Actin turnover is required for myosin-dependent mitochondrial movements in Arabidopsis root hairs

Collective mitochondrial dynamics resolve conflicting cellular tensions: From plants to general principles

Mitochondria play diverse and essential roles in eukaryotic cells, and plants are no exception. Plant mitochondria have several differences from their metazoan and fungal cousins: they often exist in a fragmented state, move rapidly on actin rather than microtubules, have many plant-specific metabolic features and roles, and usually contain only a subset of the complete mtDNA genome, which itself undergoes frequent recombination. This arrangement means that exchange and complementation is essential for plant mitochondria, and recent work has begun to reveal how their collective dynamics and resultant "social networks" of encounters support this exchange, connecting plant mitochondria in time rather than in space. This review will argue that this social network perspective can be extended to a "societal network", where mitochondrial dynamics are an essential part of the interacting cellular society of organelles and biomolecules. Evidence is emerging that mitochondrial dynamics allow optimal resolutions to competing cellular priorities; we will survey this evidence and review potential future research directions, highlighting that plant mitochondria can help reveal and test principles that apply across other kingdoms of life. In parallel with this fundamental cell biology, we also highlight the translational "One Health" importance of plant mitochondrial behaviour - which is exploited in the production of a vast amount of crops consumed worldwide - and the potential for multi-objective optimisation to understand and rationally re-engineer the evolved resolutions to these tensions.

Na+-preferential ion transporter HKT1;1 mediates salt tolerance in blueberry

Soil salinity is a major environmental factor constraining growth and productivity of highbush blueberry (Vaccinium corymbosum). Leaf Na+ content is associated with variation in salt tolerance among blueberry cultivars; however, the determinants and mechanisms conferring leaf Na+ exclusion are unknown. Here, we observed that the blueberry cultivar 'Duke' was more tolerant than 'Sweetheart' and accumulated less Na+ in leaves under salt stress conditions. Through transcript profiling, we identified a member of the high-affinity K+ transporter (HKT) family in blueberry, VcHKT1;1, as a candidate gene involved in leaf Na+ exclusion and salt tolerance. VcHKT1;1 encodes a Na+-preferential transporter localized to the plasma membrane and is preferentially expressed in the root stele. Heterologous expression of VcHKT1;1 in Arabidopsis (Arabidopsis thaliana) rescued the salt hypersensitivity phenotype of the athkt1 mutant. Decreased VcHKT1;1 transcript levels in blueberry plants expressing antisense-VcHKT1;1 led to increased Na+ concentrations in xylem sap and higher leaf Na+ contents compared with wild-type plants, indicating that VcHKT1;1 promotes leaf Na+ exclusion by retrieving Na+ from xylem sap. A naturally occurring 8-bp insertion in the promoter increased the transcription level of VcHKT1;1, thus promoting leaf Na+ exclusion and blueberry salt tolerance. Collectively, we provide evidence that VcHKT1;1 promotes leaf Na+ exclusion and propose natural variation in VcHKT1;1 will be valuable for breeding Na+-tolerant blueberry cultivars in the future.

Visualizing the dynamics of plant energy organelles

Plant organelles predominantly rely on the actin cytoskeleton and the myosin motors for long-distance trafficking, while using microtubules and the kinesin motors mostly for short-range movement. The distribution and motility of organelles in the plant cell are fundamentally important to robust plant growth and defense. Chloroplasts, mitochondria, and peroxisomes are essential organelles in plants that function independently and coordinately during energy metabolism and other key metabolic processes. In response to developmental and environmental stimuli, these energy organelles modulate their metabolism, morphology, abundance, distribution and motility in the cell to meet the need of the plant. Consistent with their metabolic links in processes like photorespiration and fatty acid mobilization is the frequently observed inter-organellar physical interaction, sometimes through organelle membranous protrusions. The development of various organelle-specific fluorescent protein tags has allowed the simultaneous visualization of organelle movement in living plant cells by confocal microscopy. These energy organelles display an array of morphology and movement patterns and redistribute within the cell in response to changes such as varying light conditions, temperature fluctuations, ROS-inducible treatments, and during pollen tube development and immune response, independently or in association with one another. Although there are more reports on the mechanism of chloroplast movement than that of peroxisomes and mitochondria, our knowledge of how and why these three energy organelles move and distribute in the plant cell is still scarce at the functional and mechanistic level. It is critical to identify factors that control organelle motility coupled with plant growth, development, and stress response.

Spatiotemporal distribution of reactive oxygen species production, delivery, and use in Arabidopsis root hairs

Fluorescent selective probes for reactive oxygen species (ROS) detection in living cells are versatile tools for the documentation of ROS production in plant developmental or stress reactions. We employed high-resolution live-cell imaging and semiquantitative analysis of Arabidopsis (Arabidopsis thaliana) stained with CM-H2DCFDA, CellROX Deep Red, and Amplex Red for functional characterization of the spatiotemporal mode of ROS production, delivery, and utilization during root hair formation. Cell viability marker fluorescein diacetate served as a positive control for dye loading and undisturbed root hair tip growth after staining. Using a colocalization analysis with subcellular molecular markers and two root hair mutants with similar phenotypes of nonelongating root hairs, but with contrasting reasons for this impairment, we found that: (i) CM-H2DCFDA is a sensitive probe for ROS generation in the cytoplasm, (ii) CellROX Deep Red labels ROS in mitochondria, (iii) Amplex Red labels apoplastic ROS and mitochondria and shows high selectivity to root hairs, (iv) the root hair defective 2-1 (rhd2-1) mutant with nonfunctional NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG PROTEIN C/ROOT HAIR-DEFECTIVE 2 (AtRBOHC/RHD2) has a low level of CM-H2DCFDA-reactive ROS in cytoplasm and lacks Amplex Red-reactive ROS in apoplast, and (v) the ACTIN2-deficient deformed root hairs1-3 (der1-3) mutant is not altered in these aspects. The sensitivity of CellROX Deep Red was documented by discrimination between larger ROS-containing mitochondria and small, yet ROS-free premature mitochondria in the growing tip of root hairs. We characterized spatial changes in ROS production and compartmentalization induced by external ROS modulators, ethylene precursor 1-aminocyclopropane-1-carboxylic acid, and ionophore valinomycin. This dynamic and high-resolution study of ROS production and utilization opens opportunities for precise speciation of particular ROS involved in root hair formation.

Camellia oleifera CoSWEET10 Is Crucial for Seed Development and Drought Resistance by Mediating Sugar Transport in Transgenic Arabidopsis

Sugar transport from the source leaf to the sink organ is critical for seed development and crop yield, as well as for responding to abiotic stress. SWEETs (sugar will eventually be exported transporters) mediate sugar efflux into the reproductive sink and are therefore considered key candidate proteins for sugar unloading during seed development. However, the specific mechanism underlying the sugar unloading to seeds in Camellia oleifera remains elusive. Here, we identified a SWEET gene named CoSWEET10, which belongs to Clade III and has high expression levels in the seeds of C. oleifera. CoSWEET10 is a plasma membrane-localized protein. The complementation assay of CoSWEET10 in SUSY7/ura3 and EBY.VW4000 yeast strains showed that CoSWEET10 has the ability to transport sucrose, glucose, and fructose. Through the C. oleifera seeds in vitro culture, we found that the expression of CoSWEET10 can be induced by hexose and sucrose, and especially glucose. By generating the restoration lines of CoSWEET10 in Arabidopsis atsweet10, we found that CoSWEET10 restored the seed defect phenotype of the mutant by regulating soluble sugar accumulation and increased plant drought tolerance. Collectively, our study demonstrates that CoSWEET10 plays a dual role in promoting seed development and enhancing plant drought resistance as a sucrose and hexose transporter.

Developmentally regulated mitochondrial biogenesis and cell death competence in maize pollen

BACKGROUND

Cytoplasmic male sterility (CMS) is a maternally inherited failure to produce functional pollen that most commonly results from expression of novel, chimeric mitochondrial genes. In Zea mays, cytoplasmic male sterility type S (CMS-S) is characterized by the collapse of immature, bi-cellular pollen. Molecular and cellular features of developing CMS-S and normal (N) cytoplasm pollen were compared to determine the role of mitochondria in these differing developmental fates.

RESULTS

Terminal deoxynucleotidyl transferase dUTP nick end labeling revealed both chromatin and nuclear fragmentation in the collapsed CMS-S pollen, demonstrating a programmed cell death (PCD) event sharing morphological features with mitochondria-signaled apoptosis in animals. Maize plants expressing mitochondria-targeted green fluorescent protein (GFP) demonstrated dynamic changes in mitochondrial morphology and association with actin filaments through the course of N-cytoplasm pollen development, whereas mitochondrial targeting of GFP was lost and actin filaments were disorganized in developing CMS-S pollen. Immunoblotting revealed significant developmental regulation of mitochondrial biogenesis in both CMS-S and N mito-types. Nuclear and mitochondrial genome encoded components of the cytochrome respiratory pathway and ATP synthase were of low abundance at the microspore stage, but microspores accumulated abundant nuclear-encoded alternative oxidase (AOX). Cytochrome pathway and ATP synthase components accumulated whereas AOX levels declined during the maturation of N bi-cellular pollen. Increased abundance of cytochrome pathway components and declining AOX also characterized collapsed CMS-S pollen. The accumulation and robust RNA editing of mitochondrial transcripts implicated translational or post-translational control for the developmentally regulated accumulation of mitochondria-encoded proteins in both mito-types.

CONCLUSIONS

CMS-S pollen collapse is a PCD event coincident with developmentally programmed mitochondrial events including the accumulation of mitochondrial respiratory proteins and declining protection against mitochondrial generation of reactive oxygen species.

Altered collective mitochondrial dynamics in the Arabidopsis msh1 mutant compromising organelle DNA maintenance

Mitochondria form highly dynamic populations in the cells of plants (and almost all eukaryotes). The characteristics and benefits of this collective behaviour, and how it is influenced by nuclear features, remain to be fully elucidated. Here, we use a recently developed quantitative approach to reveal and analyse the physical and collective 'social' dynamics of mitochondria in an Arabidopsis msh1 mutant where the organelle DNA maintenance machinery is compromised. We use a newly created line combining the msh1 mutant with mitochondrially targeted green fluorescent protein (GFP), and characterize mitochondrial dynamics with a combination of single-cell time-lapse microscopy, computational tracking, and network analysis. The collective physical behaviour of msh1 mitochondria is altered from that of the wild type in several ways: mitochondria become less evenly spread, and networks of inter-mitochondrial encounters become more connected, with greater potential efficiency for inter-organelle exchange-reflecting a potential compensatory mechanism for the genetic challenge to the mitochondrial DNA population, supporting more inter-organelle exchange. We find that these changes are similar to those observed in friendly, where mitochondrial dynamics are altered by a physical perturbation, suggesting that this shift to higher connectivity may reflect a general response to mitochondrial challenges, where physical dynamics of mitochondria may be altered to control the genetic structure of the mtDNA population.

Effects of mitochondria-selective fluorescent probes on mitochondrial movement in Arabidopsis mesophyll cells evaluated by using the quantification

Mitochondria-selective fluorescent probes such as MitoTracker are often used for mitochondria imaging in various plants. Although some of the probes are reported to induce mitochondria dysfunction in animal cells, the effect on plant cells remains to be determined. In the present study, we applied quantitative methods to analyze mitochondrial movement, speed frequency, and speed-angle changes, based on trajectory analysis of mitochondria in mesophyll protoplast cells of Arabidopsis thaliana expressing the mitochondria-localized fluorescent protein. Using the quantitative method, we assessed whether MitoTracker Red (FM and CMXRos) induce mitochondria dysfunction in A. thaliana. Although both the fluorescent probes well-stained mitochondria, the CMXRos probe, not the FM probe, gave a severe effect on mitochondrial movement at the low concentration (10 nM), indicating a MitoTracker-induced mitochondria dysfunction in A. thaliana. These results revealed that our quantitative method based on mitochondrial movement can be used to determine the appropriate concentrations of mitochondria-selective fluorescent probes in plants.

Network analysis of Arabidopsis mitochondrial dynamics reveals a resolved tradeoff between physical distribution and social connectivity

Mitochondria in plant cells exist largely as individual organelles which move, colocalize, and interact, but the cellular priorities addressed by these dynamics remain incompletely understood. Here, we elucidate these principles by studying the dynamic "social networks" of mitochondria in Arabidopsis thaliana wildtype and mutants, describing the colocalization of individuals over time. We combine single-cell live imaging of hypocotyl mitochondrial dynamics with individual-based modeling and network analysis. We identify an inevitable tradeoff between mitochondrial physical priorities (an even cellular distribution of mitochondria) and “social” priorities (individuals interacting, to facilitate the exchange of chemicals and information). This tradeoff results in a tension between maintaining mitochondrial spacing and facilitating colocalization. We find that plant cells resolve this tension to favor efficient networks with high potential for exchanging contents. We suggest that this combination of physical modeling coupled to experimental data through network analysis can shed light on the fundamental principles underlying these complex organelle dynamics. A record of this paper’s transparent peer review process is included in the supplemental information.

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Dynamic social networks of plant mitochondria reflect physical organellar encounters

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Network analysis and modeling show priorities and tradeoffs for mitochondrial motion

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Mitochondria in plant cells trade off physical spacing against social connectivity

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Plant cells favor efficient networks with high potential for information exchange

Mitochondrial movement during its association with chloroplasts in Arabidopsis thaliana

Plant mitochondria move dynamically inside cells and this movement is classified into two types: directional movement, in which mitochondria travel long distances, and wiggling, in which mitochondria travel short distances. However, the underlying mechanisms and roles of both types of mitochondrial movement, especially wiggling, remain to be determined. Here, we used confocal laser-scanning microscopy to quantitatively characterize mitochondrial movement (rate and trajectory) in Arabidopsis thaliana mesophyll cells. Directional movement leading to long-distance migration occurred at high speed with a low angle-change rate, whereas wiggling leading to short-distance migration occurred at low speed with a high angle-change rate. The mean square displacement (MSD) analysis could separate these two movements. Directional movement was dependent on filamentous actin (F-actin), whereas mitochondrial wiggling was not, but slightly influenced by F-actin. In mesophyll cells, mitochondria could migrate by wiggling, and most of these mitochondria associated with chloroplasts. Thus, mitochondria migrate via F-actin-independent wiggling under the influence of F-actin during their association with chloroplasts in Arabidopsis.

Oikawa et al. investigate the rate and trajectory of mitochondria in Arabidopsis thaliana mesophyll cells, using confocal laser-scanning microscopy. They find that mitochondria migrate via wiggling during their association with chloroplasts, providing insights into how mitochondria-chloroplast interaction affects the movement of mitochondria.

Phototropin- and photosynthesis-dependent mitochondrial positioning in Arabidopsis thaliana mesophyll cells

Mitochondria are frequently observed in the vicinity of chloroplasts in photosynthesizing cells, and this association is considered necessary for their metabolic interactions. We previously reported that, in leaf palisade cells of Arabidopsis thaliana, mitochondria exhibit blue-light-dependent redistribution together with chloroplasts, which conduct accumulation and avoidance responses under the control of blue-light receptor phototropins. In this study, precise motility analyses by fluorescent microscopy revealed that the individual mitochondria in palisade cells, labeled with green fluorescent protein, exhibit typical stop-and-go movement. When exposed to blue light, the velocity of moving mitochondria increased in 30 min, whereas after 4 h, the frequency of stoppage of mitochondrial movement markedly increased. Using different mutant plants, we concluded that the presence of both phototropin1 and phototropin2 is necessary for the early acceleration of mitochondrial movement. On the contrary, the late enhancement of stoppage of mitochondrial movement occurs only in the presence of phototropin2 and only when intact photosynthesis takes place. A plasma-membrane ghost assay suggested that the stopped mitochondria are firmly adhered to chloroplasts. These results indicate that the physical interaction between mitochondria and chloroplasts is cooperatively mediated by phototropin2- and photosynthesis-dependent signals. The present study might add novel regulatory mechanism for light-dependent plant organelle interactions.

Mitochondrial dynamics and segregation during the asymmetric division of Arabidopsis zygotes

The zygote is the first cell of a multicellular organism. In most angiosperms, the zygote divides asymmetrically to produce an embryo-precursor apical cell and a supporting basal cell. Zygotic division should properly segregate symbiotic organelles, because they cannot be synthesized de novo. In this study, we revealed the real-time dynamics of the principle source of ATP biogenesis, mitochondria, in Arabidopsis thaliana zygotes using live-cell observations and image quantifications. In the zygote, the mitochondria formed the extended structure associated with the longitudinal array of actin filaments (F-actins) and were polarly distributed along the apical-basal axis. The mitochondria were then temporally fragmented during zygotic division, and the resulting apical cells inherited mitochondria at higher concentration compared to the basal cells. Further observation of postembryonic organs showed that these mitochondrial behaviours are characteristic of the zygote. Overall, our results showed that the zygote has spatiotemporal regulation that unequally distributes the mitochondria.

Tension and Resolution: Dynamic, Evolving Populations of Organelle Genomes within Plant Cells

Mitochondria and plastids form dynamic, evolving populations physically embedded in the fluctuating environment of the plant cell. Their evolutionary heritage has shaped how the cell controls the genetic structure and the physical behavior of its organelle populations. While the specific genes involved in these processes are gradually being revealed, the governing principles underlying this controlled behavior remain poorly understood. As the genetic and physical dynamics of these organelles are central to bioenergetic performance and plant physiology, this challenges both fundamental biology and strategies to engineer better-performing plants. This article reviews current knowledge of the physical and genetic behavior of mitochondria and chloroplasts in plant cells. An overarching hypothesis is proposed whereby organelles face a tension between genetic robustness and individual control and responsiveness, and different species resolve this tension in different ways. As plants are immobile and thus subject to fluctuating environments, their organelles are proposed to favor individual responsiveness, sacrificing genetic robustness. Several notable features of plant organelles, including large genomes, mtDNA recombination, fragmented organelles, and plastid/mitochondrial differences may potentially be explained by this hypothesis. Finally, the ways that quantitative and systems biology can help shed light on the plethora of open questions in this field are highlighted.

Blobs and curves: object-based colocalisation for plant cells

Blobs and curves occur everywhere in plant bioimaging: from signals of fluorescence-labelled proteins, through cytoskeletal structures, nuclei staining and cell extensions such as root hairs. Here we look at the problem of colocalisation of blobs with blobs (protein-protein colocalisation) and blobs with curves (organelle-cytoskeleton colocalisation). This article demonstrates a clear quantitative alternative to pixel-based colocalisation methods and, using object-based methods, can quantify not only the level of colocalisation but also the distance between objects. Included in this report are computational algorithms, biological experiments and guidance for those looking to increase their use of computationally-based and quantified analysis of bioimages.

Organelle trafficking, the cytoskeleton, and pollen tube growth

The pollen tube is fundamental for the reproduction of seed plants. Characteristically, it grows relatively quickly and uni-directionally ("polarized growth") to extend the male gametophyte to reach the female gametophyte. The pollen tube forms a channel through which the sperm cells move so that they can reach their targets in the ovule. To grow quickly and directionally, the pollen tube requires an intense movement of organelles and vesicles that allows the cell's contents to be distributed to sustain the growth rate. While the various organelles distribute more or less uniformly within the pollen tube, Golgi-released secretory vesicles accumulate massively at the pollen tube apex, that is, the growing region. This intense movement of organelles and vesicles is dependent on the dynamics of the cytoskeleton, which reorganizes differentially in response to external signals and coordinates membrane trafficking with the growth rate of pollen tubes.

Imaging and analysis of mitochondrial dynamics in living cells

One of the most striking features of plant mitochondria when visualized in living tissue is their dynamism. The beauty of cytoplasmic streaming, driving, and being driven by the motility of mitochondria and other small organelles belies the complexity of the process. Equally, capturing that dynamism and investigating the genes, proteins, and mechanisms underpinning the processes using molecular cell biology and bioimaging is a complex process. It requires the generation of fluorescent protein constructs, stable transgenic plants sometimes expressing multiple fusions, and generation of mutants, even before one is ready for analytical experimentation. Here, we describe some of the key tools and methods necessary to investigate plant mitochondrial dynamics.

Arabidopsis myosin XI: a motor rules the tracks

Plant cell expansion relies on intracellular trafficking of vesicles and macromolecules, which requires myosin motors and a dynamic actin network. Arabidopsis (Arabidopsis thaliana) myosin XI powers the motility of diverse cellular organelles, including endoplasmic reticulum, Golgi, endomembrane vesicles, peroxisomes, and mitochondria. Several recent studies show that there are changes in actin organization and dynamics in myosin xi mutants, indicating that motors influence the molecular tracks they use for transport. However, the mechanism by which actin organization and dynamics are regulated by myosin XI awaits further detailed investigation. Here, using high spatiotemporal imaging of living cells, we quantitatively assessed the architecture and dynamic behavior of cortical actin arrays in a mutant with three Myosin XI (XI-1, XI-2, and XI-K) genes knocked out (xi3KO). In addition to apparent reduction of organ and cell size, the mutant showed less dense and more bundled actin filament arrays in epidermal cells. Furthermore, the overall actin dynamicity was significantly inhibited in the xi3KO mutant. Because cytoskeletal remodeling is contributed mainly by filament assembly/disassembly and translocation/buckling, we also examined the dynamic behavior of individual actin filaments. We found that the xi3KO mutant had significantly decreased actin turnover, with a 2-fold reduction in filament severing frequency. Moreover, quantitative analysis of filament shape change over time revealed that myosin XI generates the force for buckling and straightening of both single actin filaments and actin bundles. Thus, our data provide genetic evidence that three Arabidopsis class XI myosins contribute to actin remodeling by stimulating turnover and generating the force for filament shape change.

ER network dynamics are differentially controlled by myosins XI-K, XI-C, XI-E, XI-I, XI-1, and XI-2

The endoplasmic reticulum (ER) of higher plants is a complex network of tubules and cisternae. Some of the tubules and cisternae are relatively persistent, while others are dynamically moving and remodeling through growth and shrinkage, cycles of tubule elongation and retraction, and cisternal expansion and diminution. Previous work showed that transient expression in tobacco leaves of the motor-less, truncated tail of myosin XI-K increases the relative area of both persistent cisternae and tubules in the ER. Likewise, transient expression of XI-K tail diminishes the movement of organelles such as Golgi and peroxisomes. To examine whether other class XI myosins are involved in the remodeling and movement of the ER, other myosin XIs implicated in organelle movement, XI-1 (MYA1),XI-2 (MYA2), XI-C, XI-E, XI-I, and one not, XI-A, were expressed as motor-less tail constructs and their effect on ER persistent structures determined. Here, we indicate a differential effect on ER dynamics whereby certain class XI myosins may have more influence over controlling cisternalization rather than tubulation.

The endoplasmic reticulum exerts control over organelle streaming during cell expansion

Cytoplasmic streaming is crucial for cell homeostasis and expansion but the precise driving forces are largely unknown. In plants, partial loss of cytoplasmic streaming due to chemical and genetic ablation of myosins supports the existence of yet-unknown motors for organelle movement. Here we tested a role of the endoplasmic reticulum (ER) as propelling force for cytoplasmic streaming during cell expansion. Through quantitative live-cell analyses in wild-type Arabidopsis thaliana cells and mutants with compromised ER structure and streaming, we demonstrate that cytoplasmic streaming undergoes profound changes during cell expansion and that it depends on motor forces co-exerted by the ER and the cytoskeleton.

The actin-related Protein2/3 complex regulates mitochondrial-associated calcium signaling during salt stress in Arabidopsis

Microfilament and Ca(2+) dynamics play important roles in stress signaling in plants. Through genetic screening of Arabidopsis thaliana mutants that are defective in stress-induced increases in cytosolic Ca(2+) ([Ca(2+)]cyt), we identified Actin-Related Protein2 (Arp2) as a regulator of [Ca(2+)]cyt in response to salt stress. Plants lacking Arp2 or other proteins in the Arp2/3 complex exhibited enhanced salt-induced increases in [Ca(2+)]cyt, decreased mitochondria movement, and hypersensitivity to salt. In addition, mitochondria aggregated, the mitochondrial permeability transition pore opened, and mitochondrial membrane potential Ψm was impaired in the arp2 mutant, and these changes were associated with salt-induced cell death. When opening of the enhanced mitochondrial permeability transition pore was blocked or increases in [Ca(2+)]cyt were prevented, the salt-sensitive phenotype of the arp2 mutant was partially rescued. These results indicate that the Arp2/3 complex regulates mitochondrial-dependent Ca(2+) signaling in response to salt stress.

Structural and biomechanical basis of mitochondrial movement in eukaryotic cells

Mitochondria serve as energy-producing organelles in eukaryotic cells. In addition to providing the energy supply for cells, the mitochondria are also involved in other processes, such as proliferation, differentiation, information transfer, and apoptosis, and play an important role in regulation of cell growth and the cell cycle. In order to achieve these functions, the mitochondria need to move to the corresponding location. Therefore, mitochondrial movement has a crucial role in normal physiologic activity, and any mitochondrial movement disorder will cause irreparable damage to the organism. For example, recent studies have shown that abnormal movement of the mitochondria is likely to be the reason for Charcot-Marie-Tooth disease, amyotrophic lateral sclerosis, Alzheimer's disease, Huntington's disease, Parkinson's disease, and schizophrenia. So, in the cell, especially in the particular polarized cell, the appropriate distribution of mitochondria is crucial to the function and survival of the cell. Mitochondrial movement is mainly associated with the cytoskeleton and related proteins. However, those components play different roles according to cell type. In this paper, we summarize the structural basis of mitochondrial movement, including microtubules, actin filaments, motor proteins, and adaptin, and review studies of the biomechanical mechanisms of mitochondrial movement in different types of cells.

Identification of myosin XI receptors in Arabidopsis defines a distinct class of transport vesicles

To characterize the mechanism through which myosin XI-K attaches to its principal endomembrane cargo, a yeast two-hybrid library of Arabidopsis thaliana cDNAs was screened using the myosin cargo binding domain as bait. This screen identified two previously uncharacterized transmembrane proteins (hereinafter myosin binding proteins or MyoB1/2) that share a myosin binding, conserved domain of unknown function 593 (DUF593). Additional screens revealed that MyoB1/2 also bind myosin XI-1, whereas myosin XI-I interacts with the distantly related MyoB7. The in vivo interactions of MyoB1/2 with myosin XI-K were confirmed by immunoprecipitation and colocalization analyses. In epidermal cells, the yellow fluorescent protein-tagged MyoB1/2 localize to vesicles that traffic in a myosin XI-dependent manner. Similar to myosin XI-K, MyoB1/2 accumulate in the tip-growing domain of elongating root hairs. Gene knockout analysis demonstrated that functional cooperation between myosin XI-K and MyoB proteins is required for proper plant development. Unexpectedly, the MyoB1-containing vesicles did not correspond to brefeldin A-sensitive Golgi and post-Golgi or prevacuolar compartments and did not colocalize with known exocytic or endosomal compartments. Phylogenomic analysis suggests that DUF593 emerged in primitive land plants and founded a multigene family that is conserved in all flowering plants. Collectively, these findings indicate that MyoB are membrane-anchored myosin receptors that define a distinct, plant-specific transport vesicle compartment.

Phosphorylation and ubiquitination of dynamin-related proteins (AtDRP3A/3B) synergically regulate mitochondrial proliferation during mitosis

The balance between mitochondrial fission and fusion is disrupted during mitosis, but the mechanism governing this phenomenon in plant cells remains enigmatic. Here, we used mitochondrial matrix-localized Kaede protein (mt-Kaede) to analyze the dynamics of mitochondrial fission in BY-2 suspension cells. Analysis of the photoactivatable fluorescence of mt-Kaede suggested that the fission process is dominant during mitosis. This finding was confirmed by an electron microscopic analysis of the size distribution of mitochondria in BY-2 suspension cells at various stages. Cellular proteins interacting with Myc-tagged dynamin-related protein 3A/3B (AtDRP3A and AtDRP3B) were immunoprecipitated with anti-Myc antibody-conjugated beads and subsequently identified by microcapillary liquid chromatography-quadrupole time-of-flight mass spectrometry (CapLC Q-TOF) MS/MS. The identified proteins were broadly associated with cytoskeletal (microtubular), phosphorylation, or ubiquitination functions. Mitotic phosphorylation of AtDRP3A/AtDRP3B and mitochondrial fission at metaphase were inhibited by treatment of the cells with a CdkB/cyclin B inhibitor or a serine/threonine protein kinase inhibitor. The fate of AtDRP3A/3B during the cell cycle was followed by time-lapse imaging of the fluorescence of Dendra2-tagged AtDRP3A/3B after green-to-red photoconversion; this experiment showed that AtDRP3A/3B is partially degraded during interphase. Additionally, we found that microtubules are involved in mitochondrial fission during mitosis, and that mitochondria movement to daughter cell was limited as early as metaphase. Taken together, these findings suggest that mitotic phosphorylation of AtDRP3A/3B promotes mitochondrial fission during plant cell mitosis, and that AtDRP3A/3B is partially degraded at interphase, providing mechanistic insight into the mitochondrial morphological changes associated with cell-cycle transitions in BY-2 suspension cells.

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.

Quantitative analysis of organelle distribution and dynamics in Physcomitrella patens protonemal cells

BACKGROUND

In the last decade, the moss Physcomitrella patens has emerged as a powerful plant model system, amenable for genetic manipulations not possible in any other plant. This moss is particularly well suited for plant polarized cell growth studies, as in its protonemal phase, expansion is restricted to the tip of its cells. Based on pollen tube and root hair studies, it is well known that tip growth requires active secretion and high polarization of the cellular components. However, such information is still missing in Physcomitrella patens. To gain insight into the mechanisms underlying the participation of organelle organization in tip growth, it is essential to determine the distribution and the dynamics of the organelles in moss cells.

RESULTS

We used fluorescent protein fusions to visualize and track Golgi dictyosomes, mitochondria, and peroxisomes in live protonemal cells. We also visualized and tracked chloroplasts based on chlorophyll auto-fluorescence. We showed that in protonemata all four organelles are distributed in a gradient from the tip of the apical cell to the base of the sub-apical cell. For example, the density of Golgi dictyosomes is 4.7 and 3.4 times higher at the tip than at the base in caulonemata and chloronemata respectively. While Golgi stacks are concentrated at the extreme tip of the caulonemata, chloroplasts and peroxisomes are totally excluded. Interestingly, caulonemata, which grow faster than chloronemata, also contain significantly more Golgi dictyosomes and fewer chloroplasts than chloronemata. Moreover, the motility analysis revealed that organelles in protonemata move with low persistency and average instantaneous speeds ranging from 29 to 75 nm/s, which are at least three orders of magnitude slower than those of pollen tube or root hair organelles.

CONCLUSIONS

To our knowledge, this study reports the first quantitative analysis of organelles in Physcomitrella patens and will make possible comparisons of the distribution and dynamics of organelles from different tip growing plant cells, thus enhancing our understanding of the mechanisms of plant polarized cell growth.

Overexpression of S4D mutant of Leishmania donovani ADF/cofilin impairs flagellum assembly by affecting actin dynamics

Leishmania, like other eukaryotes, contains large amounts of actin and a number of actin-related and actin binding proteins. Our earlier studies have shown that deletion of the gene corresponding to Leishmania actin-depolymerizing protein (ADF/cofilin) adversely affects flagellum assembly, intracellular trafficking, and cell division. To further analyze this, we have now created ADF/cofilin site-specific point mutants and then examined (i) the actin-depolymerizing, G-actin binding, and actin-bound nucleotide exchange activities of the mutant proteins and (ii) the effect of overexpression of these proteins in wild-type cells. Here we show that S4D mutant protein failed to depolymerize F-actin but weakly bound G-actin and inhibited the exchange of G-actin-bound nucleotide. We further observed that overexpression of this protein impaired flagellum assembly and consequently cell motility by severely impairing the assembly of the paraflagellar rod, without significantly affecting vesicular trafficking or cell growth. Taken together, these results indicate that dynamic actin is essentially required in assembly of the eukaryotic flagellum.

Plant organellar calcium signalling: an emerging field

This review provides a comprehensive overview of the established and emerging roles that organelles play in calcium signalling. The function of calcium as a secondary messenger in signal transduction networks is well documented in all eukaryotic organisms, but so far existing reviews have hardly addressed the role of organelles in calcium signalling, except for the nucleus. Therefore, a brief overview on the main calcium stores in plants-the vacuole, the endoplasmic reticulum, and the apoplast-is provided and knowledge on the regulation of calcium concentrations in different cellular compartments is summarized. The main focus of the review will be the calcium handling properties of chloroplasts, mitochondria, and peroxisomes. Recently, it became clear that these organelles not only undergo calcium regulation themselves, but are able to influence the Ca(2+) signalling pathways of the cytoplasm and the entire cell. Furthermore, the relevance of recent discoveries in the animal field for the regulation of organellar calcium signals will be discussed and conclusions will be drawn regarding potential homologous mechanisms in plant cells. Finally, a short overview on bacterial calcium signalling is included to provide some ideas on the question where this typically eukaryotic signalling mechanism could have originated from during evolution.

Golgi body motility in the plant cell cortex correlates with actin cytoskeleton organization

The actin cytoskeleton is involved in the transport and positioning of Golgi bodies, but the actin-based processes that determine the positioning and motility behavior of Golgi bodies are not well understood. In this work, we have studied the relationship between Golgi body motility behavior and actin organization in intercalary growing root epidermal cells during different developmental stages. We show that in these cells two distinct actin configurations are present, depending on the developmental stage. In small cells of the early root elongation zone, fine filamentous actin (F-actin) occupies the whole cell, including the cortex. In larger cells in the late elongation zone that have almost completed cell elongation, actin filament bundles are interspersed with areas containing this fine F-actin and areas without F-actin. Golgi bodies in areas with the fine F-actin exhibit a non-directional, wiggling type of motility. Golgi bodies in areas containing actin filament bundles move up to 7 μm s⁻¹. Since the motility of Golgi bodies changes when they enter an area with a different actin configuration, we conclude that the type of movement depends on the actin organization and not on the individual organelle. Our results show that the positioning of Golgi bodies depends on the local actin organization.

Variable-angle total internal reflection fluorescence microscopy of intact cells of Arabidopsis thaliana

BACKGROUND

Total internal reflection fluorescence microscopy (TIRFM) is a powerful tool for observing fluorescently labeled molecules on the plasma membrane surface of animal cells. However, the utility of TIRFM in plant cell studies has been limited by the fact that plants have cell walls, thick peripheral layers surrounding the plasma membrane. Recently, a new technique known as variable-angle epifluorescence microscopy (VAEM) was developed to circumvent this problem. However, the lack of a detailed analysis of the optical principles underlying VAEM has limited its applications in plant-cell biology.

RESULTS

Here, we present theoretical and experimental evidence supporting the use of variable-angle TIRFM in observations of intact plant cells. We show that when total internal reflection occurs at the cell wall/cytosol interface with an appropriate angle of incidence, an evanescent wave field of constant depth is produced inside the cytosol. Results of experimental TIRFM observations of the dynamic behaviors of phototropin 1 (a membrane receptor protein) and clathrin light chain (a vesicle coat protein) support our theoretical analysis.

CONCLUSIONS

These findings demonstrate that variable-angle TIRFM is appropriate for quantitative live imaging of cells in intact tissues of Arabidopsis thaliana.

Recent advances in understanding plant myosin function: life in the fast lane

Plant myosins are required for organelle movement, and a role in actin organization has recently come to light. Myosin mutants display several gross morphological phenotypes, the most severe being dwarfism and reduced fecundity, and there is a correlation between reduced organelle movement and morphological defects. This review aims to discuss recent findings in plants relating to the role of myosins in actin dynamics, development, and organelle movement, more specifically the endoplasmic reticulum (ER). One overarching theme is that there still appear to be more questions than answers relating to plant myosin function and regulation.

Conserved and unique features of the maize (Zea mays L.) root hair proteome

Root hairs are unicellular extensions of specialized epidermis cells. Under limiting conditions, they significantly increase the water and nutrient uptake capacity of plants by enlarging their root surface. Thus far, little is known about the initiation and growth of root hairs in the monocot model species maize. To gain a first insight into the protein composition of these specialized cells, the 2573 most abundant proteins of maize root hairs attached to four-day-old primary roots of the inbred line B73 were identified by combining 1DE with nanoLC-MS/MS in a shotgun proteomic experiment. Among the identified proteins, homologues of 252 proteins have been previously associated with root hair formation and development in other species. Comparison of the root hair reference proteome of the monocot species maize with the previously published root hair proteome of the dicot species soybean revealed conserved, but also unique, protein functions in root hairs of these two major groups of flowering plants.

The conserved mobility of mitochondria during leaf senescence reflects differential regulation of the cytoskeletal components in Arabidopsis thaliana

Leaf senescence is an organized process, which requires fine tuning between nuclear gene expression, activity of proteases and the maintenance of primary metabolism. Recently, we reported that leaf senescence was accompanied by an early degradation of the microtubule cytoskeleton in Arabidopsis thaliana. As the cytoskeleton is essential for cell stability, vesicle shuttling and organelle mobility, it might be asked how the regulation of these cell functions occurs with such drastic modifications of the cytoskeleton. Based on confocal laser microscopy observations and a micro-array analysis, the following addendum shows that mitochondrial mobility is conserved until the late stages of leaf senescence and provides evidences that the actin-cytoskeleton is maintained longer than the microtubule network. This conservation of actin-filaments is discussed with regards to energy metabolism as well as calcium signaling during programmed cell death.

Why have chloroplasts developed a unique motility system?

Organelle movement in plants is dependent on actin filaments with most of the organelles being transported along the actin cables by class XI myosins. Although chloroplast movement is also actin filament-dependent, a potential role of myosin motors in this process is poorly understood. Interestingly, chloroplasts can move in any direction, and change the direction within short time periods, suggesting that chloroplasts use the newly formed actin filaments rather than preexisting actin cables. Furthermore, the data on myosin gene knockouts and knockdowns in Arabidopsis and tobacco do not support myosins' XI role in chloroplast movement. Our recent studies revealed that chloroplast movement and positioning are mediated by the short actin filaments localized at chloroplast periphery (cp-actin filaments) rather than cytoplasmic actin cables. The accumulation of cp-actin filaments depends on kinesin-like proteins, KAC1 and KAC2, as well as on a chloroplast outer membrane protein CHUP1. We propose that plants evolved a myosin XI-independent mechanism of the actin-based chloroplast movement that is distinct from the mechanism used by other organelles.

Mitochondrial fusion, division and positioning in plants

Mitochondria are involved in many fundamental processes underpinning plant growth, development and death. Owing to their multiple roles, as the sites of the tricarboxylic acid cycle and oxidative phosphorylation, as harbourers of their own genomes and as sensors of cell redox status, amongst others, mitochondria are in a unique position to act as sentinels of cell physiology. The plant chondriome is typically organized as a population of physically discrete organelles, but visualization of mitochondria in living tissues has shown that the mitochondrial population is highly interactive. Mitochondria are highly motile and movement on the cytoskeleton ensures that the physically discrete organelles come into contact with one another, which allows transient fusion, followed by division of the mitochondrial membranes. This article serves to review our current knowledge of mitochondrial fusion and division, and link this to recent discoveries regarding a putative mitochondrial 'health-check' and repair process, whereby non-repairable dysfunctional mitochondria can be removed from the chondriome. It is proposed that the unequal distribution of the multipartite plant mitochondrial genome between discrete organelles provides the driver for transient mitochondrial fusion that, in turn, is dependent on mitochondrial motility, and that both fusion and motility are necessary to maintain a healthy functional chondriome.

ADF/cofilin-driven actin dynamics in early events of Leishmania cell division

ADF/cofilin is an actin-dynamics-regulating protein that is required for several actin-based cellular processes such as cell motility and cytokinesis. A homologue of this protein has recently been identified in the protozoan parasite Leishmania, which has been shown to be essentially required in flagellum assembly and cell motility. However, the role of this protein in cytokinesis remains largely unknown. We show here that deletion of the gene encoding ADF/cofilin in these organisms results in several aberrations in the process of cell division. These aberrations include delay in basal body and kinetoplast separation, cleavage furrow progression and flagellar pocket division. In addition to these changes, the intracellular trafficking and actin dynamics are also adversely affected. All these abnormalities are, however, reversed by episomal complementation. Together, these results indicate that actin dynamics regulates early events in Leishmania cell division.

Trafficking activity of myosin XXI is required in assembly of Leishmania flagellum

Actin-based myosin motors have a pivotal role in intracellular trafficking in eukaryotic cells. The parasitic protozoan organism Leishmania expresses a novel class of myosin, myosin XXI (Myo21), which is preferentially localized at the proximal region of the flagellum. However, its function in this organism remains largely unknown. Here, we show that Myo21 interacts with actin, and its expression is dependent of the growth stage. We further reveal that depletion of Myo21 levels results in impairment of the flagellar assembly and intracellular trafficking. These defects are, however, reversed by episomal complementation. Additionally, it is shown that deletion of the Myo21 gene leads to generation of ploidy, suggesting an essential role of Myo21 in survival of Leishmania cells. Together, these results indicate that actin-dependent trafficking activity of Myo21 is essentially required during assembly of the Leishmania flagellum.

Disruption of actin filaments induces mitochondrial Ca2+ release to the cytoplasm and [Ca2+]c changes in Arabidopsis root hairs

BACKGROUND

Mitochondria are dynamic organelles that move along actin filaments, and serve as calcium stores in plant cells. The positioning and dynamics of mitochondria depend on membrane-cytoskeleton interactions, but it is not clear whether microfilament cytoskeleton has a direct effect on mitochondrial function and Ca2+ storage. Therefore, we designed a series of experiments to clarify the effects of actin filaments on mitochondrial Ca2+ storage, cytoplasmic Ca2+ concentration ([Ca2+]c), and the interaction between mitochondrial Ca2+ and cytoplasmic Ca2+ in Arabidopsis root hairs.

RESULTS

In this study, we found that treatments with latrunculin B (Lat-B) and jasplakinolide (Jas), which depolymerize and polymerize actin filaments respectively, decreased membrane potential and Ca2+ stores in the mitochondria of Arabidopsis root hairs. Simultaneously, these treatments induced an instantaneous increase of cytoplasmic Ca2+, followed by a continuous decrease. All of these effects were inhibited by pretreatment with cyclosporin A (Cs A), a representative blocker of the mitochondrial permeability transition pore (mPTP). Moreover, we found there was a Ca2+ concentration gradient in mitochondria from the tip to the base of the root hair, and this gradient could be disrupted by actin-acting drugs.

CONCLUSIONS

Based on these results, we concluded that the disruption of actin filaments caused by Lat-B or Jas promoted irreversible opening of the mPTP, resulting in mitochondrial Ca2+ release into the cytoplasm, and consequent changes in [Ca2+]c. We suggest that normal polymerization and depolymerization of actin filaments are essential for mitochondrial Ca2+ storage in root hairs.

The speed of mitochondrial movement is regulated by the cytoskeleton and myosin in Picea wilsonii pollen tubes

Strategic control of mitochondrial movements and cellular distribution is essential for correct cell function and survival. However, despite being a vital process, mitochondrial movement in plant cells is a poorly documented phenomenon. To investigate the roles of actin filaments and microtubules on mitochondrial movements, Picea wilsonii pollen tubes were treated with two microtubule-disrupting drugs, two actin-disrupting drugs and a myosin inhibitor. Following these treatments, mitochondrial movements were characterized by multiangle evanescent wave microscopy and laser-scanning confocal microscopy. The results showed that individual mitochondria underwent three classes of linear movement: high-speed movement (instantaneous velocities >5.0 microm/s), low-speed movement (instantaneous velocities <5.0 microm/s) and variable-speed movement (instantaneous velocities ranging from 0.16 to 10.35 microm/s). 10 nM latrunculin B induced fragmentation of actin filaments and completely inhibited mitochondrial vectorial movement. Jasplakinolide treatment induced a 28% reduction in chondriome motility, and dramatically inhibition of high-speed and variable-speed movements. Treatment with 2,3-butanedione 2-monoxime caused a 61% reduction of chondriome motility, and the complete inhibition of high-speed and low-speed movements. In contrast to actin-disrupting drugs, microtubule-disrupting drugs caused mild effects on mitochondrial movement. Taxol increased the speed of mitochondrial movement in cortical cytoplasm. Oryzalin induced curved mitochondrial trajectories with similar velocities as in the control pollen tubes. These results suggest that mitochondrial movement at low speeds in pollen tubes is driven by myosin, while high-speed and variable-speed movements are powered both by actin filament dynamics and myosin. In addition, microtubule dynamics has profound effects on mitochondrial velocity, trajectory and positioning via its role in directing the arrangement of actin filaments.

Spinning Disk Confocal Microscopy of Calcium Signalling in Blood Vessel Walls

Spinning disk confocal laser microscopy systems can be used for observing fast events occurring in a small volume when they include a sensitive electron-multiplying CCD camera. Such a confocal system was recently used to capture the first pictures of intracellular calcium signalling within the projections of endothelial cells to the adjacent smooth muscle cells in the blood vessel wall. Detection of these calcium signals required high spatial and temporal resolution. A newly developed calcium ion (Ca(2+)) biosensor was also used. This exclusively expressed in the endothelium and fluoresced when Ca(2+) concentrations increased during signalling. This work gives insights into blood vessel disease because Ca(2+) signalling is critical for blood flow and pressure regulation.