Phospholipase d activation correlates with microtubule reorganization in living plant cells

Lipid biosynthesis and protein concentration respond uniquely to phosphate supply during leaf development in highly phosphorus-efficient Hakea prostrata

Hakea prostrata (Proteaceae) is adapted to severely phosphorus-impoverished soils and extensively replaces phospholipids during leaf development. We investigated how polar lipid profiles change during leaf development and in response to external phosphate supply. Leaf size was unaffected by a moderate increase in phosphate supply. However, leaf protein concentration increased by more than 2-fold in young and mature leaves, indicating that phosphate stimulates protein synthesis. Orthologs of known lipid-remodeling genes in Arabidopsis (Arabidopsis thaliana) were identified in the H. prostrata transcriptome. Their transcript profiles in young and mature leaves were analyzed in response to phosphate supply alongside changes in polar lipid fractions. In young leaves of phosphate-limited plants, phosphatidylcholine/phosphatidylethanolamine and associated transcript levels were higher, while phosphatidylglycerol and sulfolipid levels were lower than in mature leaves, consistent with low photosynthetic rates and delayed chloroplast development. Phosphate reduced galactolipid and increased phospholipid concentrations in mature leaves, with concomitant changes in the expression of only four H. prostrata genes, GLYCEROPHOSPHODIESTER PHOSPHODIESTERASE1, N-METHYLTRANSFERASE2, NONSPECIFIC PHOSPHOLIPASE C4, and MONOGALACTOSYLDIACYLGLYCEROL3. Remarkably, phosphatidylglycerol levels decreased with increasing phosphate supply and were associated with lower photosynthetic rates. Levels of polar lipids with highly unsaturated 32:x (x = number of double bonds in hydrocarbon chain) and 34:x acyl chains increased. We conclude that a regulatory network with a small number of central hubs underpins extensive phospholipid replacement during leaf development in H. prostrata. This hard-wired regulatory framework allows increased photosynthetic phosphorus use efficiency and growth in a low-phosphate environment. This may have rendered H. prostrata lipid metabolism unable to adjust to higher internal phosphate concentrations.

Phosphatidic acid integrates calcium signaling and microtubule dynamics into regulating ABA-induced stomatal closure in Arabidopsis

Specific cellular components have been identified to function in abscisic acid (ABA) regulation of stomatal apertures, including calcium, the cytoskeleton, and phosphatidic acid. In this study, the regulation and dynamic organization of microtubules during ABA-induced stomatal closure by phospholipase D (PLD) and its product PA were investigated. ABA induced microtubule depolymerization and stomatal closure in wide-type (WT) Arabidopsis, whereas these processes were impaired in PLD mutant (pldα1). The microtubule-disrupting drugs oryzalin or propyzamide induced microtubule depolymerization, but did not affect the stomatal aperture, whereas their co-treatment with ABA resulted in stomatal closure in both WT and pldα1. In contrast, the microtubule-stabilizing drug paclitaxel arrested ABA-induced microtubule depolymerization and inhibited ABA-induced stomatal closure in both WT and pldα1. In pldα1, ABA-induced cytoplasmic Ca(2+) ([Ca(2+)]cyt) elevation was partially blocked, and exogenous Ca(2+)-induced microtubule depolymerization and stomatal closure were impaired. These results suggested that PLDα1 and PA regulate microtubular organization and Ca(2+) increases during ABA-induced stomatal closing and that crosstalk among signaling lipid, Ca(2+), and microtubules are essential for ABA signaling.

Phospholipids: molecules regulating cytoskeletal organization in plant abiotic stress tolerance

Cytoskeleton serves as structural, membrane-bound and highly nonlinear dynamics element that basically functions in abiotic and biotic stresses. The study of phospholipid-regulated cytoskeletal organization to strengthen plants against stresses is emerging. Phospholipids in lipid bilayers, as the main compound of cellular membranes, have roles in modulation of membrane curvature and anchoring, cross-linking or regulating particular cytoskeletal proteins to modulate cytoskeletal dynamics. In this review, we highlight the role of phospholipids and their metabolic enzymes through regulating cytoskeletal organization and dynamics in response to abiotic stresses, such as salt, drought and low/high temperature stresses.

Global analysis of transcriptome responses and gene expression profiles to cold stress of Jatropha curcas L

BACKGROUND

Jatropha curcas L., also called the Physic nut, is an oil-rich shrub with multiple uses, including biodiesel production, and is currently exploited as a renewable energy resource in many countries. Nevertheless, because of its origin from the tropical MidAmerican zone, J. curcas confers an inherent but undesirable characteristic (low cold resistance) that may seriously restrict its large-scale popularization. This adaptive flaw can be genetically improved by elucidating the mechanisms underlying plant tolerance to cold temperatures. The newly developed Illumina Hiseq™ 2000 RNA-seq and Digital Gene Expression (DGE) are deep high-throughput approaches for gene expression analysis at the transcriptome level, using which we carefully investigated the gene expression profiles in response to cold stress to gain insight into the molecular mechanisms of cold response in J. curcas.

RESULTS

In total, 45,251 unigenes were obtained by assembly of clean data generated by RNA-seq analysis of the J. curcas transcriptome. A total of 33,363 and 912 complete or partial coding sequences (CDSs) were determined by protein database alignments and ESTScan prediction, respectively. Among these unigenes, more than 41.52% were involved in approximately 128 known metabolic or signaling pathways, and 4,185 were possibly associated with cold resistance. DGE analysis was used to assess the changes in gene expression when exposed to cold condition (12°C) for 12, 24, and 48 h. The results showed that 3,178 genes were significantly upregulated and 1,244 were downregulated under cold stress. These genes were then functionally annotated based on the transcriptome data from RNA-seq analysis.

CONCLUSIONS

This study provides a global view of transcriptome response and gene expression profiling of J. curcas in response to cold stress. The results can help improve our current understanding of the mechanisms underlying plant cold resistance and favor the screening of crucial genes for genetically enhancing cold resistance in J. curcas.

Involvement of phospholipase D-related signal transduction in chemical-induced programmed cell death in tomato cell cultures

Phospholipase D (PLD) and its product phosphatidic acid (PA) are incorporated in a complex metabolic network in which the individual PLD isoforms are suggested to regulate specific developmental and stress responses, including plant programmed cell death (PCD). Despite the accumulating knowledge, the mechanisms through which PLD/PA operate during PCD are still poorly understood. In this work, the role of PLDα1 in PCD and the associated caspase-like proteolysis, ethylene and hydrogen peroxide (H(2)O(2)) synthesis in tomato suspension cells was studied. Wild-type (WT) and PLDα1-silenced cell lines were exposed to the cell death-inducing chemicals camptothecin (CPT), fumonisin B1 (FB1) and CdSO(4). A range of caspase inhibitors effectively suppressed CPT-induced PCD in WT cells, but failed to alleviate cell death in PLDα1-deficient cells. Compared to WT, in CPT-treated PLDα1 mutant cells, reduced cell death and decreased production of H(2)O(2) were observed. Application of ethylene significantly enhanced CPT-induced cell death both in WT and PLDα1 mutants. Treatments with the PA derivative lyso-phosphatidic acid and mastoparan (agonist of PLD/PLC signalling downstream of G proteins) caused severe cell death. Inhibitors, specific to PLD and PLC, remarkably decreased the chemical-induced cell death. Taken together with our previous findings, the results suggest that PLDα1 contributes to caspase-like-dependent cell death possibly communicated through PA, reactive oxygen species and ethylene. The dead cells expressed morphological features of PCD such as protoplast shrinkage and nucleus compaction. The presented findings reveal novel elements of PLD/PA-mediated cell death response and suggest that PLDα1 is an important factor in chemical-induced PCD signal transduction.

Regulation of cytoskeletal dynamics by phospholipase D and phosphatidic acid

Plants respond to diverse biotic and abiotic stimuli as well as to endogenous developmental cues. Many of these stimuli result in altered activity of phospholipase D (PLD), an enzyme that hydrolyzes structural phospholipids producing phosphatidic acid (PA). PA is a key signaling intermediate in animals, but its targets in plants are relatively uncharacterized. Recent studies have demonstrated that the cytoskeleton is a major target of PLD-PA signaling and identified a positive feedback loop between actin turnover and PLD activity. Moreover, two cytoskeletal proteins, capping protein and MAP65-1, have been identified as PA-binding proteins regulating actin and microtubule organization and dynamics. In this review, we highlight the role of the PLD-PA module as an important hub for housekeeping and stress-induced regulation of membrane-associated cytoskeletal dynamics.

Cytoskeleton-dependent endomembrane organization in plant cells: an emerging role for microtubules

Movement of secretory organelles is a fascinating yet largely mysterious feature of eukaryotic cells. Microtubule-based endomembrane and organelle motility utilizing the motor proteins dynein and kinesin is commonplace in animal cells. In contrast, it has been long accepted that intracellular motility in plant cells is predominantly driven by myosin motors dragging organelles and endomembrane-bounded cargo along actin filament bundles. Consistent with this, defects in the acto-myosin cytoskeleton compromise plant growth and development. Recent findings, however, challenge the actin-centric view of the motility of critical secretory organelles and distribution of associated protein machinery. In this review, we provide an overview of the current knowledge on actin-mediated organelle movement within the secretory pathway of plant cells, and report on recent and exciting findings that support a critical role of microtubules in plant cell development, in fine-tuning the positioning of Golgi stacks, as well as their involvement in cellulose synthesis and auxin polar transport. These emerging aspects of the biology of microtubules highlight adaptations of an ancestral machinery that plants have specifically evolved to support the functioning of the acto-myosin cytoskeleton, and mark new trends in our global appreciation of the complexity of organelle movement within the plant secretory pathway.

Microtubules, signalling and abiotic stress

Plant microtubules, in addition to their role in cell division and axial cell expansion, convey a sensory function that is relevant for the perception of mechanical membrane stress and its derivatives, such as osmotic or cold stress. During development, sensory microtubules participate in the mechanical integration of plant architecture, including the patterning of incipient organogenesis and the alignment with gravity-dependent load. The sensory function of microtubules depends on dynamic instability, and often involves a transient elimination of cortical microtubules followed by adaptive events accompanied by subsequent formation of stable microtubule bundles. It is proposed that microtubules, because of their relative rigidity in combination with their innate nonlinear dynamics, are pre-adapted for a function as mechanosensors and, in concert with the flexible actin filaments and the anisotropic cell wall, comprise a tensegral system that allows plant cells to sense geometry and to respond to fields of mechanical strains such that the load is minimized. Microtubules are proposed as elements of a sensory hub that decodes stress-related signal signatures, with phospholipase D as an important player.

Lipid profiling demonstrates that suppressing Arabidopsis phospholipase Dδ retards ABA-promoted leaf senescence by attenuating lipid degradation

Senescence is the last phase of the plant life cycle and has an important role in plant development. Degradation of membrane lipids is an essential process during leaf senescence. Several studies have reported fundamental changes in membrane lipids and phospholipase D (PLD) activity as leaves senesce. Suppression of phospholipase Dα1 (PLDα1) retards abscisic acid (ABA)-promoted senescence. However, given the absence of studies that have profiled changes in the compositions of membrane lipid molecules during leaf senescence, there is no direct evidence that PLD affects lipid composition during the process. Here, we show that application of n-butanol, an inhibitor of PLD, and N-Acylethanolamine (NAE) 12∶0, a specific inhibitor of PLDα1, retarded ABA-promoted senescence to different extents. Furthermore, phospholipase Dδ (PLDδ) was induced in leaves treated with ABA, and suppression of PLDδ retarded ABA-promoted senescence in Arabidopsis. Lipid profiling revealed that detachment-induced senescence had different effects on plastidic and extraplastidic lipids. The accelerated degradation of plastidic lipids during ABA-induced senescence in wild-type plants was attenuated in PLDδ-knockout (PLDδ-KO) plants. Dramatic increases in phosphatidic acid (PA) and decreases in phosphatidylcholine (PC) during ABA-induced senescence were also suppressed in PLDδ-KO plants. Our results suggest that PLDδ-mediated hydrolysis of PC to PA plays a positive role in ABA-promoted senescence. The attenuation of PA formation resulting from suppression of PLDδ blocks the degradation of membrane lipids, which retards ABA-promoted senescence.

α-tubulin is rapidly phosphorylated in response to hyperosmotic stress in rice and Arabidopsis

By using high-resolution two-dimensional PAGE followed by phosphoprotein-specific staining and peptide mass fingerprint analysis along with other assays, we found that α-tubulin is phosphorylated in response to hyperosmotic stress in rice and Arabidopsis. The onset of the phosphorylation response was as early as 2 min after hyperosmotic stress treatment, and a major proportion of α-tubulin was phosphorylated after 60 min in root tissues. However, the phosphorylated form of α-tubulin was readily dephosphorylated upon stress removal. The phosphorylation site was identified as Thr349 by comprehensive mutagenesis of serine/threonine residues in a rice α-tubulin isoform followed by evaluation in cultured cell protoplasts. This residue is located at the surface for the interaction with β-tubulin in polymerized α-β tubulin dimers and has been proposed to be directly involved in this interaction. Thus, α-tubulin phosphorylation was considered to occur on free tubulin dimers in response to hyperosmotic stress. The incorporation of green fluorescent protein (GFP)-α-tubulin into cortical microtubules was completely inhibited in transgenic Arabidopsis when Thr349 was substituted with glutamate or aspartate. Using transgenic Arabidopsis plants expressing GFP-α-tubulin, we found that hyperosmotic stress causes extensive cortical microtubule depolymerization. Microtubule-destabilizing treatments such as propyzamide or oryzalin and temperature stresses resulted in α-tubulin phosphorylation, whereas hyperosmotic stress-induced α-tubulin phosphorylation was partially inhibited by taxol, which stabilizes microtubules. These results and the three-dimensional location of the phosphorylation site suggested that microtubules are depolymerized in response to hyperosmotic stress via α-tubulin phosphorylation. Together, the results of the present study reveal a novel mechanism that globally regulates the microtubule polymerization.

The plant cytoskeleton controls regulatory volume increase

The ability to adjust cell volume is required for the adaptation to osmotic stress. Plant protoplasts can swell within seconds in response to hypoosmotic shock suggesting that membrane material is released from internal stores. Since the stability of plant membranes depends on submembraneous actin, we asked, whether this regulatory volume control depends on the cytoskeleton. As system we used two cell lines from grapevine which differ in their osmotic tolerance and observed that the cytoskeleton responded differently in these two cell lines. To quantify the ability for regulatory volume control, we used hydraulic conductivity (Lp) as readout and demonstrated a role of the cytoskeleton in protoplast swelling. Chelation of calcium, inhibition of calcium channels, or manipulation of membrane fluidity, did not significantly alter Lp, whereas direct manipulation of the cytoskeleton via specific chemical reagents, or indirectly, through the bacterial elicitor Harpin or activation of phospholipase D, was effective. By optochemical engineering of actin using a caged form of the phytohormone auxin we can break the symmetry of actin organisation resulting in a localised deformation of cell shape indicative of a locally increased Lp. We interpret our findings in terms of a model, where the submembraneous cytoskeleton controls the release of intracellular membrane stores during regulatory volume change.

Dissecting the mechanism of abscisic acid-induced dynamic microtubule reorientation using live cell imaging

Abscisic acid (ABA) is involved in plant development and responses to environmental stress including the formation of longitudinal microtubule arrays in elongating cells, although the underlying mechanism for this is unknown. We explored ABA-induced microtubule reorientation in leek (Allium porrum L.) leaf epidermal cells transiently expressing a GFP-MBD microtubule reporter. After 14-18h incubation with ABA, the frequency of cells with longitudinal arrays of cortical microtubules along the outer epidermal wall increased with dose-dependency until saturation at 20μM. Time-course imaging of individual cells revealed a gradual increase in the occurrence of discordant, dynamic microtubules deviating from the normal transverse microtubule array within 2-4h of exposure to ABA, followed by reorientation into a completely longitudinal array within 5-8h. Approximately one-half of the ABA-induced reorientation occurred independently of cytoplasmic streaming following the application of cytochalasin D. Reorientation occurred also in the elongation zone of Arabidopsis root tips. Transient expression of AtEB1b-GFP reporter and analysis of 'comet' velocities in Allium revealed that the microtubule growth rate increased by 55% within 3h of exposure to ABA. ABA also increased the sensitivity of microtubules to depolymerisation by oryzalin and exacerbated oryzalin-induced radial swelling of Arabidopsis root tips. The swelling was further aggravated in AtPLDδ-null mutant, suggesting PLDδ plays a role in microtubule stability. We propose that ABA-induced reorientation of transverse microtubule array initially involves destabilisation of the array combined with the formation of dynamic, discordant microtubules.

Regulation of Pollen Tube Growth by Transglutaminase

In pollen tubes, cytoskeleton proteins are involved in many aspects of pollen germination and growth, from the transport of sperm cells to the asymmetrical distribution of organelles to the deposition of cell wall material. These activities are based on the dynamics of the cytoskeleton. Changes to both actin filaments and microtubules are triggered by specific proteins, resulting in different organization levels suitable for the different functions of the cytoskeleton. Transglutaminases are enzymes ubiquitous in all plant organs and cell compartments. They catalyze the post-translational conjugation of polyamines to different protein targets, such as the cytoskeleton. Transglutaminases are suggested to have a general role in the interaction between pollen tubes and the extracellular matrix during fertilization and a specific role during the self-incompatibility response. In such processes, the activity of transglutaminases is enhanced, leading to the formation of cross-linked products (including aggregates of tubulin and actin). Consequently, transglutaminases are suggested to act as regulators of cytoskeleton dynamics. The distribution of transglutaminases in pollen tubes is affected by both membrane dynamics and the cytoskeleton. Transglutaminases are also secreted in the extracellular matrix, where they may take part in the assembly and/or strengthening of the pollen tube cell wall.

The Salt Overly Sensitive (SOS) pathway: established and emerging roles

Soil salinity is a growing problem around the world with special relevance in farmlands. The ability to sense and respond to environmental stimuli is among the most fundamental processes that enable plants to survive. At the cellular level, the Salt Overly Sensitive (SOS) signaling pathway that comprises SOS3, SOS2, and SOS1 has been proposed to mediate cellular signaling under salt stress, to maintain ion homeostasis. Less well known is how cellularly heterogenous organs couple the salt signals to homeostasis maintenance of different types of cells and to appropriate growth of the entire organ and plant. Recent evidence strongly indicates that different regulatory mechanisms are adopted by roots and shoots in response to salt stress. Several reports have stated that, in roots, the SOS proteins may have novel roles in addition to their functions in sodium homeostasis. SOS3 plays a critical role in plastic development of lateral roots through modulation of auxin gradients and maxima in roots under mild salt conditions. The SOS proteins also play a role in the dynamics of cytoskeleton under stress. These results imply a high complexity of the regulatory networks involved in plant response to salinity. This review focuses on the emerging complexity of the SOS signaling and SOS protein functions, and highlights recent understanding on how the SOS proteins contribute to different responses to salt stress besides ion homeostasis.

Arabidopsis SOS3 plays an important role in salt tolerance by mediating calcium-dependent microfilament reorganization

KEY MESSAGE : SOS3 mediates calcium dependent actin filament reorganization that plays important roles in plant responses to salt stress. Arabidopsis salt overly sensitive 3 (SOS3) plays an important role in plant salt tolerance by regulation of Na(+)/K(+) homeostasis. Plants lacking SOS3 are hypersensitive to salt stress and this phenomenon can be partially rescued by the addition of calcium. However the mechanism underlying remains elusive. We here report that the organization of actin filaments in sos3 mutant differs from that in wild-type plant. Under salt stress abnormal actin assembly and arrangement in sos3 are more pronounced, which can be partially complemented by addition of external calcium or low concentration of latrunculin A, an actin monomer-sequestering agent. The effects of calcium and Lat A on actin filament organization of sos3 mutant are accordant with their effects on sos3 salt sensitivity under salt stress. These findings indicate that the salt-hypersensitivity of sos3 mutant partially results from its disordered actin filaments, and SOS3 mediated actin filament reorganization plays important roles in plant responses to salt stress.

The non-specific lipid transfer protein N5 of Medicago truncatula is implicated in epidermal stages of rhizobium-host interaction

BACKGROUND

The symbiotic interaction between leguminous plants and rhizobia involves two processes: bacterial infection, resulting in the penetration of bacteria in epidermal and cortical cells, and root nodule organogenesis. Root nodule symbiosis is activated by rhizobial signalling molecules, called Nodulation factors (NFs). NF perception induces the expression of several genes called early nodulins. The early nodulin N5 of Medicago truncatula is a lipid transfer protein that has been shown to positively regulate nodulation although it displays in vitro inhibitory activity against Sinorhizobium meliloti. The purpose of this work was to investigate the role of MtN5 by studying its spatial and temporal pattern of expression during the symbiotic interaction, also in relation to known components of the symbiotic signalling pathway, and by analysing the phenotypic alterations displayed by rhizobia-inoculated MtN5-silenced roots.

RESULTS

We show here that MtN5 is a NF-responsive gene expressed at a very early phase of symbiosis in epidermal cells and root hairs. MtN5 expression is induced in vitro by rhizobial effector molecules and by auxin and cytokinin, phytohormones involved in nodule organogenesis. Furthermore, lipid signaling is implicated in the response of MtN5 to rhizobia, since the activity of phospholipase D is required for MtN5 induction in S. meliloti-inoculated roots. MtN5-silenced roots inoculated with rhizobia display an increased root hair curling and a reduced number of invaded primordia compared to that in wild type roots, but with no impairment to nodule primordia formation. This phenotype is associated with the stimulation of ENOD11 expression, an early marker of infection, and with the down-regulation of Flotillin 4 (FLOT4), a protein involved in rhizobial entry.

CONCLUSIONS

These data indicate that MtN5 acts downstream of NF perception and upstream of FLOT4 in regulating pre-infection events. The positive effect of MtN5 on nodule primordia invasion is linked to the restriction of bacterial spread at the epidermal level. Furthermore, MtN5 seems to be dispensable for nodule primordia formation. These findings provide new information about the complex mechanism that controls the competence of root epidermal cells for rhizobial invasion.

Phosphatidic acid regulates microtubule organization by interacting with MAP65-1 in response to salt stress in Arabidopsis

Membrane lipids play fundamental structural and regulatory roles in cell metabolism and signaling. Here, we report that phosphatidic acid (PA), a product of phospholipase D (PLD), regulates MAP65-1, a microtubule-associated protein, in response to salt stress. Knockout of the PLDα1 gene resulted in greater NaCl-induced disorganization of microtubules, which could not be recovered during or after removal of the stress. Salt affected the association of MAP65-1 with microtubules, leading to microtubule disorganization in pldα1cells, which was alleviated by exogenous PA. PA bound to MAP65-1, increasing its activity in enhancing microtubule polymerization and bundling. Overexpression of MAP65-1 improved salt tolerance of Arabidopsis thaliana cells. Mutations of eight amino acids in MAP65-1 led to the loss of its binding to PA, microtubule-bundling activity, and promotion of salt tolerance. The pldα1 map65-1 double mutant showed greater sensitivity to salt stress than did either single mutant. These results suggest that PLDα1-derived PA binds to MAP65-1, thus mediating microtubule stabilization and salt tolerance. The identification of MAP65-1 as a target of PA reveals a functional connection between membrane lipids and the cytoskeleton in environmental stress signaling.

Development of cultured microspores of maize in the presence of n-butanol and 2-aminoethanol

The aim of the present study was to examine whether the induction of maize microspore embryogenesis could be triggered by the application of biogenic alcohols, as was reported earlier in wheat. A single cross hybrid (A 18) raised in the phytotron was used as anther donor for shed microspore cultures after cold pretreatment. At the onset of culturing, anthers in liquid YP medium were treated with 0.2 or 0.4% n-butanol or with 2 mM aminoethanol (2-AE) for 6 or 18 hours.The treatments caused a drastic (approx. 50%) decrease in the viability of the microspores. After a few days of culture in medium containing neither n-butanol nor 2-AE, 9-13% of the microspores remained alive and capable of switching to the sporophytic pathway of development.Treatment with 0.2% n-butanol for 6 h considerably increased the frequency of symmetric nuclear divisions (more than 3×) and of induced microspores (2×). The embryo yield was also elevated by 10%. The results showed that n-butanol could be used to improve the androgenic response and microspore embryogenesis in maize, but not as efficiently as in wheat. Further examination will be required to find the reasons for the different behaviour of microspores of the two species.

Lipid body biogenesis and the role of microtubules in lipid synthesis in Ornithogalum umbellatum lipotubuloids

Lipid bodies present in lipotubuloids of Ornithogalum umbellatum ovary epidermis take the form of a lens between leaflets of ER (endoplasmic reticulum) membrane filled with a highly osmiophilic substance. The two enzymes, DGAT1 [DAG (diacylglycerol) acyltransferase 1] and DGAT2 (DAG acyltransferase 2), involved in this process are synthesized on rough ER and localized in the ER near a monolayer surrounding entities like lipid bodies. After reaching the appropriate size, newly formed lipid bodies transform into mature spherical lipid bodies filled with less osmiophilic content. They appear to be surrounded by a half-unit membrane, with numerous microtubules running adjacently in different directions. The ER, no longer continuous with lipid bodies, makes contact with them through microtubules. At this stage, lipid synthesis takes place at the periphery of lipid bodies. This presumption, and a hypothesis that microtubules are involved in lipid synthesis delivering necessary components to lipid bodies, is based on strong arguments: (i) silver grains first appear over microtubules after a short [3H]palmitic acid incubation and before they are observed over lipid bodies; (ii) blockade of [3H]palmitic acid incorporation into lipotubuloids by propyzamide, an inhibitor of microtubule function; and (iii) the presence of gold grains above the microtubules after DGAT1 and DGAT2 reactions, as also near microtubules after an immunogold method that identifies phospholipase D1.

Genomic analysis of phospholipase D family and characterization of GmPLDαs in soybean (Glycine max)

Phospholipase D (PLD) and its product phosphatidic acid play important roles in the regulation of plant growth, development, and stress responses. The genome database analysis has revealed PLD family in Arabidopsis, rice, poplar and grape. In this study, we report a genomic analysis of 18 putative soybean (Glycine max) PLD genes (GmPLDs), which exist in the 14 of 20 chromosomes. GmPLDs were grouped into six types, α(3), β(4), γ, δ(5), ε(2), and ζ(3), based on gene architectures, protein domains, evolutionary relationship, and sequence identity. These GmPLDs contained two HKD domains, PX/PH domains (for GmPLDζs), and C2 domain (for the other GmPLDs). The expression patterns analyzed by quantitative reverse transcription PCR demonstrated that GmPLDs were expressed differentially in various tissues. GmPLDα1, α2, and β2 were highly expressed in most tissues, whereas GmPLDδ5 was only expressed in flowers and GmPLDζ1 was predominantly expressed in flowers and early pods. The expression of GmPLDα1 and α2 was increased and that of GmPLDγ was decreased by salt stress. GmPLDα1 protein expressed in E. coli was active under the reaction conditions for both PLDα and PLDδ, hydrolyzing the common membrane phospholipids phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol. The genomic analysis for soybean PLD family provides valuable data for further identity and characterization of their functions.

Probing and tracking organelles in living plant cells

Intracellular organelle movements and positioning play pivotal roles in enabling plants to proliferate life efficiently and to survive diverse environmental stresses. The elaborate dissection of organelle dynamics and their underlying mechanisms (e.g., the role of the cytoskeleton in organelle movements) largely depends on the advancement and efficiency of organelle tracking systems. Here, we provide an overview of some recently developed tools for labeling and tracking organelle dynamics in living plant cells.

Wortmannin treatment induces changes in Arabidopsis root proteome and post-Golgi compartments

Wortmannin is a widely used pharmaceutical compound which is employed to define vesicular trafficking routes of particular proteins or cellular compounds. It targets phosphatidylinositol 3-kinase and phosphatidylinositol 4-kinases in a dose-dependent manner leading to the inhibition of protein vacuolar sorting and endocytosis. Combined proteomics and cell biological approaches have been used in this study to explore the effects of wortmannin on Arabidopsis root cells, especially on proteome and endomembrane trafficking. On the subcellular level, wortmannin caused clustering, fusion, and swelling of trans-Golgi network (TGN) vesicles and multivesicular bodies (MVBs) leading to the formation of wortmannin-induced multivesicular compartments. Appearance of wortmannin-induced compartments was associated with depletion of TGN as revealed by electron microscopy. On the proteome level, wortmannin induced massive changes in protein abundance profiles. Wortmannin-sensitive proteins belonged to various functional classes. An inhibition of vacuolar trafficking by wortmannin was related to the downregulation of proteins targeted to the vacuole, as showed for vacuolar proteases. A small GTPase, RabA1d, which regulates vesicular trafficking at TGN, was identified as a new protein negatively affected by wortmannin. In addition, Sec14 was upregulated and PLD1 alpha was downregulated by wortmannin.

The Arabidopsis TRM1-TON1 interaction reveals a recruitment network common to plant cortical microtubule arrays and eukaryotic centrosomes

Land plant cells assemble microtubule arrays without a conspicuous microtubule organizing center like a centrosome. In Arabidopsis thaliana, the TONNEAU1 (TON1) proteins, which share similarity with FOP, a human centrosomal protein, are essential for microtubule organization at the cortex. We have identified a novel superfamily of 34 proteins conserved in land plants, the TON1 Recruiting Motif (TRM) proteins, which share six short conserved motifs, including a TON1-interacting motif present in all TRMs. An archetypal member of this family, TRM1, is a microtubule-associated protein that localizes to cortical microtubules and binds microtubules in vitro. Not all TRM proteins can bind microtubules, suggesting a diversity of functions for this family. In addition, we show that TRM1 interacts in vivo with TON1 and is able to target TON1 to cortical microtubules via its C-terminal TON1 interaction motif. Interestingly, three motifs of TRMs are found in CAP350, a human centrosomal protein interacting with FOP, and the C-terminal M2 motif of CAP350 is responsible for FOP recruitment at the centrosome. Moreover, we found that TON1 can interact with the human CAP350 M2 motif in yeast. Taken together, our results suggest conservation of eukaryotic centrosomal components in plant cells.

Salt stress-induced disassembly of Arabidopsis cortical microtubule arrays involves 26S proteasome-dependent degradation of SPIRAL1

The dynamic instability of cortical microtubules (MTs) (i.e., their ability to rapidly alternate between phases of growth and shrinkage) plays an essential role in plant growth and development. In addition, recent studies have revealed a pivotal role for dynamic instability in the response to salt stress conditions. The salt stress response includes a rapid depolymerization of MTs followed by the formation of a new MT network that is believed to be better suited for surviving high salinity. Although this initial depolymerization response is essential for the adaptation to salt stress, the underlying molecular mechanism has remained largely unknown. Here, we show that the MT-associated protein SPIRAL1 (SPR1) plays a key role in salt stress-induced MT disassembly. SPR1, a microtubule stabilizing protein, is degraded by the 26S proteasome, and its degradation rate is accelerated in response to high salinity. We show that accelerated SPR1 degradation is required for a fast MT disassembly response to salt stress and for salt stress tolerance.

Proteins implicated in mediating self-incompatibility-induced alterations to the actin cytoskeleton of Papaver pollen

BACKGROUND AND AIMS

Sexual reproduction in angiosperms involves a network of signalling and interactions between pollen and pistil. To promote out-breeding, an additional layer of interactions, involving self-incompatibility (SI), is used to prevent self-fertilization. SI is generally controlled by the S-locus, and comprises allelic pollen and pistil S-determinants. This provides the basis of recognition, and consequent rejection, of incompatible pollen. In Papaver rhoeas, SI involves interaction of pistil PrsS and pollen PrpS, triggering a Ca(2+)-dependent signalling network. This results in rapid and distinctive alterations to both the actin and microtubule cytoskeleton being triggered in 'self' pollen. Some of these alterations are implicated in mediating programmed cell death, involving activation of several caspase-like proteases.

SCOPE

Here we review and discuss our current understanding of the cytoskeletal alterations induced in incompatible pollen during SI and their relationship with programmed cell death. We focus on data relating to the formation of F-actin punctate foci, which have, to date, not been well characterized. The identification of two actin-binding proteins that interact with these structures are reviewed. Using an approach that enriched for F-actin from SI-induced pollen tubes using affinity purification followed by mass spectrometry, further proteins were identified as putative interactors with the F-actin foci in an SI situation.

KEY RESULTS

Previously two important actin-binding proteins, CAP and ADF, had been identified whose localization altered with SI, both showing co-localization with the F-actin punctate foci based on immunolocalization studies. Further analysis has identified differences between proteins associated with F-actin from SI-induced pollen samples and those associated with F-actin in untreated pollen. This provides candidate proteins implicated in either the formation or stabilization of the punctate actin structures formed during SI.

CONCLUSIONS

This review brings together for the first time, our current understanding of proteins and events involved in SI-induced signalling to the actin cytoskeleton in incompatible Papaver pollen.

Molecular, cellular, and physiological responses to phosphatidic acid formation in plants

Phosphatidic acid (PA) is an essential phospholipid involved in membrane biosynthesis and signal transduction in all eukaryotes. This review focuses on its role as lipid second messenger during plant stress, metabolism, and development. The contribution of different individual isoforms of enzymes that generate and break down PA will be discussed and the downstream responses highlighted, with particular focus on proteins that bind PA. Through characterization of several of these PA targets, a molecular and genetic basis for PA's role in plant stress and development is emerging.

Arabidopsis thaliana, a plant model organism for the neuronal microtubule cytoskeleton?

The microtubule cytoskeleton is an important component of both neuronal cells and plant cells. While there are large differences in the function of microtubules between the two groups of organisms, for example plants coordinate the ordered deposition of cellulose through the microtubule cytoskeleton, there are also some notable similarities. It is suggested that Arabidopsis thaliana, with its superior availability of knockout lines, may be a suitable model organism for some aspects of the neuronal microtubule cytoskeleton. Some cellular processes that involve the neuronal microtubule cytoskeleton including neurotransmitter signalling and neurotrophic support may have homologous processes in plant cells. A number of microtubule-associated proteins (MAPs) are conserved, including katanin, EB1, CLASP, spastin, gephyrin, CRIPT, Atlastin/RHD3, and ELP3. As a demonstration of the usefulness of a plant model system for neuronal biology, an analysis of plant tubulin-binding proteins was used to show that Charcot-Marie-Tooth disease type 2D and spinal muscular atrophy may be due to microtubule dysfunction and suggest that indeed the plant microtubule cytoskeleton may be particularly similar to that of motor neurons as both are heavily reliant upon motor proteins.

Cytoskeleton and plant salt stress tolerance

The plant cytoskeleton is a highly dynamic component of plant cells and mainly based on microtubules (MTs), and actin filaments (AFs). The important functions of dynamic cytoskeletal networks have been indicated for almost every intracellular activity, from cell division to cell movement, cell morphogenesis and cell signal transduction. Recent studies have also indicated a close relationship between the plant cytoskeleton and plant salt stress tolerance. Salt stress is a significant factor that adversely affects crop productivity and quality of agricultural fields worldwide. The complicated regulatory mechanisms of plant salt tolerance have been the subject of intense research for decades. It is well accepted that cellular changes are very important in plant responses to salt stress. Because the organization and dynamics of cytoskeleton may play an important role in enhancing plant tolerance through various cell activities, study on salt stress-induced cytoskeletal network has been a vital topic in the subject of plant salt stress tolerance mechanisms. In this article, we introduce our recent work and review some current information on the dynamic changes and functions of cytoskeletal organization in response to salt stress. The accumulated data point to the existence of highly dynamic cytoskeletal arrays and the activation of complex cytoskeletal regulatory networks in response to salt stresses. The important role played by cytoskeleton in mediating the plant cell's response to salt stresses is particularly emphasized.

A mechanochemical model explains interactions between cortical microtubules in plants

Microtubules anchored to the two-dimensional cortex of plant cells collide through plus-end polymerization. Collisions can result in rapid depolymerization, directional plus-end entrainment, or crossover. These interactions are believed to give rise to cellwide self-organization of plant cortical microtubules arrays, which is required for proper cell wall growth. Although the cell-wide self-organization has been well studied, less emphasis has been placed on explaining the interactions mechanistically from the molecular scale. Here we present a model for microtubule-cortex anchoring and collision-based interactions between microtubules, based on a competition between cross-linker bonding, microtubule bending, and microtubule polymerization. Our model predicts a higher probability of entrainment at smaller collision angles and at longer unanchored lengths of plus-ends. This model addresses observed differences between collision resolutions in various cell types, including Arabidopsis cells and Tobacco cells.

The male sterile 8 mutation of maize disrupts the temporal progression of the transcriptome and results in the mis-regulation of metabolic functions

Maize anther ontogeny is complex, with the expression of more than 30,000 genes over 4 days of cell proliferation, cell fate acquisition and the start of meiosis. Although many male-sterile mutants disrupt these key steps, few have been investigated in detail. The terminal phenotypes of Zea mays (maize) male sterile 8 (ms8) are small anthers exhibiting meiotic failure. Here, we document much earlier defects: ms8 epidermal cells are normal in number but fail to elongate, and there are fewer, larger tapetal cells that retain, rather than secrete, their contents. ms8 meiocytes separate early, have extra space between them, occupied by excess callose, and the meiotic dyads abort. Thousands of transcriptome changes occur in ms8, including ectopic activation of genes not expressed in fertile siblings, failure to express some genes, differential expression compared with fertile siblings and about 40% of the differentially expressed transcripts appear precociously. There is a high correlation between mRNA accumulation assessed by microarray hybridization and quantitative real-time reverse transcriptase polymerase chain reaction. Sixty-three differentially expressed proteins were identified after two-dimensional gel electrophoresis followed by liquid chromatography tandem mass spectroscopy, including those involved in metabolism, plasmodesmatal remodeling and cell division. The majority of these were not identified by differential RNA expression, demonstrating the importance of proteomics in defining developmental mutants.

Model for the orientational ordering of the plant microtubule cortical array

The plant microtubule cortical array is a striking feature of all growing plant cells. It consists of a more or less homogeneously distributed array of highly aligned microtubules connected to the inner side of the plasma membrane and oriented transversely to the cell growth axis. Here, we formulate a continuum model to describe the origin of orientational order in such confined arrays of dynamical microtubules. The model is based on recent experimental observations that show that a growing cortical microtubule can interact through angle dependent collisions with pre-existing microtubules that can lead either to co-alignment of the growth, retraction through catastrophe induction or crossing over the encountered microtubule. We identify a single control parameter, which is fully determined by the nucleation rate and intrinsic dynamics of individual microtubules. We solve the model analytically in the stationary isotropic phase, discuss the limits of stability of this isotropic phase, and explicitly solve for the ordered stationary states in a simplified version of the model.

Silymarin secretion and its elicitation by methyl jasmonate in cell cultures of Silybum marianum is mediated by phospholipase D-phosphatidic acid

The flavonolignan silymarin is released to the extracellular medium of Silybum marianum cultures and its production can be stimulated by the elicitor methyljasmonate (MeJA). The sequence of the signalling processes leading to this response is unknown at present. It is reported in this work that MeJA increased the activity of the enzyme phospholipase D (PLD). Treatment with mastoparan (Mst), a PLD activity stimulator, also enhanced PLD and caused a substantial increase in silymarin production. The application of the product of PLD activity, phosphatidic acid (PA) promoted silymarin accumulation. Altering PLD activity by introducing in cultures n-butanol (nBuOH), which inhibits PA production by PLD, prevented silymarin elicitation by MeJA or Mst and also impeded its release in non-elicited cultures. Treatment with iso-, sec- or tert- butanol had no effect on silymarin production. The exogenous addition of PA reversed the inhibitory action of nBuOH, both in control and MeJA-treated cultures. These results suggest that the enzyme PLD and its product PA mediate silymarin secretion to the medium of S. marianum cultures.

Depletion of the membrane-associated acyl-coenzyme A-binding protein ACBP1 enhances the ability of cold acclimation in Arabidopsis

In Arabidopsis (Arabidopsis thaliana), a family of six genes encodes acyl-coenzyme A-binding proteins (ACBPs). A member of this family, ACBP1, contains an amino-terminal transmembrane domain that targets it to the plasma membrane and the endoplasmic reticulum. To investigate ACBP1 function, ACBP1-overexpressing transgenic Arabidopsis plants were characterized using lipid analysis. ACBP1 overexpressors showed reduction in several species of diunsaturated phosphatidylcholine (PC), prompting us to investigate if they were altered in response to freezing stress. ACBP1 overexpressors demonstrated increased freezing sensitivity accompanied by a decrease in PC and an increase in phosphatidic acid (PA), while acbp1 mutant plants showed enhanced freezing tolerance associated with PC accumulation and PA reduction. We also showed binding of a recombinant eukaryotic ACBP (ACBP1) to PA, indicative of the possibility of enhanced PA interaction in ACBP1 overexpressors. Since phospholipase Dalpha1 (PLDalpha1) is a major enzyme promoting the hydrolysis of PC to PA, PLDalpha1 expression was examined and was observed to be higher in ACBP1 overexpressors than in acbp1 mutant plants. In contrast, the expression of PLDdelta, which plays a positive role in freezing tolerance, declined in the ACBP1 overexpressors but increased in acbp1 mutant plants. Given that ACBP1 is localized to the endoplasmic reticulum and plasma membrane, it may regulate the expression of PLDalpha1 and PLDdelta by maintaining a membrane-associated PA pool through its ability to bind PA. Moreover, both genotypes showed no alterations in proline and soluble sugar content or in cold-regulated (COR6.6 and COR47) gene expression, suggesting that the ACBP1-mediated response is PLD associated and is independent of osmolyte accumulation.

Mechanisms of self-organization of cortical microtubules in plants revealed by computational simulations

Microtubules confined to the two-dimensional cortex of elongating plant cells must form a parallel yet dispersed array transverse to the elongation axis for proper cell wall expansion. Some of these microtubules exhibit free minus-ends, leading to migration at the cortex by hybrid treadmilling. Collisions between microtubules can result in plus-end entrainment ("zippering") or rapid depolymerization. Here, we present a computational model of cortical microtubule organization. We find that plus-end entrainment leads to self-organization of microtubules into parallel arrays, whereas catastrophe-inducing collisions do not. Catastrophe-inducing boundaries (e.g., upper and lower cross-walls) can tune the orientation of an ordered array to a direction transverse to elongation. We also find that changes in dynamic instability parameters, such as in mor1-1 mutants, can impede self-organization, in agreement with experimental data. Increased entrainment, as seen in clasp-1 mutants, conserves self-organization, but delays its onset and fails to demonstrate increased ordering. We find that branched nucleation at acute angles off existing microtubules results in distinctive sparse arrays and infer either that microtubule-independent or coparallel nucleation must dominate. Our simulations lead to several testable predictions, including the effects of reduced microtubule severing in katanin mutants.

Phospholipase D family interactions with the cytoskeleton: isoform delta promotes plasma membrane anchoring of cortical microtubules

Phospholipase D (PLD) is a key enzyme in signal transduction - mediating plant responses to various environmental stresses including drought and salinity. Isotype PLDδ interacts with the microtubule cytoskeleton, although it is unclear if, or how, each of the 12 PLD isotypes in Arabidopsis may be involved mechanistically. We employed RNA interference in epidermal cells of Allium porrum L. (leek) leaves, in which the developmental reorientation of cortical microtubule arrays to a longitudinal direction is highly sensitive to experimental manipulation. Using particle bombardment and transient transformation with synthetic siRNAs targeting AtPLDα, β, γ, δ, ॉ and ζ, we examined the effect of 'cross-target' silencing orthologous A. porrum genes on microtubule reorientation dynamics during cell elongation. Co-transformation of individual siRNAs together with a GFP-MBD microtubule-reporter gene revealed that siRNAs targeting AtPLDδ promoted, whereas siRNAs targeting AtPLDβ and γ reduced, longitudinal microtubule orientation in A. porrum. These PLD isotypes, therefore, interact, directly or indirectly, with the cytoskeleton and the microtubule-plasma membrane interface. The unique response of PLDδ to silencing, along with its exclusive localisation to the plasma membrane, indicates that this isotype is specifically involved in promoting microtubule-membrane anchorage.

Reassessing the role of phospholipase D in the Arabidopsis wounding response

Plants respond to wounding by means of a multitude of reactions, with the purpose of stifling herbivore assault. Phospholipase D (PLD) has previously been implicated in the wounding response. Arabidopsis (Arabidopsis thaliana) AtPLDalpha1 has been proposed to be activated in intact cells, and the phosphatidic acid (PA) it produces to serve as a precursor for jasmonic acid (JA) synthesis and to be required for wounding-induced gene expression. Independently, PLD activity has been reported to have a bearing on wounding-induced MAPK activation. However, which PLD isoforms are activated, where this activity takes place (in the wounded or non-wounded cells) and what exactly the consequences are is a question that has not been comprehensively addressed. Here, we show that PLD activity during the wounding response is restricted to the ruptured cells using (32)P(i)-labelled phospholipid analyses of Arabidopsis pld knock-out mutants and PLD-silenced tomato cell-suspension cultures. pldalpha1 knock-out lines have reduced wounding-induced PA production, and the remainder is completely eliminated in a pldalpha1/delta double knock-out line. Surprisingly, wounding-induced protein kinase activation, AtLOX2 gene expression and JA biosynthesis were not affected in these knock-out lines. Moreover, larvae of the Cabbage White butterfly (Pieris rapae) grew equally well on wild-type and the pld knock-out mutants.

Function and regulation of phospholipid signalling in plants

As an important metabolic pathway, phosphatidylinositol metabolism generates both constitutive and signalling molecules that are crucial for plant growth and development. Recent studies using genetic and molecular approaches reveal the important roles of phospholipid molecules and signalling in multiple processes of higher plants, including root growth, pollen and vascular development, hormone effects and cell responses to environmental stimuli plants. The present review summarizes the current progress in our understanding of the functional mechanism of phospholipid signalling, with an emphasis on the regulation of Ins(1,4,5)P3-Ca2+ oscillation, the second messenger molecule phosphatidic acid and the cytoskeleton.

Ca2+ influx and phosphoinositide signalling are essential for the establishment and maintenance of cell polarity in monospores from the red alga Porphyra yezoensis

The asymmetrical distribution of F-actin directed by cell polarity has been observed during the migration of monospores from the red alga Porphyra yezoensis. The significance of Ca2+ influx and phosphoinositide signalling during the formation of cell polarity in migrating monospores was analysed pharmacologically. The results indicate that the inhibition of the establishment of cell polarity, as judged by the ability of F-actin to localize asymmetrically, cell wall synthesis, and development into germlings, occurred when monospores were treated with inhibitors of the Ca2+ permeable channel, phospholipase C (PLC), diacylglycerol kinase, and inositol-1,4,5-trisphosphate receptor. Moreover, it was also found that light triggered the establishment of cell polarity via photosynthetic activity but not its direction, indicating that the Ca2+ influx and PLC activation required for the establishment of cell polarity are light dependent. By contrast, inhibition of phospholipase D (PLD) prevented the migration of monospores but not the asymmetrical localization of F-actin. Taken together, these findings suggest that there is functional diversity between the PLC and PLD signalling systems in terms of the formation of cell polarity; the former being critical for the light-dependent establishment of cell polarity and the latter playing a role in the maintenance of established cell polarity.

Suppression of a phospholipase D gene, OsPLDbeta1, activates defense responses and increases disease resistance in rice

Phospholipase D (PLD) plays an important role in plants, including responses to abiotic as well as biotic stresses. A survey of the rice (Oryza sativa) genome database indicated the presence of 17 PLD genes in the genome, among which OsPLDalpha1, OsPLDalpha5, and OsPLDbeta1 were highly expressed in most tissues studied. To examine the physiological function of PLD in rice, we made knockdown plants for each PLD isoform by introducing gene-specific RNA interference constructs. One of them, OsPLDbeta1-knockdown plants, showed the accumulation of reactive oxygen species in the absence of pathogen infection. Reverse transcription-polymerase chain reaction and DNA microarray analyses revealed that the knockdown of OsPLDbeta1 resulted in the up-/down-regulation of more than 1,400 genes, including the induction of defense-related genes such as pathogenesis-related protein genes and WRKY/ERF family transcription factor genes. Hypersensitive response-like cell death and phytoalexin production were also observed at a later phase of growth in the OsPLDbeta1-knockdown plants. These results indicated that the OsPLDbeta1-knockdown plants spontaneously activated the defense responses in the absence of pathogen infection. Furthermore, the OsPLDbeta1-knockdown plants exhibited increased resistance to the infection of major pathogens of rice, Pyricularia grisea and Xanthomonas oryzae pv oryzae. These results suggested that OsPLDbeta1 functions as a negative regulator of defense responses and disease resistance in rice.

Pb/Cu effects on the organization of microtubule cytoskeleton in interphase and mitotic cells of Allium sativum L

The effects of lead and copper on the arrangement of microtubule (MT) cytoskeleton in root tip cells of Allium sativum L. were investigated. Batch cultures of garlic were carried out under defined conditions in the presence 10(-4) M Pb/Cu of various duration treatments. With tubulin immunolabelling and transmission electron microscopy (TEM), we found four different types of MT structures depending on the cell cycle stage: the interphase array, preprophase band, mitotic spindle and phragmoplast were typical for the control cells. Pb/Cu affected the mechanisms controlling the organization of MT cytoskeleton, and induces the following aberrations in interphase and mitotic cells. (1) Pb/Cu induced the formation of atypical MT arrays in the cortical cytoplasm of the interphase cells, consisting of skewed, wavy MT bundles, MT fragments and ring-like tubulin aggregations. (2) Pb/Cu disordered the chromosome movements carried out by the mitotic spindle. The outcome was chromosome aberrations, for example, chromosome bridges and chromosome stickiness, as well as inhibition of cells from entering mitosis. (3) Depending on the time of exposure, MTs disintegrated into shorter fragments or they completely disappeared, indicating MT depolymerization. (4) Different metals had different effects on MT organization. MTs were more sensitive to the pressure of Cu ions than Pb. Moreover, TEM observations showed that the MTs were relatively short and in some places wavy when exposed to 10(-4) M Pb/Cu solutions for 1-2 h. In many sections MTs were no longer visible with increasing duration of treatment (>4 h). Based on these results, we suggested that MT cytoskeleton is primarily responsible for Pb/Cu-associated toxicity and tolerance in plants.