Actin polymerization processes in plant cells

Plastid DNA Barcoding and RtActin cDNA Fragment Isolation of Reutealis Trisperma: A Promising Bioresource for Biodiesel Production

Reutealis trisperma belonging to the family Euphorbiaceae is currently used for biodiesel production, and rapid development in plant-based biofuel production has led to its increasing demand. However, massive utilization of bio-industrial plants has led to conservation issues. Moreover, genetic information on R trisperma is still limited, which is crucial for developmental, physiological, and molecular studies. Studying gene expression is essential to explain plant physiological processes. Nonetheless, this technique requires sensitive and precise measurement of messenger RNA (mRNA). In addition, the presence of internal control genes is important to avoid bias. Therefore, collecting and preserving genetic data for R trisperma is indispensable. In this study, we aimed to evaluate the application of plastid loci, rbcL, and matK, to the DNA barcode of R trisperma for use in conservation programs. In addition, we isolated and cloned the RtActin (RtACT) gene fragment for use in gene expression studies. Sequence information was analyzed in silico by comparison with other Euphorbiaceae plants. For actin fragment isolation, reverse-transcription polymerase chain reaction was used. Molecular cloning of RtActin was performed using the pTA2 plasmid before sequencing. We successfully isolated and cloned 592 and 840 bp of RtrbcL and RtmatK fragment genes, respectively. The RtrbcL barcoding marker, rather than the RtmatK plastidial marker, provided discriminative molecular phylogenetic data for R Trisperma. We also isolated 986 bp of RtACT gene fragments. Our phylogenetic analysis demonstrated that R trisperma is closely related to the Vernicia fordii Actin gene (97% identity). Our results suggest that RtrbcL could be further developed and used as a barcoding marker for R trisperma. Moreover, the RtACT gene could be further investigated for use in gene expression studies of plant.

Leaf proteome modulation and cytological features of seagrass Cymodocea nodosa in response to long-term high CO2 exposure in volcanic vents

Seagrass Cymodocea nodosa was sampled off the Vulcano island, in the vicinity of a submarine volcanic vent. Leaf samples were collected from plants growing in a naturally acidified site, influenced by the long-term exposure to high CO₂ emissions, and compared with others collected in a nearby meadow living at normal pCO₂ conditions. The differential accumulated proteins in leaves growing in the two contrasting pCO₂ environments was investigated. Acidified leaf tissues had less total protein content and the semi-quantitative proteomic comparison revealed a strong general depletion of proteins belonging to the carbon metabolism and protein metabolism. A very large accumulation of proteins related to the cell respiration and to light harvesting process was found in acidified leaves in comparison with those growing in the normal pCO₂ site. The metabolic pathways linked to cytoskeleton turnover also seemed affected by the acidified condition, since a strong reduction in the concentration of cytoskeleton structural proteins was found in comparison with the normal pCO₂ leaves. Results coming from the comparative proteomics were validated by the histological and cytological measurements, suggesting that the long lasting exposure and acclimation of C. nodosa to the vents involved phenotypic adjustments that can offer physiological and structural tools to survive the suboptimal conditions at the vents vicinity.

An Arabidopsis Clathrin Assembly Protein with a Predicted Role in Plant Defense Can Function as an Adenylate Cyclase

Adenylate cyclases (ACs), much like guanylate cyclases (GCs), are increasingly recognized as essential parts of many plant processes including biotic and abiotic stress responses. In order to identify novel ACs, we have applied a search motif derived from experimentally tested GCs and identified a number of Arabidopsis thaliana candidates including a clathrin assembly protein (AT1G68110; AtClAP). AtClAP contains a catalytic centre that can complement the AC-deficient mutant cyaA in E. coli, and a recombinant AtClAP fragment (AtClAP^261–379) can produce cyclic adenosine 3′,5′ monophosphate (cAMP) from adenosine triphosphate (ATP) in vitro. Furthermore, an integrated analysis of gene expression and expression correlation implicate cAMP in pathogen defense and in actin cytoskeletal remodeling during endocytic internalization.

Microfilament Depolymerization Is a Pre-requisite for Stem Cell Formation During In vitro Shoot Regeneration in Arabidopsis

De novo shoot regeneration is widely used in fundamental studies and agricultural applications. Actin microfilaments are involved in many aspects of plant cell division, cell morphogenesis and cell signal transduction. However, the function of actin microfilaments during de novo shoot regeneration is poorly understood. Here, we investigated the organization of actin microfilaments during this process and found that stem cell formation was associated with microfilament depolymerization. Furthermore, inhibition of microfilament depolymerization by phalloidin treatment or downregulation of actin depolymerizing factors (ADFs) restrained stem cell initiation and shoot regeneration. Inhibition of ADF expression affected the architecture of microfilaments during stem cell formation, and the polar transport and distribution of auxin were also disrupted. Together, our results demonstrate that organization of the microfilament cytoskeleton play important roles in stem cell formation and shoot meristem induction during shoot regeneration.

TaARPC3, Contributes to Wheat Resistance against the Stripe Rust Fungus

The actin cytoskeleton participates in numerous cellular processes, including less-characterized processes, such as nuclear organization, chromatin remodeling, transcription, and signal transduction. As a key regulator of actin cytoskeletal dynamics, the actin related protein 2/3 complex (Arp2/3 complex) controls multiple developmental processes in a variety of tissues and cell types. To date, the role of the Arp2/3 complex in plant disease resistance signaling is largely unknown. Herein, we identified and characterized wheat ARPC3, TaARPC3, which encodes the C3 subunit of the Arp2/3 complex. Expression of TaARPC3 in the arc18 mutant of Saccharomyces cerevisiae Δarc18 resulted in complementation of stress-induced phenotypes in S. cerevisiae, as well as restore wild-type cell shape malformations. TaARPC3 was found predominantly to be localized in the nucleus and cytoplasm when expressed transiently in wheat protoplast. TaARPC3 was significantly induced in response to avirulent race of Puccinia striiformis f. sp. tritici (Pst). Knock-down of TaARPC3 by virus-induced gene silencing resulted in a reduction of resistance against Pst through a specific reduction in actin cytoskeletal organization. Interestingly, this reduction was found to coincide with a block in reactive oxygen species (ROS) accumulation, the hypersensitive response (HR), an increase in TaCAT1 mRNA accumulation, and the growth of Pst. Taken together, these findings suggest that TaARPC3 is a key subunit of the Arp2/3 complex which is required for wheat resistance against Pst, a process that is associated with the regulation of the actin cytoskeleton.

Characterization of profilin polymorphism in pollen with a focus on multifunctionality

Profilin, a multigene family involved in actin dynamics, is a multiple partners-interacting protein, as regard of the presence of at least of three binding domains encompassing actin, phosphoinositide lipids, and poly-L-proline interacting patches. In addition, pollen profilins are important allergens in several species like Olea europaea L. (Ole e 2), Betula pendula (Bet v 2), Phleum pratense (Phl p 12), Zea mays (Zea m 12) and Corylus avellana (Cor a 2). In spite of the biological and clinical importance of these molecules, variability in pollen profilin sequences has been poorly pointed out up until now. In this work, a relatively high number of pollen profilin sequences have been cloned, with the aim of carrying out an extensive characterization of their polymorphism among 24 olive cultivars and the above mentioned plant species. Our results indicate a high level of variability in the sequences analyzed. Quantitative intra-specific/varietal polymorphism was higher in comparison to inter-specific/cultivars comparisons. Multi-optional posttranslational modifications, e.g. phosphorylation sites, physicochemical properties, and partners-interacting functional residues have been shown to be affected by profilin polymorphism. As a result of this variability, profilins yielded a clear taxonomic separation between the five plant species. Profilin family multifunctionality might be inferred by natural variation through profilin isovariants generated among olive germplasm, as a result of polymorphism. The high variability might result in both differential profilin properties and differences in the regulation of the interaction with natural partners, affecting the mechanisms underlying the transmission of signals throughout signaling pathways in response to different stress environments. Moreover, elucidating the effect of profilin polymorphism in adaptive responses like actin dynamics, and cellular behavior, represents an exciting research goal for the future.

Effect of post-harvest heat treatment on proteome change of peach fruit during ripening

The extracted proteins from the heat-treated peach fruit (dipped in hot water at 48°C for 10min and then stored at room temperature (20°C-25°C) for up to 6 days) were used for proteomic analysis in order to understand the response of post-harvest peach fruit to heat treatment during ripening stage at proteomic level. After two dimensional gels electrophoresis (2-DE) was conducted, more than 600 protein spots were detected. Among them, 35 differently expressed spots (P<0.05) were selected to be excised and analyzed using MALDI-TOF/TOF, and finally 30 protein spots were confidently identified according to NCBI database. The results demonstrated that among the thirty protein spots expressed particularly induced by heat treatment, 43% were related to stress response, 17% to cell structure, 13% to protein fate, 7% to glycolytic pathway, 3% to ripening and senescence and 17% to unclassified. All of them are involved in the regulation of peach fruit development and ripening. All these indicated that the self-defense capability of peach fruit was improved by heat treatment. The study will enable future detailed investigation of gene expression and function linked with peach fruit ripening.

Characterization and expression analysis of Medicago truncatula ROP GTPase family during the early stage of symbiosis

ROPs (Rho-related GTPases of plants) are small GTPases that are plant-specific signaling proteins. They act as molecular switches in a variety of developmental processes. In this study, seven cDNA clones coding for ROP GTPases have been isolated in Medicago truncatula, and conserved and divergent domains are identified in these predicted MtROP proteins. Phylogenetic analysis has indicated that MtROPs are distributed into groups II, III, IV but group I. MtROP genes are expressed in various tissues at different levels. A quantitative reverse transcription PCR analysis indicated that these MtROP genes have different expression profiles in the roots in response to infection with rhizobia. The expression of MtROP3, MtROP5 and MtROP6 are increased, as the expression of Nod factor or rhizobial-induced marker genes--NFP, Rip1 and Enod11; MtROP10 has showed enhanced expression at a certain post-inoculation time point. No significant changes in MtROP7 and MtROP9 expression have been detected and MtROP8 expression is dramatically decreased by about 80%-90%. Additionally, ROP promoter-GUS analysis has showed that MtROP3, MtROP5 and MtROP6 have elevated expression in transgenic root hairs after rhizobial inoculation. These results might suggest a role for some ROP GTPases in the regulation of early stages during rhizobial infection in symbiosis.

Microfilament orientation constrains vesicle flow and spatial distribution in growing pollen tubes

The dynamics of cellular organelles reveals important information about their functioning. The spatio-temporal movement patterns of vesicles in growing pollen tubes are controlled by the actin cytoskeleton. Vesicle flow is crucial for morphogenesis in these cells as it ensures targeted delivery of cell wall polysaccharides. Remarkably, the target region does not contain much filamentous actin. We model the vesicular trafficking in this area using as boundary conditions the expanding cell wall and the actin array forming the apical actin fringe. The shape of the fringe was obtained by imposing a steady state and constant polymerization rate of the actin filaments. Letting vesicle flux into and out of the apical region be determined by the orientation of the actin microfilaments and by exocytosis was sufficient to generate a flux that corresponds in magnitude and orientation to that observed experimentally. This model explains how the cytoplasmic streaming pattern in the apical region of the pollen tube can be generated without the presence of actin microfilaments.

Molecular characterization of four ADF genes differentially expressed in cotton

Actin depolymerizing factor (ADF), highly conserved in all eukaryotic cells, is a low molecular mass of actin-binding protein, which plays a key role in modulating the polymerizing and depolymerizing of the actin filaments. Four cDNAs (designated GhADF2, GhADF3, GhADF4, and GhADF5, respectively) encoding ADF proteins were isolated from cotton (Gossypium hirsutum) fiber cDNA library. GhADF2 cDNA is 705 bp in length and deduces a protein with 139 amino acids. GhADF3 cDNA is 819 bp in length and encodes a protein of 139 amino acids. GhADF4 cDNA is 804 bp in length and deduces a protein with 143 amino acids. GhADF5 cDNA is 644 bp in length and encodes a protein of 141 amino acids. The molecular evolutionary relationship of these genes was analyzed by means of bioinformatics. GhADF2 is closely related to GhADF3 (99% identity) and PetADF2 (89% identity). GhADF4 is closely related to AtADF6 (78% identity), and GhADF5 is closely related to AtADF5 (83% identity). These results demonstrated that the plant ADF genes are highly conserved in structure. RT-PCR analysis showed that GhADF2 is predominantly expressed in fiber, whereas, GhADF5 is mainly expressed in cotyledons. On the other hand, it seems that GhADF3 and GhADF4 have no tissue specificity. Expression levels of different ADF genes may vary considerably in the same cell type, suggesting that they might be involved in regulating tissue development of cotton and the each ADF isoform may diverge to form the functional difference from the other ADFs during evolution.

Class-specific interaction of profilin and ADF isovariants with actin in the regulation of plant development

Two ancient and highly divergent actin-based cytoskeletal systems have evolved in angiosperms. Plant genomes encode complex actin and actin binding protein (ABP) gene families, most of which are phylogenetically grouped into gene classes with distinct vegetative or constitutive and reproductive expression patterns. In Arabidopsis thaliana, ectopic expression of high levels of a reproductive class actin, ACT1, in vegetative tissues causes severe dwarfing of plants with aberrant organization of most plant organs and cell types due to a severely altered actin cytoskeletal architecture. Overexpression of the vegetative class actin ACT2 to similar levels, however, produces insignificant phenotypic changes. We proposed that the misexpression of the pollen-specific ACT1 in vegetative cell types affects the dynamics of actin due to its inappropriate interaction with endogenous vegetative ABPs. To examine the functionally distinct interactions among the major classes of actins and ABPs, we ectopically coexpressed reproductive profilin (PRF4) or actin-depolymerizing factor (ADF) isovariants (e.g., ADF7) with ACT1. Our results demonstrated that the coexpression of these reproductive, but not vegetative, ABP isovariants suppressed the ectopic ACT1 expression phenotypes and restored wild-type stature and normal actin cytoskeletal architecture to the double transgenic plants. Thus, the actins and ABPs appear to have evolved class-specific, protein-protein interactions that are essential to the normal regulation of plant growth and development.

Verticillium dahliae toxin induced alterations of cytoskeletons and nucleoli in Arabidopsis thaliana suspension cells

In plant cells, cytoskeletons play important roles in response to biotic and abiotic stresses. However, little is known about the dynamics of cytoskeletons when cells are attacked by unphysical stress factors such as elicitors and toxins. We report here that the toxin of Verticillium dahliae (VD toxin) induced changes of microfilaments (MFs) and microtubules (MTs) in Arabidopsis thaliana suspension-cultured cells. When cells were treated with a low concentration of VD toxin, MFs were disrupted ordinally from the cortex to the perinuclear region, and then recovered spontaneously; but the MTs persisted. The MFs in the perinuclear region showed more resistance to VD toxin than the cortical ones. In contrast, when cells were treated with a high concentration of VD toxin, MFs and MTs were disrupted sooner and more severely and did not recover spontaneously. Treatments with high concentrations of VD toxin also induced changes of nucleoli. At the early stages of treatment, a nucleus had a single ring-shaped nucleolus. At the later stages, multiple smaller and more brightly fluorescing nucleoli emerged in a single nucleus. Disrupted MFs could be recovered by removing the VD toxin before the ring-shaped nucleoli appeared. All these results showed that MFs and MTs play important roles in the early defense responses against VD toxin in Arabidopsis suspension cells. The cytoskeletons may be used as sensors and effectors monitoring the defense reactions. The changes of nucleoli induced by VD toxin should be important characteristics of cell death.

Mutations in DMI3 and SUNN modify the appressorium-responsive root proteome in arbuscular mycorrhiza

Modification of the Medicago truncatula root proteome during the early stage of arbuscular mycorrhizal symbiosis was investigated by comparing, using two-dimensional electrophoresis, the protein patterns obtained from non-inoculated roots and roots synchronized for Glomus intraradices appressorium formation. This approach was conducted in wild-type (J5), mycorrhiza-defective (TRV25, dmi3), and autoregulation-defective (TR122, sunn) M. truncatula genotypes. The groups of proteins that responded to appressorium formation were further compared between wild-type and mutant genotypes; few overlaps and major differences were recorded, demonstrating that mutations in DMI3 and SUNN modified the appressorium-responsive root proteome. Except for a chalcone reductase, none of the differentially displayed proteins that could be identified using matrix-assisted laser desorption ionization time-of-flight mass spectrometry previously was known as appressorium responsive. A DMI3-dependent increased accumulation of signal transduction-related proteins (dehydroascorbate reductase, cyclophilin, and actin depolymerization factor) was found to precede mycorrhiza establishment. Differences in the accumulation of proteins related to plant defense reactions, cytoskeleton rearrangements, and auxin signaling upon symbiont contact were recorded between wild-type and hypermycorrhizal genotypes, pointing to some putative pathways by which SUNN may regulate very early arbuscule formation.

Signalling pathways in pollen germination and tube growth

Signalling is an integral component in the establishment and maintenance of cellular identity. In plants, tip-growing cells represent an ideal system to investigate signal transduction mechanisms, and among these, pollen tubes (PTs) are one of the favourite models. Many signalling pathways have been identified during germination and tip growth, namely, Ca(2+), calmodulin, phosphoinositides, protein kinases, cyclic AMP, and GTPases. These constitute a large and complex web of signalling networks that intersect at various levels such as the control of vesicle targeting and fusion and the physical state of the actin cytoskeleton. Here we discuss some of the most recent advances made in PT signal transduction cascades and their implications for our future research. For reasons of space, emphasis was given to signalling mechanisms that control PT reorientation, so naturally many other relevant works have not been cited.

ARP2 and ARP3 are localized to sites of actin filament nucleation in tobacco BY-2 cells

Complete depolymerization of actin filaments (AFs) at low temperature (0 degrees C) is followed by the formation of transient actin structures at 25 degrees C in tobacco BY-2 cells (Nicotiana tabacum L.). Using antibodies against fission yeast actin-related proteins (ARP2 and ARP3), we show here that transient actin structures (dots, dotted filaments, rods) colocalize with epitopes stained by these antibodies and thus are likely to represent sites of actin filament nucleation (SANs). In contrast to the cold-induced disassembly of AFs, no transient actin structures were detectable during recovery of AFs from latrunculin B-induced depolymerization. However, the staining pattern obtained with ARP antibodies in latrunculin B-treated cells was similar to that in controls and cold-treated cells. This suggests that, in addition to the complete depolymerization of AFs, disruption of other cellular structures is needed for the formation of transient actin structures during the early phase of recovery from cold treatment.

Characterization of three Rop GTPase genes of alfalfa (Medicago sativa L.)

Three cDNA clones coding for Medicago sativa Rop GTPases have been isolated. The represented genes could be assigned to various linkage groups by genetic mapping. They were expressed in all investigated plant organs, although at different level. Relative gene expression patterns in response to Sinorhizobium infection of roots as well as during somatic embryogenesis indicated their differential participation in these processes. DNA sequences coding for altogether six different Medicago sp. Rop GTPases could be identified in sequence databases. Based on their homology to each other and to their Arabidopsis counterparts, a unified nomenclature is suggested for Medicago Rop GTPases.

Comparative study of rice and Arabidopsis actin-depolymerizing factors gene families

Actin-depolymerizing factors (ADF) is one of the small actin-binding proteins that regulate actin dynamics in cells. Analysis of the complete rice and Arabidopsis protein sequences revealed 12 ADF proteins, respectively. A further study on the similarities and differences between ADF throughout rice and Arabidopsis genome was carried out at the level of genomic organization and protein structure. The strict conservation of essential structural features suggested that the mode of action and physiological function of these proteins, as well as the expression pattern of their coding genes, might be very similar. The ADF proteins were divided into four groups based on the phylogenetic relationships of the amino acid sequences, and was comparable with previous studies.

RNA interference identifies a calcium-dependent protein kinase involved in Medicago truncatula root development

Changes in cellular or subcellular Ca2+ concentrations play essential roles in plant development and in the responses of plants to their environment. However, the mechanisms through which Ca2+ acts, the downstream signaling components, as well as the relationships among the various Ca2+-dependent processes remain largely unknown. Using an RNA interference-based screen for gene function in Medicago truncatula, we identified a gene that is involved in root development. Silencing Ca2+-dependent protein kinase1 (CDPK1), which is predicted to encode a Ca2+-dependent protein kinase, resulted in significantly reduced root hair and root cell lengths. Inactivation of CDPK1 is also associated with significant diminution of both rhizobial and mycorrhizal symbiotic colonization. Additionally, microarray analysis revealed that silencing CDPK1 alters cell wall and defense-related gene expression. We propose that M. truncatula CDPK1 is a key component of one or more signaling pathways that directly or indirectly modulates cell expansion or cell wall synthesis, possibly altering defense gene expression and symbiotic interactions.

Aluminium causes variable responses in actin filament cytoskeleton of the root tip cells of Triticum turgidum

The effects of aluminium on the actin filament (AF) cytoskeleton of Triticum turgidum meristematic root tip cells were examined. In short treatments (up to 2 h) with 50-1000 microM AlCl3.6H2O, interphase cells displayed numerous AFs arrayed in thick bundles that lined the plasmalemma and traversed the endoplasm in different directions. Measurements using digital image analysis and assessment of the overall AF fluorescence revealed that, in short treatments, the affected cells possessed 25-30% more AFs than the untreated ones. The thick AF bundles were not formed in the Al-treated cells in the presence of the myosin inhibitors 2,3-butanedione monoxime (BDM) and 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-7), a fact suggesting that myosins are involved in AF bundling. In longer Al treatments, the AF bundles were disorganised, forming granular actin accumulations, a process that was completed after 4 h of treatment. In the Al-treated cells, increased amounts of callose were uniformly deposited along the whole surface of the cell walls. In contrast, callose formed local deposits in the Al-treated cells in the presence of cytochalasin B, BDM, or ML-7. These results favour the hypothesis that the actomyosin system in the Al-treated cells, among other roles, participates in the mechanism controlling callose deposition.

Actin-based motility of endosomes is linked to the polar tip growth of root hairs

Plant tip growth has been recognized as an actin-based cellular process requiring targeted exocytosis and compensatory endocytosis to occur at the growth cone. However, the identity of subcellular compartments involved in polarized membrane trafficking pathways remains enigmatic in plants. Here we characterize endosomal compartments in tip-growing root hair cells. We demonstrate their presence at the growing tip and differential distribution upon cessation of tip growth. We also show that both the presence of endosomes as well as their rapid movements within the tip region depends on an intact actin cytoskeleton and involves actin polymerization. In conclusion, actin-propelled endosomal motility is tightly linked to the polar tip growth of root hairs.

The formin homology 1 domain modulates the actin nucleation and bundling activity of Arabidopsis FORMIN1

The organization of actin filaments into large ordered structures is a tightly controlled feature of many cellular processes. However, the mechanisms by which actin filament polymerization is initiated from the available pool of profilin-bound actin monomers remain unknown in plants. Because the spontaneous polymerization of actin monomers bound to profilin is inhibited, the intervention of an actin promoting factor is required for efficient actin polymerization. Two such factors have been characterized from yeasts and metazoans: the Arp2/3 complex, a complex of seven highly conserved subunits including two actin-related proteins (ARP2 and ARP3), and the FORMIN family of proteins. The recent finding that Arabidopsis thaliana plants lacking a functional Arp2/3 complex exhibit rather modest morphological defects leads us to consider whether the large FORMIN family plays a central role in the regulation of actin polymerization. Here, we have characterized the mechanism of action of Arabidopsis FORMIN1 (AFH1). Overexpression of AFH1 in pollen tubes has been shown previously to induce abnormal actin cable formation. We demonstrate that AFH1 has a unique behavior when compared with nonplant formins. The activity of the formin homology domain 2 (FH2), containing the actin binding activity, is modulated by the formin homology domain 1 (FH1). Indeed, the presence of the FH1 domain switches the FH2 domain from a tight capper (Kd approximately 3.7 nM) able to nucleate actin filaments that grow only in the pointed-end direction to a leaky capper that allows barbed-end elongation and efficient nucleation of actin filaments from actin monomers bound to profilin. Another exciting feature of AFH1 is its ability to bind to the side and bundle actin filaments. We have identified an actin nucleator that is able to organize actin filaments directly into unbranched actin filament bundles. We suggest that AFH1 plays a central role in the initiation and organization of actin cables from the pool of actin monomers bound to profilin.

Arabidopsis NAP and PIR regulate actin-based cell morphogenesis and multiple developmental processes

The actin cytoskeleton mediates cellular processes through the dynamic regulation of the time, location, and extent of actin polymerization. Actin polymerization is controlled by several types of evolutionarily conserved proteins, including those comprising the ARP2/3 complex. In animal cells ARP2/3 activity is regulated by WAVE complexes that contain WAVE/SCAR proteins, PIR121, Nap125, and other proteins. The activity of the WAVE complex is regulated by Rho-GTPase-mediated signaling that leads to ARP2/3 activation by WAVE/SCAR proteins. We describe in this report Arabidopsis (Arabidopsis thaliana) genes encoding Nap and PIR proteins. Light-grown Atnap-1 and Atpir-1 mutant plants displayed altered leaf, inflorescence, silique, and seed set phenotypes. Dark-grown Atnap-1 and Atpir-1 seedlings also exhibited longer roots, enhanced skotomorphogenesis and Glc responses, and shorter thicker hypocotyls than those of wild type, showing that AtNAP and AtPIR participate in a variety of growth and developmental processes. Mutations in AtNAP and AtPIR caused cell morphology defects in cotyledon pavement cells and trichomes seen in mutants in ARP2/3 subunits and in plants expressing constitutively active Rop2 GTPase. The patterns and levels of actin polymerization observed in Atnap-1 and Atpir-1 mutant trichome cells and epidermal pavement cell morphology is consistent with Arabidopsis NAP and PIR proteins forming a WAVE complex that activates ARP2/3 activity. The multiple growth and developmental phenotypes of Atnap and Atpir mutants reveals these proteins are also required for a wider variety of cellular functions in addition to regulating trichome cell growth.

Cytomechanical properties of papaver pollen tubes are altered after self-incompatibility challenge

Self-incompatibility (SI) in Papaver rhoeas triggers a ligand-mediated signal transduction cascade, resulting in the inhibition of incompatible pollen tube growth. Using a cytomechanical approach we have demonstrated that dramatic changes to the mechanical properties of incompatible pollen tubes are stimulated by SI induction. Microindentation revealed that SI resulted in a reduction of cellular stiffness and an increase in cytoplasmic viscosity. Whereas the former cellular response is likely to be the result of a drop in cellular turgor, we hypothesize that the latter is caused by as yet unidentified cross-linking events. F-actin rearrangements, a characteristic phenomenon for SI challenge in Papaver, displayed a spatiotemporal gradient along the pollen tube; this suggests that signal propagation occurs in a basipetal direction. However, unexpectedly, local application of SI inducing S-protein did not reveal any evidence for localized signal perception in the apical or subapical regions of the pollen tube. To our knowledge this represents the first mechanospatial approach to study signal propagation and cellular responses in a well-characterized plant cell system. Our data provide the first evidence for mechanical changes induced in the cytoplasm of a plant cell stimulated by a defined ligand.

Involvement of cAMP signaling in elicitor-induced phytoalexin accumulation in Cupressus lusitanica cell cultures

•  An increasing body of evidence on plant electrophysiology, biochemistry, and molecular biology shows that cAMP exists in higher plants and plays a role in several physiological processes by affecting potassium (K⁺ ) or calcium (Ca²⁺ ) fluxes. Our study here reports that cAMP is involved in elicitor-induced accumulation of a phytoalexin, β-thujaplicin, in Cupressus lusitanica cell cultures. •  Treatment of C. lusitanica cultured cells with cAMP or its analogues stimulated β-thujaplicin accumulation. Cholera toxin and forskolin, activators of adenylyl cyclase, also stimulated β-thujaplicin accumulation. Enzyme immunoassay showed that after elicitor treatment, cAMP level in the elicited cells quickly increased to about three- to five-fold over the control. Cholera toxin and forskolin also stimulated cAMP accumulation in the absence of elicitor. •  However, K⁺ and Ca²⁺ channel blockers inhibited the β-thujaplicin accumulation induced by cAMP analogues, suggesting that the cAMP-stimulated β-thujaplicin accumulation may involve Ca²⁺ and K⁺ fluxes. Several ionophores mimicked cAMP induction of β-thujaplicin accumulation. •  Cross-talk between cAMP treatment and the ethylene signaling pathway was also observed to work in the cell cultures via Ca²⁺ signaling. The study also indicates an involvement of protein kinase cascades in cAMP signaling processes, leading to both phytoalexin and ethylene production.

Mitochondrial dynamics

Mitochondria cannot be created de novo but instead must arise from the fission (division) of a parental organelle. In addition to fission, mitochondria also fuse with one another and it is thought that a co-ordinated balance of these two processes controls mitochondrial shape, size and number. In the past 5-7 yr, molecular genetics coupled to state-of-the-art cell biology, in particular the use of mitochondrial-targeted green fluorescent protein (GFP), has enabled identification of proteins controlling mitochondrial shape, size and number in yeast and mammalian cells. Little is known about higher plant mitochondrial dynamics. Recently, however, several genes involved in the control of plant mitochondrial dynamics have been identified. The aim of this article is to bring together what is known about mitochondrial dynamics in any organisms and to relate this to our recent knowledge of the underlying processes in higher plants. Contents Summary 463 I. Introduction 464 II. Mitochondrial evolution 464 III. Mitochondria and the cytoskeleton 465 IV. Mitochondrial morphology, biogenesis, proliferation and inheritance 466 V. Mitochondrial fission and fusion 468 VI. Mitochondrial distribution 470 VII. Plant specific proteins playing a role in mitochondrial dynamics 470 VIII. Conclusions 471 Acknowledgements 475 References 475.

Arabidopsis capping protein (AtCP) is a heterodimer that regulates assembly at the barbed ends of actin filaments

The precise regulation of actin filament polymerization and depolymerization is essential for many cellular processes and is choreographed by a multitude of actin-binding proteins (ABPs). In higher plants the number of well characterized ABPs is quite limited, and some evidence points to significant differences in the biochemical properties of apparently conserved proteins. Here we provide the first evidence for the existence and biochemical properties of a heterodimeric capping protein from Arabidopsis thaliana (AtCP). The purified recombinant protein binds to actin filament barbed ends with Kd values of 12-24 nM, as assayed both kinetically and at steady state. AtCP prevents the addition of profilin actin to barbed ends during a seeded elongation reaction and suppresses dilution-mediated depolymerization. It does not, however, sever actin filaments and does not have a preference for the source of actin. During assembly from Mg-ATP-actin monomers, AtCP eliminates the initial lag period for actin polymerization and increases the maximum rate of polymerization. Indeed, the efficiency of actin nucleation of 0.042 pointed ends created per AtCP polypeptide compares favorably with mouse CapZ, which has a maximal nucleation of 0.17 pointed ends per CapZ polypeptide. AtCP activity is not affected by calcium but is sensitive to phosphatidylinositol 4,5-bisphosphate. We propose that AtCP is a major regulator of actin dynamics in plant cells that, together with abundant profilin, is responsible for maintaining a large pool of actin subunits and a surprisingly small population of F-actin.

The putative Arabidopsis arp2/3 complex controls leaf cell morphogenesis

The evolutionarily conserved Arp2/3 complex has been shown to activate actin nucleation and branching in several eukaryotes, but its biological functions are not well understood in multicellular organisms. The model plant Arabidopsis provides many advantages for genetic dissection of the function of this conserved actin-nucleating machinery, yet the existence of this complex in plants has not been determined. We have identified Arabidopsis genes encoding homologs of all of the seven Arp2/3 subunits. The function of the putative Arabidopsis Arp2/3 complex has been studied using four homozygous T-DNA insertion mutants for ARP2, ARP3, and ARPC5/p16. All four mutants display identical defects in the development of jigsaw-shaped epidermal pavement cells and branched trichomes in the leaf. These loss-of-function mutations cause mislocalization of diffuse cortical F-actin to the neck region and inhibit lobe extension in pavement cells. The mutant trichomes resemble those treated with the actin-depolymerizing drug cytochalasin D, exhibiting stunted branches but dramatically enlarged stalks due to depolarized growth suggesting defects in the formation of a fine actin network. Our data demonstrate that the putative Arabidopsis Arp2/3 complex controls cell morphogenesis through its roles in cell polarity establishment and polar cell expansion. Furthermore, our data suggest a novel function for the putative Arp2/3 complex in the modulation of the spatial distribution of cortical F-actin and provide evidence that the putative Arp2/3 complex may activate the polymerization of some types of actin filaments in specific cell types.

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.