1.0 Mitochondria in eukaryotic cells
Organelles are special compartments surrounded by a bilipid membrane which separates them from the rest of the cell which carry out specific functions. An example of an organelle is the mitochondria which are known to be the “powerhouse of the cell.” They have their own genome (mtDNA) and are semiautonomous. They are said to have evolved from a free α proteobacterium like organism (Takasugi, Yagi, et al., 2010). The word mitochondria is derived from two Greek words: “mitos” which means thread and “chondros” which means “grain”. These two words offer an apt description of the morphology of the mitochondria as observed by early cytologists. The mitochondria play a critical role in cells given that they convert carbon compounds into ATP; the form of energy that is required by the cell. The structure of the mitochondria remained a mystery until the 1950s when the electron microscope allowed for the elucidation of the structure. Mitochondria in eukaryotes are not only the sites for cellular respiration but they are also involved in oxidative phosphorylation and programmed death of cells (apoptosis) (Liesa, Palacin, et al., 2009).
2.0 Mitochondria in plants
2.1 The structure of plant mitochondria
Figure 1‑1: The image of the mitochondria showing the different components of the organelle (Arimura & Tsutsumi, 2002 )
2.2 Plant mitochondria dynamics and morphology
Mitochondrial dynamics refers to the process of “maintaining the shape, size, number and the distribution of mitochondria within the cells.” The mitochondria undergo changes in shape, size and cellular distribution depending on the prevailing conditions. For instance, in eukaryotic cells, the mitochondria can form network like structures after division and fusion. The movement or the meeting of plant mitochondria allows for the proper functioning of the mitochondria to take place. Mitochondrial dynamics in plants is determined by the following factors: the metabolism of the cell, the architecture of the cell, mechanoproteins and regulatory proteins which participate in the fusion and fission of organelles.
The dynamic nature of the plant mitochondria is attributed to the need to exchange or complement mtDNA which is facilitated by the physical meeting of discrete DNA. It has been found that mitochondria can move at speeds of up to 10µm/s in the cytosol in the root hairs of Arabidopsis, a speed that can vary depending on the position of the mitochondria within the root hairs. In BY-2 maize cells, it has been found that the speed at which the mitochondria moves varies with a single mitochondrion moving at speeds between 0.1µm/s to 0.5 µm/s which was dependent on the location within the cell. The mechanisms underlying the movement of mitochondria vary depending on the organisms. The movement of mitochondria in Aspergillus, S. cerevisiae and plants is pegged on the movement of actin on the cytoskeleton (Sheahan, Rose, et al., 2004). In most yeasts and animal cells, the movement of the mitochondria is microtubule based.
In spite the fact that the motility of the mitochondria allows for close association with other organelles, it is not known whether this association has an impact on function. In one study, the movement of chloroplasts under different lighting circumstances was found to have resulted in changes in the positioning of the mitochondria. When the plants were placed in the dark, the mitochondria were located in the palisade mesophyll cells Exposing the plants to highly intensive light resulted in the relocation of the mitochondria to an anticlinal position. Exposing the plants to low intensity blue light resulted in the relocation of the mitochondria to a periclinal position. It was however inconclusive as to whether the movement of the mitochondria was independent of the chloroplast movement; it was not clear whether the movement of the mitochondria was as a result of association with the chloroplasts via indirect or direct interactions with the cytoskeleton (Islam, Y.Niwa, et al., 2009)).
There is little information regarding the exact mechanisms that determine the cellular distribution of mitochondria in higher plants. However, it has been postulated that actin filaments may be involved given that a biased distribution of mitochondria occurs as a result of an interference with the polymerization of actin filaments (Sheahan, Rose, et al., 2004).
Mitochondrial dynamics in plants also includes fusion and fission which are important aspects in the distribution and complementing of mtDNA. The mitochondria division machinery in plants is made up of several proteins. There are about 16 dynamin homologues found in the Arabidopsis genome which have been found to have a role in the division of mitochondria. It has been found that two of these proteins DRP3A and DRP3B have a 37-41% similarity to the proteins involved in mitochondrial division in yeast and animal cells. Interruption of the DRP3A and DRP3B function as a result of genetic knockout results in an increase in the quantity of large and elongated mitochondria therefore indicating that these proteins have a role to play in the division of the mitochondria.
There are different types of adapters that direct these proteins to sites of scission. In plants, DRP3A and DRP3b proteins interact with two adapters namely: Fis 1type protein which is also referred to as BIGYIN1 and BIGYIN2 (Yoon, Krueger, et al., 2003). The two proteins may also interact directly with outer mitochondrial membrane as a result of interaction with the plant specific adapter NETWORK1. After localization by the adapters, the dynamin homologues surround the organelle and then constrict and ultimately sever the mitochondrion. Fusion in plant mitochondria remains a grossly understood process although the evidence garnered this far indicates that it is a rapid process.
The plant chondriome structure is known to undergo changes at different phases of the cell cycle. An examination of electron micrographs that were prepared from thin slices of Arabidopsis apical meristems showed that the mitochondrion underwent architectural and morphological changes at different phases of the cell cycle. At the G1 to S phase, the mitochondrion was found to have enveloped one end of the nucleus in addition to having tentaculate morphology. During the G2 phase, it was found that the large mitochondria forms a cage around the nucleus while large and small mitochondria increase in number within the cell. During the M phase, fusion of the small and large mitochondrion takes place therefore resulting in an increase in the volume. During cytokinesis, the division of the cage like mitochondria takes place giving rise to two distinct mitochondria which undergo further divisions. It was hypothesised that the formation of the tentaculate/ cage like mitochondria takes place in order to facilitate the redistribution of mtDNA (Segui-Simarro M, Coronado et al., 2008).
Figure 1.2 Typical chondriome structures
in (a) yeast, (b) human -HeLa cell (c) and plant (Arabidopsis). Scale bar in all images = 5 µm. Images adopted from.
3.0 Endoplasmic reticulum in plants
The endoplasmic reticulum in cells occurs as a system of sheets and membrane tubules which are distributed throughout the cytoplasm (Giorgi, Stefani et al., 2009 ). This elaborate network of sheets and membrane tubules is continuous with the nucleus (E.Chevet, Cameron et al., 2001). 3- Dimensional reconstructions of the endoplasmic reticulum indicate that the endoplasmic reticulum is like a basket which entirely encloses the cellular space. The Endoplasmic reticulum(ER) occupies a cortical position just outside the plasma membrane. There are two types of endoplasmic reticulum as revealed by transmission electron microscopy: cisternal endoplasmic reticulum coated with ribosomes which is also referred to as rough endoplasmic reticulum and smooth endoplasmic reticulum which is tubular and has less ribosomes (Bootman, Petersen et al., 2002). Rough endoplasmic reticulum is more common in cells that secrete proteins whilst smooth endoplasmic reticulum is more common in cells that actively secrete lipids and synthesise membranes (Borgese, Francolini et al., 2006).
Fig. 1.3: A 3-dimensional representation of the endoplasmic reticulum of plant cell. The diagram shows the continuity of the ER with the nucleus. From the diagram, Rough ER can be identified by the ribosomes on the surface while smooth ER (as the name suggests) do not have ribosomes on the surface. Source:
The smooth endoplasmic reticulum serves as a holding bay for overflowing enzymes. SER is only found in abundance in specific types of cells particularly in animal cells such as hepatocytes, neurons and muscle cells (Voeltz, Rolls et al., 2002). There are other functions that are performed by the endoplasmic reticulum which include: storage of calcium ions in the lumen of the endoplasmic reticulum and the subsequent release into the cytosol, translocation of proteins across the membrane of the endoplasmic reticulum, modification and folding of the proteins and the synthesis of phospholipids and steroids. In plants, the endoplasmic reticulum is a major component of the secretory pathway given that it is comprised of a series of membranes from the site of synthesis to the final location. Proteins that are associated with the membrane and those that are destined for destinations such as storage vesicles are synthesised as a result of the translation of the mRNA at the ribosome. These proteins are then inserted into the endoplasmic reticulum via the translocon where they undergo modification. The insertion of the proteins into the ER begins with the signalling of the proteins via an amino acid signal or transit peptide sequence which subsequently directs the proteins into the endoplasmic reticulum.
Under a light microscope, after staining with fluorescent dyes, the interphase endoplasmic reticulum is made up of two components: the peripheral and the nuclear ER. The nuclear ER or envelope is made up two sheets of membrane with a lumen and encompasses the nucleus with the inner and outer membranes only connecting at the nuclear pores. The peripheral ER is comprised of a series of interconnected tubules that extend throughout the cell (Sparkes, Frigerio et al., 2009 ). There are obvious morphological differences between smooth endoplasmic reticulum (SER) and rough endoplasmic reticulum (RER). SER is more convoluted than RER and RER has a more granular texture than SER (Prinz, Grzyb et al., 2000).
3.1 Dynamics and morphology of endoplasmic reticulum
The ER is a highly dynamic organelle whose dynamism is conserved by evolution. The exact relationship between dynamism of the ER and function is yet to be fully understood. Some of the modifications that the structure of the ER goes through include: budding and incorporation of vesicles into the ER membrane, emergence and retraction of tubules, transition of sheets into tubules and vice versa, fusion and fission of tubules. The shape of the endoplasmic reticulum and the membrane shape changes during the division of the cell, metabolic state and in the course of growth. The intrinsic curvature of the membranes is attributed to the presence of peripheral proteins which conforms the membrane to their shape. The integral membrane proteins which have specialized hydrophobic domains are responsible for the generation of the curved shape since they wedge themselves into the outer layer of the membranes (Shibata, Hu et al., 2009 ).
Another aspect that is a critical component of ER dynamics is membrane fission and fusion (Du, Ferro-Novick et al., 2004 ). The fusion of membrane is critical for the preservation of the structure of the ER in both plants and animals. Membrane fusion in ER has been found to be dependent on GTP and atlastins. Atlastins refer to integral membrane proteins which have an N terminal GTPase and two transmembrane spans. The insertion of these two components into the lipid bilayer results in the N- and C- termini protruding into the cytoplasm (M.Puhka, Vihinen et al., 2007). The exact mechanism of fission in ER and the proteins that are involved in the process is not known. However, it is speculated that ER membranes undergo fission just like the mitochondria undergo fission in order to strike a balance. An instance in which membrane fission of the ER is seen is during the process of mitosis. It is speculated that the process of fission aids in the maintenance of the shape of endoplasmic reticulum (Voeltz, Rolls et al., 2002).
Fig. 1.4: Diagram showing the morphology of the rough and smooth endoplasmic reticulum. The diagram shows the two components of the endoplasmic reticulum (categorized as per their proximity to the nucleus): nuclear ER which closely associates with the nucleus and the peripheral ER that extends throughout the cell. The nucleus (envelope) ER is linked to the nucleus by nuclear pores. The SER are found on the periphery and form a network of tubes. The two types of ER are interconnected forming a network that link the nucleus and the cytoplasm.
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