult. To bypass this limitation, we surveyed 888 genes that are expressed during caryopsis development using a novel high-throughput mRNA in situ hybridization method. This survey revealed novel distinct spatial expression patterns that either reflected the ontogeny of the developing caryopsis or indicated specialized cellular functions. We have identified both known and novel genes whose expression is cell cycle-dependent. We have identified the crease region as important in setting up the developmental patterning, because the transition from proliferation to differentiation spreads from this region to the rest of the endosperm. A comparison of this set of genes with the rice (Oryza sativa) genome shows that approximately two-thirds have rice counterparts but also suggests considerable divergence with regard to proteins involved in grain filling. We found that the wheat genes had significant homology with 350 Arabidopsis thaliana genes. At least 25 of these are already known to be essential for seed development in Arabidopsis, but many others remain to be characterized.

Genome-wide annotation and expression profiling of cell cycle regulatory genes in Chlamydomonas reinhardtii

Eukaryotic cell cycles are driven by a set of regulators that have undergone lineage-specific gene loss, duplication, or divergence in different taxa. It is not known to what extent these genomic processes contribute to differences in cell cycle regulatory programs and cell division mechanisms among different taxonomic groups. We have undertaken a genome-wide characterization of the cell cycle genes encoded by Chlamydomonas reinhardtii, a unicellular eukaryote that is part of the green algal/land plant clade. Although Chlamydomonas cells divide by a noncanonical mechanism termed multiple fission, the cell cycle regulatory proteins from Chlamydomonas are remarkably similar to those found in higher plants and metazoans, including the proteins of the RB-E2F pathway that are absent in the fungal kingdom. Unlike in higher plants and vertebrates where cell cycle regulatory genes have undergone extensive duplication, most of the cell cycle regulators in Chlamydomonas have not. The relatively small number of cell cycle genes and growing molecular genetic toolkit position Chlamydomonas to become an important model for higher plant and metazoan cell cycles.

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.

Maize cyclin D2 expression, associated kinase activity and effect of phytohormones during germination

A cDNA corresponding to 16 kDa of the maize cyclin D2 N-terminus was cloned and this polypeptide was overexpressed to produce homologous antibodies. This antibody recognized a 38 kDa protein in extracts from maize embryonic axes which corresponds to the predicted size for cyclin D2 protein. Expression of cyclin D2 was followed at the transcriptional and protein levels, and the effect of cytokinins and abscisic acid (ABA) was followed during maize germination. Cytokinins importantly stimulated cyclin D2 gene expression at late germination times and sucrose was necessary for stimulation, whereas the effect of ABA was not different from that in controls. However, cyclin D2 protein levels in control axes reached a peak at 6 h germination, declining thereafter, and neither cytokinins nor ABA modified this behavior. Two cyclic-dependent kinase A (Cdk-A)-type proteins and proliferating cell nuclear antigen (PCNA) were found co-immunoprecipitating with cyclin D2, and these immunoprecipitates were able to phosphorylate both histone H1 and the maize retinoblastoma-related protein (RBR). This protein kinase activity differed from the pattern of protein accumulation during germination, and the activity was not modified by either cytokinins or ABA. We discuss these findings in terms of the importance of the cell cycle for the germination process.

Cambial meristem dormancy in trees involves extensive remodelling of the transcriptome

The establishment of the dormant state in meristems involves considerable physiological and metabolic alterations necessary for surviving unfavourable growth conditions. However, a global molecular analysis of dormancy in meristems has been hampered by the difficulty in isolating meristem cells. We used cryosectioning to isolate purified cambial meristem cells from the woody plant Populus tremula during active growth and dormancy. These samples were used to generate meristem-specific cDNA libraries and for cDNA microarray experiments to define the global transcriptional changes underlying cambial dormancy. The results indicate a significant reduction in the complexity of the cambial transcriptome in the dormant state. Although cell division is terminated in the dormant cambium, the cell cycle machinery appears to be maintained in a skeletal state as suggested by the continued presence of transcripts for several cell cycle regulators. The downregulation of PttPIN1 and PttPIN2 transcripts explains the reduced basipetal polar auxin transport during dormancy. The induction of a member of the SINA family of ubiquitin ligases implicated in auxin signalling indicates a potential mechanism for modulation of auxin sensitivity during cambial dormancy. The metabolic alterations during dormancy are mirrored in the induction of genes involved in starch breakdown and the glyoxysomal cycle. Interestingly, the induction of RGA1 like gene suggests modification of gibberellin signalling in cambial dormancy. The induction of genes such as poplar orthologues of FIE and HAP2 indicates a potential role for these global regulators of transcription in orchestrating extensive changes in gene expression during dormancy.

CycD1, a putative G1 cyclin from Antirrhinum majus, accelerates the cell cycle in cultured tobacco BY-2 cells by enhancing both G1/S entry and progression through S and G2 phases

A putative G1 cyclin gene, Antma;CycD1;1 (CycD1), from Antirrhinum majus is known to be expressed throughout the cell cycle in the meristem and other actively proliferating cells. To test its role in cell cycle progression, we examined the effect of CycD1 expression in the tobacco (Nicotiana tabacum) cell suspension culture BY-2. Green fluorescent protein:CycD1 is located in the nucleus throughout interphase. Using epitope-tagged CycD1, we show that it interacts in vivo with CDKA, a cyclin dependent protein kinase that acts at both the G1/S and the G2/M boundaries. We examined the effect of induced expression at different stages of the cell cycle. Expression in G0 cells accelerated entry into both S-phase and mitosis, whereas expression during S-phase accelerated entry into mitosis. Consistent with acceleration of both transitions, the CycD1-associated cyclin dependent kinase can phosphorylate both histone H1 and Rb proteins. The expression of cyclinD1 led to the early activation of total CDK activity, consistent with accelerated cell cycle progression. Continuous expression of CycD1 led to moderate increases in growth rate. Therefore, in contrast with animal D cyclins, CycD1 can promote both G0/G1/S and S/G2/M progression. This indicates that D cyclin function may have diverged between plants and animals.

Differential stage-specific regulation of cyclin-dependent kinases during cambial dormancy in hybrid aspen

The cambium of woody plants cycles between active and dormant states. Dormancy can be subdivided into eco- and endodormant stages. Ecodormant trees resume growth upon exposure to growth-promotive signals, while the establishment of endodormant state results in loss of the ability to respond to these signals. In this paper, we analysed the regulation of cyclin-dependent kinases (CDKs) to understand the differential response of cell division machinery to growth-promotive signals during the distinct stages of dormancy in hybrid aspen. We show that 4 weeks of short-day (SD) treatment causes termination of the cambial cell division and establishment of the ecodormant state. This coincides with a steady decline in the histone H1 kinase activity of the PSTAIRE-type poplar CDKA (PttCDKA) and the PPTTLRE-type PttCDKB kinase complexes. However, neither the transcript nor the polypeptide levels of PttCDKA and PttCDKB are reduced during ecodormancy. In contrast, 6 weeks of SD treatment establishes endodormancy, which is marked by the reduction and disappearance of the PttCDKA and PttCDKB protein levels and the PttCDKB transcript levels. The transition to endodormancy is preceded by an elevated E2F (adenosine E2 promoter binding factor) phosphorylation activity of the PttCDKA kinase that reduces the DNA-binding activity of E2F in vitro. The transition to endodormancy is followed by a reduction of retinoblastoma (Rb) phosphorylation activity of PttCDKA protein complexes. Both phosphorylation events could contribute to block the G1 to S phase transition upon the establishment of endodormancy. Our results indicate that eco- and endodormant stages of cambial dormancy involve a stage-specific regulation of the cell cycle effectors at multiple levels.

Cell cycle function of a rice B2-type cyclin interacting with a B-type cyclin-dependent kinase

Cyclin-dependent kinases (CDKs) are involved in the control of cell cycle progression. Plant A-type CDKs are functional homologs of yeast Cdc2/Cdc28 and are expressed throughout the cell cycle. In contrast, B-type CDK (CDKB) is a family of mitotic CDKs expressed during the S/M phase, and its precise function remains unknown. Here, we identified two B2-type cyclins, CycB2;1 and CycB2;2, as a specific partner of rice CDKB2;1. The CDKB2;1-CycB2 complexes produced in insect cells showed a significant level of kinase activity in vitro, suggesting that CycB2 binds to and activates CDKB2. We then expressed green fluorescent protein (GFP)-fused CDKB2;1 and CycB2;2 in tobacco BY2 cells to investigate their subcellular localization during mitosis. Surprisingly, the fluorescence signal of CDKB2;1-GFP was tightly associated with chromosome alignment as well as with spindle structure during the metaphase. During the telophase, the signal was localized to the spindle midzone and the separating sister chromosomes, and then to the phragmoplast. On the other hand, the CycB2;2-GFP fluorescence signal was detected in nuclei during the interphase and prophase, moved to the metaphase chromosomes, and then disappeared completely after the cells passed through the metaphase. Co-localization of CDKB2;1-GFP and CycB2;2-GFP on chromosomes aligned at the center of the metaphase cells suggests that the CDKB2-CycB2 complex may function in retaining chromosomes at the metaphase plate. Overexpression of CycB2;2 in rice plants resulted in acceleration of root growth without any increase in cell size, indicating that CycB2;2 promoted cell division probably through association with CDKB2 in the root meristem.

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.

Isolation, characterization and expression of cyclin and cyclin-dependent kinase genes in Jerusalem artichoke (Helianthus tuberosus L.)

Tuber explants of Jerusalem artichoke (Helianthus tuberosus L.) are a model system for cell-cycle re-entry from a quiescent state, involving the activation of division of tuber parenchyma cells in response to exogenous auxin. To enable molecular studies of this system, two cyclin (Heltu;CYCD1;1 and Heltu; CYCD3;1) and two cyclin-dependent kinase (Heltu; CDKA;1 and Heltu;CDKB1;1) genes have been isolated from a Jerusalem artichoke cDNA library and their expression demonstrated during the activation of cell division. It was found that CDKA;1 transcripts are present in quiescent tubers, whereas CYCD1;1, CYCD3;1 and CDKB1;1 transcripts are induced during cell-cycle re-entry as well as during bud growth of whole tubers. Both CYCD1;1 and CYCD3;1 transcripts appear shortly before, or coincident with, the onset of S phase.

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.

The transcript-level-independent activation of ornithine decarboxylase in suspension-cultured BY2 cells entering the cell cycle

The regulation of ornithine decarboxylase (ODC) expression was studied in suspension-cultured tobacco (Nicotiana tabacum L.) BY2 cells. ODC activity increased rapidly 3 h after cells re-entered the cell cycle from the stationary phase, corresponding to the G1 phase, and continued to increase in the subsequent S phase, while the ODC transcript level increased only transiently. ODC activity was suppressed by sucrose-deficiency, while the ODC transcript level was not affected. U0126, a specific inhibitor of mammalian MAPK kinases (MEKs), significantly reduced ODC enzyme activity, but not the ODC transcript level. These results suggest that ODC activity is regulated independently of its transcript level in BY2 cells, and that sucrose and a U0126-sensitive protein kinase are required for the transcript-level-independent activation of ODC.