The search for the missing link: a relic plastid in Perkinsus?

Perkinsus marinus (Phylum Perkinsozoa) is a protozoan parasite that has devastated natural and farmed oyster populations in the USA, significantly affecting the shellfish industry and the estuarine environment. The other two genera in the phylum, Parvilucifera and Rastrimonas, are parasites of microeukaryotes. The Perkinsozoa occupies a key position at the base of the dinoflagellate branch, close to its divergence from the Apicomplexa, a clade that includes parasitic protista, many harbouring a relic plastid. Thus, as a taxon that has also evolved toward parasitism, the Perkinsozoa has attracted the attention of biologists interested in the evolution of this organelle, both in its ultrastructure and the conservation, loss or transfer of its genes. A review of the recent literature reveals mounting evidence in support of the presence of a relic plastid in P. marinus, including the presence of multimembrane structures, characteristic metabolic pathways and proteins with a bipartite N-terminal extension. Further, these findings raise intriguing questions regarding the potential functions and unique adaptation of the putative plastid and/or plastid genes in the Perkinsozoa. In this review we analyse the above-mentioned evidence and evaluate the potential future directions and expected benefits of addressing such questions. Given the rapidly expanding molecular/genetic resources and methodological toolbox for Perkinsus spp., these organisms should complement the currently established models for investigating plastid evolution within the Chromalveolata.     

Ultrastructural studies of isolatedZea mays L. Chloroplasts

Summary The ultrastructure of isolated mesophyll chloroplasts ofZea mays L. was studied using the shadow casting technique. Grana and interconnecting fret membranes were observed in the same basic arrangement as they appear in thin sections. Quantasome units, as previously described by other authors for other species, were detected. Portions of the peripheral reticulum and envelope were also observed. Swollen membranes, comparable to those observed when isolated plastids are placed in water, were frequent in the preparations. It can be assumed, then, that in any preparation of this nature the hydrophillic spaces will absorb a large amount of water, swelling the intramembranous spaces and giving the membranes the appearance of large vesicles which will appear as flattened sacs when dried on the electron microscope grids. As there is no indication of any quantasome pattern in these membranes it is assumed that the particle arrangement of these membranes was altered during the procedure. It was not possible to determine whether the membranes observed in the swollen vesicles belonged to the grana or frets. A change in the morphology of the membranes during the processing may be observed with the light microscope. Recognition of this change is frequently difficult in electron micrographs, consequently, membranes with similar appearance under the electron microscope may actually have arisen from different portions of the plastid membrane systems.     

Vesicles Bearing Toxoplasma Apicoplast Membrane Proteins Persist Following Loss of the Relict Plastid or Golgi Body Disruption

Toxoplasma gondii and malaria parasites contain a unique and essential relict plastid called the apicoplast. Most apicoplast proteins are encoded in the nucleus and are transported to the organelle via the endoplasmic reticulum (ER). Three trafficking routes have been proposed for apicoplast membrane proteins: (i) vesicular trafficking from the ER to the Golgi and then to the apicoplast, (ii) contiguity between the ER membrane and the apicoplast allowing direct flow of proteins, and (iii) vesicular transport directly from the ER to the apicoplast. Previously, we identified a set of membrane proteins of the T. gondii apicoplast which were also detected in large vesicles near the organelle. Data presented here show that the large vesicles bearing apicoplast membrane proteins are not the major carriers of luminal proteins. The vesicles continue to appear in parasites which have lost their plastid due to mis-segregation, indicating that the vesicles are not derived from the apicoplast. To test for a role of the Golgi body in vesicle formation, parasites were treated with brefeldin A or transiently transfected with a dominant-negative mutant of Sar1, a GTPase required for ER to Golgi trafficking. The immunofluorescence patterns showed little change. These findings were confirmed using stable transfectants, which expressed the toxic dominant-negative sar1 following Cre-loxP mediated promoter juxtaposition. Our data support the hypothesis that the large vesicles do not mediate the trafficking of luminal proteins to the apicoplast. The results further show that the large vesicles bearing apicoplast membrane proteins continue to be observed in the absence of Golgi and plastid function. These data raise the possibility that the apicoplast proteome is generated by two novel ER to plastid trafficking pathways, plus the small set of proteins encoded by the apicoplast genome.     

MEMBRANES OF MESOPHYLL CELLS OF NICOTIANA RUSTICA AND PHASEOLUS VULGARIS WITH PARTICULAR REFERENCE TO THE CHLOROPLAST

Weier , T. E., and W. W. Thomson . (U. California, Davis.) Membranes of mesophyll cells of Nicotiana rustica and Phaseolus vulgaris with particular reference to the chloroplast. Amer. Jour. Bot. 49(8): 807–820. Illus. 1962.—The endoplasmic reticulum in mesophyll cells is represented by short lengths of irregularly disposed, paired membranes. It is occasionally associated with a typically double nuclear envelope. Groups of irregularly parallel, paired membranes suggesting disorganized dictyosomes occur infrequently. Mitochondria are unevenly distributed in mesophyll; they are large and have sparse tubular cristae around their periphery. In the great majority of instances the bounding membrane is diffusely stained with KMnO₄. When it is sharp and distinct, it may be double as usually pictured, or it may have well-delineated stretches of a single membrane bounding 25–50% of its circumference. The tonoplast and ectoplast are very fragile, the former appearing as a single dark line. In young leaves the ectoplast is visualized as a continuous single membrane adjacent to the cell wall, but in our micrographs of mature leaves it is always discontinuous. The plastid membrane sometimes is distinctly double, having 2 dark components bounding a light component. In the great majority of cases, however, this membrane is either a solid dark line, or the clear component of the double membrane is crossed by delicate dark lines giving the membrane a braided, or scalariform appearance. The various appearances of the membrane may intergrade with each other. The width of the plastid membrane is variable, ranging from 200 to 400 A. The inner component may invaginate into the stroma, and bodies may form in the clear space between the 2 outer membrane components. Micrographs suggest that these bodies, and others formed by small masses of stroma, may be expelled into the hyaloplasm, where they exist as spherical single-membraned particulates. The reality of the variable structure of the plastid membrane is discussed in light of concepts of membrane activity, molecular structure, and the relation of these factors to possible artifacts.     

Friend or foe: Hybrid proline-rich proteins determine how plants respond to beneficial and pathogenic microbes

Plant plastids generate signals, including some derived from lipids, that need to be mobilized to effect signaling. We used informatics to discover potential plastid membrane proteins involved in microbial responses in Arabidopsis (Arabidopsis thaliana). Among these are proteins co-regulated with the systemic immunity component AZELAIC ACID INDUCED 1, a hybrid proline-rich protein (HyPRP), and HyPRP superfamily members. HyPRPs have a transmembrane domain, a proline-rich region (PRR), and a lipid transfer protein domain. The precise subcellular location(s) and function(s) are unknown for most HyPRP family members. As predicted by informatics, a subset of HyPRPs has a pool of proteins that target plastid outer envelope membranes via a mechanism that requires the PRR. Additionally, two HyPRPs may be associated with thylakoid membranes. Most of the plastid- and nonplastid-localized family members also have pools that localize to the endoplasmic reticulum, plasma membrane, or plasmodesmata. HyPRPs with plastid pools regulate, positively or negatively, systemic immunity against the pathogen Pseudomonas syringae. HyPRPs also regulate the interaction with the plant growth-promoting rhizobacteria Pseudomonas simiae WCS417 in the roots to influence colonization, root system architecture, and/or biomass. Thus, HyPRPs have broad and distinct roles in immunity, development, and growth responses to microbes and reside at sites that may facilitate signal molecule transport.

Hybrid proline-rich proteins that reside at plastid membranes and other sites have broad and distinct roles in immunity, development, and growth responses to microbes.     

Studies on a Maize Mutant Sensitive to Low Temperature II. Chloroplast Structure, Development, and Physiology

In the Zea mays L. mutant M11 grown in the dark at 15°, the ultrastructure of the etioplast is abnormal. The pigment content of the etioplasts is reduced but the in vivo absorption characteristics suggest that the normal protochlorophyll (ide)-holochrome is present. The lowered synthetic ability of the etioplasts is not primarily due to a reduced complement of plastid ribosomes. The plastids of mutant M11 grown in the light at 15° contain little pigment, are markedly deficient in ribosomes and their ultrastructure is abnormal. In mutant M11 grown at 15°, an extreme sensitivity of the plastid membranes to light was observed.     

Events surrounding the early development of Euglena chloroplasts. 16. Plastid thylakoid polypeptides during greening

Using sulfolipid to locate plastid thylakoid membranes in gradients from dark-grown resting cells it has been possible to study the plastid thylakoid membrane polypeptides of Euglena gracilis var. bacillaris undergoing light-induced chloroplast development. All plastid thylakoid bands seen in dark-growing wild-type cells and in mutant W₃BUL in which plastid DNA is undetectable, are observed to increase in amount during plastid development. Others, which are undetectable in dark-grown wild-type and W₃BUL increase greatly during plastid development and appear to be those associated with pigment-protein complexes. The data obtained from experiments where the polypeptides were labeled with ³⁵S during development, either continuously or in pulses, were consistent with these findings. Cycloheximide strongly inhibited the increases in amount in all bands and chloramphenicol or streptomycin produced a lower level of inhibition in all bands indicating tight control of the formation of each plastid membrane constituent by the others. The formation of a polypeptide band of 25 000 molecular weight, thought to be a part of a pigment-protein complex of the thylakoid, and chlorophyll synthesis were inhibited identically by these antibiotics.     

Distinct Pathways Mediate the Sorting of Tail-Anchored Proteins to the Plastid Outer Envelope

Background

Tail-anchored (TA) proteins are a distinct class of membrane proteins that are sorted post-translationally to various organelles and function in a number of important cellular processes, including redox reactions, vesicular trafficking and protein translocation. While the molecular targeting signals and pathways responsible for sorting TA proteins to their correct intracellular destinations in yeasts and mammals have begun to be characterized, relatively little is known about TA protein biogenesis in plant cells, especially for those sorted to the plastid outer envelope.

Methodology/Principal Findings

Here we investigated the biogenesis of three plastid TA proteins, including the 33-kDa and 34-kDa GTPases of the translocon at the outer envelope of chloroplasts (Toc33 and Toc34) and a novel 9-kDa protein of unknown function that we define here as an outer envelope TA protein (OEP9). Using a combination of in vivo and in vitro assays we show that OEP9 utilizes a different sorting pathway than that used by Toc33 and Toc34. For instance, while all three TA proteins interact with the cytosolic OEP chaperone/receptor, AKR2A, the plastid targeting information within OEP9 is distinct from that within Toc33 and Toc34. Toc33 and Toc34 also appear to differ from OEP9 in that their insertion is dependent on themselves and the unique lipid composition of the plastid outer envelope. By contrast, the insertion of OEP9 into the plastid outer envelope occurs in a proteinaceous-dependent, but Toc33/34-independent manner and membrane lipids appear to serve primarily to facilitate normal thermodynamic integration of this TA protein.

Conclusions/Significance

Collectively, the results provide evidence in support of at least two sorting pathways for plastid TA outer envelope proteins and shed light on not only the complex diversity of pathways involved in the targeting and insertion of proteins into plastids, but also the molecular mechanisms that underlie the delivery of TA proteins to their proper intracellular locations in general.     

Developmental Effects of Sandoz 6706 on Activities of Enzymes of Phenolic and General Metabolism in Barley Shoots Grown in the Dark or under Low or High Intensity Light

The activities of three enzymes of phenolic biosynthesis and six of general metabolism were studied at 24-hour intervals between the 3rd and 8th day after planting in barley shoots treated with the chlorosis-inducing herbicide Sandoz 6706 and grown in the dark or under high or low intensity light. The herbicide had no effect on fresh weight or soluble protein (per shoot) in plants grown in the dark or under low intensity light, but slightly decreased these parameters in plants grown for more than 5 days under high intensity light. In dark-grown seedlings the herbicide had no detectable effects on plastid ultrastructure or on the activity of malate dehydrogenase, cytochrome c oxidase, NADP-cytochrome c reductase, triose phosphate isomerase, peroxidase, catalase, shikimate dehydrogenase, phenylalanine ammonia-lyase, or chalcone-flavanone isomerase. Under low intensity light, Sandoz 6706-treated plants developed plastids with single thylakoids extending across the organelle, and the activity of all enzymes examined was increased to varying degrees. When the herbicide-treated plants were grown under high intensity light, plastid lamellar organization was severely disrupted. Activities of shikimate dehydrogenase and chalcone-flavanone isomerase were markedly enhanced, phenylalanine ammonia-lyase activity slightly promoted, and catalase activity severely inhibited. The other enzymes were not appreciably affected by Sandoz 6706 under high intensity light. It is concluded that the changes in plastid ultrastructure and enzyme activities of the herbicide-treated plants are largely secondary photomorphogenetic or photooxidative responses in the carotenoid-free plants in which chlorophylls accumulate in reduced amounts (low intensity light) or are completely absent (high intensity light).     

ULTRASTRUCTURE OF MUCOCYSTS AND PELLICLE OF TETRAHYMENA PYRIFORMIS

Tetrahymena pyriformis GL was fixed with glutaraldehyde and/or OsO₄ for a study of cytoplasmic ultrastructure. Many small vacuoles 0.05 to 0.5 µ in diameter were found to contain each a dense particle enveloped by a limiting membrane. This membrane is continuous with the membrane of the vacuole. The particles are irregular in shape and size, but similar to the mucocysts in the appearance of the matrix. It is suggested that they are the first morphologically distinguishable stages in the development of mucocysts. In the course of this development, amorphous material becomes crystalline with a longitudinal period of 150 A and a lateral period of 100 A. The mature mucocysts are rather uniform in size and have a spheroidal shape. During discharge, the crystalline pattern disappears and the mucocysts assume a spherical configuration. The inner limiting membrane of a mucocyst seems to disintegrate during the process of discharge while the outer membrane becomes continuous with the outermost pellicular membrane; the inner pellicular membrane is continuous with the cytoplasmic membrane. Rows of few to 15 or more microtubules were found either between the cytoplasmic membrane and the ectoplasmic layer (longitudinal fibrils) or underneath the ectoplasmic layer (transverse fibrils). The outer and inner pellicular membranes are uniformly spaced and connected by "cross-bridges." Details of these structures are described.     

Chloroplast fine structure in the japonica-2 maize mutant exposed to continuous illumination. 2. The white tissues.

Exposure to continuous illumination causes an enhancement of thylakoid swelling in the mesophyll chloroplasts of the white tissues of the japonica-2 maize mutant. In the bundle sheath plastids the effects of continuous illumination are striking and intriguing, because a regular honeycomb-like fret of membranes is formed from a provesicular body. No interpretation of this fret of membranes is, at present, possible.     

Lipid trafficking between the endoplasmic reticulum and the plastid in Arabidopsis requires the extraplastidic TGD4 protein

The development of chloroplasts in Arabidopsis thaliana requires extensive lipid trafficking between the endoplasmic reticulum (ER) and the plastid. The biosynthetic enzymes for the final steps of chloroplast lipid assembly are associated with the plastid envelope membranes. For example, during biosynthesis of the galactoglycerolipids predominant in photosynthetic membranes, galactosyltransferases associated with these membranes transfer galactosyl residues from UDP-Gal to diacylglycerol. In Arabidopsis, diacylglycerol can be derived from the ER or the plastid. Here, we describe a mutant of Arabidopsis, trigalactosyldiacylglycerol4 (tgd4), in which ER-derived diacylglycerol is not available for galactoglycerolipid biosynthesis. This mutant accumulates diagnostic oligogalactoglycerolipids, hence its name, and triacylglycerol in its tissues. The TGD4 gene encodes a protein that appears to be associated with the ER membranes. Mutant ER microsomes show a decreased transfer of lipids to isolated plastids consistent with in vivo labeling data, indicating a disruption of ER-to-plastid lipid transfer. The complex lipid phenotype of the mutant is similar to that of the tgd1,2,3 mutants disrupted in components of a lipid transporter of the inner plastid envelope membrane. However, unlike the TGD1,2,3 complex, which is proposed to transfer phosphatidic acid through the inner envelope membrane, TGD4 appears to be part of the machinery mediating lipid transfer between the ER and the outer plastid envelope membrane. The extent of direct ER-to-plastid envelope contact sites is not altered in the tgd4 mutant. However, this does not preclude a possible function of TGD4 in those contact sites as a conduit for lipid transfer between the ER and the plastid.     

Events surrounding the early development of Euglena chloroplasts. 15. Origin of plastid thylakoid polypeptides in wild-type and mutant cells

Techniques are described for the isolation of plastid thylakoid membranes from light-grown and dark-grown cells of Euglena gracilis var. bacillaris, and from mutants affecting plastid development. These membranes, which have minimal contamination with other cell fractions, are localized in sucrose gradients by using the thylakoid membrane sulfolipid as a specific marker. The plastid thylakoid membrane polypeptides isolated from these membranes were separated on SDS polyacrylamide gels and yielded patterns containing 30–40 polypeptides. Light-grown strain Z gave patterns identical with bacillaris. Since the plastid thylakoid polypeptide patterns obtained from dark-grown wild-type cells and from a bleached mutant W₃BUL in which plastid DNA is undetectable are identical, it appears that the proplastid thylakoid polypeptides of wild-type cannot be coded in plastid DNA and are probably coded in nuclear DNA. The plastid thylakoid polypeptide patterns obtained from various dark-grown mutants are identical to those obtained from dark-grown wild-type cells. Light-grown mutants, making large but abnormal chloroplasts, show a correlation between the amount of chlorophyll formed and the amount of a plastid thylakoid polypeptide thought to be associated with one of the pigment-protein light-harvesting complexes. Treatment with SAN 9789 (4-chloro-5-(methyl-amino)-2-(α,α,α,-trifluoro-m-tolyl)-3-(2H(pyridazinone) known to block carotenoid synthesis at the level of phytoene, causes a progressive loss of all plastid thylakoid polypeptides during growth in darkness and results in the establishment of a new, lower steady-state level of sulfolipid. At least ten of the plastid thylakoid polypeptides become labeled when isolated chloroplasts are supplied with radioactive amino acids; of these six are undectable in W₃BUL and are, therefore, candidates for coding by plastid DNA.     

CHLOROPLAST DEVELOPMENT IN THE GERMINATING SAFFLOWER (CARTHAMUS TINCTORIUS) COTYLEDON

Proplastids in the mesophyll cells of the cotyledons of mature seeds of safflower are irregular in shape and compressed in narrow corners between the large inclusion bodies, oil vacuoles and protein bodies. The proplastids contain a few irregular internal membranes. During dark germination, sheets or sac-like membranes are produced by the activity of the inner component of the proplastid envelope. These continuous membranes become reticulate and aggregate to the center of the proplastid to form after seven days' germination a quasicrystalline prolamellar body. The membranes are at first irregularly arranged and are of two sorts: those found in the interior of the developing prolamellar body, composed of laterally connected spherical profiles, and those on the periphery of the prolamellar body, which are continuous smooth sheets. The prolamellar body in these dark-germinated proplastids reverts after 3 hr of illumination to the irregularly arranged membranous structure of the 5-day dark germination stage. After 6 hr of illumination membranes grow from the prolamellar body forming concentric loops which, in cross section, appear as concentric circles. These membranes must be nested semi-spheroids. Small grana appear immediately on these looped membranes close to the prolamellar body. With further illumination additional grana develop along the looped membranes in close proximity to the slowly disappearing prolamellar body. Grana increase in size and number along the looped intergranal membranes. The prolamellar body disappears after 15 hr of illumination. The interconnecting fret membranes, sparse at the 15-hr stage, increase and after 24-hr illumination result in the typical grana fretwork system of the mature chloroplast. Membranes are continuously being produced by the invagination of the inner member of the plastid envelope.     

Protein trafficking to plastids: one theme, many variations

Plastids are a diverse group of essential organelles in plants that include chloroplasts. The biogenesis and maintenance of these organelles relies on the import of thousands of nucleus-encoded proteins. The complexity of plastid structure has resulted in the evolution of at least four general import pathways that target proteins into and across the double membrane of the plastid envelope. Several of these pathways can be further divided into specialty pathways that mediate and regulate the import of specific classes of proteins. The co-ordination of import by these specialized pathways with changes in gene expression is critical for plastid and plant development. Moreover, protein import is acutely regulated in response to physiological and metabolic changes within the cell. In the present review we summarize the current knowledge of the mechanism of import via these pathways and highlight the regulatory mechanisms that integrate the plastid protein-trafficking pathways with the developmental and metabolic state of the plant.     

Control of chloroplast formation by light.

Light controls the formation of plastid ultrastructure and the synthesis of chlorophyll, plastid membrane constituents and Calvin cycle enzymes. A respective light-mediated regulation of the genetic apparatus in the nucleus and the plastid compartment has been reported. Three photoreactions are involved in the regulation: (1) the protochlorophyll (ide) leads to chlorophyll (ide) a photoconversion, (2) the formation of physiologically active phytochrome and (3) light absorption by a blue light receptor (cryptochrome). The chloroplast formation in higer plants is chiefly controlled by active phytochrome, while in lower plants cryptochrome is the prevailing regulatory factor.     

THE MECHANISM OF THE INHIBITION OF HEMOLYSIS

The principal conclusion of this investigation is that the inhibitory effect of plasma or serum on hemolysis by saponin and lysins of the same type is similar in nature to the inhibitory effects of certain sugars and electrolytes, which again are similar to the acceleratory effects produced by indol, benzene, and other substances already studied. All these effects, both inhibitory and acceleratory, are the result of reactions between the inhibitors or accelerators and those components of the red cell membrane which are broken down by lysins. The inhibitory effect of plasma on saponin hemolysis has a number of properties in common with the inhibition produced by sugars and electrolytes and with accelerations in general. (a) The temperature coefficient is small and negative. (b) The extent of the inhibition depends on the type of red cell used in the hemolytic system. (c) The most satisfactory measure of the extent of the inhibition, the constant R, is a function of the concentration of lysin in the system, and (d) R is a linear function of the quantity of inhibitor present. It is also shown that the inhibitory effect of plasma, and serum is not entirely dependent on its protein content. The process underlying the phenomenon of lysis and its acceleration or inhibition seems to be one in which the lysin reacts with a component or components of the cell membrane in such a way as to break down its semipermeability to hemoglobin, and in which the accelerator or inhibitor also reacts with the same component in such a way as to increase or decrease the effectiveness of the lysin in producing breakdown. The membrane is considered as being an ultrastructure made up of small areas or spots of varying degrees of resistance to breakdown, the resistances being distributed according to a negatively skew type of frequency curve, and the process of lysis seems to begin with the least resistant spots breaking down first. These spots may be arranged in some regular spatial pattern, and the membrane has also to be regarded as possessing spots of varying rigidity of form. The accelerator or inhibitor changes the resistance of every reactive spot in the ultrastructure by a factor R, which suggests that acceleration and inhibition are results of some over-all effect, such as that of changing the extent to which lysin is concentrated at the surface or partitioned between the material of the membrane and the surrounding fluid. Some kind of combination between the accelerator or inhibitor and the material of the ultrastructure is presumably involved; at first the combination seems to be a loose one and partly reversible, but later some of the loose links are replaced by more permanent combinations involving the same types of bond as are broken down by the lysins themselves.     

A CYTOCHEMICAL INVESTIGATION OF A CHLOROPLAST INCLUSION

The chemical composition of a membrane-bound plastid inclusion observed in genetic tumor cells of the amphiploid Nicotiana suaveolens X N. langsdorffii was studied by means of electron microscope cytochemistry. Treatment with the following enzymes had no effect on the ultrastructure of the inclusion: α-amylase, DNase, lipase, papain, pronase, protease, RNase, and trypsin. The only enzyme to alter the fine structure of the inclusion was pepsin, which decreased the electron density of the granular component of the structure. These results indicate that protein is the major chemical constituent of this membrane-bound plastid inclusion. This finding is discussed in relation to the possible role of the inclusion in plastid ontogeny.     

The outer membrane Omp85‐like protein P39 influences metabolic homeostasis in mature Arabidopsis thaliana

• The Omp85 proteins form a large membrane protein family in bacteria and eukaryotes. Omp85 proteins are composed of a C‐terminal β‐barrel‐shaped membrane domain and one or more N‐terminal polypeptide transport‐associated (POTRA) domains. However, Arabidopsis thaliana contains two genes coding for Omp85 proteins without a POTRA domain. One gene is designated P39, according to the molecular weight of the encoded protein. The protein is targeted to plastids and it was established that p39 has electrophysiological properties similar to other Omp85 family members, particularly to that designated as Toc75V/Oep80.
• We analysed expression of the gene and characterised two T‐DNA insertion mutants, focusing on alterations in photosynthetic activity, plastid ultrastructure, global expression profile and metabolome.
• We observed pronounced expression of P39, especially in veins. Mutants of P39 show growth aberrations, reduced photosynthetic activity and changes in plastid ultrastructure, particularly in the leaf tip. Further, they display global alteration of gene expression and metabolite content in leaves of mature plants.
• We conclude that the function of the plastid‐localised and vein‐specific Omp85 family protein p39 is important, but not essential, for maintenance of metabolic homeostasis of full‐grown A. thaliana plants. Further, the function of p39 in veins influences the functionality of other plant tissues. The link connecting p39 function with metabolic regulation in mature A. thaliana is discussed.