Tobacco BY-2 Cell Line as the “HeLa” Cell in the Cell Biology of Higher Plants

Sugar-regulated expression of a putative hexose transport gene in grape

Different lengths of the promoter of grape (Vitis vinifera) VvHT1 (Hexose Transporter 1) gene, which encodes a putative hexose transporter expressed during the ripening of grape, have been transcriptionally fused to the beta-glucuronidase reporter gene. In transgenic tobacco (Nicotiana tabacum) transformed with these constructs, VvHT1 promoters were clearly responsible for the sink organ preferential expression. The potential sugar effectors of VvHT1 promoter were studied in tobacco cv Bright-Yellow 2 cells transformed with chimeric constructs. Glucose (56 mM), sucrose (Suc; 58 mM), and the non-transported Suc isomer palatinose doubled the beta-glucuronidase activity conferred by the VvHT1 promoter, whereas fructose did not affect it. These effects were the strongest with the 2.4-kb promoter, which contains all putative sugar-responsive elements (activating and repressing), but they were also significant with the 0.3-kb promoter, which contains only activating sugar boxes. The induction of VvHT1 expression by both Suc and palatinose was confirmed in the homologous grape berry cell culture. The data provide the first example of a putative sugar transporter, which is induced by both glucose and Suc in higher plants. Although induction of VvHT1 expression by Suc does not require transport, the presence of glucosyl moiety is necessary for Suc sensing. These results provide new insights into sugar sensing and signaling in plants.

Amyloplast formation in cultured tobacco BY-2 cells requires a high cytokinin content

When cytokinin-autonomous tobacco BY-2 cell cultures are transferred into 2,4-dichlorophenoxyacetic acid (2,4-D)-deprived medium, amyloplast development is initiated. Using this in vitro amyloplast-inducing system, the role of cytokinins in amyloplast formation was investigated. We show that addition of lovastatin, an inhibitor of mevalonate synthesis, to amyloplast-inducing medium reduced starch accumulation. Microscopic observation also revealed that lovastatin treatment decreased starch deposition; however, the overall morphologies of cells and plastids were less affected than control cell cultures. In addition, lovastatin lowered the transcription level of the ADP-glucose pyrophosphorylase small subunit (AgpS) gene. Application of mevalonate or zeatin dramatically restored the decrease in starch deposition, and restored AgpS mRNA accumulation. Moreover, addition of other molecules with cytokinin activity, such as adenine- and phenylurea-type compounds, restored starch accumulation and AgpS transcript levels, whereas other isopentenyl pyrophosphate-derived phytohormones did not. Liquid chromatography-mass spectrometry/mass spectrometry quantification of endogenous cytokinins revealed that endogenous cytokinins increased when BY-2 cells were transferred into 2,4-D-deprived medium from conventional medium containing 2,4-D. In addition, lovastatin treatment decreased endogenous cytokinins to some extent when cultured under 2,4-D-deprived conditions. Our results suggest that both 2,4-D deprivation and an increase in endogenous cytokinins have important roles in accelerating the changes in plastid morphology, starch accumulation, and AgpS gene expression.

Comparative molecular and functional analyses of the tobacco cyclin-dependent kinase inhibitor NtKIS1a and its spliced variant NtKIS1b

In all eukaryotes, cell cycle progression is controlled by cyclin-dependent kinases (CDKs) whose activity is regulated at several levels including inhibition by CDK inhibitors. Here, we report a comparative molecular and functional analysis of the tobacco (Nicotiana tomentosiformis) CDK inhibitor, NtKIS1a, and its spliced variant, NtKIS1b. The C-terminal end of NtKIS1a shares strong sequence similarity with mammalian CIP/KIP inhibitors, which is not the case for NtKIS1b. Consistent with this, NtKIS1a but not NtKIS1b inhibits in vitro the kinase activity of CDK/cyclin complexes, and tobacco (Nicotiana tabacum) D-type cyclins and an A-type CDK are NtKIS1a, but not NtKIS1b, interacting partners. Although both NtKIS1a and NtKIS1b transcripts are mainly found in flowers and more precisely in stamens, NtKIS1b transcript levels are cell cycle regulated, whereas those of NtKIS1a remain constant during the cell cycle. NtKIS1a and NtKIS1b fused to fluorescent proteins are localized in the nucleus when transiently expressed in onion epidermal cells. Furthermore, there is no competition for their nuclear localization when they are simultaneously overexpressed. In vitro competition toward CDK kinase activity suggests that NtKIS1b is a strong competitor of NtKIS1a. Arabidopsis plants overexpressing NtKIS1a-green fluorescent protein (GFP) or NtKIS1b-GFP fusion proteins were obtained. In these plants, the fusion proteins are still localized in the nucleus. Interestingly, NtKIS1a-GFP-overexpressing plants display strong morphological modifications and a reduced CDK kinase activity, whereas NtKIS1b-GFP-overexpressing plants display a wild-type phenotype including a wild-type CDK kinase activity. Our results strongly suggest that the inhibition of the kinase activity is responsible for the phenotypic modifications.

Dynamic changes and the role of the cytoskeleton during the cell cycle in higher plant cells

In higher plant cells microtubules (MTs) show dynamic structural changes during cell cycle progression and play significant roles in cell morphogenesis. The cortical MT (CMT), preprophase band (PPB), and phragmoplast, all of which are plant-specific MT structures, can be observed during interphase, from the late G2 phase to prophase, and from anaphase to telophase, respectively. The CMT controls cell shape, either irreversibly or reversibly, by orientating cellulose microfibril (CMF) deposition in the cell wall; the PPB is involved in determining the site of division; and the phragmoplast forms the cell plate at cytokinesis. The appearance and disappearance of these MT structures during the cell cycle have been extensively studied by immunofluorescence microscopy using highly synchronized tobacco BY-2 cells. Indeed, these studies, together with visualization of MT dynamics in living plant cells using the green fluorescent protein, have revealed much about the modes of MT structural organization, for example, of CMTs at the M/G1 interphase. The microfilaments which also show dynamic changes during the cell cycle, being similar to MTs at particular stages and different at other stages, appear to play roles in supporting MTs. In this article, we summarize our ongoing research and that of related studies of the structure and function of the plant cytoskeleton during cell cycle progression.

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.

Cyclic nucleotide content of tobacco BY-2 cells

The cyclic nucleotide content of cultured tobacco bright yellow-2 (BY-2) cells was determined, after freeze-killing, perchlorate extraction and sequential chromatography, by radioimmunoassay. The identities of the putative cyclic nucleotides, adenosine 3',5'-cyclic monophosphate (cyclic AMP), guanosine 3',5'-cyclic monophosphate (cyclic GMP) and cytidine 3',5'-cyclic monophosphate (cyclic CMP) were unambiguously confirmed by tandem mass spectrometry. The potential of BY-2 cell cultures as a model system for future investigations of cyclic nucleotide function in higher plants is discussed.

Molecular characterization of plant ubiquitin-conjugating enzymes belonging to the UbcP4/E2-C/UBCx/UbcH10 gene family

The anaphase promoting complex or cyclosome is the ubiquitin-ligase that targets destruction box-containing proteins for proteolysis during the cell cycle. Anaphase promoting complex or cyclosome and its activator (the fizzy and fizzy-related) proteins work together with ubiquitin-conjugating enzymes (UBCs) (E2s). One class of E2s (called E2-C) seems specifically involved in cyclin B1 degradation. Although it has recently been shown that mammalian E2-C is regulated at the protein level during the cell cycle, not much is known concerning the expression of these genes. Arabidopsis encodes two genes belonging to the E2-C gene family (called UBC19 and UBC20). We found that UBC19 is able to complement fission yeast (Schizosaccharomyces pombe) UbcP4-140 mutant, indicating that the plant protein can functionally replace its yeast ortholog for protein degradation during mitosis. In situ hybridization experiments were performed to study the expression of the E2-C genes in various tissues of plants. Their transcripts were always, but not exclusively, found in tissues active for cell division. Thus, the UBC19/20 E2s may have a key function during cell cycle, but may also be involved in ubiquitylation reactions occurring during differentiation and/or in differentiated cells. Finally, we showed that a translational fusion protein between UBC19 and green fluorescent protein localized both in the cytosol and the nucleus in stable transformed tobacco (Nicotiana tabacum cv Bright Yellow 2) cells.

Gene expression of ascorbic acid-related enzymes in tobacco

GDP-D-mannose pyrophosphorylase (GMPase) and L-galactono-1, 4-lactone dehydrogenase (GalLDH) are key enzymes in L-ascorbic acid (AsA) biosynthesis of plants, and a full-length cDNA for GMPase was isolated from tobacco using PCR. Additionally, expression of GMPase, GalLDH and other AsA-related enzymes was examined in tobacco tissues and cultured BY-2 cells, and the relationship between their expression patterns and AsA content is discussed. It was found that the expression of GalLDH and GMPase mRNAs was markedly suppressed by loading AsA, suggesting that AsA concentration in the cells may regulate AsA biosynthesis. Moreover, the expression of GMPase and GalLDH mRNAs in tobacco leaf also suggested that AsA biosynthesis may be induced by light.

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.

Ralstonia solanacearum requires type 4 pili to adhere to multiple surfaces and for natural transformation and virulence

As reported previously for Ralstonia solanacearum strain GMI1000, wild-type strains AW1 and K60 were shown to produce Hrp pili. AW1 and K60 mutants lacking Hrp pili still exhibited twitching motility, which requires type 4 pili (Tfp), and electron microscopy revealed that they still made flexuous polar pili. Twitching-positive cells had an extracellular 17 kDa protein that was associated with piliation, and an internal 43-amino-acid sequence of this protein was typical of type 4 pilins. This amino acid sequence is encoded by an open reading frame, designated pilA, in the genomic sequence of GMI1000. PilA is 46% identical to a Pseudomonas aeruginosa type 4 pilin over its entire length and has all the conserved residues and motifs characteristic of type 4 group A pilins. pilA mutants did not make the 17 kDa PilA protein and did not exhibit twitching motility. When compared with its parent, an AW1 pilA mutant was reduced in virulence on tomato plants and in autoaggregation and biofilm formation in broth culture. Unlike AW1, a pilA mutant did not exhibit polar attachment to tobacco suspension culture cells or to tomato roots; it was also not naturally competent for transformation. We reported previously that twitching motility ceases in maturing AW1 colonies and that inactivation of PhcA, a global transcriptional regulator, results in colonies that continue to exhibit twitching motility. Similarly, in broth culture, expression of a pilA::lacZ fusion in AW1 decreased 10-fold at high cell density, but expression remained high in a phcA mutant. In addition, pilA::lacZ expression was positively regulated 10-fold by PehR, a response regulator that is known to be repressed by PhcA. This signal cascade is sufficient to explain why pilA expression, and thus twitching motility, decreases at high cell densities.

Changes in the antioxidant systems as part of the signaling pathway responsible for the programmed cell death activated by nitric oxide and reactive oxygen species in tobacco Bright-Yellow 2 cells

Nitric oxide (NO) has been postulated to be required, together with reactive oxygen species (ROS), for the activation of the hypersensitive reaction, a defense response induced in the noncompatible plant-pathogen interaction. However, its involvement in activating programmed cell death (PCD) in plant cells has been questioned. In this paper, the involvement of the cellular antioxidant metabolism in the signal transduction triggered by these bioactive molecules has been investigated. NO and ROS levels were singularly or simultaneously increased in tobacco (Nicotiana tabacum cv Bright-Yellow 2) cells by the addition to the culture medium of NO and/or ROS generators. The individual increase in NO or ROS had different effects on the studied parameters than the simultaneous increase in the two reactive species. NO generation did not cause an increase in phenylalanine ammonia-lyase (PAL) activity or induction of cellular death. It only induced minor changes in ascorbate (ASC) and glutathione (GSH) metabolisms. An increase in ROS induced oxidative stress in the cells, causing an oxidation of the ASC and GSH redox pairs; however, it had no effect on PAL activity and did not induce cell death when it was generated at low concentrations. In contrast, the simultaneous increase of NO and ROS activated a process of death with the typical cytological and biochemical features of hypersensitive PCD and a remarkable rise in PAL activity. Under the simultaneous generation of NO and ROS, the cellular antioxidant capabilities were also suppressed. The involvement of ASC and GSH as part of the transduction pathway leading to PCD is discussed.

Chemical inactivation of the cinnamate 4-hydroxylase allows for the accumulation of salicylic acid in elicited cells

The cinnamate (CA) 4-hydroxylase (C4H) is a cytochrome P450 that catalyzes the second step of the main phenylpropanoid pathway, leading to the synthesis of lignin, pigments, and many defense molecules. Salicylic acid (SA) is an essential trigger of plant disease resistance. Some plant species can synthesize SA from CA by a mechanism not yet understood. A set of specific inhibitors of the C4H, including competitive, tight-binding, mechanism-based irreversible, and quasi-irreversible inhibitors have been developed with the main objective to redirect cinnamic acid to the synthesis of SA. Competitive inhibitors such as 2-hydroxy-1-naphthoic acid and the heme-coordinating compound 3-(4-pyridyl)-acrylic acid allowed strong inhibition of C4H activity in a tobacco (Nicotiana tabacum cv Bright Yellow [BY]) cell suspension culture. This inhibition was however rapidly relieved either because of substrate accumulation or because of inhibitor metabolism. Substrate analogs bearing a methylenedioxo function such as piperonylic acid (PIP) or a terminal acetylene such as 4-propynyloxybenzoic acid (4PB), 3-propynyloxybenzoic acid, and 4-propynyloxymethylbenzoic acid are potent mechanism-based inactivators of the C4H. PIP and 4PB, the best inactivators in vitro, were also efficient inhibitors of the enzyme in BY cells. Inhibition was not reversed 46 h after cell treatment. Cotreatment of BY cells with the fungal elicitor beta-megaspermin and PIP or 4PB led to a dramatic increase in SA accumulation. PIP and 4PB do not trigger SA accumulation in nonelicited cells in which the SA biosynthetic pathway is not activated. Mechanism-based C4H inactivators, thus, are promising tools for the elucidation of the CA-derived SA biosynthetic pathway and for the potentiation of plant defense.

Two E2F sites in the Arabidopsis MCM3 promoter have different roles in cell cycle activation and meristematic expression

The commitment to DNA replication is a key step in cell division control. The Arabidopsis MCM3 homologue forms part of the mini chromosome maintenance (MCM) complex involved in the initiation of DNA replication at the transition G(1)/S. Consistent with its role at the G(1)/S transition we show that the AtMCM3 gene is transcriptionally regulated at S phase. The 5' region of this gene contains several E2F consensus binding sites, two of which match the human consensus closely and whose roles have been studied here. The identity of the two sequences as E2F binding sites has been confirmed by electrophoretic mobility shift assay analyses. Furthermore the promoter is activated by AtE2F-a and AtDP-a factors in transient expression studies. One of the E2F binding sites is shown to be responsible for the G(2)-specific repression of the promoter in synchronized cell suspension cultures. In contrast, the second E2F binding site has a role in meristem-specific expression in planta as deletion of this site eliminates the expression of a reporter gene in root and apical meristems. Thus two highly similar E2F binding sites in the promoter of the MCM3 gene are responsible for different cell cycle regulation or developmental expression patterns depending on the cellular environment.

De-etiolated 1 and damaged DNA binding protein 1 interact to regulate Arabidopsis photomorphogenesis

BACKGROUND

Plant development is exquisitely sensitive to light. Seedlings grown in the dark have a developmentally arrested etiolated phenotype, whereas in the light they develop leaves and complete their life cycle. Arabidopsis de-etiolated 1 (det1) mutants develop like light-grown seedlings even when grown in the dark. DET1 encodes a nuclear protein that appears to act downstream from multiple photoreceptors to regulate morphogenesis and gene expression in response to light. However, its function has remained unknown.

RESULTS

We used microarrays to examine defects in transcription in dark-grown det1 seedlings. We found extensive changes in gene expression, including many of the transcriptional responses observed in light-treated wild-type seedlings. We used an epitope-tagging approach to determine the basis of DET1 function. GFP-DET1 rescues the det1 phenotype, is localized to the nucleus, and forms an approximately 350 kDa complex, which is required for full DET1 activity. We affinity-purified the DET1 complex and identified an approximately 120 kDa copurifying protein that is the plant homolog of UV-Damaged DNA Binding Protein 1 (DDB1), a protein implicated in the human disease xeroderma pigmentosa. A null mutation in Arabidopsis DDB1A results in no obvious phenotype on its own, yet it enhances the phenotype of a weak det1 allele.

CONCLUSIONS

DET1 and DDB1 interact both biochemically and genetically. In animal cells, DDB1 interacts with histone acetyltransferase complexes. The DET1/DDB1 complex may regulate gene expression in response to light via recruitment of HAT activity. Thus, DET1, whose sequence is conserved in both animals and plants, may play a direct role in the regulation of many genes.

Stimulation of the cell cycle and maize transformation by disruption of the plant retinoblastoma pathway

The genome of the Mastreviruses encodes a replication-associated protein (RepA) that interacts with members of the plant retinoblastoma-related protein family, which are putative cell cycle regulators. Expression of ZmRb1, a maize retinoblastoma-related gene, and RepA inhibited and stimulated, respectively, cell division in tobacco cell cultures. The effect of RepA was mitigated by over-expression of ZmRb1. RepA increased transformation frequency and callus growth rate of high type II maize germplasm. RepA-containing transgenic maize calli remained embryogenic, were readily regenerable, and produced fertile plants that transmitted transgene expression in a Mendelian fashion. In high type II, transformation frequency increased with the strength of the promoter driving RepA expression. When a construct in which RepA was expressed behind its native LIR promoter was used, primary transformation frequencies did not improve for two elite Pioneer maize inbreds. However, when LIR:RepA-containing transgenic embryos were used in subsequent rounds of transformation, frequencies were higher in the RepA+ embryos. These data demonstrate that RepA can stimulate cell division and callus growth in culture, and improve maize transformation.

Inhibition of squalene synthase and squalene epoxidase in tobacco cells triggers an up-regulation of 3-hydroxy-3-methylglutaryl coenzyme a reductase

To get some insight into the regulatory mechanisms controlling the sterol branch of the mevalonate pathway, tobacco (Nicotiana tabacum cv Bright Yellow-2) cell suspensions were treated with squalestatin-1 and terbinafine, two specific inhibitors of squalene synthase (SQS) and squalene epoxidase, respectively. These two enzymes catalyze the first two steps involved in sterol biosynthesis. In highly dividing cells, SQS was actively expressed concomitantly with 3-hydroxy-3-methylglutaryl coenzyme A reductase and both sterol methyltransferases. At nanomolar concentrations, squalestatin was found to inhibit efficiently sterol biosynthesis as attested by the rapid decrease in SQS activity and [(14)C]radioactivity from acetate incorporated into sterols. A parallel dose-dependent accumulation of farnesol, the dephosphorylated form of the SQS substrate, was observed without affecting farnesyl diphosphate synthase steady-state mRNA levels. Treatment of tobacco cells with terbinafine is also shown to inhibit sterol synthesis. In addition, this inhibitor induced an impressive accumulation of squalene and a dose-dependent stimulation of the triacylglycerol content and synthesis, suggesting the occurrence of regulatory relationships between sterol and triacylglycerol biosynthetic pathways. We demonstrate that squalene was stored in cytosolic lipid particles, but could be redirected toward sterol synthesis if required. Inhibition of either SQS or squalene epoxidase was found to trigger a severalfold increase in enzyme activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase, giving first evidence for a positive feedback regulation of this key enzyme in response to a selective depletion of endogenous sterols. At the same time, no compensatory responses mediated by SQS were observed, in sharp contrast to the situation in mammalian cells.

Dynamic organization of vacuolar and microtubule structures during cell cycle progression in synchronized tobacco BY-2 cells

In higher plant cells, vacuoles show considerable diversity in their shapes and functions. The roles of vacuoles in the storage, osmoregulation, digestion and secretory pathway are well established; however, their functions in cell morphogenesis and cell division are still unclear. To observe the dynamic changes of vacuoles in living plant cells, we attempted to visualize the vacuolar membrane (VM) by pulse-labeling tobacco BY-2 cells with a styryl fluorescent dye, FM4-64. By time-sequence observations using confocal laser scanning microscopy (CLSM), we could follow the dynamics of vacuolar structures throughout the cell cycle in living higher plant cells. We also confirmed the dynamic changes of VM structures by the observation using transgenic BY-2 cells expressing GFP-AtVam3p fusion protein (BY-GV). Furthermore, by using transgenic BY-2 cells that stably express a GFP-tubulin fusion protein [BY-GT16, Kumagai et al. (2001) Plant Cell Physiol. 42: 723], we could study the relationship between the dynamics of vacuoles and microtubules. From these observations, we identified, for the first time, some remarkable events: (1) at the late G(2) phase, tubular structures of the vacuolar membrane developed in the central region of the cell, probably in the premitotic cytoplasmic band (phragmosome), surrounding the mitotic apparatus; (2) from anaphase to telophase, these tubular structures invaded the region of the phragmoplast within which the cell plate was being formed; (3) at the early G(1) phase, some of the tubular structures expanded rapidly between the cell plate and daughter nuclei, and subsequently developed into large vacuoles at interphase.

Arf1 GTPase plays roles in the protein traffic between the endoplasmic reticulum and the Golgi apparatus in tobacco and Arabidopsis cultured cells

Arf GTPases are known to be key regulators of vesicle budding in various steps of membrane traffic in yeast and animal cells. We cloned the Arabidopsis Arf1 homologue, AtArf1, and examined its function. AtArf1 complements yeast arf1 arf2 mutants and its GFP-fusion is localized to the Golgi apparatus in plant cells like its animal counterpart. The expression of dominant negative mutants of AtArf1 in tobacco and Arabidopsis cultured cells affected the localization of co-expressed GFP-tagged proteins in a variety of ways. AtArf1 Q71L and AtArf1 T31N, GTP- and GDP-fixed mutants, respectively, changed the localization of a cis-Golgi marker, AtErd2-GFP, from the Golgi apparatus to the endoplasmic reticulum but not that of GFP-AtRer1B or GFP-AtSed5. GFP-AtRer1B and GFP-AtSed5 were accumulated in aberrant structures of the Golgi by AtArf1 Q71L. A soluble vacuolar protein, sporamin-GFP, was also located to the ER by AtArf1 Q71L. These results indicate that AtArf1 play roles in the vesicular transport between the ER and the Golgi and in the maintenance of the normal Golgi organization in plant cells.

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.

Null mutation of AtCUL1 causes arrest in early embryogenesis in Arabidopsis

The SCF (for SKP1, Cullin/CDC53, F-box protein) ubiquitin ligase targets a number of cell cycle regulators, transcription factors, and other proteins for degradation in yeast and mammalian cells. Recent genetic studies demonstrate that plant F-box proteins are involved in auxin responses, jasmonate signaling, flower morphogenesis, photocontrol of circadian clocks, and leaf senescence, implying a large spectrum of functions for the SCF pathway in plant development. Here, we present a molecular and functional characterization of plant cullins. The Arabidopsis genome contains 11 cullin-related genes. Complementation assays revealed that AtCUL1 but not AtCUL4 can functionally complement the yeast cdc53 mutant. Arabidopsis mutants containing transfer DNA (T-DNA) insertions in the AtCUL1 gene were shown to display an arrest in early embryogenesis. Consistently, both the transcript and the protein of the AtCUL1 gene were found to accumulate in embryos. The AtCUL1 protein localized mainly in the nucleus but also weakly in the cytoplasm during interphase and colocalized with the mitotic spindle in metaphase. Our results demonstrate a critical role for the SCF ubiquitin ligase in Arabidopsis embryogenesis.

Cell-cycle dependent dynamic change of 26S proteasome distribution in tobacco BY-2 cells

The 26S proteasome is known to play pivotal roles in cell-cycle progression in various eukaryotic cells; however, little is known about its role in higher plants. Here we report that the subcellular distribution of the 26S proteasome is dynamically changed in a cell-cycle dependent manner in tobacco BY-2 cells as determined by immunostaining with anti-Rpn10 (a regulatory PA700 subunit) and anti-20S catalytic proteasome antibodies. The 26S proteasome was found to localize not only in nuclear envelopes and mitotic spindles but also in preprophase bands (PPBs) and phragmoplasts appearing in G(2) and M phases, respectively. MG132, a proteasome inhibitor, exclusively caused cell-cycle arrest not only at the metaphase but also the early stage of PPB formation at the G(2) phase and the collapse of the phragmoplast, which seems to be closely related to proteasome distribution in the cells.

S phase and meristem-specific expression of the tobacco RNR1b gene is mediated by an E2F element located in the 5' leader sequence

The RB/E2F pathway is involved in the control of the G(1)/S transition of the eukaryotic cell cycle where various S phase genes are activated by specific E2F factors. Ribonucleotide reductase (RNR) plays an essential role in the DNA synthesis pathway. Earlier studies showed that there are at least two RNR1 genes (RNR1a and RNR1b) and one RNR2 gene in tobacco. In synchronized tobacco BY2 cells, RNR1b gene expression is at its highest level in S phase. To investigate transcriptional regulation of the RNR1b gene, its promoter region was cloned and sequenced. Unlike its animal counterparts, the tobacco RNR1b promoter contains a consensus E2F-binding site. Surprisingly, this site is found in the leader sequence of the gene. We show here by gel shift analysis and antibody competition that one nuclear complex specifically binds this motif, and an E2F factor is part of this complex. Using reporter gene analysis, tobacco RNR1b promoter activity was detected during S phase in synchronized cells and in plant meristematic tissues. Mutation of the E2F element substantially reduced both activities. For the first time in plants, a single E2F motif found in the leader sequence plays an important role in the meristem and S phase-specific expression of the tobacco RNR1b gene.

Formation of protoplasts from cultured tobacco cells and Arabidopsis thaliana by the action of cellulosomes and pectate lyase from Clostridium cellulovorans

The crude culture supernatants from Clostridium cellulovorans were tested for their ability to convert plant cells to protoplasts. The supernatants readily released protoplasts from cultured tobacco cells and Arabidopsis thaliana. The crude culture supernatant from pectin-grown cells was more active than supernatants from glucose-, cellobiose-, xylan-, and locust bean gum-grown cells. After removal of cellulosomes, the crude culture supernatant lost its protoplast formation activity. The protoplast formation activity of the crude culture supernatant from C. cellulovorans was more effective than those of commercial enzymes based on protein content.

A dynamin-like protein (ADL2b), rather than FtsZ, is involved in Arabidopsis mitochondrial division

Recently, the FtsZ protein, which is known as a key component in bacterial cell division, was reported to be involved in mitochondrial division in algae. In yeast and animals, however, mitochondrial fission depends on the dynamin-like proteins Dnm1p and Drp1, respectively, whereas in green plants, no potential mitochondrial division genes have been identified. BLAST searches of the nuclear and mitochondrial genome sequences of Arabidopsis thaliana did not find any obvious homologue of the alpha-proteobacterial-type ftsZ genes. To determine whether mitochondrial division of higher plants depends on a dynamin-like protein, we cloned a cDNA for ADL2b, an Arabidopsis homologue of Dnm1p, and tested its subcellular localization and its dominant-negative effect on mitochondrial division. The fusion protein of green fluorescent protein and ADL2b was observed as punctate structures localized at the tips and at the constriction sites of mitochondria in live plant cells. Cells expressing dominant-negative mutant ADL2b proteins (K56A and T77F) showed a significant fusion, aggregation, and/or tubulation of mitochondria. We propose that mitochondrial division in higher plants is conducted by dynamin-like proteins similar to ADL2b in Arabidopsis. The evolutional points of loss of mitochondrial FtsZ and the functional acquisition of dynamin-like proteins in mitochondrial division are discussed.

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

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

IDI7, a new iron-regulated ABC transporter from barley roots, localizes to the tonoplast

A new Fe-deficiency-induced cDNA, IDI7, was isolated from the roots of Fe-deficient barley (Hordeum vulgare L. cv. Ehimehadaka no. 1). The transcript levels of IDI7 in roots strongly correlated with iron nutritional status, and induction by Fe-deficiency was restricted to roots. Excess treatment with heavy metal ions, such as copper, manganese, and zinc, did not cause obvious IDI7 induction in either leaves or roots. IDI7 encodes a 644 amino acid protein, and has features typical of ATP-binding cassette (ABC) transporters. Phylogenetic analysis revealed that IDI7 is closely related to the half-type ABC protein subfamily, which includes mammalian transporters associated with antigen processing (TAPs). A transiently expressed fusion protein of IDI7 to green fluorescent protein (GFP) was localized to tonoplasts in suspension-cultured tobacco (Nicotiana tabacum L.) cells. IDI7 and its orthologues are thought to comprise a new class of ABC transporters, located in the tonoplasts of higher plants. A possible Fe-deficiency adaptation role for IDI7 in barley root cells, involving transport across the tonoplast, is proposed.

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

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

Activation of the promoters of Arabidopsis genes for the branched-chain alpha-keto acid dehydrogenase complex in transgenic tobacco BY-2 cells under sugar starvation

Sugar starvation exerted by sub-10 mM levels of sucrose on Arabidopsis T87 suspension-cultured cells triggered marked accumulation of the transcripts of genes for E1beta and E2 subunit of the branched-chain alpha-keto acid dehydrogenase complex. Similar levels of sugar starvation increased the luciferase activity in transgenic tobacco BY-2 lines expressing the Arabidopsis E1beta- or E2-promoter-luciferase fusion gene. These results indicate that sugar levels tightly regulate the E1beta and E2 promoter activity in the heterologous plant system. We further showed in the transgenic tobacco BY-2 lines that sugar-starvation-induced activation of the E1beta and E2 promoters was prevented by K-252a, an inhibitor of Ser/Thr protein kinase, and was enhanced by okadaic acid, an inhibitor of protein phosphatases. By contrast, the cauliflower mosaic virus 35S promoter activity in sugar-starved BY-2 cells was not significantly affected by K-252a and only slightly enhanced by okadaic acid. Taken together, we propose that transcriptional activation of genes for the branched-chain alpha-keto acid dehydrogenase complex and its modulation by specific protein kinases/phosphatases are of critical importance in branched-chain amino acid catabolism in plant cells under sugar starvation.

Aluminum-induced rapid changes in the microtubular cytoskeleton of tobacco cell lines

Aluminum (Al) is a major factor that limits plant growth in acid soils. It causes a cessation of root growth and changes in root morphology suggesting a role of the root cytoskeleton as a target of Al-toxicity. Here we report a rapid effect of Al on the microtubular cytoskeleton of the suspension tobacco cell lines BY-2 and VBI-0. Viability studies showed that the cells were more sensitive to Al during exponential phase as compared to stationary cells. During the first hours of exposure, Al induced the formation of additional bundles of cortical microtubules (cMTs), whereas the thickness of the individual bundles decreased. Prolonged exposure resulted in disorientation of cMTs. These changes of cMTs preceded the decrease of cell viability by several hours and were accompanied by an increase in the levels of alpha-tubulin (in its tyrosinated form) and elements of the tubulin-folding chaperone CCT. These findings suggest that the microtubular cytoskeleton is one of the early targets of Al toxicity.

Evolution and structure of 5S rDNA loci in allotetraploid Nicotiana tabacum and its putative parental species

Nicotiana tabacum (tobacco) is an allotetraploid derived from ancestors of the modern diploids, N. sylvestris and N. tomentosiformis. We identified and characterized two distinct families of 5S ribosomal DNA (rDNA) in N. tabacum; one family had an average 431 bp unit length and the other a 646 bp unit length. In the diploid species, N. sylvestris and N. tomentosiformis, the 5S rDNA unit lengths are 431 bp and 644 bp respectively. The non-coding spacer sequence of the short unit in tobacco had high sequence homology to the spacer of N. sylvestris5S rDNA, while the longer spacer of tobacco had high homology with the 5S spacer of N. tomentosiformis. This suggests that the two 5S families in tobacco have their origin in the diploid ancestors. The longer spacer sequence had a GC rich sub-region (called the T-genome sub-region) that was absent in the short spacer. Pulsed field gel analysis and fluorescent in situ hybridization to tobacco metaphase chromosomes showed that the two families of 5S rDNA units are spatially separate at two chromosomal loci, on chromosomes S8 (short family) and T8 (long family). The repeat copy number at each chromosomal locus showed heterogeneity between different tobacco cultivars, with a tendency for a decrease in the copy number of one family to be compensated by an increase in the copy number of the second family. Sequence analysis reveals there is as much diversity in 5S family units within the diploid species as there is within the T and S-genome 5S family units respectively, suggesting 5S diversification within each family had occurred before tobacco speciation. There is no evidence of interlocus homogenization of the two 5S families in tobacco. This is therefore substantially different to 18-26S rDNA where interlocus gene conversion has substantially influenced most sequences of S and T genome origin; possible reasons are discussed.

Intracellular localization of the peanut clump virus replication complex in tobacco BY-2 protoplasts containing green fluorescent protein-labeled endoplasmic reticulum or Golgi apparatus

RNA-1 of Peanut clump virus (PCV) encodes the proteins P131 and P191, containing the signature motifs of replication proteins, and P15, which regulates viral RNA accumulation. In PCV-infected protoplasts both P131 and P191 were immunodetected in the perinuclear region. Laser scanning confocal microscopy (LSCM) showed that P131 and P191 colocalized with neosynthesized 5-bromouridine 5'-triphosphate-labeled RNA and double-stranded RNA, demonstrating that they belong to the replication complex. On the contrary, the P15 fused to the enhanced green fluorescent protein (EGFP) never colocalized with the two proteins. In endoplasmic reticulum (ER)-GFP transgenic BY-2 protoplasts, the distribution of the green fluorescent-labeled ER was strongly modified by PCV infection. LSCM showed that both P131 and P191 colocalized with ER green fluorescent bodies accumulating around the nucleus during infection. The replication process was not inhibited by cerulenin and brefeldin A, suggesting that PCV replication does not depend on de novo-synthesized membrane and does not require transport through the Golgi apparatus. Electron microscopy of ultrathin sections of infected protoplasts showed aggregates of broken ER but also visualized vesicles, some of which resembled modified peroxisomes. The results suggest that accumulation of PCV during infection is accompanied by specific association of PCV RNA-1-encoded proteins with membranes of the ER and other organelles. The concomitant extensive rearrangement of these membranous structures leads to the formation of intracellular compartments in which synthesis and accumulation of the viral RNA occur in defined areas.

Analysis of the 5S RNA pool in Arabidopsis thaliana: RNAs are heterogeneous and only two of the genomic 5S loci produce mature 5S RNA

One major 5S RNA, 120 bases long, was revealed by an analysis of mature 5S RNA from tissues, developmental stages, and polysomes in Arabidopsis thaliana. Minor 5S RNA were also found, varying from the major one by one or two base substitutions; 5S rDNA units from each 5S array of the Arabidopsis genome were isolated by PCR using CIC yeast artificial chromosomes (YACs) mapped on the different loci. By using a comparison of the 5S DNA and RNA sequences, we could show that both major and minor 5S transcripts come from only two of the genomic 5S loci: chromosome 4 and chromosome 5 major block. Other 5S loci are either not transcribed or produce rapidly degraded 5S transcripts. Analysis of the 5'- and 3'-DNA flanking sequence has permitted the definition of specific signatures for each 5S rDNA array.

The possible involvement of a phosphate-induced transcription factor encoded by phi-2 gene from tobacco in ABA-signaling pathways

A novel phosphate-induced gene, phi-2, has been identified by its induction on addition of phosphate to phosphate-starved tobacco BY-2 cells. The predicted gene product of phi-2 has significant homology to a group of bZIP proteins involved in ABA-signaling pathways, and phi-2 also responded to ABA treatment. A previously isolated phosphate-induced gene, phi-1, (Sano et al. (1999) Plant Cell Physiol. 40: 1) was also responsive to ABA. Although phosphate addition induced semi-synchronous cell division in phosphate-starved tobacco BY-2 cells, ABA adversely affected cell division. Detailed examination revealed that the high levels of phosphate required to induce semi-synchronous cell division seemed to be perceived as indicators of stress by the cells. One of the stress indicators perceived by the cells is a cytoplasmic pH change, to which phi-2 and phi-1 genes respond. The different components of the cell's response to phosphate induction are discussed.

Differential effect of jasmonic acid and abscisic acid on cell cycle progression in tobacco BY-2 cells

Environmental stress affects plant growth and development. Several plant hormones, such as salicylic acid, abscisic acid (ABA), jasmonic acid (JA), and ethylene play a crucial role in altering plant morphology in response to stress. Developmental regulation often has the cell cycle machinery among its targets. We analyzed the effect of JA and ABA on cell cycle progression in synchronized tobacco (Nicotiana tabacum) BY-2 cells. Both compounds were found to prevent DNA replication, keeping the cells in the G1 stage, when applied just before the G1/S transition. However, ABA did not have any effect on subsequent phases of the cell cycle when applied at a later stage, whereas JA effectively prevented mitosis on application during DNA synthesis. This demonstrates that JA treatment can freeze synchronized BY-2 cells in both the G1 and G2 stages of the cell cycle. Jasmonate administered after the S-phase was less effective in decreasing the mitotic index, suggesting that cell sensitivity toward JA is dependent on the cell cycle phase. In cultures detained in the G2-phase, we observed a reduced histone H1 kinase activity of kinases associated with the p13(suc1) protein.

Suppression of gene expression by RNA interference in cultured plant cells

Suppression by double-stranded RNA (dsRNA) of the expression of a target gene is known as RNA interference (RNAi). No quantitative analysis of the effects of RNAi on the expression of specific genes in cultured plant cells has been reported. However, as it is possible to produce populations of cultured plant cells that are uniform and divide synchronously for functional analysis of genes of interest, we performed a quantitative study of the effects of RNAi in such cells. We constructed dsRNA expression plasmids for a luciferase gene under the control of the cauliflower mosaic virus (CaMV) 35S promoter by simply connecting sense and antisense sequences in a head-to-head manner. An RNAi effect was observed 24 hours after the introduction of dsRNA expression plasmids into tobacco BY-2 cells by electroporation. The simple system for suppression of specific genes in plant cells should be useful in attempts to elucidate the roles of individual genes in plant cells.

The role of auxin-binding protein 1 in the expansion of tobacco leaf cells

Tobacco leaf was used to investigate the mechanism of action of auxin-binding protein 1 (ABP1). The distributions of free auxin, ABP1, percentage of leaf nuclei in G2 and the amount of auxin-inducible growth were each determined in control tobacco leaves and leaves over-expressing Arabidopsis ABP1. These parameters were compared with growth of tobacco leaves, measured both spatially and temporally throughout the entire expansion phase. Within a defined window of leaf development, juvenile leaf cells that inducibly expressed Arabidopsis ABP1 prematurely advanced nuclei to the G2 phase. The ABP1-induced increase in cell expansion occured before the advance to the G2 phase, indicating that the ABP1-induced G2 phase advance is an indirect effect of cell expansion. The level of ABP1 was highest at the position of maximum cell expansion, maximum auxin-inducible growth and where the free auxin level was the lowest. In contrast, the position of maximum cell division correlated with higher auxin levels and lower ABP1 levels. Consistent with the correlations observed in leaves, tobacco cells (BY-2) in culture displayed two dose-dependent responses to auxin. At a low auxin concentration, cells expanded, while at a relatively higher concentration, cells divided and incorporated [3H]-thymidine. Antisense suppression of ABP1 in these cells dramatically reduced cell expansion with negligible effect on cell division. Taken together, the data suggest that ABP1 acts at a relatively low level of auxin to mediate cell expansion, whereas high auxin levels stimulate cell division via an unidentified receptor.

Characterization of a family of Arabidopsis genes related to xyloglucan fucosyltransferase1

To understand primary cell wall assembly in Arabidopsis, we have focused on identifying and characterizing enzymes involved in xyloglucan biosynthesis. Nine genes (AtFUT2-10) were identified that share between 47% and 62% amino acid similarity with the xyloglucan-specific fucosyltransferase AtFUT1. Reverse transcriptase-PCR analysis indicates that all these genes are expressed. Bioinformatic analysis predicts that these family members are fucosyltransferases, and we first hypothesized that some may also be involved in xyloglucan biosynthesis. AtFUT3, AtFUT4, and AtFUT5 were expressed in tobacco (Nicotiana tabacum L. cv BY2) suspension culture cells, and the resulting proteins did not transfer fucose (Fuc) from GDP-Fuc to tamarind xyloglucan. AtFUT3, AtFUT4, and AtFUT5 were overexpressed in Arabidopsis plants. Leaves of plants overexpressing AtFUT4 or AtFUT5 contained more Fuc than wild-type plants. Stems of plants overexpressing AtFUT4 or AtFUT5 contained more xylose, less arabinose, and less galactose than wild-type plants. We suggest that the AtFUT family is likely to include fucosyltransferases important for the synthesis of wall carbohydrates. A targeted analysis of isolated cell wall matrix components from plants altered in expression of these proteins will help determine their specificity and biological function.

Signal transduction in maize and Arabidopsis mesophyll protoplasts

Plant protoplasts show physiological perceptions and responses to hormones, metabolites, environmental cues, and pathogen-derived elicitors, similar to cell-autonomous responses in intact tissues and plants. The development of defined protoplast transient expression systems for high-throughput screening and systematic characterization of gene functions has greatly contributed to elucidating plant signal transduction pathways, in combination with genetic, genomic, and transgenic approaches.

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.

Cyclic AMP affinity purification and ESI-QTOF MS-MS identification of cytosolic glyceraldehyde 3-phosphate dehydrogenase and two nucleoside diphosphate kinase isoforms from tobacco BY-2 cells

The soluble protein fraction of tobacco bright yellow 2 cells contained adenosine 3',5'-cyclic monophosphate (cAMP)-binding activity, detected with both a conventional binding assay and a surface plasmon resonance biosensor. A cAMP-agarose-based affinity purification procedure yielded three proteins which were identified by mass spectrometry as glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and two nucleoside diphosphate kinases (NDPKs). This is the first report describing an interaction between cAMP and these proteins in higher plants. Our findings are discussed in view of the reported role of the interaction of cAMP with GAPDH and NDPK in animals and yeast. In addition, we provide a rapid method to isolate both proteins from higher plants.

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.

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

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

N-terminal domains of plant poly(ADP-ribose) polymerases define their association with mitotic chromosomes

Poly(ADP-ribos)ylation is a reversible protein modification that in higher plants is catalyzed by two structurally different poly(ADP-ribose) polymerases, App and Zap. In vivo imaging of green-fluorescent protein (GPF) fusions showed that both Zap and App were associated with chromatin through the cell cycle progression. The in vivo behaviour of the App-GFP protein fusions can be attributed to the activity of two NASA motifs that mediate protein-protein interactions and nucleic acid binding. Expression of Zap deletion variants revealed that both Zn fingers and helix-turn-helix domains contributed to the association with chromosomes, whereas the localization in the nucleoplasm was mostly determined by the Zn fingers. The results highlight novel properties of protein sequences found in plant poly(ADP-ribose) polymerases and suggest important functions for this class of nuclear enzymes in chromosome dynamics.

Linear DNA intermediates of the Tto1 retrotransposon in Gag particles accumulated in stressed tobacco and Arabidopsis thaliana

The active transcription of some plant retrotransposons under diverse stress conditions suggests active transposition. However, transposition has been demonstrated only during tissue/cell culture. To examine whether transposition is activated under conditions other than tissue/cell culture, DNA intermediates for retrotransposition of the tobacco retrotransposon Tto1 were analysed. Using transgenic Arabidopsis callus expressing high levels of Tto1 RNA in a ddm1 hypomethylation mutant background, the existence of extrachromosomal Tto1 linear DNA molecules in a Gag-particle fraction was demonstrated. By combination with ligation-mediated PCR amplification, we detected Tto1 linear DNA molecules in particle fractions from callus and methyl jasmonate-treated leaves of tobacco, but not from non-stressed leaves. Tto1 DNA intermediates could not be detected in the tobacco corolla where Tto1 is expressed. These results indicate that the transcriptional activation of Tto1 by defence-related stresses leads to the synthesis of DNA intermediates, whereas post-transcriptional suppression of Tto1 activity is suggested in the corolla.

A plant-specific cyclin-dependent kinase is involved in the control of G2/M progression in plants

Cyclin-dependent kinases (CDKs) control the key transitions in the eukaryotic cell cycle. All the CDKs known to control G(2)/M progression in yeast and animals are distinguished by the characteristic PSTAIRE motif in their cyclin-binding domain and are closely related. Higher plants contain in addition a number of more divergent non-PSTAIRE CDKs with still obscure functions. We show that a plant-specific type of non-PSTAIRE CDKs is involved in the control of the G(2)/M progression. In synchronized tobacco BY-2 cells, the corresponding protein, accumulated in a cell cycle-regulated fashion, peaking at the G(2)/M transition. The associated histone H1 kinase activity reached a maximum in mitosis and required a yet unidentified subunit to be fully active. Down-regulation of the associated kinase activity in transgenic tobacco plants using a dominant-negative mutation delayed G(2)/M transition. These results provide the first evidence that non-PSTAIRE CDKs are involved in the control of the G(2)/M progression in plants.

Plant gene expression response to Agrobacterium tumefaciens

To elucidate the nature of plant response to infection and transformation by Agrobacterium tumefaciens, we compared the cDNA-amplified fragment length polymorphism (AFLP) pattern of Agrobacterium- and mock-inoculated Ageratum conyzoides plant cell cultures. From 16,000 cDNA fragments analyzed, 251 (1.6%) were differentially regulated (0.5% down-regulated) 48 h after cocultivation with Agrobacterium. From 75 strongly regulated fragments, 56 were already regulated 24 h after cocultivation. Sequence similarities were obtained for 20 of these fragments, and reverse transcription-PCR analysis was carried out with seven to confirm their cDNA-AFLP differential pattern. Their sequence similarities suggest a role for these genes in signal perception, transduction, and plant defense. Reverse transcription-PCR analysis indicated that four genes involved in defense response are regulated in a similar manner by nonpathogenic bacteria, whereas one gene putatively involved in signal transduction appeared to respond more strongly to Agrobacterium. A nodulin-like gene was regulated only by Agrobacterium. These results demonstrate a rapid plant cell response to Agrobacterium infection, which overlaps a general response to bacteria but also has Agrobacterium-specific features.

Srchi24, a chitinase homolog lacking an essential glutamic acid residue for hydrolytic activity, is induced during nodule development on Sesbania rostrata

The interaction between the tropical legume Sesbania rostrata and the bacterium Azorhizobium caulinodans results in the formation of nodules on both stem and roots. Stem nodulation was used as a model system to isolate early markers by differential display. One of them, Srchi24 is a novel early nodulin whose transcript level increased already 4 h after inoculation. This enhancement depended on Nod factor-producing bacteria. Srchi24 transcript levels were induced also by exogenous cytokinins. In situ hybridization and immunolocalization experiments showed that Srchi24 transcripts and proteins were present in the outermost cortical cell layers of the developing nodules. Sequence analyses revealed that Srchi24 is similar to class III chitinases, but lacks an important catalytic glutamate residue. A fusion between a maltose-binding protein and Srchi24 had no detectable hydrolytic activity. A function in nodulation is proposed for the Srchi24 protein.

Cell cycle regulation of cyclin-dependent kinases in tobacco cultivar Bright Yellow-2 cells

Plants possess two major classes of cyclin-dependent kinases (CDK) with cyclin-binding motifs PSTAIRE (CDK-a) and PPTA/TLRE (CDK-b). Tobacco (Nicotiana tabacum L. cv Bright Yellow-2) cells are the most highly synchronizable plant culture, but no detailed analysis of CDK activities has been reported in this system. Here we describe isolation of new PPTALRE CDKs (Nicta;CdkB1) from Bright Yellow-2 cells and present detailed analysis of the mRNA, protein and kinase activity levels of CdkB1, and the PSTAIRE CDKA during the growth and cell cycles. CdkA and CdkB1 transcripts are more abundant in exponential than in stationary phase cells, but the two genes show strikingly different regulation during the cell cycle. CdkA mRNA and protein accumulate during G1 in cells re-entering the cell cycle, and immunoprecipitated histone H1 kinase activity increases at the G1/S boundary. Aphidicolin synchronized cells show the highest CDKA-associated histone H1 kinase activity during S-G2 phases, although CdkA mRNA and protein levels are not significantly regulated. In contrast, CdkB1 transcripts are present at very low levels until S phase and CDKB1 protein and kinase activity is almost undetectable in G1. CdkB1 mRNA accumulates through S until M phase and its associated kinase activity peaks at the G2/M boundary, confirming that transcription of PPTALRE CDKs is cell cycle regulated. We suggest that CDKA kinase activity likely plays roles at the G1/S phase boundary, during S phase, and at the G2/M phase transition, and that CDKB1 kinase activity is present only at G2/M.