Plastids

 

Plastid is another important energy transducing cell organelle found only in plants. Shimper coined the name Plastids for those structures responsible for photosynthesis. In actuality, photosynthesis provides chemical energy directly or indirectly, for all other living organism, Chloroplasts are unique organells for they are capable of capturing, converting and conserving solar energy in the form of chemical energy. Plastids are found in almost all cells of the plant body either in the form of colorless plastids or colored plastids or proplastids.

Classification:

On the basis of presence or absence of pigments, and the stage of development, plastids have been classified into proplastids, leucoplasts and chromoplasts.

Proplastids:

Small vesicular structures present in meristematic cells are called proplastids. They are colorless and undeveloped. As cells mature into different cell types, depending upon the organs and presence or absence of light, proplastids undergo transformation and develop into either colorless leucoplasts or colored chromoplasts including green chloroplasts. Proplastids continuously divide and redivide and provide them for cells undergoing differentiation into various types.

Leucoplasts:

Colorless plastids that are found in storage parenchyma and other colorless tissues are refereed to as leucoplasts. Most of them act as storage organelles. Based on the kind of substance they store they are further classified into amyloplasts. If such leucoplasts are exposed to sunlight they will be transformed into colored plastids, which suggests that these plastids have retained all the genetic potentiality to develop and perform photosynthesis.

Chromoplasts:

All plastids containing different colored pigments are grouped under chromoplasts, of which green colored ones are called chloroplasts. Depending upon the dominant pigments present in plastids, they are further classified into Rhodoplasts rich in red pigment i.e. phycoerythrin. Phaeoplasts and Xanthoplasts contain yellow pigments i.e. xanthophylls, carotinoids. Along with the above pigments phycocyanin and other pigments are also present in other colored plastids.

Other plastids: Such colored plastids, other than chloroplasts are predominantly found in certain class of plants and plant organs including floral parts. Though floral parts are derived from the same set of proplastids, produce different pigments in petals. The exact process differentiation is not known for different plants do produce different colored petals and it is genetically programmed.

Interconversion:

Proplastids divide and redivide in meristematic cells, and then they are distributed to cell derivatives on exposure to light, depending upon the structures in which they found and also depending upon the intra cellular factors they develop into colorless plastids or colored plastids. Leucoplasts on exposure to light develop into green plastids. Similarly chloroplasts may become leucoplasts; but colored plastids as in petals are mostly terminally differentiated.

Chromatophores:

Prokarytic photosynthetic bacteria contain photosynthetic organelles called chromatophores. Bacterial chromatophores are made up on membranous vesicles in which photo synthetic pigments and other factors are located. In these structure photosynthetic pigments, associated with light harvesting proteins and other required enzymes are aggregated into photosynthetic units. Apart from that, chromatophores do now show any specialized structural differentiation. However, in blue green algae, the entire cell apart from its cell wall acts as a giant chloroplast with its grana and stromatic fluid.

Chloroplasts:

Chloroplasts are green colored plastids for they contain greater amounts of chlorophyll pigments. They are ubiquitously present in green plants and they are mainly responsible for providing food for themselves and to other animals in the world. These structures are mostly restricted to photosynthetic parts of the plants.

Number and Shapes:

In lower plants, such as Chlamydomonas, spirogyra, diatoms, Hdrodictyon, etc, the number of plastids present in each cell is always constant and characteristic. So also their shapes. Chlorella cells contains a single plate shaped chloroplast; where as Chlamydomonas possesses only one cup shaped chloroplast. One the other hand chloroplasts in spirogyra are ribbon shaped spirally coiled. But Zygnema cells contain two star shapes chloroplasts. In Hydrodictyon and cladophora chloroplasts are highly diffused and this is in the form of a network. In all the above said plants, plastids contain pyrenoids.

In higher land plants, the number of chloroplasts varies from cell to cell and from organ to organ, i.e. 30-200 per cell and most of them are nearly spherical or ovoidal in shape.

Distribution:

 

In their disposition within the cell they show some variations. In certain algae like Caulerpa and Vaucheria, chloroplasts are generally clustered in the region of the cell where it is exposed to sunlight. Even in higher plants such as tropical grasses etc, chloroplasts found in bundle sheath cells and those found in the surrounding mesophyll cells are crowded towards each other. Generally the position of chloroplasts in photosynthetic cells is not fixed, because they are constantly swept along with the protoplasmic movement.

Size of eukaryotic chloroplasts is 4-5 nm in size but size may vary from plant to plant. Plants growing in shade contain large chloroplasts in their cells than that of growing in intense light.

Multiplication of plastids:

Proplastids are constantly dividing in meristematic cells and keep pace with cell division. Once proplastids develop into fully developed chloroplasts, in higher plants, they rarely divide. But in spirogyra and many algae fully developed plastids, divide at the time of cell division and they are equally distributed among the daughter cells. In Chlamydomonas, chloroplasts divide into two four or more at the time of reproduction. The mode of division is typical of bacterial cleavage. The time taken for such division is should take 10-18 hours. In all developing organs or plant structures plastids keep on multiplying

Structure:

Al green plastids, for that matter every kind of plastids are bounded by two units membranes; i.e. outer and inner of 7 nm thick membranes and they are separated by periplastid space of 8-10 mm thick. Unlike mitochondria, the inner membrane of fully developed plastids does not show any inward foldings; but it plays active role during the development proplastids into mature plastids. Chloroplast is filled with a liquid called stroma, in which highly organized membrane structures are found they are called grana. Besides grana, the stromatic fluid contains a host of enzymes, plastid DNA, RNAs and 70s ribosomes.

Grana:

Grana are highly specialized membrane clusters. Each chloroplast may contain 10-30 such granal clusters, a single granum can be compared to a flat circular membranous discs called thylakoids stacked one above the other. Such 20-60 thylakoids together constitute a granum. Moreover the grana are interconnected by another membrane structure called intergranal lamellae or stromal lamellae, which is located in between the stacks of thylakoid membrane, but it extends laterally, so as to form a kind of network of interconnecting membranes.

Though the above described structure hold good for most of the chloroplasts, but certain plants belonging to tropical grass members like sugarcane, zea mays, sorghum, crab grass and even some dicots like amaranthus contain chloroplasts of two types. Chloroplasts found in mesophyll cells have the same granal organization as described above, but lack in thylakoids. In its place only stromal lamellae are found. Such dimorphic chloroplasts though found in the same leaf and exhibit different functions.

Chemical Composition:

Chloroplast limiting membranes contain normal lipid protein composition. But the granal membranes including both thylakoid and intergranal membranes, posses 20-30% of the lipids and the rest of it is all proteins. The most common lipids found in these membranes are ethanolamine, sulfolipids, phytosterols and glycolipids and pigments.

Pigments:

Plant pigments are basically simple lipids containing many isoprenoid units. Association with other compounds brings about variations in them. Among the plant pigments present in plastids, chlorophylls and carotinoids are found in 3:1 proportions. The composition of pigments varies significantly among different groups of plant kingdom. Chl a, Chl b, β-carotene and xanthophylls are found in most of the green plants. But lower plants contain diverse pigments composition. Nonetheless, Chl a is found in all photosynthetic plants. In fact, it acts as a primary pigment and all others including phycoerythrin and phycocyanin are considered as accessory pigments. Different kinds of pigments can be identified by their absorption spectrum.

 

Chlorophylls:

Chlorophyll molecules are unique molecules of the biological world. They are made up of hydrophilic head and a hydrophobic tail. The head consists of four pyrole rings joined to each other by a single master ring of CH brides. The inorganic Mg2+ ion is found in the centre of the ring. If Mg2+ is replaced with Fe2+, this ring becomes a heme group found in animal hemoglobins. However, chlorophyll also contains an additional long chain of saturated hydrocarbons called phytol chain. But Chl a is distinguished from Chl b in having CH3 group in II ring, while Chl b possesses CHO. Pheophytin is similar to chlorophyll, but lacks Mg2+.

Carotinoids:

Carotinoids are simple lipids containing long chain of isoprenoid hydrocarbons. They are soluble in lipids. There are of two types of carotinoids namely carotenes and xanthophylls. Carotene pigments are made up of short chains of unsaturated hydrocarbons with hexane rings at each end. Xanthophylls also have similar hydrocarbon chains with hexane rings, but contain quite a number of hydroxyl groups.

Proteins:

Analysis of granal membrane proteins on SDS slab gel-electrophoresis reveals the presence of more than 200 polypeptides. A most of them are structural proteins, light harvesting antenna proteins , electron transporting proteins. oxygen releasing Z-proteins, etc. Along with them ATP synthase and RUBP carboxylase are also found associated with granal membranes. Majority of them are imported from cytosol

Light harvesting proteins are a group of proteins that are associated with various pigments and they are responsible for harvesting and transferring solar energy. The mol wt of these proteins ranges from 11 KD to 46 KD. The core LHP proteins however has mol wt. of 11-16 KD and the peripheral LHP proteins have a mol. Wt. of 24-46 KD. Most of the proteins exhibit hydrophilic as well as intrinsic hydrophobic properties.

The oxygen releasing Z-protein is found associated with photosystem II and it has a mol wt. of 64 kd. It is made up of two subunits of 32 KD each. The Z-protein is associated with 4 Mn2+ ions, which are tightly bound to the protein moiety. The Z-mn2+ protein complex is a hydrophobic protein and it is mainly responsible for oxygen liberation. Similar to the above mentioned protein complexes, electron transporting proteins, ferrodoxin reducing proteins, NADP reductase proteins and others are grouped into complexes. The cyt.b6-cyt.f protein complex has non heme iron and also possesses phospholipids and carotinoids but no chlorophylls. The mol. Wt. of this protein is 103 KD.

The enzymatic complex required for ATP synthesis and its associated coupling factor proteins are situated at the outer surfaces of thylakoids. They are half buried in the membranes in such a way that the head is exposed to stromatic fluid. This ATP synthetase complex called CF 1 has 5 subunits of mol. wt 350KD. But the protons secreting hydrophobic protein called HF (mol. Wt 32KD), has two subunits and it is buried in the core of the thylakoid membrane, at the same time it is also in contact with CF1 structure.

Another enzyme complex which is mainly responsible for the fixation of CO2 is RUBP carboxylase. Its mol. Wt is 55.8KD. It consists of 8 large subunits and 8 small subunits. This enzyme is located at the outer surface of thylakoid membranes. Interestingly RUBP carboxylase is one of the most abundant proteins found in the biological world. More than 50% per cent of the total proteins found in leaves is RUBP carboxylase.

Structural Organization of Thylakoid membranes:

Freeze fracture electron microscopic studies, combined with biochemical analysis of various components of granal membranes reveal, that thylakoid membrananes contain a large number of granular structures of various dimensions. Most of them are embedded in the core of membranes. The components are not fixed in a position, but exhibit lateral mobility true to the nature of dynamic fluid nature of membranes. The granular particulates were observed by Briggs and Park. Later they were isolated and purified into different fractions and chemical analysis was made. Such granular structures are called Quantosomes, because they are mainly responsible for capturing quantum of photons of solar electromagnetic radiation.

The quantosomes found in thylakoid membranes are of different sizes and dimensions but they show vectorial disposition within the membrane. The bigger quantosomes are located in the core of thylakoid lipid layers spanning the entire cross section projecting towards both stromal side and lumen side. On the other hand, smaller quantosomes are placed in the core of membrane but more towards stromatic surface in such a way of the structure is embedded in the lipid core. The granular particles of both sizes are organized in such a way each of them fit well into the spaces found in between them.

The larger quantosomes are called photosynthetic units or photosystem-II and the smaller as PS I. Nonetheless, there are other particulate which are of the same size as PS I, but have different composition and functions. However intergranal lamellar organization is quite different from that of thylakoid membranes. The intergranal lamella is almost lacking in PS II quantosomes, but contain mostly PS I systems.

The organization of quantosomes found in chloroplasts of bundle sheaths of C4 is quite different from that of other normal chloroplasts. These chloroplasts contain only stromal lamellae. Even though PS II is more or less absent from intergranal membranes, one may find few of them, but contain PSI systems.

Photosynthetic units like PS I and PS II have different chemical composition. Still each of these units are made up of 250-300 Chl molecules complexes with LHP antenna proteins and other cofactors required for specific photochemical reactions.

More details about their composition and structure have been described in the chapter photosynthesis. The role of PS I ad PS II present in thylakoid membranes is to perform NADP reduction, noncyclic photophosphorylation and oxygen liberation. On the contrary, the intergranal lamellae containing just PS I systems perform just cyclic photophosphorylation.

The structural and functional specificity applies to C4 chloroplasts also, where chloroplasts of mesophyll cells perform noncyclic photophosphorylation, NADP reduction and liberate oxygen. But the stromal lamellae found in the chloroplasts of bundle sheath cells perform cyclic photophosphorylation only for they contain only PS I. systems.

Stroma:

Amorphous, often semi viscous fluid present within the chloroplast membrane is called Stroma. A large number of enzymes responsible for carbon fixation, amino acid synthesis, protein synthesis, nucleic acid metabolism, pigment synthesis, N2 metabolism and fatty acid synthesis are present in stromatic fluid. It has also enzymes for the synthesis of Gibberellic acid and Abscisic acid. Some of the biosynthetic pathways in the stroma are under the control of various factors like light, phytochromes, temperature and photoperiods. Though chloroplasts have their own genetic material, the synthesis of various components required for chloroplast is under the dual control of nuclear genome and plastogenome.

In the case of C4 plants, the stromatic fluid of chloroplasts found in mesophyll cells contain enzymes for Hatch and Slack pathway. Such enzymes are totally absent in the stroma of C3 chloroplasts. On the other hand, C4 chloroplasts found in bundle sheath cells possess enzymes for C3 pathway and also contain malate dehydrogenase. Thus C4 and C3 chloroplasts show dimorphism both in their structure and in function.

Another important feature of chloroplasts is the presence of circular DNA, various species of tRNAs and 70s ribosomes. Having its own genetic material and translation machinery chloroplasts enjoy semiautonomous state in the cell, a feature similar to that of mitochondria.

Chloroplast DNA:

Plastid DNAs are circular duplex molecules with a total length of 45 mm. But in some cases DNA of 15 mm have been isolated. However each plastid consists of 6-30 copies of circular DNAs and most of them are in super coiled state. Based on its genomic size, it has been calculated that each cp DNA molecule can code for about 110-120 proteins. Chloroplast DNA when subjected to ultra centrifugation, settles as satellite DNA, because of high GC content.

Gene mapping of chloroplast DNA has shown that it has two sets of genes for ribosomal RNAs. It also has genes for the large subunit of RUBP carboxylase, hydrogen secreting proteins, all tRNAs, all aminoacyl synthetases, RNA polymerases, DNA polymerase, some LHPs, ALA synthetase etc.

Chloroplast DNAs replicate by D-loop mechanism by a DNA polymerase coded for by its own genome. Transcription is performed by its own RNA polymerases which is sensitive to rifampicin. But plastogenome expression is controlled by light, nuclear factors and other environmental factors. Phytochrome which is also present in plastids control or assist gene expression during greening and development of plastids. Added to this, plastids are also involved in the expression certain gene products which are responsible for the synthesis of GA ad ABA under certain environmental conditions. The plastogenome also contains genes for male sterility in specific plants. Recent studies indicate plastogenome expression has a direct bearing on floral induction.

Chloroplast ribosomes are more or less similar to prokaryotes in terms of size is 70s and rRNA content. Their functional activity is also inhibited by chloramphenicol similar to that of bacteria and mitochondria.

Biogenesis:

Plastids are generally inherited through maternal side as in the case of mitochondria. During the development of plant structures, proplastids multiply and they are evenly or unevenly distributed among daughter cells. Every cell in the plant body possesses plastids. The pattern of inheritance itself indicates that plastids are derived from pre existing plastids.

To begin with, proplastids contain a paracrystalline lattice in stroma. With the onset of light as the stimulus (red light is enough), the gene for aminoleuvulinic acid synthetase is derepressed, in the sense it is activated. The product of gene expression is ALA synthetase. The production of this enzyme can be inhibited by CAP but not with CHI, which suggests that they are light sensitive late gene products, which in turn activate light insensitive constitutive genes. Though the development of chloroplasts is interdependent of nuclear genes and plastogenome expression at certain stages of development, chloroplasts show autonomy. For example, the division of proplastids in Euglena is independent of cell division, but further development requires the products of both nuclear genes and plastogenes.

When proplastids are exposed to light they gradually turn green and enlarge in size. This is accompanied with the development of granal structures. During these stages, the inner chloroplast membrane produces a number of finger shaped invaginations. They in turn pinch off a number of membranous vesicles, which accumulate in the centre. The vesicles start fusing with one another and finally organize into clusters of thylakoid membranes called Grana. Once the development of chloroplast is completed the invaginations of inner membranes disappear.

Plastids by virtue of having its own genetic material and ribosomal translating machinery exhibits semi autonomous state within the cells. The inheritances pattern also demonstrates the same. Inheritance of chloroplasts is maternal and non Mendelian cytoplasmic type. The Mendelian pattern is controlled by nuclear genome, but the cytoplasmic inheritance is controlled by plastogenome. Though plastids have their own genome, they need the co-ordination of nuclear genes and its products for the completion of the development of chloroplasts.

Such interactions between plastogenome and nuclear genome can be observed during the development of proplastids into green chloroplasts. It is very well known that chloramphenicol, an antibiotic, inhibits the translation of 70s ribosome mediated protein synthesis. On the other hand cycloheximide inhibits cytosolic 80s ribosome mediated protein synthesis. If CAP is added during the greening of proplastids, pigments continue to accumulate in the thylakoid membranes, but electron transport activity is inhibited. In addition to this the membranes vesicles generated by inner plastid membrane do into fuse with one another to form thylakoid membranes. On the contrary, if CHI is added during the development of proplastids, greening is inhibited but the thylakoid membrane formation takes place partially (50%). The above results suggest but there is an interaction between the nuclear genome and plastogenome products in the biogenesis of thylakoids and its components.

Studies in this regard show that a large number of the protein complexes found in chloroplasts are found to be nuclear gene products and 120 or so proteins are coded for by the plastogenome. For example, ferrodoxin and plastoquinones associated proteins, 32KD protein of photosystem II, some of the LHP proteins for Chl. A/b of PS II small subunit protein part of RUB carboxylase is coded for by the nuclear genome. On the other hand the production of small subunits of RUBP carboxylase which is essential for the expression of large RUBP subunit is the product of nuclear gene. Another interesting observation is that one of nuclear gene product affects the binding of RNA polymerase to plastid DNA. The RNA polymerase itself is a product of plastogenome. However there are no clear cut reports to show that plastogenome products control the nuclear gene expression required for plastid development.