Mitochondrial dynamics

Dynamic motion of mitochondria, plastids, and NAD(P)H zoning in Arabidopsis pollen tubes

Pollen tubes consume a tremendous amount of energy and are the fastest-growing cells known in plants. Mitochondria are key organelles that supply energy and play important roles in modulating cellular redox homeostasis. Here, we found that endogenous NAD(P)H in Arabidopsis pollen tubes was spatially highly correlated with the distribution of mitochondria, both peaking in the subapex region. A weak association was also observed between the NAD(P)H levels and pollen plastids. Further studies using Class XI myosin mutants confirmed that altered mitochondrial distribution and trafficking concomitantly affected intracellular NAD(P)H zoning in pollen tubes. By targeting the NADPH- and NADH/NAD+-specific biosensors to the pollen tube cytosol of the myo11c1/myo11c2 double mutants, we showed that the growing pollen tubes in the double mutants possessed a lower level of cytosolic NADPH but a higher cytosolic NADH/NAD+ ratio than the WT. We also found that the knockout of Myo11C1 and Myo11C2 led to fragmented mitochondria with reduced motility. Therefore, altered cytosolic NAD(P)H levels may be secondary to changes in mitochondrial mobility, positioning, or morphology. Our results suggest that the spatial distribution and movement of mitochondria and plastids affect NAD(P)H zoning in Arabidopsis growing pollen tubes and that their movements depend on Class XI myosins.

Dynamics of mitochondrial distribution during development and asymmetric division of rice zygotes

Mitochondria change their distribution from nuclear peripheral to uniformly distributed in cytoplasm during zygotic development of rice, and the mitochondria re-distribute around nucleus for even segregation into daughter cells. Mitochondria are highly dynamic organelles that actively move and change their localization along with actin filaments during the cell cycle. Studies of mitochondrial dynamics and distribution in plant cells have mainly been conducted on somatic cells, and our understanding about these aspects during the formation and development of zygotes remains limited. In this study, mitochondrial nucleoids of rice egg cells and zygotes were successfully stained by using N-aryl pyrido cyanine 3 (PC3), and their intracellular localization and distribution were demonstrated. Mitochondria in rice egg cells were small and coccoid in shape and were primarily distributed around the nucleus. Upon gamete fusion, the resulting zygotes showed mitochondrial dispersion and accumulation equivalent to those in rice egg cells until 8 h after fusion (HAF). Around 12 HAF, the mitochondria started to disperse throughout the cytoplasm of the zygotes, and this dispersive distribution pattern continued until the zygotes entered the mitotic phase. At early prophase, the mitochondria redistributed from dispersive to densely accumulated around the nucleus, and during the metaphase and anaphase, the mitochondria were depleted from possible mitotic spindle region. Thereafter, during cell plate formation between daughter nuclei, the mitochondria distributed along the phragmoplast, where the new cell wall was formed. Finally, relatively equivalent amounts of mitochondria were detected in the apical and basal cells which were produced through asymmetric division of the zygotes. Further observation by treating the egg cell with latrunculin B revealed that the accumulation of mitochondria around the nuclear periphery in egg cells and early zygotes depended on the actin meshwork converging toward the egg or zygote nucleus.

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

Effects of Salicylic Acid on the Metabolism of Mitochondrial Reactive Oxygen Species in Plants

Different abiotic and biotic stresses lead to the production and accumulation of reactive oxygen species (ROS) in various cell organelles such as in mitochondria, resulting in oxidative stress, inducing defense responses or programmed cell death (PCD) in plants. In response to oxidative stress, cells activate various cytoprotective responses, enhancing the antioxidant system, increasing the activity of alternative oxidase and degrading the oxidized proteins. Oxidative stress responses are orchestrated by several phytohormones such as salicylic acid (SA). The biomolecule SA is a key regulator in mitochondria-mediated defense signaling and PCD, but the mode of its action is not known in full detail. In this review, the current knowledge on the multifaceted role of SA in mitochondrial ROS metabolism is summarized to gain a better understanding of SA-regulated processes at the subcellular level in plant defense responses.

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.

Witnessing Genome Evolution: Experimental Reconstruction of Endosymbiotic and Horizontal Gene Transfer

Present day mitochondria and plastids (chloroplasts) evolved from formerly free-living bacteria that were acquired through endosymbiosis more than a billion years ago. Conversion of the bacterial endosymbionts into cell organelles involved the massive translocation of genetic material from the organellar genomes to the nucleus. The development of transformation technologies for organellar genomes has made it possible to reconstruct this endosymbiotic gene transfer in laboratory experiments and study the mechanisms involved. Recently, the horizontal transfer of genetic information between organisms has also become amenable to experimental investigation. It led to the discovery of horizontal genome transfer as an asexual process generating new species and new combinations of nuclear and organellar genomes. This review describes experimental approaches towards studying endosymbiotic and horizontal gene transfer processes, discusses the new knowledge gained from these approaches about both the evolutionary significance of gene transfer and the underlying molecular mechanisms, and highlights exciting possibilities to exploit gene and genome transfer in biotechnology and synthetic biology.

DRP3 and ELM1 are required for mitochondrial fission in the liverwort Marchantia polymorpha

Mitochondria increase in number by the fission of existing mitochondria. Mitochondrial fission is needed to provide mitochondria to daughter cells during cell division. In Arabidopsis thaliana, four kinds of genes have been reported to be involved in mitochondrial fission. Two of them, DRP3 (dynamin-related protein3) and FIS1 (FISSION1), are well conserved in eukaryotes. The other two are plant-specific ELM1 (elongated mitochondria1) and PMD (peroxisomal and mitochondrial division). To better understand the commonality and diversity of mitochondrial fission factors in land plants, we examined mitochondrial fission-related genes in a liverwort, Marchantia polymorpha. As a bryophyte, M. polymorpha has features distinct from those of the other land plant lineages. We found that M. polymorpha has single copies of homologues for DRP3, FIS1 and ELM1, but does not appear to have a homologue of PMD. Citrine-fusion proteins with MpDRP3, MpFIS1 and MpELM1 were localized to mitochondria in M. polymorpha. MpDRP3- and MpELM1-defective mutants grew slowly and had networked mitochondria, indicating that mitochondrial fission was blocked in the mutants, as expected. However, knockout of MpFIS1 did not affect growth or mitochondrial morphology. These results suggest that MpDRP3 and MpELM1 but neither MpFIS1 nor PMD are needed for mitochondrial fission in M. polymorpha.

Qualitative and quantitative modifications of root mitochondria during senescence of above-ground parts of Arabidopis thaliana

This work studied modifications experienced by root mitochondria during whole plant senescence or under light deprivation, using Arabidopsis thaliana plants with YFP tagged to mitochondria. During post-bolting development, root respiratory activity started to decline after aboveground organs (i.e., rosette leaves) had senesced. This suggests that carbohydrate starvation may induce root senescence. Similarly, darkening the whole plant induced a decrease in respiration of roots. This was partially due to a decrease in the number of total mitochondria (YFP-labelled mitochondria) and most probably to a decrease in the quantity of mitochondria with a developed inner membrane potential (ΔΨm, i.e., Mitotracker red- labelled mitochondria). Also, the lower amount of mitochondria with ΔΨm compared to YFP-labelled mitochondria at 10d of whole darkened plant, suggests the presence of mitochondria in a "standby state". The experiments also suggest that small mitochondria made the main contribution to the respiratory activity that was lost during root senescence. Sugar supplementation partially restored the respiration of mitochondria after 10d of whole plant dark treatment. These results suggest that root senescence is triggered by carbohydrate starvation, with loss of ΔΨm mitochondria and changes in mitochondrial size distribution.

Contributions of photosynthetic and non-photosynthetic cell types to leaf respiration in Vicia faba L. and their responses to growth temperature

In intact leaves, mitochondrial populations are highly heterogeneous among contrasting cell types; how such contrasting populations respond to sustained changes in the environment remains, however, unclear. Here, we examined respiratory rates, mitochondrial protein composition and response to growth temperature in photosynthetic (mesophyll) and non-photosynthetic (epidermal) cells from fully expanded leaves of warm-developed (WD) and cold-developed (CD) broad bean (Vicia faba L.). Rates of respiration were significantly higher in mesophyll cell protoplasts (MCPs) than epidermal cell protoplasts (ECPs), with both protoplast types exhibiting capacity for cytochrome and alternative oxidase activity. Compared with ECPs, MCPs contained greater relative quantities of porin, suggesting higher mitochondrial surface area in mesophyll cells. Nevertheless, the relative quantities of respiratory proteins (normalized to porin) were similar in MCPs and ECPs, suggesting that ECPs have lower numbers of mitochondria yet similar protein complement to MCP mitochondria (albeit with lower abundance serine hydroxymethyltransferase). Several mitochondrial proteins (both non-photorespiratory and photorespiratory) exhibited an increased abundance in response to cold in both protoplast types. Based on estimates of individual protoplast respiration rates, combined with leaf cell abundance data, epidermal cells make a small but significant (2%) contribution to overall leaf respiration which increases twofold in the cold. Taken together, our data highlight the heterogeneous nature of mitochondrial populations in leaves, both among contrasting cell types and in how those populations respond to growth temperature.

Does serum cause lipid-droplet accumulation in bovine embryos produced in vitro, during developmental days 1 to 4?

PURPOSE

Serum supplementation has shown to have beneficial effects on in vitro bovine embryo development. However, it is often assumed that serum supplementation may produce mitochondrial damage and this damage would generate lipid accumulation, a major obstacle for cryopreservation. The aim of the present study is to investigate the previous assumptions in early embryonic stages.

METHODS

We considered in vitro produced bovine embryos from day 1 to 4 of development, which were grown in presence of serum from days 1, 2 or 3 or in absence of it. Electron transmission micrographs allowed us to quantify the area occupied by lipid droplets and by the different mitochondrial types to evaluate serum effect. Using confocal microscopy we analyzed mitochondrial activity and location.

RESULTS

We found no evidence of lipid droplets accumulation or mitochondrial degeneration or reduction of mitochondrial area in serum supplemented media. Further, our results suggest that events of mitochondrial proliferation are taking place even in serum supplemented media.

CONCLUSIONS

Serum does not produce lipid accumulation or mitochondrial damage in bovine embryos from 2 to 16 cells. When serum was added to embryo culture medium on day 3 of development, there were ultrastructural signs of a beneficial effect for embryo development. The lack of serum until day 3 may also avoid the unnecessary exposure to potentially inhibitory factors present on it.

Effect of serum on the mitochondrial active area on developmental days 1 to 4 in in vitro-produced bovine embryos

Certain morphological changes at the subcellular level caused by the current techniques for in vitro embryo production seem to affect mitochondria. Many of these, including dysfunctional changes, have been associated with the presence of serum in the culture medium. Thus, the aim of the present work was to assess the mitochondrial dynamics occurring in embryos during the first 4 days of development, in order to analyze the most appropriate time for adding the serum. We used transmission electron microscopy (TEM) micrographs to calculate the embryo area occupied by the different morphological types of mitochondria, and analyzed them with Image Pro Plus analyzer. The results showed hooded mitochondria as the most representative type in 1- to 4-day-old embryos. Swollen, on-fusion, orthodox and vacuolated types were also present. When analyzed in embryos cultured without serum, the dynamics of the different mitochondrial types appeared to be similar, a fact that may provide evidence that the developmental changes control the mitochondrial dynamics, and that swollen mitochondria may not be completely inactive. In contrast, in culture medium supplemented with serum from estrous cows, we observed an increased area of hooded mitochondria by developmental day 4, a fact that may indicate an increased production of energy compared with previous days. According to these results, the bovine serum added to the culture medium seems not to be responsible for the functional changes in mitochondria.

Steps towards a mechanistic understanding of respiratory temperature responses

Temperature crucially affects the speed of metabolic processes in poikilotherm organisms, including plants. The instantaneous temperature responses of O(2)-reduction and CO(2)-release can be approximated by Arrhenius kinetics, even though respiratory gas exchange of plants is the net effect of many constituent biochemical processes. Nonetheless, the classical Arrhenius equation must be modified to account for a dynamic response to measurement temperatures. We show that this dynamic response is readily explained by combining Arrhenius and Michaelis-Menten kinetics, as part of a fresh appraisal of metabolic interpretations of instantaneous temperature responses. In combination with recent experimental findings, we argue that control of mitochondrial electron flow is shared among cytochrome oxidase and alternative oxidase under in vivo conditions, and is continuously coordinated. In this way, upstream carbohydrate metabolism and downstream electron transport appear to be optimized according to the demand of ATP, TCA-cycle intermediates and anabolic reducing power under differing metabolic states. We provide a link to the 'Growth and Maintenance Paradigm' of respiration and argue that respiratory temperature responses can be used as a tool to probe metabolic states of plant tissue, such that we can learn more about the mechanisms that govern longer-term acclimatization responses of plant metabolism.

Co-localization of mitochondria with chloroplasts is a light-dependent reversible response

Co-localization of mitochondria with chloroplasts in plant cells has long been noticed as beneficial interactions of the organelles to active photosynthesis. Recently, we have found that mitochondria in mesophyll cells of Arabidopsis thaliana expressing mitochondrion-targeted green fluorescent protein (GFP) change their distribution in a light-dependent manner. Mitochondria occupy the periclinal and anticlinal regions of palisade cells under weak and strong blue light, respectively. Redistributed mitochondria seem to be rendered static through co-localization with chloroplasts. Here we further demonstrated that distribution patterns of mitochondria, together with chloroplasts, returned back to those of dark-adapted state during dark incubation after blue-light illumination. Reversible association of the two organelles may underlie flexible adaptation of plants to environmental fluctuations.

Light-dependent intracellular positioning of mitochondria in Arabidopsis thaliana mesophyll cells

Mitochondria, the power house of the cell, are one of the most dynamic cell organelles. Although there are several reports on actin- or microtubule-dependent movement of mitochondria in plant cells, intracellular positioning and motility of mitochondria under different light conditions remain open questions. Mitochondria were visualized in living Arabidopsis thaliana leaf cells using green fluorescent protein fused to a mitochondrion-targeting signal. In darkness, mitochondria were distributed randomly in palisade cells. In contrast, mitochondria accumulated along the periclinal walls, similar to the accumulation response of chloroplasts, when treated with weak blue light (470 nm, 4 micromol m(-2) s(-1)). Under strong blue light (100 micromol m(-2) s(-1)), mitochondria occupied the anticlinal positions similar to the avoidance response of chloroplasts and nuclei. While strong red light (660 nm, 100 micromol m(-2) s(-1)) induced the accumulation of mitochondria along the inner periclinal walls, green light exhibited little effect on the distribution of mitochondria. In addition, the mode of movement of individual mitochondria along the outer periclinal walls under different light conditions was precisely analyzed by time-lapse fluorescence microscopy. A gradual increase in the number of static mitochondria located in the vicinity of chloroplasts with a time period of blue light illumination clearly demonstrated the accumulation response of mitochondria. Light-induced co-localization of mitochondria with chloroplasts strongly suggested their mutual metabolic interactions. This is the first characterization of the light-dependent redistribution of mitochondria in plant cells.

Mitochondrial reticulation in shoot apical meristem cells of Arabidopsis provides a mechanism for homogenization of mtDNA prior to gamete formation

Plant mitochondria are typically portrayed as being small, oval organelles. However, a recent study has demonstrated that the chondriome of shoot apical meristem (SAM) cells of Arabidopsis thaliana is unique in having two types of mitochondria, a large, central, tentaculate mitochondrion and variable numbers of small, oval mitochondria in the cell cortex that fuse with and fission from the tentaculate mitochondrion. The tentaculate mitochondrion wraps around the nucleus and persists throughout the cell cycle, undergoing distinct changes in morphology and size in a cell cycle-dependent manner. Here we demonstrate that SAM cell plastids, which also contain DNA, do not reticulate, and address the question as to why SAM cell mitochondria but not plastids form reticulate structures. We postulate that the presence of a large, tentaculate mitochondrion in SAM cells provides an efficient means for homogenizing the mitochondrial DNA and proteins during vegetative life prior to gamete production, and that this mitochondrial architecture prevents speciation. The lack of plastid reticulation in the same cells most likely reflects on the fact that the individual plastids are much larger than the small mitochondria and therefore do not need to fuse to achieve efficient intermixing of their genomes.

Characterization of mitochondrial dynamics and subcellular localization of ROS reveal that HsfA2 alleviates oxidative damage caused by heat stress in Arabidopsis

Heat shock transcription factor A2 (HsfA2) participates in multiple stress responses. To provide new insights into the role of HsfA2 in the heat stress (HS) response, in vivo production and localization of reactive oxygen species (ROS) and mitochondrial dynamics were investigated during the onset of cell death induced by an HS (40 degrees C, 10 min) applied after a 2 d recovery at 24 degrees C following a conditioning treatment at 37 degrees C for 1 h. In response to the HS, generated ROS were significantly higher in hsfA2 than in wild-type (WT) protoplasts and did not return to the baseline level when compared with WT protoplasts. The uncontrolled ROS in hsfA2 protoplasts localized not only to mitochondria but also to chloroplasts. Microscopic observations also revealed that, prior to cell death, hsfA2 protoplasts underwent more severe alterations in mitochondrial dynamics than WT protoplasts, including mitochondrial swelling, transmembrane potential loss, and the cessation of mitochondrial movement. The lower cell viability in hsfA2 than in WT protoplasts suggested that--combined with the findings that antioxidants only partially blocked ROS generation and arrested cell death in hsfA2 protoplasts relative to WT protoplasts--ROS participated in HS-induced cell death. Also the disruption of HsfA2 resulted in more severe oxidative stress and more cell death which, together with the more severe alterations in mitochondrial dynamics, could be complemented by introducing a WT copy of HsfA2. These results represent the first subcellular evidence that HsfA2 protects plants against HS-induced oxidative damage, organelle dysfunction, and subsequent cell death.

Methyl jasmonate induces production of reactive oxygen species and alterations in mitochondrial dynamics that precede photosynthetic dysfunction and subsequent cell death

Methyl jasmonate (MeJa) is a well-known plant stress hormone. Upon exposure to stress, MeJa is produced and causes activation of programmed cell death (PCD) and defense mechanisms in plants. However, the early events and the signaling mechanisms of MeJa-induced cell death have yet to be fully elucidated. To obtain some insights into the early events of this cell death process, we investigated mitochondrial dynamics, chloroplast morphology and function, production and localization of reactive oxygen species (ROS) at the single-cell level as well as photosynthetic capacity at the whole-seedling level under MeJa stimulation. Our results demonstrated that MeJa induction of ROS production, which first occurred in mitochondria after 1 h of MeJa treatment and subsequently in chloroplasts by 3 h of treatment, caused a series of alterations in mitochondrial dynamics including the cessation of mitochondrial movement, the loss of mitochondrial transmembrane potential (MPT), and the morphological transition and aberrant distribution of mitochondria. Thereafter, photochemical efficiency dramatically declined before obvious distortion in chloroplast morphology, which is prior to MeJa-induced cell death in protoplasts or intact seedlings. Moreover, treatment of protoplasts with ascorbic acid or catalase prevented ROS production, organelle change, photosynthetic dysfunction and subsequent cell death. The permeability transition pore inhibitor cyclosporin A gave significant protection against MPT loss, mitochondrial swelling and subsequent cell death. These results suggested that MeJa induces ROS production and alterations of mitochondrial dynamics as well as subsequent photosynthetic collapse, which occur upstream of cell death and are necessary components of the cell death process.

Recombination and linkage disequilibrium among mitochondrial genes in structured populations of the gynodioecious plant Silene vulgaris

The impact of intergenic recombination on the population genetics of plant mitochondrial genomes is unknown. In an effort to study this in the gynodioecious plant Silene vulgaris three-locus PCR/RFLP genotypes (based on the mitochondrial genes atpA, cox1, and cob) were determined for 239 individuals collected from 20 North American populations. Seventeen three-locus PCR/RFLP genotypes were found. Recombination was indicated by observation of each of the four two-locus genotypes possible when the two most common alleles are considered for each of two loci. Based on these common alleles the absolute values of standardized linkage disequilibrium |D'| between pairs of loci range from 0.17 to 0.78. This indicates modest disequilibrium, rather than the maximum value expected in the absence of recombination |D'=1|, or the linkage equilibrium expected if recombination is pervasive (D'=0). Values of D' did not depend on which pair of loci contributed alleles to the analysis. The direction of D' obtained for the common atpA and cox1 alleles was comparable in sign and magnitude to that obtained by examining similar information obtained in a prior study of European samples. All three loci indicated a high degree of population structure (average FST=0.63), which would limit the within-population genetic diversity required for intergenic recombination to create novel genotypes, if most mating is local. Thus, population structure acts as a constraint on the approach to linkage equilibrium.

Heterogeneity of plant mitochondrial responses underpinning respiratory acclimation to the cold in Arabidopsis thaliana leaves

In this study, we investigated whether changes in mitochondrial abundance, ultrastructure and activity are involved in the respiratory cold acclimation response in leaves of the cold-hardy plant Arabidopsis thaliana. Confocal microscopy [using plants with green fluorescence protein (GFP) targeted to the mitochondria] and transmission electron microscopy (TEM) were used to visualize changes in mitochondrial morphology, abundance and ultrastructure. Measurements of respiratory flux in isolated mitochondria and intact leaf tissue were also made. Warm-grown (WG, 25/ 20 degrees C day/night), 3-week cold-treated (CT) and cold-developed (CD) leaves were sampled. Although CT leaves exhibited some evidence of acclimation (as evidenced by higher rates of respiration at moderate measurement temperatures), it was only the CD leaves that were able to re-establish respiratory flux within the cold. Associated with the recovery of respiratory flux in the CD leaves were: (1) an increase in the total volume of mitochondria per unit volume of tissue in epidermal cells; (2) an increase in the ratio of cristae to matrix within mesophyll cell mitochondria; and (3) an increase in the capacity of the energy-producing cytochrome pathway in mitochondria isolated from whole leaf homogenates. Regardless of growth temperature, we found that contrasting cell types exhibited distinct differences in mitochondrial ultrastructure, morphology and abundance. Collectively, our data demonstrated the diversity and tissue-specific nature of mitochondrial responses that underpin respiratory acclimation to the cold, and revealed the heterogeneity of mitochondrial structure and abundance that exists within leaves.

Plant mitochondrial dynamics

Higher plant mitochondria are dynamic, pleomorphic organelles. The higher plant chondriome (all mitochondria in a cell collectively) is typically composed of numerous, physically discrete, mitochondria. However, frequent inter-mitochondrial fusion, enabling the mixing and recombination of mtDNA, ensures that the higher plant chondriome functions, at least genetically, as a discontinuous whole. Nothing is known about the genes controlling mitochondrial fusion in plants; there are no plant homologues of most of the genes known to be involved in fusion in other organisms. In contrast, the mitochondrial fission apparatus is generally conserved. Higher plant mitochondria use dynamin-like and Fis-type proteins for division; like yeast and animals, higher plants have lost the mitochondrial-specific form of the prokaryote-derived protein, FtsZ. In addition to being providers of energy for life, mitochondria provide a trigger for death. The role of mitochondrial dynamics in the initiation and promulgation of cell death is conserved in higher plants although there are specific differences in the genes and mechanisms involved relative to other higher eukaryotes.

Growth and senescence of Medicago truncatula cultured cells are associated with characteristic mitochondrial morphology

Here mitochondrial morphology and dynamics were investigated in Medicago truncatula cell-suspension cultures during growth and senescence. Cell biology techniques were used to measure cell growth and death in culture. Mitochondrial morphology was investigated in vivo using a membrane potential sensor probe coupled with confocal microscopy. Expression of a senescence-associated gene (MtSAG) was evaluated in different cell-growth phases. Mitochondria appeared as numerous, punctuate organelles in cells at the beginning of the subculture cycle, while interconnected networks were observed in actively growing cells. In senescent cells, giant mitochondria were associated with dying cells. The release of cytochrome c from mitochondria was detected in different growth phases of cultured cells. Studies on plant cell cultures allowed us to identify physiological and molecular markers of senescence and cell death, and to associate distinct mitochondrial morphology with cells under different physiological conditions.

BIGYIN, an orthologue of human and yeast FIS1 genes functions in the control of mitochondrial size and number in Arabidopsis thaliana

Reverse-genetics was used to evaluate the role of an Arabidopsis homologue of the human and yeast FIS1 genes, which are both involved in mitochondrial fission. Two independent T-DNA insertion mutants of gene At3g57090 were identified and genetically transformed to express mitochondria-targeted GFP to enable visualization of mitochondria in vivo. Plants homozygous for either of the recessive T-DNA mutant alleles, termed bigyin1-1 (bgy1-1) and bigyin1-2 (bgy1-2), displayed an abnormal mitochondrial morphology. Disruption of BIGYIN leads to a reduced number of mitochondria per cell, coupled to a large increase in the size of individual mitochondria, relative to wild-type. It is concluded that BIGYIN is an Arabidopsis FIS orthologue and is part of the Arabidopsis mitochondrial division apparatus.

Mitochondria as a connected population: ensuring continuity of the mitochondrial genome during plant cell dedifferentiation through massive mitochondrial fusion

Mitochondrial fusion in plants and its role in development are poorly understood. Cultured tobacco mesophyll protoplasts provide an excellent experimental system for visualizing mitochondrial dynamics. Before protoplasts first divide, mitochondria undergo a phase of extensive elongation before fission causes an increase in number, followed by actin filament (AF)-dependent dispersion that distributes mitochondria uniformly throughout the cytoplasm. Here, by fusing protoplasts containing either green fluorescent protein- or MitoTracker-labelled mitochondria, we show that elongation results from fusion during early (4-8 h) protoplast culture. This massive mitochondrial fusion (MMF) leads to near-complete mixing of the mitochondrial population within 24 h. Staining isolated mitochondria with 4',6-diamidino-2-phenylindole (DAPI) revealed that in freshly prepared protoplasts mitochondrial nucleoids were unequally distributed, with many mitochondria failing to stain with DAPI, suggesting the presence of an incomplete mitochondrial genome. Following MMF, nucleoids were distributed evenly throughout the population, thereby ensuring continuity of the mitochondrial genome in daughter cells. Massive mitochondrial fusion appears to be specific to dedifferentiation, since it also occurs in mesophyll protoplasts of Arabidopsis and Medicago but not in protoplasts from already dedifferentiated cells such as BY-2 or callus cultures. Efficient MMF requires an inner membrane electrical gradient, cytoplasmic protein synthesis, microtubules and functional kinesin but not ATP or AFs, indicating fundamental differences from mitochondrial fusion in non-plant systems. Our studies reveal that individual mitochondria are connected over time by fusion events, a finding that allows a clearer interpretation of how novel mitochondrial genotypes develop following cell fusion, and indicates that developmentally regulated fusion ensures continuity of the mitochondrial genome.

Organelle interactions and possible degradation pathways visualized in high-pressure frozen algal cells

Summary Organelle interactions, although essential for both anabolic and catabolic pathways in plant cells have not been examined in detail so far. In the present study the structure of different organelle-organelle, organelle-vesicle and organelle-membrane interactions were investigated in growing and nongrowing cells of the green alga Micrasterias denticulata by use of high pressure freeze fixation and energy filtering transmission electron microscopy. It became clear that contacts between mitochondria always occur by formation of a cone-shaped protuberance of one of the mitochondria which penetrates into its fusion partner. In the same way, structural interactions between mitochondria and mucilage vesicles and between microbodies and mucilage vesicles are achieved. Lytic compartments contact mitochondria or mucilage vesicles again by forming protuberances and by extending their contents into the respective compartment. Detached portions of mitochondria are found inside lytic compartments as a consequence of such interactions. Mitochondria found in contact with the plasma membrane reveal structural disintegration. Our study shows that interactions of organelles and vesicles are frequent events in Micrasterias cells of different ages. The interactive contacts between lytic compartments and organelles or vesicles suggest a degradation pathway different from autophagy processes described in the literature. Both the interactions between vesicles and organelles and the degradation pathways occur independently from cytoskeleton function as demonstrated by use of cytochalasin D and the microtubule inhibitor amiprophos-methyl.

Isolation of mutants with aberrant mitochondrial morphology from Arabidopsis thaliana

To identify genes related to plant mitochondrial morphology and dynamics, novel mutants with respect to mitochondrial morphology were isolated from an ethyl methane sulphonate (EMS)-mutated population of Arabidopsis thaliana. Mitochondria were visualized by transforming Arabidopsis with a gene for a fusion protein consisting of GFP and a mitochondria-targeting pre-sequence. From 19,000 M2 populations, 17 mutants were isolated by fluorescent microscopic observations. All mitochondria in these mutants were longer and/or larger than wild-type mitochondria. The approximate chromosomal loci of the mutations of seven mutants that grew well were determined. The mitochondrial phenotypes of six of the mutants were recessive but the mitochondrial phenotype of the seventh mutant was dominant. Chromosomal rough mapping of the seven mutants showed that the mutations occurred at four different loci. At least one of these loci was novel, i.e., it was different from loci of other known mitochondrial morphology mutants of Arabidopsis and different from loci of Arabidopsis homologues of yeast genes related to mitochondrial morphology.

Frequent fusion and fission of plant mitochondria with unequal nucleoid distribution

The balance between mitochondrial fusion and fission influences the reticular shape of mitochondria in yeasts. Little is known about whether mitochondria fusion occurs in plants. Plant mitochondria are usually more numerous and more grain-shaped than animal mitochondria. BLAST searches of the nuclear and mitochondrial genome sequences of Arabidopsis thaliana did not find any obvious homologue of mitochondrial fusion genes found in animals and yeasts. To determine whether mitochondrial fusion actually occurs in plants, we labeled mitochondria in onion epidermal cells with a mitochondria-targeted, photoconvertible fluorescent protein Kaede and then altered the fluorescence of some of the mitochondria within a cell from green to red. Frequent and transient fusion of red and green mitochondria was demonstrated by the appearance of yellow mitochondria that subsequently redivided. We also show that mitochondrial fission occasionally occurs without an equal distribution of the nucleoid (DNA-protein complex in mitochondria), resulting in the coexistence of mitochondria containing various amounts of DNA within a single cell. The heterogeneity of DNA contents in mitochondria may be overcome by the frequent and transient fusion of mitochondria.

ADL2a, like ADL2b, is involved in the control of higher plant mitochondrial morphology

A mitochondrial-GFP construct was used to tag mitochondria fluorescently in a T-DNA knockout line for the Arabidopsis dynamin ADL2a. Visualization of mitochondria in vivo demonstrated that disruption of ADL2a affected mitochondrial morphology. Mitochondria in the mutant had a complex morphology; occasionally large spherical organelles could be seen, but, more frequently, the mitochondria adopted a tubular morphology with many constrictions along their length. Mitochondria in the mutant also frequently possessed long protuberances that were named matrixules, extending to many micrometres in length.