Stromules: Mobile Protrusions and Interconnections Between Plastids

Abstract: Stroma-filled tubules, recently named stromules, extend from the surface of plastids in most cell types and plant species examined. Stromules are highly dynamic structures, continuously and rapidly changing shape. They have been shown to interconnect plastids and permit the exchange of green fluorescent protein (GFP) between plastids. Stromules are enclosed by the inner and outer plastid envelope membranes and are 0.4 - 0.8 μm in diameter and up to 65 μm long. Movement of stromules is dependent on the actin cytoskeleton and the ATPase activity of myosin. Stromules are more abundant in cells containing a relatively small plastid volume and provide a means of enormously increasing the plastid surface area. Many important questions on the structure, function and mobility of stromules remain unanswered.     

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

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

Visualisation of stromules in transgenic wheat expressing a plastid-targeted yellow fluorescent protein

Stromules are stroma-filled tubules that extend from the plastids in all multicellular plants examined to date. To facilitate the visualisation of stromules on different plastid types in various tissues of bread wheat (Triticum aestivum L.), a chimeric gene construct encoding enhanced yellow fluorescent protein (EYFP) targeted to plastids with the transit peptide of wheat granule-bound starch synthase I was introduced by Agrobacterium-mediated transformation. The gene construct was under the control of the rice Actin1 promoter, and EYFP fluorescence was detected in plastids in all cell types throughout the transgenic plants. Stromules were observed on all plastid types, although the stromule length and abundance varied markedly in different tissues. The longest stromules (up to 40 μm) were observed in epidermal cells of leaves, whereas only short beak-like stromules were observed on chloroplasts in mesophyll cells. Epidermal cells in leaves and roots contained the highest proportion of plastids with stromules, and stromules were also abundant on amyloplasts in the endosperm tissue of developing seeds. The general features of stromule morphology and distribution were similar to those shown previously for tobacco (Nicotiana tabacum L.) and arabidopsis (Arabidopsis thaliana (L.) Heynh.).     

Stromuli. Plastiden‐Brücken im Netzwerk der Zelle

Plastidic bridges in the plant cell network: Stromules Stromules are mobile protrusions emanating from plastids. They might form bridges between plastids and connect them also with other compartments of the plant cell. They could be involved in coordination of plastid activities and in signalling. Stromules have been first observed in the water fern Selaginella more than 100 years ago. Later improved light microscopy enabled the visualization of stromules in higher plant plastids. 15 years ago, since plants accumulating the green fluorescing proteins (GPF) in the stroma became available they have been newly detected and are now studied intensively. Formation of stromules differs among plant tissues, developmental stages and environmental situations. Actin and myosin are required for the formation of stromules.     

Trafficking of Proteins through Plastid Stromules

Stromules are thin projections from plastids that are generally longer and more abundant on non-green plastids than on chloroplasts. Occasionally stromules can be observed to connect two plastid bodies with one another. However, photobleaching of GFP-labeled plastids and stromules in 2000 demonstrated that plastids do not form a network like the endoplasmic reticulum, resulting in the proposal that stromules have major functions other than transfer of material from one plastid to another. The absence of a network was confirmed in 2012 with the use of a photoconvertible fluorescent protein, but the prior observations of movement of proteins between plastids were challenged. We review published evidence and provide new experiments that demonstrate trafficking of fluorescent protein between plastids as well as movement of proteins within stromules that emanate from a single plastid and discuss the possible function of stromules.Projections from chloroplasts have been reported sporadically in the literature for over a hundred years (reviewed in Gray et al., 2001; Kwok and Hanson, 2004a) and became established as genuine features of plastids when they were observed by the targeting of green fluorescent protein (GFP) to the stromal compartment (Köhler et al., 1997). This study showed that these projections sometimes appeared to connect discrete plastid bodies, and photobleaching experiments demonstrated flow of GFP from one plastid body to another. After GFP in one plastid body was bleached, fluorescence rapidly recovered as a result of flow from GFP from the unbleached plastids. By continuous bleaching of a stromule connecting two plastids, fluorescence was lost from both plastids. This led to the speculation that there could be an interplastid communication system (Köhler et al., 1997). In a follow-up study to test the degree of interplastid connectedness, the term “stromule” was coined to prevent confusion with other tubular structures in the cell (Köhler and Hanson, 2000). The existence of a stromule-based plastid network was ruled out by these experiments, but movement of protein through stromules was confirmed, and it was proposed that stromules might function to facilitate transport of substances in and out of the plastid by increasing surface area and by placing the plastid compartment in close proximity to other organelles or subcellular structures (Köhler and Hanson, 2000). A study by Schattat et al. (2012) confirmed the absence of a plastid network with the use of a photoconvertible fluorescent protein. These authors also describe photoconversion experiments that appear to contradict our prior work demonstrating flow of GFP between two plastid bodies connected by a stromule. Here, we confirm our prior fluorescence recovery after photobleaching (FRAP) results, showing that proteins can move through stromules between individual plastids, and we demonstrate that a red photoconverted protein can also move into a region where photoconversion has not occurred, provided that potentially damaging levels of light are not used during the photoconversion experiment. We review previous studies showing the lack of an interconnected plastid network and consider other functions for stromules, such as facilitating the transport of enzymes and metabolites to and from the plastid to the vicinity of other organelles or regions of the cell.     

The Xanthomonas effector XopL uncovers the role of microtubules in stromule extension and dynamics in Nicotiana benthamiana

Xanthomonas campestris pv. vesicatoria type III‐secreted effectors were screened for candidates influencing plant cell processes relevant to the formation and maintenance of stromules in Nicotiana benthamiana lower leaf epidermis. Transient expression of XopL, a unique type of E3 ubiquitin ligase, led to a nearly complete elimination of stromules and the relocation of plastids to the nucleus. Further characterization of XopL revealed that the E3 ligase activity is essential for the two plastid phenotypes. In contrast to the XopL wild type, a mutant XopL lacking E3 ligase activity specifically localized to microtubules. Interestingly, mutant XopL‐labeled filaments frequently aligned with stromules, suggesting an important, yet unexplored, microtubule–stromule relationship. High time‐resolution movies confirmed that microtubules provide a scaffold for stromule movement and contribute to stromule shape. Taken together, this study has defined two populations of stromules: microtubule‐dependent stromules, which were found to move slower and persist longer, and microtubule‐independent stromules, which move faster and are transient. Our results provide the basis for a new model of stromule dynamics including interactions with both actin and microtubules.     

GFP-labelled Rubisco and aspartate aminotransferase are present in plastid stromules and traffic between plastids

Plastid stromules are membrane-bound protrusions of the plastid envelope that contain soluble stroma. Stromules are often found connecting plastids within a cell and fluorescence recovery after photobleaching (FRAP) experiments have demonstrated that green fluorescent protein (GFP) can move between plastids via these connections. In this report, the ability of endogenous plastid proteins to travel through stromules was investigated. The motility of GFP-labelled plastid aspartate aminotransferase and the Rubisco small subunit was studied in stromules by FRAP. Both fusion proteins assemble into protein complexes that appear to behave similarly to their endogenous counterparts. In addition, both enzymes are capable of trafficking between plastids via stromules.     

Regulation of leucoplast morphology in roots: Interorganellar signaling from mitochondria?

The appearance of leaf mesophyll chloroplasts in angiosperms is characterized by their uniform and static shape, which is molded by symmetric division of the preexisting organelles, involving three prokaryote-derived proteins: the division executor protein, FtsZ, and the division site positioning proteins, MinD and MinE. On the other hand, noncolored plastids in roots, where the involvement of the known chloroplast division factors in plastid morphogenesis is yet unclear, are morphologically heterogeneous and transform dynamically. This is further emphasized by the active formation of long tubular protrusions called stromules from the main body of those plastids. Molecular regulation and physiological significance of such dynamic morphology of root plastids also remain unknown. In this context, we have recently demonstrated that the mitochondrial respiratory inhibitor antimycin A induces rapid and reversible filamentation of root plastids (leucoplasts) in Arabidopsis thaliana. In contrast, the same treatment with antimycin A did not affect the morphology of amyloplasts in the columella cells at the root tip. The alternative oxidase inhibitor salicylhydroxamic acid suppresses the antimycin-induced plastid filamentation, perhaps implying an alternative oxidase-mediated interorganellar signaling between the mitochondria and the leucoplasts in the root cells. Our data may provide some clues as to how the formation of stromules is initiated.Key words: antimycin A, interorganellar crosstalk, plastid morphology, respiration, stress response, stromule     

Exclusion of plastid nucleoids and ribosomes from stromules in tobacco and Arabidopsis

Stromules are stroma-filled tubules that extend from the surface of plastids and allow the transfer of proteins as large as 550 kDa between interconnected plastids. The aim of the present study was to determine if plastid DNA or plastid ribosomes are able to enter stromules, potentially permitting the transfer of genetic information between plastids. Plastid DNA and ribosomes were marked with green fluorescent protein (GFP) fusions to LacI, the lac repressor, which binds to lacO-related sequences in plastid DNA, and to plastid ribosomal proteins Rpl1 and Rps2, respectively. Fluorescence from GFP-LacI co-localised with plastid DNA in nucleoids in all tissues of transgenic tobacco (Nicotiana tabacum L.) examined and there was no indication of its presence in stromules, not even in hypocotyl epidermal cells, which contain abundant stromules. Fluorescence from Rpl1-GFP and Rps2-GFP was also observed in a punctate pattern in chloroplasts of tobacco and Arabidopsis [Arabidopsis thaliana (L.) Heynh.], and fluorescent stromules were not detected. Rpl1-GFP was shown to assemble into ribosomes and was co-localised with plastid DNA. In contrast, in hypocotyl epidermal cells of dark-grown Arabidopsis seedlings, fluorescence from Rpl1-GFP was more evenly distributed in plastids and was observed in stromules on a total of only four plastids (<0.02% of the plastids observed). These observations indicate that plastid DNA and plastid ribosomes do not routinely move into stromules in tobacco and Arabidopsis, and suggest that transfer of genetic information by this route is likely to be a very rare event, if it occurs at all.     

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.     

Plastid stromule branching coincides with contiguous endoplasmic reticulum dynamics

Stromules are stroma-filled tubules extending from plastids whose rapid extension toward or retraction from other plastids has suggested a role in interplastidic communication and exchange of metabolites. Several studies point to sporadic dilations, kinks, and branches occurring along stromule length but have not elucidated the underlying basis for these occurrences. Similarly, although specific details on interacting partners have been missing, a consensus viewpoint suggests that stromules increase the interactive surface of a plastid with its cytoplasmic surroundings. Here, using live imaging, we show that the behavior of dynamic, pleomorphic stromules strongly coincides with that of cortical endoplasmic reticulum (ER) tubules. Covisualization of fluorescent protein-highlighted stromules and the ER in diverse cell types clearly suggests correlative dynamics of the two membrane-bound compartments. The extension and retraction, as well as directional changes in stromule branches occur in tandem with the behavior of neighboring ER tubules. Three-dimensional and four-dimensional volume rendering reveals that stromules that extend into cortical regions occupy channels between ER tubules possibly through multiple membrane contact sites. Our observations clearly depict coincidental stromule-ER behavior and suggest that either the neighboring ER tubules shape stromules directly or the behavior of both ER and stromules is simultaneously dictated by a shared cytoskeleton-based mechanism. These new observations strongly implicate the ER membrane in interactions with stromules and suggest that their interacting surfaces might serve as major conduits for bidirectional exchange of ions, lipids, and metabolites between the two organelles.     

New insights on stromules: Stroma filled tubules extended by independent plastids

The recognition of stromules as sporadically extended stroma filled tubules from all kinds of plastids constitutes one of the major insights that resulted from the direct application of green fluorescent protein aided imaging of living plant cells. Observations of dynamic green fluorescent stromules strongly suggested that plastids frequently interact with each other while photo-bleaching of interconnected plastids indicated that proteins can move within the stroma filled tubules. These observations readily fit into the prevailing concept of the endosymbiogenic origins of plastids and provided stromules the status of conduits for inter-plastid communication and macromolecule transfer. However, experimental evidence obtained recently through the use of photoconvertible protein labeled stromules strongly supports plastid independence rather than their interconnectivity. Additional information on stress conditions inducing stromules and observations on their alignment with other organelles suggests that the major role of stromules is to increase the interactive surface of a plastid with the rest of the cytoplasm.     

Regulation of actin cytoskeleton architecture by Eps8 and Abi1

Background  

The actin cytoskeleton participates in many fundamental processes including the regulation of cell shape, motility, and adhesion. The remodeling of the actin cytoskeleton is dependent on actin binding proteins, which organize actin filaments into specific structures that allow them to perform various specialized functions. The Eps8 family of proteins is implicated in the regulation of actin cytoskeleton remodeling during cell migration, yet the precise mechanism by which Eps8 regulates actin organization and remodeling remains elusive.     

Metabolic pathways of the wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) endosperm amyloplast revealed by proteomics

Background  

By definition, amyloplasts are plastids specialized for starch production. However, a proteomic study of amyloplasts isolated from wheat (Triticum aestivum Butte 86) endosperm at 10 days after anthesis (DPA) detected enzymes from many other metabolic and biosynthetic pathways. To better understand the role of amyloplasts in food production, the data from that study were evaluated in detail and an amyloplast metabolic map was outlined.     

Correlated behavior implicates stromules in increasing the interactive surface between plastids and ER tubules

Stromules are extended by plastids but the underlying basis for their extension and retraction had not been understood until recently. Our live-imaging aided observations on coincident plastid stromule branching and ER tubule dynamics open out new areas of investigation relating to these rapid subcellular interactions. This addendum provides a testable hypothesis on the formation of stromules, which argues against the need for new membrane incorporation and suggests that stromal extensions might result from a remodeling of the plastid envelope membrane in an ER aided manner.Key words: stromules, plastids, endoplasmic reticulum, fluorescent proteins, subcellular interactions, FNR-EGFP, RFP-ERThe extension and retraction of stromules (stroma-filled tubules) from both chlorophyll containing and achlorophyllous plastids is well established for diverse plant species.¹ Many different conditions such as increased subcellular redox stress,² symbiotic interactions,^3–5 elevated temperatures,⁶ viral infection⁷ and alterations in plastid size and density^8,9 have been associated with stromule formation. Stromules extended from different plastids have been observed as forming connecting bridges^10–13 through which an exchange of proteins has been demonstrated.^{10,11,14–16} The latter observations strongly suggested stromules as transient communication channels between plastids for exchanging metabolites.^10,11 The connectivity of plastids via stromules also led to the suggestion that plant cells might contain a plastidom, defined as a well-connected plastid-compartment.¹² However, as pointed out by Netasan and co-workers¹⁷ most stromules do not appear to interconnect plastids and thus the movement of macromolecules between plastids might not be their sole function. In concurrence with this viewpoint the observations of Gunning¹⁸ and Lütz and Engel¹⁹ suggest that stromules have a role in increasing plastid interactions with mitochondria and peroxisomes. Thus a more generalized and acceptable statement is that stromules serve to increase the plastid stromal surface area that is exposed to the subcellular environment.Notably, the largest membrane surface area within a cell is provided by the endoplasmic reticulum (ER). Transmission electron micrographs often suggest a close proximity between plastids and the ER^20,21 and the presence of a chloroplast envelope associated-ER has been demonstrated.²² However, studies aimed at uncovering possible dynamic interactions between stromules and the ER in living plant cells had not been carried out. Our recent work²³ investigated this possible relationship by simultaneously visualizing stromules (labeled using a ferredoxin NADP(H) oxidoreductase (FNR) transit peptide fused to enhanced GFP; hereafter referred to as FNR-EGFP) and the ER (highlighted using a chimeric red fluorescent protein (RFP) carrying a basic chitinase signal sequence and an ER retention sequence; referred to as RFP-ER). Our observations clearly identified the ER as providing the nearest set of membranes with which the plastid envelope can interact. The observations are most pertinent for the narrow cortical sleeve where the relatively large sized plastids (including chloroplasts) are pressed against the cell boundary (plasma membrane and comparatively rigid cell wall) on one side and a turgid vacuole on the inner side. This space is also traversed by the cortical ER mesh created by constantly reorganizing ER-tubules. As shown in Figure 1A the side of a plastid pressed against the plasma membrane is surrounded by a loose mesh of ER tubules while the side pressed close to the vacuolar membrane (Fig. 1B) is free of the ER cradle. The cortically located plastids display strong behavioral correlations between their stromules and the neighboring ER tubules (Fig. 1C–E). Notably stromules form triangular junctions that appear very similar to the 3-way junctions displayed by the cortical ER. The three-way junctions of the ER are cisternal locations from which tubules are extended to create ER polygons. Stromule branches are extended from the stromal triangles in a very similar manner along comparable angles. Interestingly the extension and retraction of stromules and their branches occurs in tandem with contiguous ER tubules. These observations on the coincident behavior of the two organelles indicate that stromule branching and dynamic behavior might rely upon the creation of multiple membrane contact sites (MCSs) with the ER. The MCSs could aid in stretching stromules along ER tubules while loss of contact between the two membranes could cause rapid stromule retraction. While the possibility of identifying MCSs on the stromule envelope is being explored further by us our initial observations evoke a long-standing but very relevant question. Where does the membrane for forming stromules come from?Open in a separate windowFigure 1Confocal laser scanning microscopy based imaging of living Nicotiana benthamiana cells co-expressing FNR-EGFP (labels stromules) and RFP-ER (labels ER), 3D isosurface rendering and a model based on the observations. (A) 3D iso-surface rendering of a chloroplast facing the plasma membrane side shows the plastid situated in a loose cradle of ER tubules. (B) The 3D rendered chloroplast ER reconstruction seen in (A) observed from the vacuolar side after rotation by 180° shows the chloroplast appears smooth and free of the ER mesh. (C–E) Sequential frames from a time-lapse movie depict a FNR-EGFP labeled stromule undergoing a branching event. Note that in (C) the stromule extending along an ER channel with possible contacts at several points along the stromule length. (D) depicts a stromal triangle (arrowhead) that forms a branch initial. (E) depicts a branched stromule with the branches aligned with the ER (size bar = 5 µm). (F) A plastid exhibiting a short, wide stroma-filled area after stromule retraction has occured. (G) The same plastid shown in (F) without the FNR-EGFP labeled stroma (arrowhead) provides an appreciation of the “mobile jacket” created by the stromal contents around the chlorophyll containing thylakoids (size bar = 5 µm). (H and I) A diagrammatic depiction based on our observations and pertinent literature. (H) The narrow cortical region between the vacuolar membrane and the cell boundary is shared by large plastids and a dynamic ER. A depiction of the loose stroma filled jacket (arrowhead) of a plastid along with contiguous ER. Note the relative positions of the vacuolar membrane and the plasma membrane. The plastid size cannot increase isotropically within the narrow confines of the cortex and thus stromule formation along ER tubules is favored (Arrow pointing direction of stromule extension along the ER). (I) Multiple contact sites might be created between the extending stromule and the neighboring ER tubules. The diagrammatic depiction emphasizes the remodeling of the loose plastid jacket for stromule extension along ER. The schematic does not depict the strong possibility that both organelles might share an F-actin based mechanism for their extension.Stromule extension visibly enlarges the area occupied by the stromal contents of a plastid. The general conclusion of stromules increasing the plastid surface is clearly validated. Indeed Hanson and coworkers¹ calculated that for a model plastid of 3 µm diameter extending a 0.2 µm diameter and 10 µm long stromule the stromule represents about 20% of the overall plastid envelope surface area. A point that remains unclear is whether stromule extension involves a net increase in plastidic membrane or merely involves a remodeling of the existing membrane into an elongated shape. In the first scenario stromule formation would involve the creation of fresh membrane vesicles and their incorporation into an existing bi-layered plastid coat. This would be followed by their dispersal into inner and outer plastid envelopes along with complete complements of protein import and export machinery. Currently there is no experimental evidence to support this scenario. Moreover, this viewpoint does not consider observations of stromule retraction at all. If new membranes have been added during stromule extension then what happens to them during the retraction stage? Further, live imaging clearly shows that the time involved in sporadic stromule extension, branching and retraction is in the order of seconds to minutes. Such short periods do not favor the viewpoint involving creation of new membrane and its incorporation into stromules.Interestingly, observations of plastids in living cells often suggest that they have a rather wobbly form than a tight compact shape. The unstable form suggests the presence of relaxed membranes enveloping the main plastid body (Fig. 1F and G). These loose fitting membranes have been described as a “mobile jacket”^1,24 whose presence and irregular protrusions can be clearly distinguished from long stromules using a shape index.⁶ Hanson and co-workers¹ speculated that the “mobile jacket” might be stretched out and could thus provide the extra membrane needed for stromule formation. The notion is supported by the work of Gunning, which shows stromules retracting into short amorphous protrusions.¹⁸ Our observations of short protrusions or “beaks”²³ being created sporadically on a relatively regular-appearing plastid surface support these ideas. According to our observations one or two of the beaks might develop into long stromules that would be extended along neighboring ER tubules (Fig. 1H and I).Moreover, in vitro experiments on sucrose filled liposomes and giant vesicles^25,26 are particularly interesting in this context. Plasmolytic shrinkage of the sucrose filled vesicles creates states in which excess membrane surrounds a small volume. Over time the membrane exhibits small protrusions that are very similar in shape to the protrusions formed by plastid. Taken together the above-mentioned observations support behavior that might be expected upon a reshaping of already existing membranes. Notably the membrane-remodeling scenario for the formation of a stromule does not involve an increase in the net membrane of a plastid but it does achieve an increase in the surface area over which plastid stromal contents can interact with their surroundings. The viewpoint is also able to explain stromule elasticity that is suggested by their rapid extension and retraction.The new observations and ensuing discussion suggesting stromules being formed by stretching of the plastid envelope and identifying the ER as an interactive membrane partner in the process lay down the foundation for a number of other questions. These include investigations relating to the role of stromules in metabolite import and export between the two organelles, understanding the relationship between stromule and the ER behavior in relation to the underlying actin cytoskeleton as well as assessing the specificity of myosin motors that might be involved in these dynamic processes.     

Stromules: a characteristic cell-specific feature of plastid morphology

Stromules (stroma-filled tubules) are highly dynamic structures extending from the surface of all plastid types examined so far, including proplastids, chloroplasts, etioplasts, leucoplasts, amyloplasts, and chromoplasts. Stromules are usually 0.35-0.85 microm in diameter and of variable length, from short beak-like projections to linear or branched structures up to 220 mum long. They are enclosed by the inner and outer plastid envelope membranes and enable the transfer of molecules as large as Rubisco (approximately 560 kDa) between interconnected plastids. Stromules occur in all cell types, but stromule morphology and the proportion of plastids with stromules vary from tissue to tissue and at different stages of plant development. In general, stromules are more abundant in tissues containing non-green plastids, and in cells containing smaller plastids. The primary function of stromules is still unresolved, although the presence of stromules markedly increases the plastid surface area, potentially increasing transport to and from the cytosol. Other functions of stromules, such as transfer of macromolecules between plastids and starch granule formation in cereal endosperm, may be restricted to particular tissues and cell types.     

Organelle extensions in plant cells

The life strategy of plants includes their ability to respond quickly at the cellular level to changes in their environment. The use of targeted fluorescent protein probes and imaging of living cells has revealed several rapidly induced organelle responses that create the efficient sub-cellular machinery for maintaining homeostasis in the plant cell. Several organelles, including plastids, mitochondria, and peroxisomes, extend and retract thin tubules that have been named stromules, matrixules, and peroxules, respectively. Here, I combine all these thin tubular forms under the common head of organelle extensions. All extensions change shape continuously and in their elongated form considerably increase organelle outreach into the surrounding cytoplasm. Their pleomorphy reflects their interactions with the dynamic endoplasmic reticulum and cytoskeletal elements. Here, using foundational images and time-lapse movies, and providing salient information on some molecular and biochemically characterized mutants with increased organelle extensions, I draw attention to their common role in maintaining homeostasis in plant cells.

Dynamic tubules extended from different organelles are integral components of the rapid subcellular response machinery involved in maintaining optimal working conditions within plant cells.     

A mutation in the cytosolic O-acetylserine (thiol) lyase induces a genome-dependent early leaf death phenotype in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Background  

Cysteine is a component in organic compounds including glutathione that have been implicated in the adaptation of plants to stresses. O-acetylserine (thiol) lyase (OAS-TL) catalyses the final step of cysteine biosynthesis. OAS-TL enzyme isoforms are localised in the cytoplasm, the plastids and mitochondria but the contribution of individual OAS-TL isoforms to plant sulphur metabolism has not yet been fully clarified.     

The influence of acetyl phosphate on DspA signalling in the Cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC6803

Background  

The dspA (hik33) gene, coding for a putative sensory histidine kinase, is conserved in plastids (ycf26) and cyanobacteria. It has been linked with a number of different stress responses in cyanobacteria.