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

Production of reactive oxygen species, alteration of cytosolic ascorbate peroxidase, and impairment of mitochondrial metabolism are early events in heat shock-induced programmed cell death in tobacco Bright-Yellow 2 cells

To gain some insight into the mechanisms by which plant cells die as a result of abiotic stress, we exposed tobacco (Nicotiana tabacum) Bright-Yellow 2 cells to heat shock and investigated cell survival as a function of time after heat shock induction. Heat treatment at 55 degrees C triggered processes leading to programmed cell death (PCD) that was complete after 72 h. In the early phase, cells undergoing PCD showed an immediate burst in hydrogen peroxide (H2O2) and superoxide (O2*-) anion production. Consistently, death was prevented by the antioxidants ascorbate (ASC) and superoxide dismutase (SOD). Actinomycin D and cycloheximide, inhibitors of transcription and translation, respectively, also prevented cell death, but with a lower efficiency. Induction of PCD resulted in gradual oxidation of endogenous ASC; this was accompanied by a decrease in both the amount and the specific activity of the cytosolic ASC peroxidase (cAPX). A reduction in cAPX gene expression was also found in the late PCD phase. Moreover, changes of cAPX kinetic properties were found in PCD cells. Production of ROS in PCD cells was accompanied by early inhibition of glucose (Glc) oxidation, with a strong impairment of mitochondrial function as shown by an increase in cellular NAD(P)H fluorescence, and by failure of mitochondria isolated from cells undergoing PCD to generate membrane potential and to oxidize succinate in a manner controlled by ADP. Thus, we propose that in the early phase of tobacco Bright-Yellow 2 cell PCD, ROS production occurs, perhaps because of damage of the cell antioxidant system, with impairment of the mitochondrial oxidative phosphorylation.

Characterization of CXIP4, a novel Arabidopsis protein that activates the H+/Ca2+ antiporter, CAX1

Precise regulation of calcium transporters is essential for modulating the Ca2+ signaling network that is involved in the growth and adaptation of all organisms. The Arabidopsis H+/Ca2+ antiporter, CAX1, is a high capacity and low affinity Ca2+ transporter and several CAX1-like transporters are found in Arabidopsis. When heterologously expressed in yeast, CAX1 is unable to suppress the Ca2+ hypersensitivity of yeast vacuolar Ca2+ transporter mutants due to an N-terminal autoinhibition mechanism that prevents Ca2+ transport. Using a yeast screen, we have identified CAX nteracting Protein 4 (CXIP4) that activated full-length CAX1, but not full-length CAX2, CAX3 or CAX4. CXIP4 encodes a novel plant protein with no bacterial, fungal, animal, or mammalian homologs. Expression of a GFP-CXIP4 fusion in yeast and plant cells suggests that CXIP4 is targeted predominantly to the nucleus. Using a yeast growth assay, CXIP4 activated a chimeric CAX construct that contained specific portions of the N-terminus of CAX1. Together with other recent studies, these results suggest that CAX1 is regulated by several signaling molecules that converge on the N-terminus of CAX1 to regulate H+/Ca2+ antiport.

Replication of plant RNA virus genomes in a cell-free extract of evacuolated plant protoplasts

The replication of eukaryotic positive-strand RNA virus genomes occurs through a complex process involving multiple viral and host proteins and intracellular membranes. Here we report a cell-free system that reproduces this process in vitro. This system uses a membrane-containing extract of uninfected plant protoplasts from which the vacuoles had been removed by Percoll gradient centrifugation. We demonstrate that the system supported translation, negative-strand RNA synthesis, genomic RNA replication, and subgenomic RNA transcription of tomato mosaic virus and two other plant positive-strand RNA viruses. The RNA synthesis, which depended on translation of the genomic RNA, produced virus-related RNA species similar to those that are generated in vivo. This system will aid in the elucidation of the mechanisms of genome replication in these viruses.

Identification and Molecular Characterization of the β-Ketoacyl-[Acyl Carrier Protein] Synthase Component of the Arabidopsis Mitochondrial Fatty Acid Synthase

Substrate specificity of condensing enzymes is a predominant factor determining the nature of fatty acyl chains synthesized by type II fatty acid synthase (FAS) enzyme complexes composed of discrete enzymes. The gene (mtKAS) encoding the condensing enzyme, beta-ketoacyl-[acyl carrier protein] (ACP) synthase (KAS), constituent of the mitochondrial FAS was cloned from Arabidopsis thaliana, and its product was purified and characterized. The mtKAS cDNA complemented the KAS II defect in the E. coli CY244 strain mutated in both fabB and fabF encoding KAS I and KAS II, respectively, demonstrating its ability to catalyze the condensation reaction in fatty acid synthesis. In vitro assays using extracts of CY244 containing all E. coli FAS components, except that KAS I and II were replaced by mtKAS, gave C(4)-C(18) fatty acids exhibiting a bimodal distribution with peaks at C(8) and C(14)-C(16). Previously observed bimodal distributions obtained using mitochondrial extracts appear attributable to the mtKAS enzyme in the extracts. Although the mtKAS sequence is most similar to that of bacterial KAS IIs, sensitivity of mtKAS to the antibiotic cerulenin resembles that of E. coli KAS I. In the first or priming condensation reaction of de novo fatty acid synthesis, purified His-tagged mtKAS efficiently utilized malonyl-ACP, but not acetyl-CoA as primer substrate. Intracellular targeting using green fluorescent protein, Western blot, and deletion analyses identified an N-terminal signal conveying mtKAS into mitochondria. Thus, mtKAS with its broad chain length specificity accomplishes all condensation steps in mitochondrial fatty acid synthesis, whereas in plastids three KAS enzymes are required.

Arabidopsis dynamin-like protein 2a (ADL2a), like ADL2b, is involved in plant mitochondrial division

The Arabidopsis genome has two similar dynamin-like proteins, ADL2a and ADL2b (76.7% identity). ADL2a is reported to be localized in chloroplasts [Kang et al. (1998) Plant Mol. Biol. 38: 437], while ADL2b functions in mitochondrial division [Arimura and Tsutsumi (2002) PROC: Natl. Acad. Sci. USA 99: 5727]. Using GFP fusion proteins, we observed both ADL2a and ADL2b in portions of mitochondria but not in chloroplasts. Furthermore, cells transformed with ADL2a and ADL2b with a defective GTPase domain had normal chloroplasts but elongated mitochondria. These results imply that both ADL2b and ADL2a are involved in the division of plant mitochondria.

Cytological changes and alterations in polyamine contents induced by cadmium in tobacco BY-2 cells

Changes in cell viability, proliferation, cell and nuclear morphology including nuclear and DNA fragmentation induced by 0.05 and 1 mM CdSO4 (Cd2+) in tobacco BY-2 cell line (Nicotiana tabacum L.) were studied in the course of 7 days. Simultaneously changes in endogenous contents of both free and conjugated forms of polyamines (PAs) were investigated for 3 days. The application of 0.05 mM Cd2+ evoked decline of cell viability to approximately 60% during the first 24 h of treatment. Later on degradation of cytoplasmic strands, formation of the stress granules and vesicles, modifications in size and shape of the nuclei, including their fragmentation, were observed in the surviving cells. Their proliferation was blocked and cells elongated. Beginning the first day of treatment TUNEL-positive nuclei were detected in cells cultivated in medium containing 0.05 mM Cd2+. Treatment with highly toxic 1 mM Cd2+ induced fast decrease of cell viability (no viable cells remained after 6-h treatment) and cell death occurred before DNA cleavage might be initiated. The exposure of tobacco BY-2 cells to 0.05 mM Cd2+ resulted in a marked accumulation of total PAs (represented by the sum of free PAs and their perchloric acid (PCA)-soluble and PCA-insoluble conjugates) during 3-day treatment. The increase in total PA contents was primarily caused by the increase in putrescine (Put) concentration. The accumulation of free spermidine (Spd) and spermine (Spm) at 12 and 24 h in 0.05 mM Cd2+ treated BY-2 cells and high contents of Spd and especially Spm determined in dead cells after I mM Cd2+ application was observed. The participation of PA conjugation with hydroxycinnamic acids and PA oxidative deamination in maintaining of free PA levels in BY-2 cells under Cd2+-induced oxidative stress is discussed.

Cryptogein-induced initial events in tobacco BY-2 cells: pharmacological characterization of molecular relationship among cytosolic Ca(2+) transients, anion efflux and production of reactive oxygen species

Ion fluxes and the production of reactive oxygen species (ROS) are early events that follow elicitor treatment or microbial infection. However, molecular mechanisms for these responses as well as their relationship have been controversial and still largely unknown. We here simultaneously monitored the temporal sequence of initial events at the plasma membrane in suspension-cultured tobacco cells (cell line BY-2) in response to a purified proteinaceous elicitor, cryptogein, which induced hypersensitive cell death. The elicitor induced transient rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) showing two distinct peaks, followed by biphasic (rapid/transient and slow/prolonged) Cl(-) efflux and H(+) influx. Pharmacological analyses suggested that the two phases of the [Ca(2+)](cyt) response correspond to Ca(2+) influx through the plasma membrane and an inositol 1,4,5-trisphophate-mediated release of Ca(2+) from intracellular Ca(2+) stores, respectively, and the [Ca(2+)](cyt) transients and the Cl(-) efflux were mutually dependent events regulated by protein phosphorylation. The elicitor also induced production of ROS including (*)O(2)(-) and H(2)O(2), which initiated after the [Ca(2+)](cyt) rise and required Ca(2+) influx, Cl(-) efflux and protein phosphorylation. An inhibitor of NADPH oxidase, diphenylene iodonium, completely inhibited the elicitor-induced production of (*)O(2)(-) and H(2)O(2), but did not affect the [Ca(2+)](cyt) transients. These results suggest that cryptogein-induced plasma membrane Ca(2+) influx is independent of ROS, and NADPH oxidase dependent ROS production is regulated by these series of ion fluxes.

Cryopreservation of transformed and wild-type Arabidopsis and tobacco cell suspension cultures

We have recently described Arabidopsis cell suspension cultures that can be effectively synchronised. Here, we describe procedures that allow clonal-transformed cell suspension lines to be produced using Agrobacterium-mediated transformation, and an optimised and straightforward procedure for the cryopreservation and recovery of both parental and transformed lines. Frozen cultures show 90% viability and rapid re-growth after recovery. We show that the cryopreservation procedure is equally applicable to the frequently used tobacco bright yellow (BY)2 cell suspension culture, and that cell cycle synchronisation capacity of parental lines is maintained after both transformation and recovery from cryopreservation. The techniques require no specialised equipment, and are suitable for routine laboratory use, greatly facilitating the handling and maintenance of cell cultures and providing security against both contamination and cumulative somaclonal variation. Finally, the ability to store easily large numbers of transformed lines opens the possibility of using Arabidopsis cell suspension cultures for high-throughput analysis.

Overexpression of NtHAL3 genes confers increased levels of proline biosynthesis and the enhancement of salt tolerance in cultured tobacco cells

The Hal3 protein of Saccharomyces cerevisiae inhibits the activity of PPZ1 type-1 protein phosphatases and functions as a regulator of salt tolerance and cell cycle control. In plants, two HAL3 homologue genes in Arabidopsis thaliana, AtHAL3a and AtHAl3b, have been isolated and the function of AtHAL3a has been investigated through the use of transgenic plants. Expressions of both AtHAL3 genes are induced by salt stress. AtHAL3a overexpressing transgenic plants exhibit improved salt and sorbitol tolerance. In vitro studies have demonstrated that AtHAL3 protein possessed 4'-phosphopantothenoylcysteine decarboxylase activity. This result suggests that the molecular function of plant HAL3 genes is different from that of yeast HAL3. To understand the function of plant HAL3 genes in salt tolerance more clearly, three tobacco HAL3 genes, NtHAL3a, NtHAL3b, and NtHAL3c, from Nicotiana tabacum were identified. NtHAL3 genes were constitutively expressed in all organs and under all conditions of stress examined. Overexpression of NtHAL3a improved salt, osmotic, and lithium tolerance in cultured tobacco cells. NtHAL3 genes could complement the temperature-sensitive mutation in the E. coli dfp gene encoding 4'-phosphopantothenoyl-cysteine decarboxylase in the coenzyme A biosynthetic pathway. Cells overexpressing NtHAL3a had an increased intracellular ratio of proline. Taken together, these results suggest that NtHAL3 proteins are involved in the coenzyme A biosynthetic pathway in tobacco cells.

Patterns of fructan synthesized by onion fructan : fructan 6G-fructosyltransferase expressed in tobacco BY2 cells - is fructan : fructan 1-fructosyltransferase needed in onion?

•  Fructan : fructan 6G-fructosyltransferase (6G-FFT) has been proposed to be the enzyme essential for the production of neo-series inulin. Transfer of a fructose unit from short chain inulins to the C6 of the glucose residue of sucrose or inulin was proposed to be its most important characteristic. Here, we investigate the activity of 6G-FFT from onion (Allium cepa) more thoroughly. •  Tobacco BY2 suspension cultures were employed as an expression system for the fructosyltransferase 6G-FFT. Activity was measured using 1-kestose as a substrate and products were detected using high-performance anion exchange chromatography (HPAEC). •  6G-FFT showed multiple activities. An array of fructans of the inulin series and inulin neo-series were produced. First 1,1-kestotetraose and 1 and 6G-kestotetraose were synthesized, as well as 6G-kestotriose. Prolonged incubation produces a complex fructan series with a higher degree of polymerization. •  The fructan pattern observed after incubation of onion 6G-FFT with 1-kestose closely resembles the complex fructan pattern found in onion. These results questions the need for a separate fructan : fructan 1-fructosyltransferase (1-FFT) activity in onion.

Three-dimensional reconstruction of tubular structure of vacuolar membrane throughout mitosis in living tobacco cells

Plant vacuoles are the largest of organelles, performing various functions in cellular metabolism, morphogenesis and cell division. Dynamic changes in vacuoles during mitosis were studied by monitoring tubular structure of vacuolar membrane (TVM) in living transgenic tobacco BY-2 cells stably expressing a GFP-AtVam3p fusion protein (BY-GV). Comprehensive images of the complicated TVM configurations were obtained by reconstructing three-dimensional (3-D) surface structures from sequential confocal sections, using newly developed software, SSR (stereo-structure reconstructor). Using the surface modeling technique, we succeeded for the first time in clarifying the development process of TVMs and the topological relationship between TVMs and large vacuoles. TVMs, initially organized from large vacuoles, elongated to encircle the spindle at metaphase. Subsequently, the TVMs invaded the equatorial region from anaphase to telophase, and then they were divided to the two daughter cells by the cell plate at cytokinesis. When the daughter nuclei were separating from the cell plate, some TVMs enlarged to form large vacuoles near the division site. Spatial analysis revealed that from anaphase until cytokinesis, TVMs connected the two large vacuoles and functioned as a route for inter-vacuolar transport. Furthermore, the experiments using the inhibitor for actin microfilaments indicated that the microfilaments were indispensable for the development and the maintenance of TVMs.

Mapping of viral RNA sequences required for assembly of peanut clump virus particles

RNA sequences required for assembly into rod-shaped virions of RNA-1 and RNA-2 of Peanut clump virus (PCV) were mapped by testing the ability of different RNA-1 and -2 deletion mutants to be encapsidated in vivo in an RNase-resistant form. Encapsidation of RNA-1 was found to require a sequence domain in the 5'-proximal part of the P15 gene, the 3'-proximal gene of RNA-1. On the other hand, the subgenomic RNA which encodes P15 was not encapsidated, suggesting that other features of RNA-1 are important as well. Two sequences which could drive encapsidation of RNA-2 deletion mutants were located. One was in the 5'-proximal coat protein gene and the other in the P14 gene near the RNA 3' terminus. There were no obvious sequence homologies between the different assembly initiation sequences.

Dehydroascorbate uptake activity correlates with cell growth and cell division of tobacco bright yellow-2 cell cultures

Recently, ascorbate (ASC) concentration and the activity of a number of enzymes from the ASC metabolism have been proven to correlate with differences in growth or cell cycle progression. Here, a possible correlation between growth and the activity of a plasma membrane dehydroascorbate (DHA) transporter was investigated. Protoplasts were isolated from a tobacco (Nicotiana tabacum) Bright Yellow-2 cell culture at different intervals after inoculation and the activity of DHA transport was tested with (14)C-labeled ASC. Ferricyanide (1 mM) or dithiothreitol (1 mM) was included in the test to keep the external (14)C-ASC in its oxidized respectively reduced form. Differential uptake activity was observed, correlating with growth phases of the cell culture. Uptake of DHA in cells showed a peak in exponential growth phase, whereas uptake in the presence of dithiothreitol did not. The enhanced DHA uptake was not due to higher endogenous ASC levels that are normally present in exponential phase because preloading of protoplasts of different ages did not affect DHA uptake. Preloading was achieved by incubating cells before protoplastation for 4 h in a medium supplemented with 1 mM DHA. In addition to testing cells at different growth phases, uptake of DHA into the cells was also followed during the cell cycle. An increase in uptake activity was observed during M phase and the M/G1 transition. These experiments are the first to show that DHA transport activity into plant cells differs with cell growth. The relevance of the data to the action of DHA and ASC in cell growth will be discussed.

Origin of cortical microtubules organized at M/G1 interface: recruitment of tubulin from phragmoplast to nascent microtubules

The origin of cortical microtubules (CMTs) was investigated in transgenic BY-2 cells stably expressing a GFP (green fluorescent protein) -tubulin fusion protein (BY-GT16). In a previous study, we found that CMTs were initially organized in the perinuclear regions but then elongated to reach the cell cortex where they formed bright spots, and that the appearance of parallel MTs from the bright spots was followed by the appearance of transverse MTs (Kumagai et al., Plant Cell Physiol. 42, 723-732, 2001). In this study, we investigated the migration of tubulin to the reorganization sites of CMTs at the M/G1 interface. After synchronization of the BY-GT16 cells by aphidicolin, the localization of GFP-tubulin was monitored and analyzed by deconvolution microscopy. GFP-tubulin was found to accumulate on the nuclear surface near the cell plate at the final stage of phragmoplast collapse. Subsequently, GFP-tubulin accumulated again on the nuclear surface opposite the cell plate, where nascent MTs elongated to the cell cortex. The significance of these observations on the mode of CMT organization is discussed.

Short interfering RNA-mediated interference of gene expression and viral DNA accumulation in cultured plant cells

Gene silencing mediated by double-stranded RNA is a sequence-specific RNA degradation mechanism highly conserved in eukaryotes that serves as an antiviral defense pathway in both plants and Drosophila. Short interfering RNAs (siRNAs), the 21- to 23-nt double-stranded intermediates of this natural defense mechanism, are becoming powerful tools for reducing gene expression and countering viral infection in a variety of mammalian cells. Here we report the use of siRNAs to target reporter gene expression and viral DNA accumulation in cultured plant cells. Transient expression of reporter genes encoding either GFP or red fluorescent protein from Discosoma was specifically reduced by 58% and 47%, respectively, at 24 h after codelivery of cognate siRNAs in BY2 protoplasts. In contrast to mammalian systems, the siRNA-induced silencing of GFP expression was transitive as indicated by the presence of siRNAs representing parts of the target RNA outside the region homologous to the triggering siRNA. Codelivery of an siRNA designed to target the mRNA encoding the replication-associated protein (AC1) of the geminivirus African cassava mosaic virus (ACMV) from Cameroon blocked AC1 mRNA accumulation by approximately 91% and inhibited accumulation of the ACMV genomic DNA by approximately 66% at 36 and 48 h after transfection. As with siRNA-induced reporter gene silencing, the siRNA targeting ACMV AC1 was specific and did not affect the replication of East African cassava mosaic Cameroon virus. This report demonstrates the occurrence of siRNA-mediated suppression of gene expression in cultured plant cells and that siRNA can interfere with and suppress accumulation of a nuclear-replicated DNA virus.

LET dependence of lethality of carbon ion irradiation to single tobacco cells

PURPOSE

To determine the radiation sensitivity and relationship between linear energy transfer (LET) and relative biological effectiveness (RBE) in single plant cells irradiated with heavy ions.

MATERIALS AND METHODS

Single cells were isolated from the tobacco BY-2 cell line and irradiated with carbon ions (78.6-309 keV microm(-1)) and gamma-rays (0.2 keV microm(-1)). Two weeks after irradiation, colonies with 16 cells or more derived from the irradiated cells were counted as survivors. The surviving fraction was fitted using the single-hit, multitarget theory.

RESULTS

The doses needed to reduce the surviving fraction of the cells to 0.1 (D10) of gamma-rays and carbon ions were 47.2 and 10.5-12.6 Gy, respectively. The RBE based on the D10 peaked at an LET of 247 keV microm(-1). The inactivation cross-section of carbon ions reached a plateau of 11.3 microm2 at an LET of 247 keV microm(-1).

CONCLUSIONS

The radiation sensitivity of single tobacco cells was much lower than that of mammalian cells, although the mean number of base pairs per chromosome in the two cell types was similar. The RBE peak based on the D10 of carbon ions in single tobacco cells occurred at a higher LET than it does in other organisms.

Exopolysaccharides produced by plant pathogenic bacteria affect ascorbate metabolism in Nicotiana tabacum

The role of the exopolysaccharides (EPSs) produced by plant pathogenic bacteria has not completely clarified, they are considered either molecules able to avoid or delay the activation of plant defences, or acting as signal in the plant-pathogen cross-talk. In order to understand whether EPSs are recognized by infected plant cells and are able to induce the activation of plant defence responses, their capability to induce metabolic alteration in tobacco cells has been analysed. The results indicate that several EPSs, even if not chemically related, induce increases in phenylalanine ammonia-lyase, a marker enzyme of defence responses of plants against stress; but others are completely ineffective. The EPSs affecting phenylalanine ammonia-lyase also induce an increase in hydrogen peroxide production. Moreover, they alter the metabolism of ascorbate, another parameter indicative of the presence of stress conditions and the involvement of which in the hypersensitive reaction has been recently reported. The possibility that specific EPSs could act as signals in the plant-pathogenic bacteria interaction is discussed.

Thermotolerant guard cell protoplasts of tree tobacco do not require exogenous hormones to survive in culture and are blocked from reentering the cell cycle at the G1-to-S transition

When guard cell protoplasts (GCPs) of tree tobacco [Nicotiana glauca (Graham)] are cultured at 32 degrees C with an auxin (1-napthaleneacetic acid) and a cytokinin (6-benzylaminopurine), they reenter the cell cycle, dedifferentiate, and divide. GCPs cultured similarly but at 38 degrees C and with 0.1 micro M +/- -cis,trans-abscisic acid (ABA) remain differentiated. GCPs cultured at 38 degrees C without ABA dedifferentiate partially but do not divide. Cell survival after 1 week is 70% to 80% under all of these conditions. In this study, we show that GCPs cultured for 12 to 24 h at 38 degrees C accumulate heat shock protein 70 and develop a thermotolerance that, upon transfer of cells to 32 degrees C, enhances cell survival but inhibits cell cycle reentry, dedifferentiation, and division. GCPs dedifferentiating at 32 degrees C require both 1-napthaleneacetic acid and 6-benzylaminopurine to survive, but thermotolerant GCPs cultured at 38 degrees C +/- ABA do not require either hormone for survival. Pulse-labeling experiments using 5-bromo-2-deoxyuridine indicate that culture at 38 degrees C +/- ABA prevents dedifferentiation of GCPs by blocking cell cycle reentry at G1/S. Cell cycle reentry at 32 degrees C is accompanied by loss of a 41-kD polypeptide that cross-reacts with antibodies to rat (Rattus norvegicus) extracellular signal-regulated kinase 1; thermotolerant GCPs retain this polypeptide. A number of polypeptides unique to thermotolerant cells have been uncovered by Boolean analysis of two-dimensional gels and are targets for further analysis. GCPs of tree tobacco can be isolated in sufficient numbers and with the purity required to study plant cell thermotolerance and its relationship to plant cell survival, growth, dedifferentiation, and division in vitro.

BY-2 cells: culture and transformation for live cell imaging

Tobacco Bright Yellow-2 (BY-2) suspension cells are a widely used biological material for studying plant cell morphology and physiology. These cells are easy to transform and maintain in culture and tolerate transformation with fluorescent proteins such as the green fluorescent protein and its derivatives. These, by the addition of plant or mammalian targeting sequences, can be directed to specific subcellular locations for the study of cell dynamics in vivo. This unit describes the production of BY-2 cell stable transformants via an Agrobacterium based method to permit the visualisation of cellular components in vivo by epifluorescence or confocal microscopy.

Cross-talk between the cytosolic mevalonate and the plastidial methylerythritol phosphate pathways in tobacco bright yellow-2 cells

In plants, two pathways are utilized for the synthesis of isopentenyl diphosphate, the universal precursor for isoprenoid biosynthesis. The key enzyme of the cytoplasmic mevalonic acid (MVA) pathway is 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR). Treatment of Tobacco Bright Yellow-2 (TBY-2) cells by the HMGR-specific inhibitor mevinolin led to growth reduction and induction of apparent HMGR activity, in parallel to an increase in protein representing two HMGR isozymes. Maximum induction was observed at 24 h. 1-Deoxy-d-xylulose (DX), the dephosphorylated first precursor of the plastidial 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway, complemented growth inhibition by mevinolin in the low millimolar concentration range. Furthermore, DX partially re-established feedback repression of mevinolin-induced HMGR activity. Incorporation studies with [1,1,1,4-2H4]DX showed that sterols, normally derived from MVA, in the presence of mevinolin are synthesized via the MEP pathway. Fosmidomycin, an inhibitor of 1-deoxy-d-xylulose-5-phosphate reductoisomerase, the second enzyme of the MEP pathway, was utilized to study the reverse complementation. Growth inhibition by fosmidomycin of TBY-2 cells could be partially overcome by MVA. Chemical complementation was further substantiated by incorporation of [2-13C]MVA into plastoquinone, representative of plastidial isoprenoids. Best rates of incorporation of exogenous stably labeled precursors were observed in the presence of both inhibitors, thereby avoiding internal isotope dilution.

A functional genomics approach toward the understanding of secondary metabolism in plant cells

Despite the tremendous importance of secondary metabolites for humans as for the plant itself, plant secondary metabolism remains poorly characterized. Here, we present an experimental approach, based on functional genomics, to facilitate gene discovery in plant secondary metabolism. Targeted metabolite analysis was combined with cDNA-amplified fragment length polymorphism-based transcript profiling of jasmonate-elicited tobacco Bright yellow 2 cells. Transcriptome analysis suggested an extensive jasmonate-mediated genetic reprogramming of metabolism, which correlated well with the observed shifts in the biosynthesis of the metabolites investigated. This method, which in addition to transcriptome data also generates gene tags, in the future might lead to the creation of novel tools for metabolic engineering of medicinal plant systems in general.

The division of pleomorphic plastids with multiple FtsZ rings in tobacco BY-2 cells

Plastids, an essential group of plant cellular organelles, proliferate by division to maintain continuity through cell lineages in plants. In recent years, it was revealed that the bacterial cell division protein FtsZ is encoded in the nuclear genome of plant cells, and plays a major role in the plastid division process forming a ring along the center of plastids. Although the best-characterized type of plastid division so far is the division with a single FtsZ ring at the plastid midpoint, it was recently reported that in some plant organs and tissues, plastids are pleomorphic and form multiple FtsZ rings. However, the pleomorphic plastid division mechanism, such as the formation of multiple FtsZ rings, the constriction of plastids and the behavior of plastid (pt) nucleoids, remains totally unclear. To elucidate these points, we used the cultured cell line, tobacco (Nicotiana tabacum L.) Bright Yellow-2, in which plastids are pleomorphic and show dynamic morphological changes during culture. As a result, it was revealed that as the plastid elongates from an ellipsoid shape to a string shape after medium renewal, FtsZ rings are multiplied almost orderly and perpendicularly to the long axis of plastids. Active DNA synthesis of pt nucleoids is induced by medium transfer, and the division and the distribution of pt nucleoids occur along with plastid elongation. Although it was thought that the plastid divides with simultaneous multiple constrictions at all the FtsZ ring sites, giving rise to many small plastids, we found that the plastids generally divide constricting at only one FtsZ ring site. Moreover, using electron microscopy, we revealed that plastid-dividing (PD) rings are observed only at the constriction site, and not at swollen regions. These results indicate that in the pleomorphic plastid division with multiple FtsZ rings, the formation of PD rings occurs at a limited FtsZ ring site for one division. Multiplied FtsZ rings seem to localize in advance at the expected sites of division, and the formation of a PD ring at each FtsZ ring site occurs in a certain order, not simultaneously. Based on these results, a novel model for the pleomorphic plastid division with multiple FtsZ rings is proposed.

Actin-related defense mechanism to reject penetration attempt by a non-pathogen is maintained in tobacco BY-2 cells

The actin cytoskeleton is a key player in defense responses during early stages of infection by fungal pathogens. To investigate molecular mechanisms of actin-related defense responses, a cultured tobacco ( Nicotiana tabacum L.) BY-2 cell system was devised. When conidia were directly deposited on BY-2 cells, neither a pathogen, Erysiphe cichoracearum, nor a non-pathogen, Erysiphe pisi, was able to form appressoria or haustoria on BY-2 cells. On the other hand, conidia of the powdery mildews formed appressoria on BY-2 cells if they were covered with a thin hydrophobic membrane of Formvar. Percentages of appressoria formation of the powdery mildews on the Formvar-covered BY-2 cells were mostly the same as those on leaf epidermal cells. The pathogen successfully penetrated through the membrane into BY-2 cells and formed haustoria, whereas penetration attempts of the non-pathogen were completely rejected by the BY-2 cells similar to attempts on leaf epidermal cells. On the other hand, when BY-2 cells were treated with actin cytoskeleton-depolymerizing agents, cytochalasins, the non-pathogen became able to penetrate and form haustoria in BY-2 cells. Simultaneously, cytochalasin inhibited callose deposition at penetration sites of the non-pathogen. These results demonstrated that the actin cytoskeleton plays an important role in defense mechanisms against fungal penetration, even in the dedifferentiated cultured cells. The newly devised Formvar-covered cultured cell system will be a useful tool for molecular dissection of signal perception and defense mechanisms of plant cells during the early stage of fungal attack.

Purine synthesis and increased Agrobacterium tumefaciens transformation of yeast and plants

The bacterium Agrobacterium tumefaciens transforms eukaryotic hosts by transferring DNA to the recipient cell where it is integrated and expressed. Bacterial factors involved in this interkingdom gene transfer have been described, but less is known about host-cell factors. Using the yeast Saccharomyces cerevisiae as a model host, we devised a genetic screen to identify yeast mutants with altered transformation sensitivities. Twenty-four adenine auxotrophs were identified that exhibited supersensitivity to A. tumefaciens-mediated transformation when deprived of adenine. We extended these results to plants by showing that purine synthesis inhibitors cause supersensitivity to A. tumefaciens transformation in three plant species. The magnitude of this effect is large and does not depend on prior genetic manipulations of host cells. These data indicate the utility of yeast as a model for the transformation process and identify purine biosynthesis as a key determinant of transformation efficiency. These findings should increase the utility of A. tumefaciens in genetic engineering.

The gene encoding glutathione-dependent formaldehyde dehydrogenase/GSNO reductase is responsive to wounding, jasmonic acid and salicylic acid

It has recently been discovered that glutathione-dependent formaldehyde dehydrogenase (FALDH) exhibits a strong S-nitrosoglutathione reductase activity. Plants use NO and S-nitrosothiols as signaling molecules to activate defense mechanisms. Therefore, it is interesting to investigate the regulation of FALDH by mechanical wounding and plant hormones involved in signal transduction. Our results show that the gene encoding FALDH in Arabidopsis (ADH2) is down-regulated by wounding and activated by salicylic acid (SA). In tobacco, FALDH levels and enzymatic activity decreased after jasmonate treatment, and increased in response to SA. This is the first time that regulation of FALDH in response to signals associated with plant defense has been demonstrated.

Quantitative cDNA-AFLP analysis for genome-wide expression studies

An improved cDNA-AFLP method for genome-wide expression analysis has been developed. We demonstrate that this method is an efficient tool for quantitative transcript profiling and a valid alternative to microarrays. Unique transcript tags, generated from reverse-transcribed messenger RNA by restriction enzymes, were screened through a series of selective PCR amplifications. Based on in silico analysis, an enzyme combination was chosen that ensures that at least 60% of all the mRNAs were represented by an informative sequence tag. The sensitivity and specificity of the method allows one to detect poorly expressed genes and distinguish between homologous sequences. Accurate gene expression profiles were determined by quantitative analysis of band intensities, and subtle differences in transcriptional activity were revealed. A detailed screen for cell cycle-modulated genes in tobacco demonstrates the usefulness of the technology for genome-wide expression analysis.

A novel alpha-helical protein, specific to and highly conserved in plants, is associated with the nuclear matrix fraction

A cDNA for a novel plant protein was isolated from tomato. Nuclear Matrix Protein 1 (NMP1) is a ubiquitously expressed 36 kDa protein, which has no homologues in animals and fungi, but is highly conserved among flowering and non-flowering plants, including gymnosperms, moss, and the liverwort Marchantia polymorpha. NMP1 is predominantly alpha-helical with multiple stretches of short amphipathic regions. Cell fractionation, immunofluorescence, and GFP localization experiments showed that NMP1 is located both in the cytoplasm and nucleus and that the nuclear fraction is associated with the nuclear matrix. NMP1 is a candidate for a plant-specific structural protein with a function both in the nucleus and cytoplasm.

Isolation of tobacco ubiquitin-conjugating enzyme cDNA in a yeast two-hybrid system with tobacco ERF3 as bait and its characterization of specific interaction

Tobacco ETHYLENE-RESPONSIVE FACTOR3 (ERF3) is a member of the ERF-domain transcription factors and has a transcriptional repressor activity, whereas other ERF proteins show activation activity. To understand the regulation of ERF3-repressor activity, protein(s) were screened which interact with ERF3 in a yeast two-hybrid system. A partial sequence (B8) of NtUBC2, a tobacco ubiquitin-conjugating enzyme was isolated. This B8 specifically interacted with ERF3 in the yeast two-hybrid system. Further analyses revealed that the region unique to ERF3 interacted with B8. The physiological functions of NtUBC2 and the stability of ERF3 are discussed in relation to the regulation of the repression activity of ERF3.

Characterization of an ascorbate peroxidase in plastids of tobacco BY-2 cells

In higher plants, ascorbate peroxidase (APX; EC 1.11.1.11), the major H2O2-scavenging enzyme, occurs in several distinct isoenzymes that are localized in cytosol and various cell organelles. Here, we have purified and characterized an APX from the soluble fraction of plastids of non-photosynthetic tobacco BY-2 cells. The plastidic APX was a monomer with a molecular weight of 34 000. The enzymatic properties of the plastidic APX, including the rapid inactivation by H2O2 in ascorbate-depleted medium, were highly comparable with those of the chloroplastic stromal APX of spinach and tea leaves. However, the other chloroplastic APX isoenzyme, the thylakoid-membrane bound APX, was not detected in the plastids of the BY-2 cells. The N-terminal amino acid sequence of the plastidic APX was completely identical with the deduced amino acid sequence of a previously identified cDNA sequence of tobacco chloroplastic APX. When a green fluorescence protein gene tagged with the chloroplast-targeting signal sequence of APX was expressed in the BY-2 cells, the fluorescence protein exclusively localized into plastids, and not into mitochondria. We conclude that plastidic APX in non-photosynthetic tissues is the same as the chloroplastic APX that occurs in leaves.

Phytotoxicity of the tetramic acid metabolite trichosetin

Trichosetin, a tetramic acid-containing metabolite produced in the dual culture of Trichoderma harzianum and Catharanthus roseus (L.) G. Don callus, was subjected to phytotoxicity assays. In seedling growth assays, trichosetin inhibited root and shoot growth of all five plant species tested by damaging the cell membrane, as evidenced by the dose-dependent increase in electrolyte leakage and lipid peroxidation. Vital staining of trichosetin-treated Nicotiana tabacum BY-2 cells, with rhodamine 123, showed a weaker green fluorescence compared to controls indicating damaging effects on mitochondria. FDA-PI staining, to determine cell viability, indicated that cells of the trichosetin-treated roots were mostly dead.

Identification of photorespiratory glutamate:glyoxylate aminotransferase (GGAT) gene in Arabidopsis

In the photorespiratory process, peroxisomal glutamate:glyoxylate aminotransferase (GGAT) catalyzes the reaction of glutamate and glyoxylate to 2-oxoglutarate and glycine. Although GGAT has been assumed to play important roles for the transamination in photorespiratory carbon cycles, the gene encoding GGAT has not been identified. Here, we report that an alanine:2-oxoglutarate aminotransferase (AOAT)-like protein functions as GGAT in peroxisomes. Arabidopsis has four genes encoding AOAT-like proteins and two of them (namely AOAT1 and AOAT2) contain peroxisomal targeting signal 1 (PTS1). The expression analysis of mRNA encoding AOATs and EST information suggested that AOAT1 was the major protein in green leaves. When AOAT1 fused to green fluorescent protein (GFP) was expressed in BY-2 cells, it was found to be localized to peroxisomes depending on PTS1. By screening of Arabidopsis T-DNA insertion lines, an AOAT1 knockout line (aoat1-1) was isolated. The activity of GGAT and alanine:glyoxylate aminotransferase (AGAT) in the above-ground tissues of aoat1-1 was reduced drastically and, AOAT and glutamate:pyruvate aminotransferase (GPAT) activity also decreased. Peroxisomal GGAT was detected in the wild type but not in aoat1-1. The growth rate was repressed in aoat1-1 grown under high irradiation or without sugar, though differences were slight in aoat1-1 grown under low irradiation, high-CO2 (0.3%) or high-sugar (3% sucrose) conditions. These phenotypes resembled those of photorespiration-deficient mutants. Glutamate levels increased and serine levels decreased in aoat1-1 grown in normal air conditions. Based on these results, it was concluded that AOAT1 is targeted to peroxisomes, functions as a photorespiratory GGAT, plays a markedly important role for plant growth and the metabolism of amino acids.

Cloning of a pine germin-like protein (GLP) gene promoter and analysis of its activity in transgenic tobacco Bright Yellow 2 cells

Germins and germin-like proteins (GLPs) constitute a large and highly diverse family of ubiquitous plant cell wall proteins. These proteins seem to be involved in many developmental stages and stress-related processes, but their exact participation in these processes generally remains obscure. In Pinus caribaea Morelet, the PcGER1 gene is expressed uniquely in embryo tissues, and encodes a GLP ionically bound to the walls of pine embryo cells maintained in 2,4-D-containing medium. We have cloned a genomic fragment including the 1520 bp 5'-upstream promoter region of PcGER1. This sequence contains, in its 1200 bp distal part, several cis elements (e.g. SEF4, 60 kDa protein, ABA RE and Dof recognition sites) present in genes responding to hormones and/or expressed in embryo or seed tissues, or during germination. The PcGER1 promoter sequence was cloned upstream of the GUS (beta-glucuronidase) reporter gene and transferred to tobacco Bright Yellow 2 (BY-2) cells via Agrobacterium tumefaciens-mediated transformation. Promoter activity and growth performances of transgenic asynchronous cell suspensions were analysed in the presence or absence of 2,4-D and/or BA. Optimal growth, maximum cell-wall yield and PcGER1 promoter activity were observed in the presence of 2,4-D and BA at day 4, the end of the exponential growth phase where 70-75% cells have a 2C DNA content. Analysis of promoter activity during the cell cycle in an aphidicoline-synchronized culture suggested that the expression is maximum in G1 cells. We also showed that under optimal growth conditions, 5' promoter deletions decreased the activity of the reporter gene. We discuss the function of this gene with regards to cell growth. Accession number: The PcGER1 promoter sequence was submitted to the genbank database under the accession number AY077704.

Cytokinin affinity purification and identification of a tobacco BY-2 adenosine kinase

Adenosine kinase is one of the enzymes potentially responsible for the formation of cytokinin nucleotides in plants. Using a zeatin affinity column a 40 kDa protein was isolated from tobacco Bright Yellow 2 (TBY-2) and identified by mass spectrometry as adenosine kinase. The ligand interaction reported here can be disrupted by several other adenine- but not guanine-based purine derivatives. The observed interaction with cytokinins is discussed in view of a putative role for adenosine kinase in TBY-2 cytokinin metabolism. The presented results show for the first time a plant adenosine kinase affinity-purified to homogeneity that was identified by primary structure analysis.

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.

Do phytotropins inhibit auxin efflux by impairing vesicle traffic?

Phytotropins such as 1-N-naphthylphthalamic acid (NPA) strongly inhibit auxin efflux, but the mechanism of this inhibition remains unknown. Auxin efflux is also strongly decreased by the vesicle trafficking inhibitor brefeldin A (BFA). Using suspension-cultured interphase cells of the BY-2 tobacco (Nicotiana tabacum L. cv Bright-Yellow 2) cell line, we compared the effects of NPA and BFA on auxin accumulation and on the arrangement of the cytoskeleton and endoplasmic reticulum (ER). The inhibition of auxin efflux (stimulation of net accumulation) by both NPA and BFA occurred rapidly with no measurable lag. NPA had no observable effect on the arrangement of microtubules, actin filaments, or ER. Thus, its inhibitory effect on auxin efflux was not mediated by perturbation of the cytoskeletal system and ER. BFA, however, caused substantial alterations to the arrangement of actin filaments and ER, including a characteristic accumulation of actin in the perinuclear cytoplasm. Even at saturating concentrations, NPA inhibited net auxin efflux far more effectively than did BFA. Therefore, a proportion of the NPA-sensitive auxin efflux carriers may be protected from the action of BFA. Maximum inhibition of auxin efflux occurred at concentrations of NPA substantially below those previously reported to be necessary to perturb vesicle trafficking. We found no evidence to support recent suggestions that the action of auxin transport inhibitors is mediated by a general inhibition of vesicle-mediated protein traffic to the plasma membrane.

Identification of peroxisomal targeting signal of pumpkin catalase and the binding analysis with PTS1 receptor

Many peroxisomal proteins are imported into peroxisomes via recognition of the peroxisomal targeting signal (PTS1) present at the C-termini by the PTS1 receptor (Pex5p). Catalase, a peroxisomal protein, has PTS1-like motifs around or at the C-terminus. However, it remains unclear whether catalase is imported into peroxisome via the PTS1 system. In this work, we analyzed the PTS of pumpkin catalase (Cat1). A full or truncated pumpkin Cat1 cDNA fused at the 3' end of the green fluorescent protein (GFP) coding sequence was introduced and stably expressed in tobacco BY-2 (Nicotiana tabacum cv. Bright Yellow 2) cells or Arabidopsis thaliana by Agrobacterium-mediated transformation. The cellular localization of GFP was analyzed by fluorescence microscopy. The results showed that the C-terminal 10-amino acid region containing an SKL motif-like tripeptide (SHL) was not required for the import into peroxisomes. Surprisingly, the C-terminal 3-amino acid region was required for the import when the fusion proteins were transiently expressed by using particle gun bombardment, suggesting that the transient expression system is inadequate to analyze the targeting signal. We proposed that the C-terminal amino acid region from 13 to 11 (QKL), which corresponds with the PTS1 consensus sequence, may function as an internal PTS1. Analysis of the binding of Cat1 to PTS1 receptor (Pex5p) by the yeast two-hybrid system revealed that Cat1 can bind with the PTS1 receptor (Pex5p), indicating that Cat1 is imported into peroxisomes by the PTS1 system.

Early steps in the oxidative burst induced by cadmium in cultured tobacco cells (BY-2 line)

The rapid generation of H(2)O(2) by Cd(2+)-treated plant cells was investigated in cultured tobacco (Nicotiana tabacum L.) BY-2 cells. The starting point for the generation of H(2)O(2) has been located at the cell plasma membrane using cytochemical methods. Treatment of the cells with diphenyleneiodonium (DPI) and imidazol, both inhibitors of the neutrophil NADPH oxidase, prevented the generation of H(2)O(2) induced by Cd(2+). These data suggest the involvement of an NADPH oxidase-like enzyme leading to H(2)O(2) production through O(2)(*-) dismutation by superoxide dismutase enzymes. To investigate the implication of Ca(2+) channels in a Cd(2+)-induced oxidative burst, different inhibitors of Ca(2+) channels were used. Only La(3+) totally inhibited the generation of H(2)O(2) induced by Cd(2+). However, verapamil and nifedipine, inhibitors of Ca(2+) channels, were not effective. Calmodulin or a Ca(2+)-dependent protein kinase is also implicated in the signal transduction sequence, based on the results obtained with two types of calmodulin antagonists, fluphenazine and N-(-6-amino-hexyl)-5-chloro-1-naphthalenesulphonamide (W-7) and staurosporine, an inhibitor of protein kinases. However, neomycin, an inhibitor of the phosphoinositide cycle, did not inhibit the generation of H(2)O(2) induced by Cd(2+), suggesting mainly an induction of the oxidative burst mediated by calmodulin and/or calmodulin-dependent proteins.

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.

Fast liquid chromatography coupled to electrospray tandem mass spectrometry peptide sequencing for cross-species protein identification

Using a parallel microcolumn switching liquid chromatography set-up coupled to a quadrupole time-of-flight mass spectrometer, a rapid liquid chromatography/mass spectrometric (LC/MS) protein identification method is presented. Without prior sample clean-up up to 300 protein digest samples a day can be processed. Using data-directed acquisition, up to 10 fragmentation analyses for each protein sample can be acquired in the same chromatographic run that can be used for database searching. Using internal peptide sequence information, protein databases and the various nucleic acid databases can both be queried for cross-species identification of the protein sample. The method was evaluated and put into force to generate data for a tobacco cell culture protein database.