Simultaneous visualization of peroxisomes and cytoskeletal elements reveals actin and not microtubule-based peroxisome motility in plants

Growth and Division of Peroxisomes

Peroxisomes are ubiquitous subcellular organelles, which are highly dynamic and display large plasticity in response to cellular and environmental conditions. Novel proteins and pathways that mediate and control peroxisome formation, growth, and division continue to be discovered, and the cellular machineries that act together to regulate peroxisome number and size are under active investigation. Here, advances in the field of peroxisomal dynamics and proliferation in mammals and yeast are reviewed. The authors address the signals, conditions, and proteins that affect, regulate, and control the number and size of this essential organelle, especially the components involved in the division of peroxisomes. Special emphasis is on the function of dynamin-related proteins (DRPs), on Fis1, a putative adaptor for DRPs, on the role of the Pex11 family of peroxisomal membrane proteins, and the cytoskeleton.

Arabidopsis group Ie formins localize to specific cell membrane domains, interact with actin-binding proteins and cause defects in cell expansion upon aberrant expression

The closely related proteins AtFH4 and AtFH8 represent the group Ie clade of Arabidopsis formin homologues. The subcellular localization of these proteins and their ability to affect the actin cytoskeleton were examined. AtFH4 protein activity was identified using fluorimetric techniques. Interactions between Arabidopsis profilin isoforms and AtFH4 were assayed in vitro and in vivo using pull-down assays and yeast-2-hybrid. The subcellular localization of group Ie formins was observed with indirect immunofluorescence (AtFH4) and an ethanol-inducible green fluorescent protein (GFP) fusion construct (AtFH8). AtFH4 protein affected actin dynamics in vitro, and yeast-2-hybrid assays suggested isoform-specific interactions with the actin-binding protein profilin in vivo. Indirect immunofluorescence showed that AtFH4 localized specifically to the cell membrane at borders between adjoining cells. Expression of an AtFH8 fusion protein resulted in GFP localization to cell membrane zones, similar to AtFH4. Furthermore, aberrant expression of AtFH8 resulted in the inhibition of root hair elongation. Taken together, these data suggest that the group Ie formins act with profilin to regulate actin polymerization at specific sites associated with the cell membrane.

The peroxisomal multifunctional protein interacts with cortical microtubules in plant cells

Background

The plant peroxisomal multifunctional protein (MFP) possesses up to four enzymatic activities that are involved in catalyzing different reactions of fatty acid β-oxidation in the peroxisome matrix. In addition to these peroxisomal activities, in vitro assays revealed that rice MFP possesses microtubule- and RNA-binding activities suggesting that this protein also has important functions in the cytosol.

Results

We demonstrate that MFP is an authentic microtubule-binding protein, as it localized to the cortical microtubule array in vivo, in addition to its expected targeting to the peroxisome matrix. MFP does not, however, interact with the three mitotic microtubule arrays. Microtubule co-sedimentation assays of truncated versions of MFP revealed that multiple microtubule-binding domains are present on the MFP polypeptide. This indicates that these regions function together to achieve high-affinity binding of the full-length protein. Real-time imaging of a transiently expressed green fluorescent protein-MFP chimera in living plant cells illustrated that a dynamic, spatial interaction exits between peroxisomes and cortical microtubules as peroxisomes move along actin filaments or oscillate at fixed locations.

Conclusion

Plant MFP is associated with the cortical microtubule array, in addition to its expected localization in the peroxisome. This observation, coupled with apparent interactions that frequently occur between microtubules and peroxisomes in the cell cortex, supports the hypothesis that MFP is concentrated on microtubules in order to facilitate the regulated import of MFP into peroxisomes.

Recruitment of myosin VIII towards plastid surfaces is root-cap specific and provides the evidence for actomyosin involvement in root osmosensing

The existence of a cell wall-plasma membrane-cytoskeleton (WMC) continuum in plants has long been postulated. However, the individual molecules building such a continuum are still largely unknown. We test here the hypothesis that the integrin-based multiprotein complexes of animal cells have been replaced in plants with more dynamic entities. Using an experimental approach based on protoplast digestion mixtures, and utilising specific antibodies against Arabidopsis ATM1 myosin, we reveal possible roles played by plant-specific unconventional myosin VIII in the functioning of WMC continuum. We demonstrate rapid relocation (less than 5 min) of myosin VIII to statolith surfaces in maize root-cap cells, which is accompanied by the reorganisation of actin cytoskeleton. Upon prolonged stimulation, myosin VIII is also recruited to plasmodesmata and pit-fields of plasmolysing root cap statocytes. The osmotic stimulus is the major factor inducing relocation, but the cell wall-cytoskeleton interactions also play an important role. In addition, we demonstrate the tight association of myosin VIII with the surfaces of chloroplasts, and provide an indication for the differences in the mechanisms of plastid movement in roots and leaves of plants. Overall, our data provide evidence for the active involvement of actomyosin complexes, rooted in the WMC continuum, in the cellular volume control and maintenance of spatial relationships between cellular compartments.

The control of peroxisome number and size during division and proliferation

Like other subcellular organelles, peroxisomes divide and segregate to daughter cells during cell division, but this organelle can also proliferate or be degraded in response to environmental cues. Although the mechanisms and genes involved in these processes are still under active investigation, an important player in peroxisome proliferation is a dynamin-related protein (DRP) that is recruited to the organelle membrane by a DRP receptor. Related DRPs also function in the division of mitochondria and chloroplasts. Many other proteins and signals regulate peroxisome division and proliferation, but their modes of action are still being studied.

Limkain b1, a novel human autoantigen localized to a subset of ABCD3 and PXF marked peroxisomes

Detection of self-reactive antibodies has an established role in the diagnosis and monitoring of many human autoimmune diseases. Autoantibodies with restricted reactivity to cytoplasmic compartments and structures are an occasional incidental finding following routine examination of serum for antinuclear antibody reactivity. A prerequisite for rational exploitation of self-reactive antibodies, in either clinical or research settings, is the establishment of the molecular identity of the target autoantigen(s). Here we report on the identification of a novel autoantigen that co-localizes with a subset of cytoplasmic microbodies marked by ABCD3 (PMP-70) and/or PXF (PEX19). Immunoscreening a HeLa cell cDNA expression library with a human autoimmune serum identified two clones that encode fragments of limkain b1 (LKAP). We demonstrate that mouse polyclonal antibodies raised against a bacterially expressed fragment of limkain b1 mark the same cytoplasmic structures as human serum, as does an EGFP:LKAPCT429 fusion protein expressed in HeLa cells. An immunoblot screen against a bacterially expressed MBP:LKAPCT429 fusion protein substrate, using a cohort of 16 additional human sera that display Hep 2 cell cytoplasmic staining patterns similar to the prototype serum, identified three additional sera reactive to limkain b1. This is the first report establishing the molecular identity of a peroxisomal autoantigen. Preliminary results suggest that limkain b1 may be a relatively common target of human autoantibodies reactive to cytoplasmic vesicle-like structures.

GFP-FABD2 fusion construct allows in vivo visualization of the dynamic actin cytoskeleton in all cells of Arabidopsis seedlings

In vivo visualization of filamentous actin in all cells of Arabidopsis thaliana seedlings is essential for understanding the numerous roles of the actin cytoskeleton in diverse processes of cell differentiation. A previously introduced reporter construct based on the actin-binding domain of mouse talin proved to be useful for unravelling some of these aspects in cell layers close to the organ surface. However, cells more deeply embedded, especially stelar cells active in polar transport of auxin, show either diffuse or no fluorescence at all due to the lack of expression of the fusion protein. The same problem is encountered in the root meristem. Recently introduced actin reporters based on fusions between A. thaliana fimbrin 1 and GFP gave brilliant results in organs from the root differentiation zone upwards to the leaves, however failed to depict the filamentous actin cytoskeleton in the transition zone of the root, in the apical meristem and the root cap. To overcome these problems, we have prepared new transgenic lines for the visualization of F-actin in vivo. We report here that a construct consisting of GFP fused to the C-terminal half of A. thaliana fimbrin 1 reveals dynamic arrays of F-actin in all cells of stably transformed A. thaliana seedlings.

Peroxisomal localization of a myosin XI isoform in Arabidopsis thaliana

The genome of Arabidopsis thaliana contains 13 myosin XI isoforms. Here we prepared a specific antibody against a peptide that mimics a unique C-terminal region from the myosin XI isoform, MYA2. The resulting antibody was used to demonstrate that MYA2 in Arabidopsis protein extracts co-sedimented with actin filaments and dissociated from the filaments with ATP treatment. Immunolocalization studies showed that MYA2 co-localized predominantly with actin filaments in clustered punctuate dots in leaf epidermal cells, root hair cells and suspension-cultured cells. In a transgenic plant in which peroxisomes are labeled with green fluorescent protein, some MYA2 signals were localized on peroxisomes in an actin-dependent manner. We propose that the peroxisome is one of the cargos translocated by MYA2 on actin filaments.

Visualization of autophagy in Arabidopsis using the fluorescent dye monodansylcadaverine and a GFP-AtATG8e fusion protein

Autophagy is a process that is thought to occur in all eukaryotes in which cells recycle cytoplasmic contents when subjected to environmental stress conditions or during certain stages of development. Upon induction of autophagy, double membrane-bound structures called autophagosomes engulf portions of the cytoplasm and transfer them to the vacuole or lysosome for degradation. In this study, we have characterized two potential markers for autophagy in plants, the fluorescent dye monodansylcadaverine (MDC) and a green fluorescent protein (GFP)-AtATG8e fusion protein, and propose that they both label autophagosomes in Arabidopsis. Both markers label the same small, apparently membrane-bound structures found in cells under conditions that are known to induce autophagy such as starvation and senescence. They are usually seen in the cytoplasm, but occasionally can be observed within the vacuole, consistent with a function in the transfer of cytoplasmic material into the vacuole for degradation. MDC-staining and the GFP-AtATG8e fusion protein can now be used as very effective tools to complement biochemical and genetic approaches to the study of autophagy in plant systems.

Peroxisome elongation and constriction but not fission can occur independently of dynamin-like protein 1

The mammalian dynamin-like protein DLP1 belongs to the dynamin family of large GTPases, which have been implicated in tubulation and fission events of cellular membranes. We have previously shown that the expression of a dominant-negative DLP1 mutant deficient in GTP hydrolysis (K38A) inhibited peroxisomal division in mammalian cells. In this study, we conducted RNA interference experiments to 'knock down' the expression of DLP1 in COS-7 cells stably expressing a GFP construct bearing the C-terminal peroxisomal targeting signal 1. The peroxisomes in DLP1-silenced cells were highly elongated with a segmented morphology. Ultrastructural and quantitative studies confirmed that the tubular peroxisomes induced by DLP1-silencing retained the ability to constrict their membranes but were not able to divide into spherical organelles. Co-transfection of DLP1 siRNA with Pex11pbeta, a peroxisomal membrane protein involved in peroxisome proliferation, induced further elongation and network formation of the peroxisomal compartment. Time-lapse microscopy of living cells silenced for DLP1 revealed that the elongated peroxisomes moved in a microtubule-dependent manner and emanated tubular projections. DLP1-silencing in COS-7 cells also resulted in a pronounced elongation of mitochondria, and in more dispersed, elongated Golgi structures, whereas morphological changes of the rER, lysosomes and the cytoskeleton were not detected. These observations clearly demonstrate that DLP1 acts on multiple membranous organelles. They further indicate that peroxisomal elongation, constriction and fission require distinct sets of proteins, and that the dynamin-like protein DLP1 functions primarily in the latter process.

Plant peroxisomes

Peroxisomes, one of single membrane-bound organelles, are present ubiquitously in eukaryotic cells. They were originally identified as organelles for production of hydrogen peroxide, the degradation of its hydrogen peroxide, and metabolism of fatty acids, which are functions common to almost all the organisms. Meanwhile, photorespiration and assimilation of symbiotically induced nitrogen are plant-specific functions. Recent postgenetic approaches such as transcriptome and proteome showed that plant peroxisomes are differentiated in various tissues, and revealed that peroxisomes have more important roles in various metabolic processes including biosynthesis of plant hormones than we speculated. All peroxisomal proteins, including metabolic enzymes in the matrix, membrane proteins, and factors responsible for peroxisome biogenesis, are nuclear encoded, and are provided from the outside of peroxisomes. Peroxisome biogenesis, such as protein transport, division, and enlargement, requires various complicated steps and is one of the most intriguing topics. Analyses using peroxisome biogenesis mutants and the whole-scale sequencing projects among several organisms revealed the existence of essential factors responsible for peroxisome biogenesis such as peroxins. This review addresses a comprehensive issue relating to function and biogenesis of plant peroxisomes and Arabidopsis mutants that have been accelerating our understanding of peroxisomes in planta.

Subcellular localization of Arabidopsis 3-hydroxy-3-methylglutaryl-coenzyme A reductase

Plants produce diverse isoprenoids, which are synthesized in plastids, mitochondria, endoplasmic reticulum (ER), and the nonorganellar cytoplasm. 3-Hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) catalyzes the synthesis of mevalonate, a rate-limiting step in the cytoplasmic pathway. Several branches of the pathway lead to the synthesis of structurally and functionally varied, yet essential, isoprenoids. Several HMGR isoforms have been identified in all plants examined. Studies based on gene expression and on fractionation of enzyme activity suggested that subcellular compartmentalization of HMGR is an important intracellular channeling mechanism for the production of the specific classes of isoprenoids. Plant HMGR has been shown previously to insert in vitro into the membrane of microsomal vesicles, but the final in vivo subcellular localization(s) remains controversial. To address the latter in Arabidopsis (Arabidopsis thaliana) cells, we conducted a multipronged microscopy and cell fractionation approach that included imaging of chimeric HMGR green fluorescent protein localizations in transiently transformed cell leaves, immunofluorescence confocal microscopy in wild-type and stably transformed seedlings, immunogold electron microscopy examinations of endogenous HMGR in seedling cotyledons, and sucrose density gradient analyses of HMGR-containing organelles. Taken together, the results reveal that endogenous Arabidopsis HMGR is localized at steady state within ER as expected, but surprisingly also predominantly within spherical, vesicular structures that range from 0.2- to 0.6-microm diameter, located in the cytoplasm and within the central vacuole in differentiated cotyledon cells. The N-terminal region, including the transmembrane domain of HMGR, was found to be necessary and sufficient for directing HMGR to ER and the spherical structures. It is believed, although not directly demonstrated, that these vesicle-like structures are derived from segments of HMGR-ER. Nevertheless, they represent a previously undescribed subcellular compartment likely capable of synthesizing mevalonate, which provides new evidence for multiorganelle compartmentalization of the isoprenoid biosynthetic pathways in plants.

Peroxisomal localization of sulfite oxidase separates it from chloroplast-based sulfur assimilation

Recently, we isolated the sulfite oxidase (SO) gene from Arabidopsis thaliana and characterized the purified SO protein. The purpose of the present study was to determine the subcellular localization of this novel plant enzyme. Immunogold electron-microscopic analysis showed the gold labels nearly exclusively in the peroxisomes. To verify this finding, green fluorescent protein was fused to full-length plant SO including the putative peroxisomal targeting signal 1 (PTS1) 'SNL' and expressed in tobacco leaves. Our results showed a punctate fluorescence pattern resembling that of peroxisomes. Co-labelling with MitoTracker-Red excluded that the observed fluorescence was due to mitochondrial sorting. By investigation of deleted or mutated PTS1, no functional peroxisomal targeting signal 2 (PTS2) could be detected in plant SO. This conclusion is supported by expression studies in Pichia pastoris mutants with defined defects either in PTS1- or PTS2-mediated peroxisomal import.

Cell shape development in plants

The shape of a plant cell has long been the cornerstone of diverse areas of plant research but it is only recently that molecular-genetic and cell-biological tools have been effectively combined for dissecting plant cell morphogenesis. Increased understanding of the polar growth characteristics of model cell types, the availability of many morphological mutants and significant advances in fluorescent-protein-aided live-cell visualization have provided the major impetus for these analyses. The cytoskeleton and its regulators have emerged as essential components of the scaffold involved in fabricating plant cell shape. In this article, I collate information from recent discoveries to derive a simple cytoskeleton-based operational framework for plant cell morphogenesis.

Plant organelle positioning

Correct positioning and active movement of organelles within cells are essential for cellular homeostasis and adaptation to external stresses. Unlike animal and fungal systems, plant organelle positioning has not yet been revealed at the molecular level. The recent development of organelle-targeting green fluorescent protein (GFP) constructs and genetic analyses using Arabidopsis thaliana have shed new light on the field of plant organelle positioning, which has been found to be regulated by mechanisms that are similar to and/or distinct from those used by animals and fungi.

Green fluorescent protein fusions to Arabidopsis fimbrin 1 for spatio-temporal imaging of F-actin dynamics in roots

The visualization of green fluorescent protein (GFP) fusions with microtubule or actin filament (F-actin) binding proteins has provided new insights into the function of the cytoskeleton during plant development. For studies on actin, GFP fusions to talin have been the most generally used reporters. Although GFP-Talin has allowed in vivo F-actin imaging in a variety of plant cells, its utility in monitoring F-actin in stably transformed plants is limited particularly in developing roots where interesting actin dependent cell processes are occurring. In this study, we created a variety of GFP fusions to Arabidopsis Fimbrin 1 (AtFim1) to explore their utility for in vivo F-actin imaging in root cells and to better understand the actin binding properties of AtFim1 in living plant cells. Translational fusions of GFP to full-length AtFim1 or to some truncated variants of AtFim1 showed filamentous labeling in transient expression assays. One truncated fimbrin-GFP fusion was capable of labeling distinct filaments in stably transformed Arabidopsis roots. The filaments decorated by this construct were highly dynamic in growing root hairs and elongating root cells and were sensitive to actin disrupting drugs. Therefore, the fimbrin-GFP reporters we describe in this study provide additional tools for studying the actin cytoskeleton during root cell development. Moreover, the localization of AtFim1-GFP offers insights into the regulation of actin organization in developing roots by this class of actin cross-linking proteins.

Mammalian peroxisomes and reactive oxygen species

The central role of peroxisomes in the generation and scavenging of hydrogen peroxide has been well known ever since their discovery almost four decades ago. Recent studies have revealed their involvement in metabolism of oxygen free radicals and nitric oxide that have important functions in intra- and intercellular signaling. The analysis of the role of mammalian peroxisomes in a variety of physiological and pathological processes involving reactive oxygen species (ROS) is the subject of this review. The general characteristics of peroxisomes and their enzymes involved in the metabolism of ROS are briefly reviewed. An expansion of the peroxisomal compartment with proliferation of tubular peroxisomes is observed in cells exposed to UV irradiation and various oxidants and is apparently accompanied by upregulation of PEX genes. Significant reduction of peroxisomes and their enzymes is observed in inflammatory processes including infections, ischemia-reperfusion injury, and allograft rejection and seems to be related to the suppressive effect of tumor necrosis factor-alpha on peroxisome function and peroxisome proliferator activated receptor-alpha. Xenobiotic-induced proliferation of peroxisomes in rodents is accompanied by the formation of hepatic tumors, and evidently the imbalance in generation and decomposition of ROS plays an important role in this process. In PEX5-/- knockout mice lacking functional peroxisomes severe alterations of mitochondria in various organs are observed which seem to be due to a generalized increase in oxidative stress confirming the important role of peroxisomes in homeostasis of ROS and the implications of its disturbances for cell pathology.

In vivo analysis of interactions between GFP-labeled microfilaments and plastid stromules

BACKGROUND

Plastid stromules are stroma-filled tubules that extend from the surface of plastids in higher plants and allow the exchange of protein molecules between plastids. These structures are highly dynamic; stromules change both their shape and position in the cytoplasm very rapidly. Previous studies with microfilament inhibitors indicated that stromule shape and movement are dependent on the actin cytoskeleton. To learn more about the nature of the interactions of stromules and the cytoskeleton, we imaged fluorescently-labeled microfilaments and plastids.

RESULTS

We have used Arabidopsis thaliana plants expressing green fluorescent protein fused to the human actin-binding protein talin to observe microfilaments and their relationship to stromules in vivo. Microfilaments were observed in close contact with stromules and plastid bodies of hypocotyl epidermis. Time-lapse confocal microscopy revealed that microfilament rearrangements were associated with changes in plastid and stromule morphology and position. We also observed close interactions between mitochondria and stromules in double-labeled cells.

CONCLUSION

Our results indicate a correlation between the rearrangement of microfilaments and changes in the shape and position of plastids and stromules. Stromules interact with microfilaments that may also be utilized by mitochondria and other organelles. The interaction of microfilaments and plastids is likely to be mediated by actin-binding proteins on the plastid envelope membrane.

Organelle inheritance in plant cell division: the actin cytoskeleton is required for unbiased inheritance of chloroplasts, mitochondria and endoplasmic reticulum in dividing protoplasts

Nuclear inheritance is highly ordered, ensuring stringent, unbiased partitioning of chromosomes before cell division. In plants, however, little is known about the analogous cellular processes that might ensure unbiased inheritance of non-nuclear organelles, either in meristematic cell divisions or those induced during the acquisition of totipotency. We have investigated organelle redistribution and inheritance mechanisms during cell division in cultured tobacco mesophyll protoplasts. Quantitative analysis of organelle repositioning observed by autofluorescence of chloroplasts or green fluorescent protein (GFP), targeted to mitochondria or endoplasmic reticulum (ER), demonstrated that these organelles redistribute in an ordered manner before division. Treating protoplasts with cytoskeleton-disrupting drugs showed that redistribution depended on actin filaments (AFs), but not on microtubules (MTs), and furthermore, that an intact actin cytoskeleton was required to achieve unbiased organelle inheritance. Labelling the actin cytoskeleton with a novel GFP-fusion protein revealed a highly dynamic actin network, with local reorganisation of this network itself, appearing to contribute substantially to repositioning of chloroplasts and mitochondria. Our observations show that each organelle exploits a different strategy of redistribution to ensure unbiased partitioning. We conclude that inheritance of chloroplasts, mitochondria and ER in totipotent plant cells is an ordered process, requiring complex interactions with the actin cytoskeleton.

An Arabidopsis pex10 null mutant is embryo lethal, implicating peroxisomes in an essential role during plant embryogenesis

Peroxisomes participate in many important functions in plants, including seed reserve mobilization, photorespiration, defense against oxidative stress, and auxin and jasmonate signaling. In mammals, defects in peroxisome biogenesis result in multiple system abnormalities, severe developmental delay, and death, whereas in unicellular yeasts, peroxisomes are dispensable unless required for growth of specific substrates. PEX10 encodes an integral membrane protein required for peroxisome biogenesis in mammals and yeast. To investigate the importance of PEX10 in plants, we characterized a Ds insertion mutant in the PEX10 gene of Arabidopsis (AtPEX10). Heterozygous AtPEX10::dissociation element mutants show normal vegetative phenotypes under optimal growth conditions, but produce about 20% abnormal seeds. The embryos in the abnormal seeds are predominantly homozygous for the disruption allele. They show retarded development and some morphological abnormalities. No viable homozygous mutant plants were obtained. AtPEX10 fused to yellow fluorescent protein colocalized with green fluorescent protein-serine-lysine-leucine, a well-documented peroxisomal marker, suggesting that AtPEX10 encodes a peroxisomal protein that is essential for normal embryo development and viability.

Chloroplast unusual positioning1 is essential for proper chloroplast positioning

The intracellular distribution of organelles is a crucial aspect of effective cell function. Chloroplasts change their intracellular positions to optimize photosynthetic activity in response to ambient light conditions. Through screening of mutants of Arabidopsis defective in chloroplast photorelocation movement, we isolated six mutant clones in which chloroplasts gathered at the bottom of the cells and did not distribute throughout cells. These mutants, termed chloroplast unusual positioning (chup), were shown to belong to a single genetic locus by complementation tests. Observation of the positioning of other organelles, such as mitochondria, peroxisomes, and nuclei, revealed that chloroplast positioning and movement are impaired specifically in this mutant, although peroxisomes are distributed along with chloroplasts. The CHUP1 gene encodes a novel protein containing multiple domains, including a coiled-coil domain, an actin binding domain, a Pro-rich region, and two Leu zipper domains. The N-terminal hydrophobic segment of CHUP1 was expressed transiently in leaf cells of Arabidopsis as a fusion protein with the green fluorescent protein. The fusion protein was targeted to envelope membranes of chloroplasts in mesophyll cells, suggesting that CHUP1 may localize in chloroplasts. A glutathione S-transferase fusion protein containing the actin binding domain of CHUP1 was found to bind F-actin in vitro. CHUP1 is a unique gene identified that encodes a protein required for organellar positioning and movement in plant cells.

The association of peroxisomes with the developing cell plate in dividing onion root cells depends on actin microfilaments and myosin

We have investigated changes in the distribution of peroxisomes through the cell cycle in onion ( Allium cepa L.) root meristem cells with immunofluorescence and electron microscopy, and in leek ( Allium porrum L.) epidermal cells with immunofluorescence and peroxisomal-targeted green fluorescent protein. During interphase and mitosis, peroxisomes distribute randomly throughout the cytoplasm, but beginning late in anaphase, they accumulate at the division plane. Initially, peroxisomes occur within the microtubule phragmoplast in two zones on either side of the developing cell plate. However, as the phragmoplast expands outwards to form an annulus, peroxisomes redistribute into a ring immediately inside the location of the microtubules. Peroxisome aggregation depends on actin microfilaments and myosin. Peroxisomes first accumulate in the division plane prior to the formation of the microtubule phragmoplast, and throughout cytokinesis, always co-localise with microfilaments. Microfilament-disrupting drugs (cytochalasin and latrunculin), and a putative inhibitor of myosin (2,3-butanedione monoxime), inhibit aggregation. We propose that aggregated peroxisomes function in the formation of the cell plate, either by regulating hydrogen peroxide production within the developing cell plate, or by their involvement in recycling of excess membranes from secretory vesicles via the beta-oxidation pathway. Differences in aggregation, a phenomenon which occurs in onion, some other monocots and to a lesser extent in tobacco BY-2 suspension cells, but which is not obvious in the roots of Arabidopsis thaliana (L.) Heynh., may reflect differences within the primary cell walls of these plants.

Overexpression of a human and a fungal ABC transporter similarly suppresses the differentiation defects of a fungal peroxisomal mutant but introduces pleiotropic cellular effects

Among the peroxisome membrane proteins, some are required for peroxisome biogenesis (e.g. PEX2) while others are not, e.g. ABC (ATP-binding cassette) transporters. Unexpectedly, overproduction of the peroxisomal ABC transporter PMP70 was found to be able to restore peroxisome biogenesis in mammalian pex2 mutant cell lines. In the filamentous fungus Podospora anserina, pex2 mutations not only impair peroxisome biogenesis but also cause a precise cell differentiation defect. Here, we show that both defects are partially suppressed by expression of the human cDNA encoding PMP70. In addition, PMP70 expression causes new developmental defects, different from those induced by pex2 mutations. We also show that overexpression of the P. anserina pABC1 gene, which encodes a peroxisomal ABC transporter, leads to similar effects. Taken together, our results demonstrate that: (i) the genetic relationship between PEX2 and PMP70, initially observed in mammals, has been conserved through evolution; (ii) the cell differentiation defect observed in the P. anserina pex2 mutants is indeed linked to impairment in peroxisome biogenesis; and (iii) unexpected detrimental cellular defects result from overproduction of peroxisomal ABC transporters.

Peroxisome biogenesis: advances and conundrums

Investigations of peroxisome biogenesis in diverse organisms reveal new details of this unique process and its evolutionary conservation. Interactions among soluble receptors and the membrane peroxins that catalyze protein translocation are being mapped. Ubiquitination is observed. A receptor enters the organelle carrying folded cargo and recycles back to the cytosol. Tiny peroxisome remnants - vesicles and tubules - are discovered in pex3 mutants that lack the organelle. When the mutant is transfected with a good PEX3 gene, these protoperoxisomes acquire additional membrane peroxins and then import the matrix enzymes to reform peroxisomes. Thus, de novo formation need not be postulated. Dynamic imaging of yeast reveals dynamin-dependent peroxisome division and regulated actin-dependent segregation of the organelle before cell division. These results are consistent with biogenesis by growth and division of pre-existing peroxisomes.

Arabidopsis CROOKED encodes for the smallest subunit of the ARP2/3 complex and controls cell shape by region specific fine F-actin formation

The generation of a specific cell shape requires differential growth, whereby specific regions of the cell expand more relative to others. The Arabidopsis crooked mutant exhibits aberrant cell shapes that develop because of mis-directed expansion, especially during a rapid growth phase. GFP-aided visualization of the F-actin cytoskeleton and the behavior of subcellular organelles in different cell-types in crooked and wild-type Arabidopsis revealed that localized expansion is promoted in cellular regions with fine F-actin arrays but is restricted in areas that maintain dense F-actin. This suggested that a spatiotemporal distinction between fine versus dense F-actin in a growing cell could determine the final shape of the cell. CROOKED was molecularly identified as the plant homolog of ARPC5, the smallest sub-unit of the ARP2/3 complex that in other organisms is renowned for its role in creating dendritic arrays of fine F-actin. Rescue of crooked phenotype by the human ortholog provides the first molecular evidence for the presence and functional conservation of the complex in higher plants. Our cell-biological and molecular characterization of CROOKED suggests a general actin-based mechanism for regulating differential growth and generating cell shape diversity.

Mutations in actin-related proteins 2 and 3 affect cell shape development in Arabidopsis

ACTIN-RELATED PROTEINS 2 and 3 form the major subunits of the ARP2/3 complex, which is known as an important regulator of actin organization in diverse organisms. Here, we report that two genes, WURM and DISTORTED1, which are important for cell shape control in Arabidopsis, encode the plant ARP2 and ARP3 orthologs, respectively. Mutations in these genes result in misdirected expansion of various cell types: trichome expansion is randomized, pavement cells fail to produce lobes, hypocotyl cells curl out of the normal epidermal plane, and root hairs are sinuous. At the subcellular level, cell shape changes are linked to severe filamentous actin aggregation and compromised vacuole fusion. Because all seven subunits of the ARP2/3 complex are present in plants, our data indicate that this complex may play a pivotal role during plant cell morphogenesis.

Microfilaments and microtubules control the morphology and movement of non-green plastids and stromules in Nicotiana tabacum

Plastid stromules are stroma-filled tubular extensions of the plastid envelope membrane. These structures, which have been observed in a number of species, allow transfer of proteins between interconnected plastids. The dramatic shape of stromules and their dynamic movement within the cell provide an opportunity to study the control of morphology and motion of plastids. Using inhibitors of actin and tubulin, we found that both microfilaments and microtubules affect the shape and motility of non-green plastids. Actin and tubulin control plastid and stromule structure by independent mechanisms, while plastid movement is promoted by microfilaments but inhibited by microtubules. The presence or absence of stromules does not affect the motility of plastids. Photobleaching experiments indicate that actin and tubulin are not necessary for the bulk of green fluorescent protein (GFP) movement between plastids via stromules.

Overexpression and mislocalization of a tail-anchored GFP redefines the identity of peroxisomal ER

Peroxisomal ascorbate peroxidase (APX) sorts indirectly via a subdomain of the ER (peroxisomal ER) to the boundary membrane of peroxisomes in tobacco Bright Yellow 2 cells. This novel subdomain characteristically appears as fluorescent reticular/circular compartments distributed variously in the cytoplasm. Further characterizations are presented herein. A peptide possessing the membrane targeting information for peroxisomal APX was fused to GFP (GFP-APX). Transiently expressed GFP-APX sorted to peroxisomes and to reticular/circular compartments; in both cases, the GFP moiety faced the cytosol. Of particular interest, both homotypic and heterotypic aggregates of peroxisomes, mitochondria, and/or plastids were formed. The latter two organelles comprised the circular portion of the reticular/circular compartments, apparently as a consequence of oligomerization (zippering) of the GFP moieties after insertion into the outer membranes of the affected organelles. These results, coupled with the accumulation of endogenous peroxisomal APX in cytoplasmic, noncircular compartment(s) following treatment with brefeldin A, indicate that authentic peroxisomal ER is composed only of a reticular compartment(s). Equally important, the data show that overexpressed, membrane-targeted GFP fusion proteins have a propensity to form organelle aggregates that may lead to misinterpretations of sorting pathways of trafficked proteins.

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.

Remodeling the cytoskeleton for growth and form: an overview with some new views

The cytoskeleton coordinates all aspects of growth in plant cells, including exocytosis of membrane and wall components during cell expansion. This review seeks to integrate current information about cytoskeletal components in plants and the role they play in generating cell form. Advances in genome analysis have fundamentally changed the nature of research strategies and generated an explosion of new information on the cytoskeleton-associated proteins, their regulation, and their role in signaling to the cytoskeleton. Some of these proteins appear novel to plants, but many have close homologues in other eukaryotic systems. It is becoming clear that the mechanisms behind cell growth are essentially similar across the growth continuum, which ranges from tip growth to diffuse expansion. Remodeling of the actin cytoskeleton at sites of exocytosis is an especially critical feature of polarized and may also contribute to axial growth. We evaluate the most recent work on the signaling mechanisms that continually remodel the actin cytoskeleton via the activation of actin-binding proteins (ABPs) and consider the role the microtubule cytoskeleton plays in this process.

In vitro assays demonstrate that pollen tube organelles use kinesin-related motor proteins to move along microtubules

The movement of pollen tube organelles relies on cytoskeletal elements. Although the movement of organelles along actin filaments in the pollen tube has been studied widely and is becoming progressively clear, it remains unclear what role microtubules play. Many uncertainties about the role of microtubules in the active transport of pollen tube organelles and/or in the control of this process remain to be resolved. In an effort to determine if organelles are capable of moving along microtubules in the absence of actin, we extracted organelles from tobacco pollen tubes and analyzed their ability to move along in vitro-polymerized microtubules under different experimental conditions. Regardless of their size, the organelles moved at different rates along microtubules in the presence of ATP. Cytochalasin D did not inhibit organelle movement, indicating that actin filaments are not required for organelle transport in our assay. The movement of organelles was cytosol independent, which suggests that soluble factors are not necessary for the organelle movement to occur and that microtubule-based motor proteins are present on the organelle surface. By washing organelles with KI, it was possible to release proteins capable of gliding carboxylated beads along microtubules. Several membrane fractions, which were separated by Suc density gradient centrifugation, showed microtubule-based movement. Proteins were extracted by KI treatment from the most active organelle fraction and then analyzed with an ATP-sensitive microtubule binding assay. Proteins isolated by the selective binding to microtubules were tested for the ability to glide microtubules in the in vitro motility assay, for the presence of microtubule-stimulated ATPase activity, and for cross-reactivity with anti-kinesin antibodies. We identified and characterized a 105-kD organelle-associated motor protein that is functionally, biochemically, and immunologically related to kinesin. This work provides clear evidence that the movement of pollen tube organelles is not just actin based; rather, they show a microtubule-based motion as well. This unexpected finding suggests new insights into the use of pollen tube microtubules, which could be used for short-range transport, as actin filaments are in animal cells.

Microtubule cytoskeleton: a track record

The plant microtubule cytoskeleton forms unique arrays during cell division and morphogenesis. Recent studies have addressed the biogenesis, turnover, spatio-temporal organisation and cellular function of microtubules. The results suggest that both conserved eukaryotic mechanisms and plant-specific modifications determine microtubule dynamics and function.

Actin polymerization processes in plant cells

Growing evidence shows that the actin cytoskeleton is a key effector of signal transduction, which controls and maintains the shape of plant cells, as well as playing roles in plant morphogenesis. Recently, several signaling pathways, including those triggered by hormones, Ca(2+), and cAMP, have been reported to be connected to the reorganization of the actin cytoskeleton. The molecular mechanisms involved in such signaling cascades are, however, largely unknown. The Arabidopsis genome sequence is a valuable tool for identifying some of the highly conserved molecules that are involved in such signaling cascades. Recent work has begun to unravel these complex pathways using a panoply of techniques, including genetic analysis, live-cell imaging of intracellular actin dynamics, in vivo localization of factors that are involved in the control of actin dynamics, and the biochemical characterization of how these factors function.

Microtubules and microfilaments in cell morphogenesis in higher plants

Microtubules and microfilaments play important roles in cell morphogenesis. The picture emerging from drug studies and molecular-genetic analyses of mutant higher plants defective in cell morphogenesis shows that the roles played by them remain the same in both tip-growing and diffuse-growing cells. Microtubules are important for establishing and maintaining growth polarity whereas actin microfilaments deliver the materials required for growth to specified sites. The recent cloning of several cell morphogenesis genes has revealed that conserved mechanisms as well as novel signal transduction pathways spatially organize the plant cytoskeleton.

A 75-kDa plant protein isolated by tubulin-affinity chromatography is a peroxisomal matrix enzyme

The function of microtubules in plant cells relies on their interactions with various, largely unidentified, proteins. A 75-kDa polypeptide (p75) was isolated previously by tubulin affinity chromatography of tobacco BY-2 proteins and is further characterized here. We have obtained two peptide sequences of 13 and 8 amino acid residues from the p75, which have 77% and 87% similarity to two putative dTDP-glucose 4,6-dehydratase proteins of 74.9 and 75.3 kDa in Arabidopsis. The proteins contain a peroxisomal matrix targeting signal SKL or AKL near their carboxyl terminus, an epimerase-dehydratase motif, and a region of 321 amino acid residues sharing 41% similarity with a dTDP-glucose 4,6-dehydratase from Aneurinibacillus thermoaerophilus. A monoclonal antibody 4B9 against the tobacco p75 reacted with a 75-kDa polypeptide on immunoblots of tobacco BY-2 and Arabidopsis protein extracts, and in immunofluorescence microscopy it revealed small organelle-like structures in tobacco BY-2 and Arabidopsis root-tip cells. Double labelling with an antibody against the peroxisomal marker enzyme, catalase, showed that the organelles are indeed peroxisomes. The peroxisomes were in closer association with actin filaments than microtubules. This observation supports recent findings that plant peroxisomes move on actin filaments. We propose that the peroxisomal dTDP-glucose 4,6-dehydratases may be involved in plant defence responses to oxidative stress.Key words: actin, antibodies, microtubules.

The role of peroxisomes in the integration of metabolism and evolutionary diversity of photosynthetic organisms

The peroxisome is a metabolic compartment serving for the rapid oxidation of substrates, a process that is not coupled to energy conservation. In plants and algae, peroxisomes connect biosynthetic and oxidative metabolic routes and compartmentalize potentially lethal steps of metabolism such as the formation of reactive oxygen species and glyoxylate, thus preventing poisoning of the cell and futile recycling. Peroxisomes exhibit properties resembling inside-out vesicles and possess special systems for the import of specific proteins, which form multi-enzyme complexes (metabolons) linking numerous reactions to flavin-dependent oxidation, coupled to the decomposition of hydrogen peroxide by catalase. Hydrogen peroxide and superoxide originating in peroxisomes are important mediators in signal transduction pathways, particularly those involving salicylic acid. By contributing to the synthesis of oxalate, formate and other organic acids, peroxisomes regulate major fluxes of primary and secondary metabolism. The evolutionary diversity of algae has led to the presence of a wide range of enzymes in the peroxisomes that are only similar to higher plants in their direct predecessors, the Charophyceae. The appearance of seed plants was connected to the acquirement by storage tissues, of a peroxisomal fatty acid oxidation function linked to the glyoxylate cycle, which is induced during seed germination and maturation. Rearrangement of the peroxisomal photorespiratory function between different tissues of higher plants led to the appearance of different types of photosynthetic metabolism. The peroxisome may therefore have played a key role in the evolutionary formation of metabolic networks, via establishing interconnections between different metabolic compartments.