Membrane continuities involving chloroplasts and other organelles in plant cells

Organelle Interactions in Plant Cells

The sequestration of enzymes and associated processes into sub-cellular domains, called organelles, is considered a defining feature of eukaryotic cells. However, what leads to specific outcomes and allows a eukaryotic cell to function singularly is the interactivity and exchanges between discrete organelles. Our ability to observe and assess sub-cellular interactions in living plant cells has expanded greatly following the creation of fluorescent fusion proteins targeted to different organelles. Notably, organelle interactivity changes quickly in response to stress and reverts to a normal less interactive state as homeostasis is re-established. Using key observations of some of the organelles present in a plant cell, this chapter provides a brief overview of our present understanding of organelle interactions in plant cells.

Membrane contacts with the endoplasmic reticulum modulate plastid morphology and behaviour

Plastid behaviour often occurs in tandem with endoplasmic reticulum (ER) dynamics. In order to understand the underlying basis for such linked behaviour we have used time-lapse imaging-based analysis of plastid movement and pleomorphy, including the extension and retraction of stromules. Stable transgenic plants that simultaneously express fluorescent fusion proteins targeted to the plastid stroma, and the ER along with BnCLIP1-eGFP, an independent plastid envelope localized membrane contact site (MCS) marker were utilized. Our experiments strongly suggest that transient MCS formed between the plastid envelope and the ER are responsible for their concomitant behaviour.

The ER Is a Common Mediator for the Behavior and Interactions of Other Organelles

Optimal functioning of a plant cell depends upon the efficient exchange of genetic information, ions, proteins and metabolites between the different organelles. Intuitively, increased proximity between organelles would be expected to play an important role in facilitating exchanges between them. However, it remains to be seen whether under normal, relatively non-stressed conditions organelles maintain close proximity at all. Moreover, does interactivity involve direct and frequent physical contact between the different organelles? Further, many organelles transition between spherical and tubular forms or sporadically produce thin tubular extensions, but it remains unclear whether changes in organelle morphology play a role in increasing their interactivity. Here, using targeted multicolored fluorescent fusion proteins, we report observations on the spatiotemporal relationship between plastids, mitochondria, peroxisomes and the endoplasmic reticulum in living plant cells. Under normal conditions of growth, we observe that the smaller organelles do not establish direct, physical contacts with each other but, irrespective of their individual form they all maintain intimate connectivity with the ER. Proximity between organelles does increase in response to stress through concomitant alterations in ER dynamics. Significantly, even under increased proximity the ER still remains sandwiched between the different organelles. Our observations provide strong live-imaging-based evidence for the ER acting as a common mediator in interactions between other organelles.

The Passage of H2O2 from Chloroplasts to Their Associated Nucleus during Retrograde Signalling: Reflections on the Role of the Nuclear Envelope

The response of chloroplasts to adverse environmental cues, principally increases in light intensity, stimulates chloroplast-to-nucleus retrograde signalling, which leads to the induction of immediate protective responses and longer-term acclimation. Hydrogen peroxide (H2O2), generated during photosynthesis, is proposed to both initiate and transduce a retrograde signal in response to photoinhibitory light intensities. Signalling specificity achieved by chloroplast-sourced H2O2 for signal transduction may be dependent upon the oft-observed close association of a proportion of these organelles with the nucleus. In this review, we consider more precisely the nature of the close association between a chloroplast appressed to the nucleus and the requirement for H2O2 to cross both the double membranes of the chloroplast and nuclear envelopes. Of particular relevance is that the endoplasmic reticulum (ER) has close physical contact with chloroplasts and is contiguous with the nuclear envelope. Therefore, the perinuclear space, which transducing H2O2 molecules would have to cross, may have an oxidising environment the same as the ER lumen. Based on studies in animal cells, the ER lumen may be a significant source of H2O2 in plant cells arising from the oxidative folding of proteins. If this is the case, then there is potential for the ER lumen/perinuclear space to be an important location to modify chloroplast-to-nucleus H2O2 signal transduction and thereby introduce modulation of it by additional different environmental cues. These would include for example, heat stress and pathogen infection, which induce the unfolded protein response characterised by an increased H2O2 level in the ER lumen.

Lipid transport required to make lipids of photosynthetic membranes

Photosynthetic membranes provide much of the usable energy for life on earth. To produce photosynthetic membrane lipids, multiple transport steps are required, including fatty acid export from the chloroplast stroma to the endoplasmic reticulum, and lipid transport from the endoplasmic reticulum to the chloroplast envelope membranes. Transport of hydrophobic molecules through aqueous space is energetically unfavorable and must be catalyzed by dedicated enzymes, frequently on specialized membrane structures. Here, we review photosynthetic membrane lipid transport to the chloroplast in the context of photosynthetic membrane lipid synthesis. We independently consider the identity of transported lipids, the proteinaceous transport components, and membrane structures which may allow efficient transport. Recent advances in lipid transport of chloroplasts, bacteria, and other systems strongly suggest that lipid transport is achieved by multiple mechanisms which include membrane contact sites with specialized protein machinery. This machinery is likely to include the TGD1, 2, 3 complex with the TGD5 and TGD4/LPTD1 systems, and may also include a number of proteins with domains similar to other membrane contact site lipid-binding proteins. Importantly, the likelihood of membrane contact sites does not preclude lipid transport by other mechanisms including vectorial acylation and vesicle transport. Substantial progress is needed to fully understand all photosynthetic membrane lipid transport processes and how they are integrated.

Structural associations between organelle membranes in nectary parenchyma cells

The close association between membranes and organelles, and the intense chloroplast remodeling in parenchyma cells of extrafloral nectaries occurred only at the secretion time and suggest a relationship with the nectar secretion. Associations between membranes and organelles have been well documented in different tissues and cells of plants, but poorly explored in secretory cells. Here, we described the close physical juxtaposition between membranes and organelles, mainly with chloroplasts, in parenchyma cells of Citharexylum myrianthum (Verbenaeceae) extrafloral nectaries under transmission electron microscopy, using conventional and microwave fixation. At the time of nectar secretion, nectary parenchyma cells exhibit a multitude of different organelle and membrane associations as mitochondria-mitochondria, mitochondria-endoplasmic reticulum, mitochondria-chloroplast, chloroplast-nuclear envelope, mitochondria-nuclear envelope, chloroplast-plasmalemma, chloroplast-chloroplast, chloroplast-tonoplast, chloroplast-peroxisome, and mitochondria-peroxisome. These associations were visualized as amorphous electron-dense material, a network of dense fibrillar material and/or dense bridges. Chloroplasts exhibited protrusions variable in shape and extension, which bring them closer to each other and to plasmalemma, tonoplast, and nuclear envelope. Parenchyma cells in the pre- and post-secretory stages did not exhibit any association or juxtaposition of membranes and organelles, and chloroplast protrusions were absent. Chloroplasts had peripheral reticulum that was more developed in the secretory stage. We propose that such subcellular phenomena during the time of nectar secretion optimize the movement of signaling molecules and the exchange of metabolites. Our results open new avenues on the potential mechanisms of organelle contact in parenchyma nectary cells, and reveal new attributes of the secretory cells on the subcellular level.

Spatial distribution of organelles in leaf cells and soybean root nodules revealed by focused ion beam-scanning electron microscopy

Analysis of cellular ultrastructure has been dominated by transmission electron microscopy (TEM), so images collected by this technique have shaped our current understanding of cellular structure. More recently, three-dimensional (3D) analysis of organelle structures has typically been conducted using TEM tomography. However, TEM tomography application is limited by sample thickness. Focused ion beam-scanning electron microscopy (FIB-SEM) uses a dual beam system to perform serial sectioning and imaging of a sample. Thus FIB-SEM is an excellent alternative to TEM tomography and serial section TEM tomography. Animal tissue samples have been more intensively investigated by this technique than plant tissues. Here, we show that FIB-SEM can be used to study the 3D ultrastructure of plant tissues in samples previously prepared for TEM via commonly used fixation and embedding protocols. Reconstruction of FIB-SEM sections revealed ultra-structural details of the plant tissues examined. We observed that organelles packed tightly together in Nicotiana benthamiana Domin leaf cells may form membrane contacts. 3D models of soybean nodule cells suggest that the bacteroids in infected cells are contained within one large membrane-bound structure and not the many individual symbiosomes that TEM thin-sections suggest. We consider the implications of these organelle arrangements for intercellular signalling.

Twin anchors of the soybean isoflavonoid metabolon: evidence for tethering of the complex to the endoplasmic reticulum by IFS and C4H

Isoflavonoids are specialized plant metabolites, almost exclusive to legumes, and their biosynthesis forms a branch of the diverse phenylpropanoid pathway. Plant metabolism may be coordinated at many levels, including formation of protein complexes, or 'metabolons', which represent the molecular level of organization. Here, we have confirmed the existence of the long-postulated isoflavonoid metabolon by identifying elements of the complex, their subcellular localizations and their interactions. Isoflavone synthase (IFS) and cinnamate 4-hydroxylase (C4H) have been shown to be tandem P450 enzymes that are anchored in the ER, interacting with soluble enzymes of the phenylpropanoid and isoflavonoid pathways (chalcone synthase, chalcone reductase and chalcone isomerase). The soluble enzymes of these pathways, whether localized to the cytoplasm or nucleus, are tethered to the ER through interaction with these P450s. The complex is also held together by interactions between the soluble elements. We provide evidence for IFS interaction with upstream and non-consecutive enzymes. The existence of such a protein complex suggests a possible mechanism for flux of metabolites into the isoflavonoid pathway. Further, through interaction studies, we identified several candidates that are associated with GmIFS2, an isoform of IFS, in soybean hairy roots. This list provides additional candidates for various biosynthetic and structural elements that are involved in isoflavonoid production. Our interaction studies provide valuable information about isoform specificity among isoflavonoid enzymes, which may guide future engineering of the pathway in legumes or help overcome bottlenecks in heterologous expression.

Evaluation of the Acquisition of the Aerobic Metabolic Capacity by Myelin, during its Development

Our previous reports indicate that the electron transfer chain and F_oF₁-ATP synthase are functionally expressed in myelin sheath, performing an extra-mitochondrial oxidative phosphorylation (OXPHOS), which would provide energy to the nerve axon. This supports the idea that myelin plays a trophic role for the axon. Although the four ETC complexes and ATP synthase are considered exquisite mitochondrial proteins, they are found ectopically expressed in several membranous structures. This study was designed to understand when and how the mitochondrial OXPHOS machinery is embedded in myelin, following myelinogenesis in the rat, which starts at birth and continues until the first month of age. Rats were sacrificed at different time points (from day 5 to 90 post birth). Western blot, immunofluorescence microscopy, luminometric, and oximetric analyses show that the isolated myelin starts to show OXPHOS components around the 11th day after birth and increases proportionally to the rat age, becoming similar to those of adult rat around the 30-third day. Interestingly, WB data show the same temporal relationship between myelinogenesis and appearance of proteins involved in mitochondrial fusion and cellular trafficking. It may be speculated that the OXPHOS complexes may be transferred to the endoplasmic reticulum membrane (known to interact with mitochondria) and from there through the Golgi apparatus to the forming myelin membrane.

Chloroplast signaling within, between and beyond cells

The most conspicuous function of plastids is the oxygenic photosynthesis of chloroplasts, yet plastids are super-factories that produce a plethora of compounds that are indispensable for proper plant physiology and development. Given their origins as free-living prokaryotes, it is not surprising that plastids possess their own genomes whose expression is essential to plastid function. This semi-autonomous character of plastids requires the existence of sophisticated regulatory mechanisms that provide reliable communication between them and other cellular compartments. Such intracellular signaling is necessary for coordinating whole-cell responses to constantly varying environmental cues and cellular metabolic needs. This is achieved by plastids acting as receivers and transmitters of specific signals that coordinate expression of the nuclear and plastid genomes according to particular needs. In this review we will consider the so-called retrograde signaling occurring between plastids and nuclei, and between plastids and other organelles. Another important role of the plastid we will discuss is the involvement of plastid signaling in biotic and abiotic stress that, in addition to influencing retrograde signaling, has direct effects on several cellular compartments including the cell wall. We will also review recent evidence pointing to an intriguing function of chloroplasts in regulating intercellular symplasmic transport. Finally, we consider an intriguing yet less widely known aspect of plant biology, chloroplast signaling from the perspective of the entire plant. Thus, accumulating evidence highlights that chloroplasts, with their complex signaling pathways, provide a mechanism for exquisite regulation of plant development, metabolism and responses to the environment. As chloroplast processes are targeted for engineering for improved productivity the effect of such modifications on chloroplast signaling will have to be carefully considered in order to avoid unintended consequences on plant growth and development.

Transorganellar complementation redefines the biochemical continuity of endoplasmic reticulum and chloroplasts

Tocopherols are nonpolar compounds synthesized and localized in plastids but whose genetic elimination specifically impacts fatty acid desaturation in the endoplasmic reticulum (ER), suggesting a direct interaction with ER-resident enzymes. To functionally probe for such interactions, we developed transorganellar complementation, where mutated pathway activities in one organelle are experimentally tested for substrate accessibility and complementation by active enzymes retargeted to a companion organelle. Mutations disrupting three plastid-resident activities in tocopherol and carotenoid synthesis were complemented from the ER in this fashion, demonstrating transorganellar access to at least seven nonpolar, plastid envelope-localized substrates from the lumen of the ER, likely through plastid:ER membrane interaction domains. The ability of enzymes in either organelle to access shared, nonpolar plastid metabolite pools redefines our understanding of the biochemical continuity of the ER and chloroplast with profound implications for the integration and regulation of organelle-spanning pathways that synthesize nonpolar metabolites in plants.

Oxydative phosphorylation in sciatic nerve myelin and its impairment in a model of dysmyelinating peripheral neuropathy

Myelin sheath is the proteolipid membrane wrapping the axons of CNS and PNS. We have shown data suggesting that CNS myelin conducts oxidative phosphorylation (OXPHOS), challenging its role in limiting the axonal energy expenditure. Here, we focused on PNS myelin. Samples were: (i) isolated myelin vesicles (IMV) from sciatic nerves, (ii) mitochondria from primary Schwann cell cultures, and (iii) sciatic nerve sections, from wild type or Charcot-Marie-Tooth type 1A (CMT1A) rats. The latter used as a model of dys-demyelination. O₂ consumption and activity of OXPHOS proteins from wild type (Wt) or CMT1A sciatic nerves showed some differences. In particular, O₂ consumption by IMV from Wt and CMT1A 1-month-old rats was comparable, while it was severely impaired in IMV from adult affected animals. Mitochondria extracted from CMT1A Schwann cell did not show any dysfunction. Transmission electron microscopy studies demonstrated an increased mitochondrial density in dys-demyelinated axons, as to compensate for the loss of respiration by myelin. Confocal immunohistochemistry showed the expression of OXPHOS proteins in the myelin sheath, both in Wt and dys-demyelinated nerves. These revealed an abnormal morphology. Taken together these results support the idea that also PNS myelin conducts OXPHOS to sustain axonal function.

Dual targeting and retrograde translocation: regulators of plant nuclear gene expression can be sequestered by plastids

Changes in the developmental or metabolic state of plastids can trigger profound changes in the transcript profiles of nuclear genes. Many nuclear transcription factors were shown to be controlled by signals generated in the organelles. In addition to the many different compounds for which an involvement in retrograde signaling is discussed, accumulating evidence suggests a role for proteins in plastid-to-nucleus communication. These proteins might be sequestered in the plastids before they act as transcriptional regulators in the nucleus. Indeed, several proteins exhibiting a dual localization in the plastids and the nucleus are promising candidates for such a direct signal transduction involving regulatory protein storage in the plastids. Among such proteins, the nuclear transcription factor WHIRLY1 stands out as being the only protein for which an export from plastids and translocation to the nucleus has been experimentally demonstrated. Other proteins, however, strongly support the notion that this pathway might be more common than currently believed.

Evidence for ectopic aerobic ATP production on C6 glioma cell plasma membrane

Extracellular ATP plays a pivotal role as a signaling molecule in physiological and pathological conditions in the CNS. In several glioma cell lines, ATP is a positive factor for one or more characteristics important for the abnormal growth and survival of these cells. This work presents immunofluorescence and biochemical analyses suggesting that an aerobic metabolism, besides mitochondria, is located also on the plasma membrane of C6 glioma cells. An ATP synthesis coupled to oxygen consumption was measured in plasma membrane isolated from C6 cells, sensitive to common inhibitors of respiratory chain complexes, suggesting the involvement of a putative surface ATP synthase complex. Immunofluorescence imaging showed that Cytochrome c oxydase colocalized with WGA, a typical plasma membrane protein, on the plasma membrane of glioma cells. Cytochrome c oxydase staining pattern appeared punctuate, suggesting the intriguing possibility that the redox chains may be expressed in discrete sites on C6 glioma cell membrane. Data suggest that the whole respiratory chain is localized on C6 glioma cell surface. Moreover, when resveratrol, an ATP synthase inhibitor, was added to culture medium, a cytostatic effect was observed, suggesting a correlation among the ectopic ATP synthesis and the tumor growth. So, a potential direction for the design of new targets for future therapies may arise.

Extensive and widespread homologies between mitochondrial DNA and chloroplast DNA in plants

We used hybridization techniques to demonstrate that numerous sequence homologies exist between cloned mung bean and spinach chloroplast DNA (ctDNA) restriction fragments and mtDNAs from corn, mung bean, spinach, and pea. The strongest cross-homologies are between clones derived from the ctDNA inverted repeat and mtDNA from corn and pea, although all the ctDNA clones tested hybridized to at least one mtDNA restriction fragment. Known chloroplast genes showing strong mtDNA homologies include those for the large subunit of ribulosebisphosphate carboxylase, which hybridizes to corn mtDNA, and the beta subunit of the chloroplast ATPase, which hybridizes to mung bean mtDNA. Certain of these homologies were confirmed by using cloned spinach mtDNA restriction fragments as probes in reciprocal hybridizations to ctDNA. Several of these ctDNA-homologous mtDNA sequences were shown to be much more closely related to ctDNA from the same species than to that of a distantly related species. We interpret these differential homologies as evidence for relatively recent DNA sequence transfer events, suggesting that transpostion between the two genomes is an ongoing evolutionary process.

Evidence for aerobic ATP synthesis in isolated myelin vesicles

Even though brain represents only 2-3% of the body weight, it consumes 20% of total body oxygen, and 25% of total body glucose. This sounds surprising, in that mitochondrial density in brain is low, while mitochondria are thought to be the sole site of aerobic energy supply. These data would suggest that structures other than mitochondria are involved in aerobic ATP production. Considering that a sustained aerobic metabolism needs a great surface extension and that the oxygen solubility is higher in neutral lipids, we have focused our attention on myelin sheath, the multilayered membrane produced by oligodendrocytes, hypothesizing it to be an ATP production site. Myelin has long been supposed to augment the speed of conduction, however, there is growing evidence that it exerts an as yet unexplained neuro-trophic role. In this work, by biochemical assays, Western Blot analysis, confocal laser microscopy, we present evidence that isolated myelin vesicles (IMV) are able to consume O(2) and produce ATP through the operation of a proton gradient across their membranes. Living optic nerve sections were exposed to MitoTracker, a classical mitochondrial dye, by a technique that we have developed and it was found that structures closely resembling nerve axons were stained. By immunohistochemistry we show that ATP synthase and myelin basic protein colocalize on both IMV and optic nerves. The complex of data suggests that myelin sheath may be the site of oxygen absorption and aerobic metabolism for the axons.

Targeting of nucleus-encoded proteins to chloroplasts in plants

Most chloroplast proteins are encoded in the nucleus and synthesized on free, cytosolic ribosomes in precursor form. Each precursor has an amino-terminal extension called a transit peptide, which directs the protein through a post-translational targeting pathway and is removed upon arrival inside the organelle. This 'protein import' process is mediated by the coordinate action of two multiprotein complexes, one in each of the envelope membranes: the TOC and TIC (Translocon at the Outer/ Inner envelope membrane of Chloroplasts) machines. Many components of these complexes have been identified biochemically in pea; these include transit peptide receptors, channel proteins, and molecular chaperones. Intriguingly, the Arabidopsis genome encodes multiple, homologous genes for receptor components of the TOC complex. Careful analysis indicated that the different receptor isoforms operate in different import pathways with distinct precursor recognition specificities. These 'substrate-specific' import pathways might play a role in the differentiation of different plastid types, and/or act to prevent deleterious competition effects between abundant and nonabundant precursors. Until recently, all proteins destined for internal chloroplast compartments were thought to possess a cleavable transit peptide, and to engage the TOC/TIC machinery. New studies using proteomics and other approaches have revealed that this is far from true. Remarkably, a significant number of chloroplast proteins are transported via a pathway that involves the endoplasmic reticulum and Golgi apparatus. Other recent reports have elucidated an intriguing array of protein targeting routes leading to the envelope membranes themselves.

Plastids and stromules interact with the nucleus and cell membrane in vascular plants

The various metabolic activities of plastids require continuous exchange of reactants and products with other organelles of the plant cell. Physical interactions between plastids and other organelles might therefore enhance the efficiency of plant metabolism. We have observed a close apposition of plastids and nuclei in various organs of Nicotiana tabacum and Arabidopsis thaliana. In hypocotyl epidermal cells, plastids and stromules, stroma-filled tubular extensions of the plastid envelope membrane, were observed to reside in grooves and infoldings of the nuclear envelope, indicating a high level of contact between the two organelle membranes. In a number of non-green tissues, including suspension-cultured cells, perinuclear plastids were frequently associated with long stromules that extended from the cell center to the cell membrane. In cotyledon petioles, cells lying adjacent to one another frequently contained stromules that met on either side of the shared cell wall, suggesting a means of intercellular communication. Our results therefore suggest that stromules have diverse roles within plant cells, perhaps serving as pathways between nuclei and more distant regions of the cell and possibly even other cells.

Oxidative remodeling of plastid carotenoids

Carotenoids are isoprenoid pigmented compounds that are present in representatives from practically all eukaryotic and prokaryotic taxa. In plants, carotenoids are synthesized and normally sequestered in plastids as lipophilic C40 constituents. However, they are also subjected to oxidative remodeling initiated by specific carotenoid cleavage dioxygenases. Primary products resulting from these reactions undergo modifications involving oxido-reduction, dehydratation rearrangement, and glycosylation. This review focuses on only a few of these derivatives for which the enzymes and genes involved have been characterized. The compartmentation of this metabolism and its significance have also been considered.

Mitochondrial proteins at unexpected cellular locations: export of proteins from mitochondria from an evolutionary perspective

Researchers in a wide variety of unrelated areas studying functions of different proteins are unexpectedly finding that their proteins of interest are actually mitochondrial proteins, although functions would appear to be extramitochondrial. We review the leading current examples of mitochondrial macromolecules indicated to be also present outside of mitochondria that apparently exit from mitochondria to arrive at their destinations. Mitochondrial chaperones, which have been implicated in growth and development, autoimmune diseases, cell mortality, antigen presentation, apoptosis, and resistance to antimitotic drugs, provide some of the best studied examples pointing to roles for mitochondria and mitochondrial proteins in diverse cellular phenomena. To explain the observations, we propose that specific export mechanisms exist by which certain proteins exit mitochondria, allowing these proteins to have additional functions at specific extramitochondrial sites. Several possible mechanisms by which mitochondrial proteins could be exported are discussed. Gram-negative proteobacteria, from which mitochondria evolved, contain a number of different mechanisms for protein export. It is likely that mitochondria either retained or evolved export mechanisms for certain specific proteins.

Mitochondrial-matrix proteins at unexpected locations: are they exported?

Many proteins that were originally characterized on the basis of non-mitochondrial functions have unexpectedly been shown to be identical to mitochondrial-matrix proteins. Most of these proteins are encoded by single nuclear genes and are initially targeted to the mitochondrial matrix. We suggest that mitochondria, as organelles of bacterial origin, possess specific mechanisms for export of proteins to other compartments.

Escape and migration of nucleic acids between chloroplasts, mitochondria, and the nucleus

The escape and migration of genetic information between mitochondria, chloroplasts, and nuclei have been an integral part of evolution and has a continuing impact on the biology of cells. The evolutionary transfer of functional genes and fragments of genes from chloroplasts to mitochondria, from chloroplasts to nuclei, and from mitochondria to nuclei has been documented for numerous organisms. Most documented instances of genetic material transfer have involved the transfer of information from mitochondria and chloroplasts to the nucleus. The pathways for the escape of DNA from organelles may include transient breaches in organellar membranes during fusion and/or budding processes, terminal degradation of organelles by autophagy coupled with the subsequent release of nucleic acids to the cytoplasm, illicit use of nucleic acid or protein import machinery, or fusion between heterotypic membranes. Some or all of these pathways may lead to the escape of DNA or RNA from organellar compartments with subsequent uptake of nucleic acids from the cytoplasm into the nucleus. Investigations into the escape of DNA from mitochondria in yeast have shown the rate of escape for gene-sized fragments of DNA from mitochondria and its subsequent migration to the nucleus to be roughly equivalent to the rate of spontaneous mutation of nuclear genes. Smaller fragments of mitochondrial DNA may appear in the nucleus even more frequently. Mutations of nuclear genes that define gene products important in controlling the rate of DNA escape from mitochondria in yeast also have been described. The escape of genetic material from mitochondria and chloroplasts has clearly had an impact on nuclear genetic organization throughout evolution and may also affect cellular metabolic processes.

Movement of DNA across the chloroplast envelope: Implications for the transfer of promiscuous DNA

Little is known about the mechanistic basis for the movement of promiscuous nucleic acids across cell membranes. To address this problem we sought conditions that would permit the entry of plasmid DNA into isolated, intact pea chloroplasts. DNA uptake did not occur normally, but was induced by hypotonic treatments, by incubation with millimolar levels of Mg(2+), or by heat shock at 42 °C. These results are consistent with DNA movement being permitted by conditions that transiently alter the permeability of the chloroplast envelope. Plant cells are subject to osmotic tensions and/or conditions inducing polymorphic changes in the membranes, such as those used in the present study, under several environmental stresses. In an evolutionary time frame, these phenomena may provide a mechanism for the transfer of promiscuous nucleic acids between organelles.

Biochemistry and molecular biology of chromoplast development

Plant cells contain a unique class of organelles, designated the plastids, which distinguish them from animal cells. According to the largely accepted endosymbiotic theory of evolution, plastids are descendants of prokaryotes. This process requires several adaptative changes which involve the maintenance and the expression of part of the plastid genome, as well as the integration of the plastid activity to the cellular metabolism. This is illustrated by the diversity of plastids encountered in plant cells. For instance, in tissues undergoing color changes, i.e., flowers and fruits, the chromoplasts produce and accumulate excess carotenoids. In this paper we attempt to review the basic aspects of chromoplast development.

On the mechanism of rapid plasma membrane and chloroplast envelope expansion in Dunaliella salina exposed to hypoosmotic shock

Dunaliella salina cells rapidly diluted from their normal 1.71 M NaCl-containing growth medium into medium containing 0.86 M NaCl swelled within 2--4 min to an average volume 1.76 X larger and a surface area 1.53 X larger than found in control cells. Morphometric analysis of thin section electron micrographs revealed that certain organelles, including the chloroplast, nucleus, and some types of vacuoles, also expanded in surface area as much or more than did the entire cell. It is likely that glycerol, the most important osmotically active intracellular solute, was present in high concentration within these organelles as well as in the cytoplasm itself. Thin section and freeze-fracture electron microscopy were utilized to trace the origin of membrane material whose addition permitted the large increase in plasma membrane surface area and the equally large growth of the chloroplast outer envelope. The findings indicated that the plasma membrane's expansion resulted from its selective fusion with numerous small (less than or equal to 0.25 micron diam) vesicles prevalent throughout the cytoplasm. In contrast, new membrane added to the chloroplast outer envelope was drawn from an entirely different source, namely, elements of the endoplasmic reticulum.

Development and enzyme activity of protein bodies in proteinoplasts of tobacco root cells

The development of protein bodies in proteinoplasts of tobacco (Nicotiana tabacum L. var. Wis. 38) roots was investigated with TEM, HVEM, and enzyme cytochemistry. These plastids contain a three-dimensional network of fenestrated tubules which originate from invaginations of the inner membrane of the plastid envelope. Elaboration of the network occurs in parallel with cell differentiation: slender tubules common to plastids in meristematic cells undergo dilation as protein accumulates during cell differentiation; proteinoplasts of vacuolate and root cap cells usually contain a large protein body. The contents of the peripheral tubules, originating from the inner membrane, are less electron dense than the tubules making up the central network. Localized dilations within the tubular network result in the formation of dense spheroidal structures, protein bodies, apparently as a result of continued protein accumulation via tubules connecting to the central network. Protein might be imported from segments of rough ER attached to or apposed to the outer membrane of the proteinoplast envelope. The presence of catalase (E.C. 1.11.1.6), peroxidase (E.C. 1.11.1.7), and cytochrome oxidase (E.C. 1.9.3.1) was demonstrated by cytochemistry with diaminobenzidine (DAB) as substrate. Oxidized DAB was found in protein bodies after incubation in each of the specific reaction media. While aminotriazole and sodium azide inhibited oxidation of DAB by catalase and peroxidase, respectively, only potassium cyanide completely inhibited oxidation of DAB in protein bodies. We conclude that protein bodies of proteinoplasts in tobacco roots are not sites for storage of protein, rather protein bodies contain heme protein(s) with strong oxidase activity that may convey a specific function to proteinoplasts.

Mitochondrial and chloroplast genomes of maize have a 12-kilobase DNA sequence in common

A 12-kilobase DNA sequence has been identified in the maize mitochondrial genome which is homologous to part of the inverted repeat of the maize chloroplast genome. In chloroplasts the sequence contains a 16S rRNA gene, and also the coding sequences for tRNAIle and tRNAVal. Mitochondrial DNA from the male-sterile cytoplasms of maize is altered in this region.

Paramylon synthesis by Euglena gracilis photoheterotrophically grown under low O2 pressure : Description of a mitochloroplast complex

Special culture conditions for Euglena gracilis Z and ZR are described. They induce interactions between the chloroplast and mitochondrial metabolisms leading to paramylon synthesis. When grown in continuous light under pure nitrogen and in the presence of lactate as the sole carbon source, sugar synthesis occurs during the first 24 h of culture with the participation of both mitochondria (using lactate) and of chloroplasts (fixing CO2 from lactate decarboxylation). The activities of ribulose bisphosphate carboxylase, phosphoenolpyruvate carboxylase, and phosphoenolpyruvate carboxykinase are very high and mitochondria and chloroplasts develop then a common network of vesicles in which paramylon grains can be seen. Electron micrographs demonstrate membrane continuity between the two types of organelles. Occasionally the mitochondrial matrix and the chloroplast stroma are separated by only a unit membrane.

Conjoined mitochondria and plastids in the barley mutant 'albostrians'

The intercalary meristem and surrounding tissues of the gene induced plastome mutant 'albostrians' of Hordeum vulgare L. were examined in the electron microscope for ultrastructural evidence of membrane continuities between plastids and mitochondria. In well developed tissues the ribosome-deficient plastids were usually in close proximity or appressed to mitochondria of normal appearance. In some sections through the meristemmatic region however the relationship between the two organelles was observed to be of a fused nature. These conjoinings are thought to be similar to those reported in normal living cells using cinephotomicrography but never before observed by transmission electron microscopy.