Plastids

Plastids - Types, Structure & Functions of Plastids

The plastid is a membrane-bound organelle found in the cells of plants, algae, and various other eukaryotic organisms. Plastids were discovered and named by E. Haeckel, but A. F. W. Schimper was the first to deliver a clear definition. Plastids are the location for manufacture and storage of important chemical compounds used by the cells of autotrophic eukaryotes. They often comprise pigments used in photosynthesis, and the kinds of pigments in a plastid determine the cell's color. They have a mutual evolutionary origin and possess a double-stranded DNA molecule that is circular, like that of prokaryotic cells.

Plastid is one more important energy-transducing cell organelle found only in plants.  Shimper invented the name Plastids for those structures responsible for photosynthesis.  In reality, photosynthesis transports chemical energy straight or indirectly, for all other living organism, Chloroplasts are exclusive organelles for they are capable of capturing, converting and conserving solar energy in the form of chemical energy.  Plastids are commonly found in almost all cells of the plant body either one in the form of colorless plastids or colored plastids or proplastids.

In Plants

Plastids that have chlorophyll can carry out photosynthesis and are called chloroplasts. Plastids can also accumulate products like starch and can produce fatty acids and terpenes, which can be used for producing energy and as raw material for the synthesis for other molecules. For example, the constituents of the plant cuticle and its epicuticular wax are made by the epidermal cells from palmitic acid, which is synthesized in the chloroplasts of the mesophyll tissue. All plastids are derived from proplastids, which are existing in the meristematic regions of the plant. Proplastids and young chloroplasts normally divide by binary fission, but more mature chloroplasts also have this capacity.

Plastids may distinguish into several forms, depending upon which purpose they play in the cell. Identical plastids (proplastids) may develop into any of the following variants:

·         Chloroplasts: green plastids for photosynthesis; 
        ·         Chromoplasts: colored plastids for pigment is synthesis and storage
·         Gerontoplasts: regulate the dismantling of the photosynthetic apparatus during plant senescence
·         Leucoplasts: colorless plastids for monoterpene combination; leucoplasts sometimes differentiate into more specialized plastids:
        ·         Amyloplasts: for starch storage and become aware of gravity (for geotropism)
        ·         Elaioplasts: for fat storing
        ·         Proteinoplasts: for storing and altering protein
        ·         Tannosomes: for synthesizing and creating tannins and polyphenols

Depending on their morphology and function, plastids have the skill to differentiate or redifferentiate, between these and other forms.

Each plastid produces multiple copies of a circular 75–250 kilobase plastome. The number of genome copies per plastid is inconstant, ranging from more than 1000 in quickly dividing cells, which, in general, it contains few plastids, to 100 or less in mature cells, where plastid separations have given rise to many plastids. The plastome contains about 100 genes encoding ribosomal and transfer ribonucleic acids (rRNAs and tRNAs) as well as proteins which are involved in the photosynthesis and plastid gene transcription and translation. On the other hand, these proteins only characterize a small fraction of the total protein set-up necessary to build and to keep the structure and function of a particular type of plastid. Plant nuclear genes convert the huge bulk of plastid proteins, and the expression of plastid genes and nuclear genes is strongly co-regulated to coordinate proper growth of plastids in relation to cell differentiation.

Plastid DNA survives as large protein-DNA complexes associated with the internal envelope membrane and is called 'plastid nucleoids'. Each nucleoid particle may cover more than 10 copies of the plastid DNA. The proplastid contains a particular nucleoid located in the center of the plastid. The rising plastid has many nucleoids, local at the periphery of the plastid, unlikely to the inner envelope membrane. During the development of proplastids to chloroplasts, plastids convert from one type to another, nucleoids change in morphology, size, and location within the organelle. The remodeling of nucleoids is believed to happen by modifications to the composition and large quantity of nucleoid proteins.

Several plastids, mainly those responsible for photosynthesis, possess numerous internal membrane layers.

In Algae

In algae, the name leucoplast is used for all unpigmented plastids. Their role is to differ from the leucoplasts of plants. Etioplasts, amyloplasts, and chromoplasts are plant-specific and do not take place in algae. Plastids in algae and hornworts may also vary from plant plastids in that they contain pyrenoids.

Glaucophyte algae contain muroplasts, which are alike to chloroplasts but that they have a peptidoglycan cell wall that is parallel to that of prokaryotes. Red algae comprise rhodoplasts, which are red chloroplasts that permit them to photosynthesise to a depth of up to 268 m. The chloroplasts of plants vary from the rhodoplasts of red algae in their skill to synthesize starch, which is deposited in the form of granules within the plastids. In red algae, floridean starch is produced and stored outside the plastids in the cytosol.

Classification:

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

Proplastids:

Minor vesicular structures present in meristematic cells are called proplastids.  They are colorless and immature.  As cells mature into other cell types, depending upon the organs and presence or absence of light, proplastids undergo change and develop into either colorless leucoplasts or colored chromoplasts together with green chloroplasts. Proplastids constantly divide and redivide and provide them for cells undergoing differentiation into various types.    

Leucoplasts:

Colorless plastids that are present in storage parenchyma and other colorless tissues are mentioned as leucoplasts.  Most of them behave as storage organelles.  Based on the kind of chemicals they store they are further divided into amyloplasts.  If such leucoplasts are open to sunlight they will be altered into colored plastids, which advise that these plastids have recollected all the genetic potentiality to develop and perform photosynthesis. 

Chromoplasts:

All plastids containing several colored pigments are band together under chromoplasts, in which green colored ones are known as chloroplasts.  Depending upon the main pigments present in plastids, they are further divided into Rhodoplasts rich in red pigment i.e. phycoerythrin.  Phaeoplasts and Xanthoplasts have yellow pigments i.e. xanthophylls, carotinoids.  Along with that above pigments phycocyanin and other pigments are also present in other colored plastids. 

Additional plastids:  Such colored plastids, other than chloroplasts are mainly found in the certain class of plants and plant organs containing floral parts. Though floral parts are derived from the same type of proplastids, it produces different pigments in petals.  The exact method to differentiation is not known for different plants to make different colored petals and it is genetically encoded. 

Interconversion:

Proplastids split and redivide in meristematic cells, and then they are spread to cell derivatives on exposure to light, depending upon the buildings in which they were found and also depending upon the intra cellular aspects they grow into colorless plastids or colored plastids.  Leucoplasts on exposure to light change into green plastids.  Similarly, chloroplasts can become leucoplasts; but colored plastids as in petals are mostly terminally distinguished.



Structure:

All green plastids, for that matter every kind of plastids are surrounded by two-unit membranes; i.e. outer and inner of 7 nm thick membranes and they are separated by a periplastid space of 8-10 nm thick.  Nothing like mitochondria, the inner membrane of fully developed plastids does not show any inward foldings; but it plays an active role during the development proplastids into mature plastids.  Chloroplast is completely filled with a liquid called stroma, in which highly organized membrane structures are found they are called grana.  Apart from grana, the stromatic fluid contains a host of enzymes, plastid DNA, RNAs, and 70s ribosomes.



Biogenesis:

Plastids are usually inherited through the parental side as in the case of mitochondria.  During the development of plant structures, proplastids multiply and they are evenly or unevenly spread among daughter cells.  Every cell in the plant body has plastids.  The arrangement of inheritance itself indicates that plastids are derived from pre-existing plastids. 

To begin with, proplastids have a paracrystalline lattice in stroma.  With the start of light as the stimulus (red light is enough), the gene for aminoleuvulinic acid synthetase is down, and in the sense, it is activated.  The product of gene appearance is ALA synthetase.  Though out the development of chloroplasts it is interdependent of nuclear genes and plastogenome expression at several stages of development, chloroplasts show autonomy.  For example, the separation of proplastids in Euglena is free of cell division, but further development needs the products of both nuclear genes and plastogenes.

When proplastids are exposed to light they slowly turn green and enlarge in size.  This is accompanied by the development of granal assemblies.  During these periods, the inner chloroplast membrane produces numerous of finger-shaped invaginations. They, in turn, pinch off several membranous vesicles, which are gathered at the center.  The vesicles start merging 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 dissolve.

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

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

Studies in this respect show that a huge number of protein complexes found in chloroplasts are also found to be nuclear gene products and 120 or so proteins are coded for by the plastogenome.  For example, ferrodoxin and plastoquinones linked 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 making of small subunits of RUBP carboxylase which is vital for the expression of large RUBP subunit is the product of nuclear gene.  Another exciting observation is that one of nuclear gene product disturbs the binding of RNA polymerase to plastid DNA.  The RNA polymerase is a product of plastogenome.  However, there is no clear-cut information to show that plastogenome products regulate the nuclear gene expression which is required for plastid development.



Perhaps the organelle genetic systems are an evolutionary end. In the context of the endosymbiont hypothesis, this would mean that the method whereby the endosymbionts transferred most of their genes to the nucleus inhibits before it was complete. Further transfers may have been ruled out, for mitochondria, by new alterations in the mitochondrial genetic code that made the remaining mitochondrial genes nonfunctional if they were transported to the nucleus.