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- Structure and Function of Chloroplasts
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This volume provides a comprehensive look at the biology of plastids, the multifunctional biosynthetic factories that are unique to plants and algae. Fifty-six international experts have contributed 28 chapters that cover all aspects of this large and diverse family of plant and algal organelles. Each chapter includes an integrated view of plant biology from the standpoint of the plastid. The book is intended for a wide audience, but is specifically designed for advanced undergraduate and graduate students and scientists in the fields of photosynthesis, biochemistry, molecular biology, physiology, and plant biology. Skip to main content Skip to table of contents.
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These two organisms became inextricably associated due to several processes that occurred during their co-evolution including the transfer of genetic information from the endosymbiont to the nuclear genome of the host.
This way, although retaining part of their own DNA, and their own machinery for nucleic acid and protein synthesis, the plastids got fully integrated in the host cell, became semi-autonomic and therefore intensive coregulation and communication occur between the host nucleus and the organelle.
In contrast with most plant cells, algae often contain only a few or a single, but relatively large giant plastid in their cell s. These plastids are usually not specialized, but can fulfill simultaneously and under unstressed, natural conditions all metabolic functions related to this organelle.
However, in land plants and some algae especially those with complex organization -for details see e. This specialization and diversification of plastids is acquired by intensive co-regulation between the host nucleus and the organelle. In nonphotosynthetic organs, tissues and cells, the most ancient plastid-type, the chloroplast has lost partially or completely its photosynthetic activity and distinct plastid types responsible for other vital physiological functions and metabolites differentiated.
Plastid differentiation, therefore, occurs concomitantly with cellular differentiation and is under the control of genetic and environmental factors. However, the functional and morphological differences between plastid types should be considered only as an acquired but not definitive and irreversible specialization. Consistently with this, the term plastid derived from the Greek word meaning 'formed, mouldable' also reflects the high plasticity of these organelles that probably contributed in several ways to the fitness and the succesful adaptation of higher plants to the environment.
Although plastid types are highly and dynamically interconvertible reviewed e. For instance, proplastids defined as small, undifferentiated organelles with poorly developed or no internal membrane system, have at least two basically different functional forms.
Proplastids present in meristematic and embryonic tissues as well as in dedifferentiated cells e. On the other hand, functionally special plastid types have been characterized recently.
For instance, the so-called desiccoplasts Fig. But these plastids are formed by dedifferentiation in the cells of fully differentiated leaves of poikilochlorophyllous desiccation-tolerant plants upon dehydration.
During dehydration, the thylakoid system of the chloroplasts of such leaves are dismantled and their photosynthetic pigments both chlorophylls and carotenoids are degraded. Upon rehydration of the same, fully differentiated leaves of such resurrection plants, the desiccoplasts synthesize Chl-s and photosynthetic membranes, and get this way transformed into functionally active chloroplasts reviewed in e.
Please note the clusters of electron-dense plastoglobuli white asterisk , the nucleoids black asterisk , the poorly developed single thylakoid membranes black arrowheads that are sometimes perforated e. Plastid nomenclature is also complicated when considering the plastids in the different algal groups reviewed in . In this review, we focus on ultrastructural and functional features characterizing and distinguishing the different plastid types of land plants and on the possible interconversions of the different plastids.
Due to extensive literature reviewed in this paper we had to make strong selection of literature data and had to prioritize review papers and few examples. Therefore we apologize to all authors whose important contributions could not be cited in this work.
The number of plastids per cell is in general relatively high around or even in diploid leaf parenchyma cells of higher plants , but is quite variable in the different taxa reviewed e. The presence of a single chloroplast in each cell of the gametophyte and in some species also in the sporophyte is only characteristic for the ancestral taxon of anthocerotes reviewed in  or may be observed in some special cells e.
Plastid Morphology in Land PlantsIn contrast with algae discussed in  , plastid morphology is quite simple in plants. Plastids are in general either spherical, discoid, ellipsoidal or lens-shaped. Not surprisingly, anthocerotes are special also in this respect, because besides ellipsoidal plastids they can also have dumbbell-shaped and irregular plastids reviewed in . However, at distinct developmental stages of proplastid-tochloroplast and proplastid-to-etioplast development amoeboid plastids may be observed also in higher plants Fig.
Similarly, leucoplasts involved in secretion of terpenoids may have amoeboid shape e. Cytoplasmic inclusions or the presence of mitochondria in the plastids by intrusion inclusion only may be observed especially under stressful conditions observed during our works e. In these cases, the chloroplast may surround the mitochondrium in a cup-shaped manner as observed often in amoeboid plastids. Although plastids of land plants are in general quite similar morphologically, stromules represent a highly dynamic system of plastid shape and function, and may have important roles in connecting the plastidome into one network within a plant cell, and also in the intracellular communication of the plastids with the nucleus, the endoplasmic reticulum and the mitochondria , reviewed in , details about algae are discussed by .
Stromules extend from the surface of all plastid types examined so far, including proplastids, chloroplasts, etioplasts, leucoplasts, amyloplasts, and chromoplasts, but are more characteristic in tissues having in general small and non-green plastids. These structures are sometimes only characteristic for distinct developmental stages, but can also appear under different biotic or abiotic stresses reviewed in .
Whether or not the protrusions of amoeboid plastids and plastid stromules are related in any fashion is not clear reviewed in . For more details about plastid stromules the readers are kindly directed towards the above-cited recent reviews. Amoeboid plastid from the cotyledon of 2-day-old darkgerminated Helianthus annuus seedlings. Please note the densely staining stroma and the well-developed peripheral reticulum black arrowheads. A small starch grain asterisk can be also observed in the plastid.
Ultrastructural Features of Plastids in Land PlantsDespite their large diversity in land plants, plastids have several common features. Simply put, they have three major structural regions: i a pair of outer membranes, that is, the plastid envelope, ii an amorphous background rich in soluble proteins and ribosomes, i. Below, we briefly discuss these three different regions.
Plastid EnvelopesThe two limiting envelope membranes are actually the only permanent membrane structure of the different plastid types reviewed in . This double-membraned envelope varies in the closeness of the two membranes from well separated to completely appressed, and contains the galactolipids, monogalactosyl diacylglycerol MGDG and digalactosyl diacylglycerol DGDG along with other lipids , but also different pigments, especially violaxanthin and some chlorophyll precursors, and special proteins reviewed e.
Due to the reduction of plastid genome and gene transfer towards the nucleus of the host cell during endosymbiogenesis, it is evident that plastids depend strongly on proteins imported from the cytoplasm. In fact, plastids encode only about proteins in their genome, while approximately nuclear-encoded proteins are predicted to be plastid-targeted reviewed in .
Therefore, the envelope has a crucial role in protein transport, but also in the signaling processes between the plastid and the nucleus involving coregulation or mutual regulation of gene expression in these two compartments reviewed e. The protein import machinery of plastid envelopes reviewed in  as well as their role in other transport processes e. In addition, the envelope has a crucial role in carotenoid, chlorophyll, prenylquinone, fatty acid and lipid metabolism in biosynthesis, transfer, etc.
The envelope is the site of the synthesis of galactolipids, sulfoquinovosyldiacylglycerol and phosphatidylglycerol reviewed in . During plastid differentiation from proplastids, the internal membrane system of the plastids is formed from invaginations of the inner envelope, whereas in mature plastids a vesicle-based transport system was suggested to supply galactolipids and other membrane components maybe proteins, pigments or their precursors, etc.
Similarly, the outer envelope membrane is involved in different interactions e. Strong interaction of the endoplasmic reticulum and plastid envelopes has been established since long time, and these plastid-associated membranes may play a role in supplying lipid precursors to the plastids reviewed in [4,20].
Components of the Plastid StromaProkaryotic-type, 70S ribosomes are characteristic compounds of the plastid stroma. Sometimes they are present as polyribosomes reviewed e. They are responsible for protein synthesis translation in the chloroplast and this way their presence indicates active protein synthesis within this compartment. The plastid stroma also contains nucleoids Fig. Nucleoids consist of multiple copies of plastid DNA, as well as various proteins and RNA, and thus represent the functional unit of the plastid genome plastome.
Nucleoids are duplicated in meristematic and young developing tissues at a high rate, and they are separated during plastid division, however, at later stages of development e. The number of nucleoids, DNA quantity and plastid size usually correlate well, i.
Chromoplasts may contain up to 35 copies of plastome per plastid , and in general, non-green plastid types often possess fewer plastome copies than chloroplasts reviewed in .
Nucleoid arrangement within the plastids varies with species, plastid types and even developmental stages. Nucleoids are membrane bound, i. For instance, in proplastids the nucleoids are attached to the envelope , while in mature chloroplasts they are often scattered in the stroma and have no particular arrangement except that they may be connected to thylakoids Fig. The nucleoids may be also located along a ring at the periphery of the proplastid or the young chloroplast during the proplastid-to-chloroplast development of leaves, with developmental stages having either a fairly continuous peripheral ring of DNA or discrete nucleoids arranged in a ring .
It has to be mentioned, that in addition to circular DNA, both electron microscopic investigations and pulsedfield gel electrophoretic analyses have identified various linear genome conformations within the plastids, the function and significance of which is still not completely clear reviewed in . The plastid genome of parasitic plants suffers dramatic size reductions, mainly caused by the loss of photosynthesis genes or their degeneration to pseudogenes reviewed in .
For details about the sample preparation see . Scale bar: 0. As in the Chlorophyta, plastids represent the major site for intracellular storage of polysaccharides in land plants in the form of starch. Smaller or larger starch grains may be present in almost all plastid types irrespectively from the arrangement of the internal membrane system, although they are not characteristic for some kinds of leucoplasts and the chromoplasts.
Primary or transitory starch accumulates in chloroplasts during the day during the light phase of photosynthesis and persists only until nightfall , while massive starch accumulation and storage on a longer time scale occur in the amyloplasts of parenchymatic tissues specialized for storage reviewed e. Single, perforated thylakoids are often concentrically arranged around starch grains even in proplastid-like plastids or etioplasts e.
This property is intimately linked to the well-known role of starch containing plastids amyloplasts in graviperception in the columella region of the root cap reviewed in  , and similarly in the statoliths present in the cells around the vascular tissues of the shoot . Chromoplasts can also harbor starch in some species e. Starch grains are in general rounded, but can have special shapes as well, e. In contrast with algae, pyrenoids are absent from the plastids of land plants with the exception of the ancestral group of anthocerotes in which pyrenoid morphology is quite special and is discussed in details by Vaughn et al.
Similarly to algae, plastids of land plants also often contain plastoglobuli, i. The plastoglobuli might also contain some proteins , reviewed in  and are involved in membrane repair processes, in senescence or even membrane resynthesis [56,57]. Plastoglobuli in chloroplasts form a functional metabolic link between the inner envelope and thylakoid membranes, and play a role in breakdown of carotenoids and oxidative stress defense, whereas plastoglobuli in chromoplasts are also an active site for carotenoid conversions , and in elaioplasts they are responsible for oil storage.
The number and size of plastogobuli may change during the development of plastids in case of etioplasts it increases with etioplast senescence -reviewed in  , and is also strongly influenced by growth conditions i. For instance metal imbalance in plastids, i. The size and sometimes also the number of plastoglobuli increases with senescence in gerontoplasts, in which huge plastoglobuli sometimes fusing with the envelopes, and being extruded from the chloroplast are also characteristic e.
In electron micrographs the plastoglobuli are often electron-dense in young, unstressed tissues, but have often more or less electron-transparent staining in gerontoplasts. Plastoglobuli occur in general as single lipid droplets or in groups of , but in some cases they are arranged into larger clusters e.
Compared to the chloroplasts of green algae e.
Structure and Function of Chloroplasts
The most studied plastid type is the chloroplast, which carries out the ancestral plastid function of photosynthesis. During the course of evolution, plastid activities were increasingly integrated with cellular metabolism and functions, and plant developmental processes, and this led to the creation of new types of non-photosynthetic plastids. These include the chromoplast, a carotenoid-rich organelle typically found in flowers and fruits. Here, we provide an introduction to non-photosynthetic plastids, and then review the structures and functions of chromoplasts in detail. The discovery of SP1 suppressor of ppi1 locus1 , which encodes a RING-type ubiquitin E3 ligase localized in the plastid outer envelope membrane, revealed that plastid protein import is regulated through the selective targeting of TOC complexes for degradation by the ubiquitin—proteasome system. This suggests the possibility of engineering plastid protein import in novel crop improvement strategies. Approximately 1.
Plastid Origin and Development. Front Matter. Pages PDF.
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Plastids perform many essential functions in plant metabolism including photosynthesis, synthesis of metabolites, and stress signaling. The most prominent type in green leaves is the chloroplast which contains thylakoids, plastoglobules, and starch. As these structures are closely linked to the metabolism of chloroplasts, changes during plant growth and development and during environmental stress situations are likely to occur.
Plastids are a group of phylogenetically and physiologically-related organelles found in all types of plants and algae. In their roles, the different types of plastids contribute to plant metabolism thus promoting plant growth and development. One of the main characteristics of these organelles is the fact that they have a double membrane. In the cells, plastids are primarily involved in the manufacture and storage of food. They are therefore involved in such processes as photosynthesis, synthesis of amino acids and lipids as well as storage of various materials among a few other functions. Apart from plants and algae, plastids can also be found in a number of other organisms including:. Like all plant cells, plastids are derived from meristem cells within the plant.
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