Movement or locomotion is an integral part of life. The plant kingdom is not bestowed with the ability to move, per se, but all of its species have an established system of intercellular contact and intercellular communication.
And all of these movements are done by the cytoskeleton. The cytoskeleton is the chief organelle of the cell interior involved with these functions.
What Is The Cytoskeleton
‘Cyto’ means cell/cell-like, so cytoskeleton refers to a complex network of proteins that constitute the skeletal system of a cell. Now, this is different from what we usually know as a skeleton in a literal form and is more of a physical skeleton than an anatomical one.
The cytoskeleton is practically comprised of three components, namely- microtubules, intermediate filament, and microfilament. Each of these is assigned with unique activities and has a different structural constitution.
Their location in a cell can be detected in an interspersed manner, extending from the cell wall to the nucleus.
Does Cytoskeleton Exist In Plant Cell
Cytoskeleton exists in plant cells. It exists in all eukaryotic cells except for bacteria. Plant cells’ cytoskeleton differs from that of the animal cell in terms of the building block. The cytoskeleton of plant cells originates greatly from cellulose while the animal cytoskeleton is mostly actin filaments.
Another important thing to remember for the plant cell cytoskeleton is its three constituents (mentioned above). Meanwhile, animal cell cytoskeleton is grouped under centrosome and centriole (later turned into astral rays during cell division).
Structure And Function Of Cytoskeleton
The cytoskeleton is rather versatile and multipurpose when it comes to its properties. Some of the basic structure and functions are discussed briefly:
Microtubules
Structure: Microtubules are microscopic structures visualized under an electron microscope. These are tubular and straw shaped, made up of thirteen protofilaments. The protofilaments are made up of alpha and beta tubulins.
This is the largest component of the cytoskeleton triad and ranges in size from 23 nm to 27 nm (average 24 nm) in diameter, and their length can be of unlimited value.
The cross-section of a microtubule confirms the arrangement of alpha and beta subunits due to polymerization assembly.
Functions:
- Microtubules are meant to provide mechanical support and strengthen the shape of a cell. They protect against the distortion of the cells under mechanical and thermal injuries.
- Microtubules have a plus-end and a minus-end. The minus-end is anchored inside the cell while the plus-end is free and is responsible for signal transduction and cell signaling, regarded with endocrine functions.
- Plant cell cytoskeleton is exclusive for intercellular movement of the component. This happens as it doesn’t have cilia, flagella, and centriole like an animal cell. The intercellular movement includes vesicles, vacuoles, and sometimes the entire organelle.
Intermediate Filaments
Structure:
Intermediate filaments are a molecular aggregation of actin protein filaments bound together by hydrophobic interactions. Two blocks of proteins make up the building block of intermediate filaments, named as coiled-coil structure. The helical structure consists of 1A, 1B, 2A, and 2B subunits separated by linker regions.
These subunits in turn, form a twisted helix called a dimer, two dimers make a tetramer. (Di: 2, tetra: 4). The tetramers then form protofilaments (previously discussed under microtubules). Its diameter is 10 nm, making it an in-between the other two components.
Functions:
- Intermediate filaments are known to be involved with axoplasmic transport of signal molecules and chemical mediators as a slow component. Axoplasm is the part of the neuronal process under cover of axolemma.
- This functions as an essential component in the contraction of smooth muscle. Since plant cells are devoid of muscle, this is implied under the sense of cellular contraction and movement.
- Filaments are concerned with the movement of pigment granules across cells, as well as provide strength to and contact among neighboring cells. This ensures the exchange of materials to and fro.
- The junctional complex structural element ensures the proper cell spreading of chemical and biophysical components, the junctional components can be replaced by tight junctions instead of gap junctions to prevent the spreading and movement of substances when required.
Microfilaments
Structure:
Initially, actin filaments make up the central building block of microfilaments. But in the cellular environment, the microfilaments can react with other sorts of proteins.
Actin subunits aggregate to form globular subunits that will condense later on to form longer chains by the process of polymerization. This will give rise to the filamentous structure of microfilaments.
These are about only 7 nm in diameter, have polarity, and possess the flexibility and tensile elasticity. This is also the smallest component of the cytoskeleton.
Functions:
- As the structure suggests the abundance of actin (one of the essential proteins of muscle contraction, another being the myosin), it performs greatly in muscle contraction from a cellular level. (Or in terms of the plant cell, cellular locomotion).
- This is locomotory, that is, it is concerned with the movement and shifting of cytoplasmic components within and among cells.
- The actin filaments provide a structural framework to the cytoskeleton of the cell, being the thinnest and finely dispersed all over the cytoplasm.
- These can polarize rapidly, causing the shape-shifting of cells. This feature protects the cell from external factors and thus prevents premature destruction.
How The Cytoskeleton Helps Plants To Grow And Develop
Cytoskeleton influences the growth and development of a plant by means of internal organization and cell division (to be discussed later).
Internal organization:
- The cytoskeleton components are known to interact with the plasma membrane of the cell. It creates a pathway for external substances to permeate inside and for waste materials and debris from the inside to leave the extracellular space.
- The intracellular organelles are commanded to move in the desired direction to perform certain activities. This formation, destruction, and rearrangement of vacuoles and enzymatic vesicles is an essential part of plant growth since these acts heavily regulate the development process.
- Chromosomal aggregation and cellular polarity are another way the cytoskeleton affects growth in plants by enhancing proliferation.
What Role Does Cytoskeleton Play In Plant Cell Division
Plant cell division is vastly mitotic for the division of somatic cells that determines physiological growth. On the other hand, meiosis cell division converts sporophytes to gametophytes for reproduction. The cytoskeleton helps in both of these processes.
Mitosis
- The initial step is the formation of the preprophase band, which is a ring of cortical microtubules and actin filaments exclusive to plant cells. It determines the axis and planes in the cell division that will take place. (For example, meiosis 1 and meiosis 2 take place in altered planes).
- Spindle formation is the next necessary step. The spindle apparatus is a framework formed by a cytoskeleton extending from one pole of the cell to the other. Then, the sister chromatids are fused to the spindle apparatus throughout the mitosis.
- Cytoskeleton is involved in the formation of the phragmoplast, which is the guiding agent of chromosomes during mitosis. This instructs the movement of an equal number of chromosomes to each pole, preventing disjunction. This step ensures the equational division that maintains the integrity of each plant species.
Meiosis
- Just as found in mitosis, the spindle apparatus of meiosis is formed by the cytoskeleton. The cytoskeleton also determines the chromatin movement and specific dynamics of chromosomes.
- Cytoskeleton also influences genetic inheritance and transmission and modification of information of the genetic blueprint.
Conclusion
The cytoskeleton is one of the significant organelles of a cell, although not discussed as frequently as others. All the functions and contributions of the cytoskeleton to the life cycle of a plant are seen from the very beginning till the last cell deceases.
Recent studies show the possible presence of cytoskeleton in species and sub-species that were previously considered to not have any cytoplasm. This discovery may play a role in developing advanced horizons in genetic engineering in plants to increase food production.
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