Plant Cytoskeleton

The cytoskeleton organizes the cell and helps traffic organelles.

The plant cytoskeleton has two important components: microtubules and microfilaments.

Each type is filamentous, with fix diameter and variable lengths, some micrometers. They are assemblies of globular proteins.

Microtubules has a diameter of 25 nm, they formed from polymers of the protein tubulin. The tubulin monomers are heterodimer composed of two polypeptide chains ( and

 tubulin).

Hundreds or thousands of tubulin monomers can arrange a column called protofilaments.

Microfilaments have a dimeter of 7 nm and they build up from globular actin, called G-actin.

These monomers can be polymerized in order to form protofilaments. In the polymerized protofilaments, the actin has a name, F-actin. One microfilament contains two actin protofilaments in a double helical pattern.

Assembly Events of Microtubules and Microfilaments

Actin and tubulin subunits can exist as a pool of free proteins and their polymerized forms.

Every monomer contains a bound nucleotide, ATP in the actin and GTP in the case of tubulin.

Microtubules and microfilaments are polarized, so the ends are different. Microfilaments and microtubules have half-lives, some minutes, and determined by accessory proteins, actin-binding proteins (ABPs) in microfilaments and microtubule-associated proteins (MAPs) in microtubules.

Cortical microtubules are moving around the cell by “tread milling”. Microtubules in the

cortical cytoplasm migrate laterally around the cell periphery by a process called tread milling.

During this process, tubulin heterodimers are added to the growing end at the same rate as tubulin is being lost from the other end, it seems like microtubules can move through the cytoplasm. Microtubules can play an important role in the polarity of the plant.

Cytoplasmic streaming and organelle trafficking

Molecular motors can help the movements of organelles through the cytoplasm. These motor proteins have a similar structure. They consist of a large globular head, a neck region and a tail which can connect to the cargo-binding domain. The head has the microtubule/actin-binding and the motor domains. The motor domain has ATPase activity; ATP hydrolysis provides the energy for the unidirectional movement via ATP-dependent cyclic conformation changes in the cytoskeleton-binding head. Molecular motors differ from each other considering the cytoskeletal elements they bind and the direction of their movement. Myosins move along the actin microfilaments toward the plus end of the F-actin. Kinesins can move along the microtubules, however, two-third of them are moving toward to the plus ends, while one-third of toward the minus end. Dyneins are absent from plant cells; in animal cells they are negative end motors of the microtubules.

Cytoplasmic streaming is a coordinated movement of particles and organelles through the cytosol, and occurs in all plant cells, but with variable velocities. One example of the developmentally regulated cytoplasmic streaming is the migration of the nucleus to the site of root hair formation in the root epidermal cells. However, environmental conditions also can induce cytoplasmic streaming, e.g. chloroplasts can be repositioned by light in the leaf cells in order to maximize or reduce exposure of light.

Summary

1. Plant cell walls are typical plant cell structures.

2. Plant cells contain compartments derived from the endomembrane system.

3. Chloroplasts and mitochondria are not derived from the endomembrane system. The

endomembrane system plays a central role in secretory processes, membrane

recycling, and the cell cycle.

4. The composition and fluid-mosaic structure of the plasma membrane permits regulation of transport into and out of the cell.

5. The endomembrane system conveys both membrane and cargo proteins to diverse organelles.

6. The specialized membranes of the nuclear envelope are derived from the endoplasmic reticulum (ER), a component of the endomembrane system.

7. The nucleus is the site of storage, replication, and transcription of the chromatin, as well as being the site for the synthesis of ribosomes.

8. The ER is a system of membrane-bound tubules that form a complex and dynamic structure.

9. The rough ER (ER) is involved in synthesis of proteins that enter the lumen of the ER, the smooth ER is the site of lipid biosynthesis.

10. The ER provides the membrane and internal cargo for the other compartments of the endomembrane system.

11. Secretion of proteins from cells begins with the RER.

12. Glycoproteins and polysaccharides destined for secretion are processed in the Golgi apparatus.

13. During endocytosis, membrane is removed from the plasma membrane by formation of small chlatrin-coated vesicles.

14. Endocytosis from the plasma membrane provides membrane recycling.

15. Oil bodies, peroxisomes, and glyoxysomes grow and proliferate independently of the endomembrane system.

16. Mitochondria and chloroplasts each have an inner and an outer membrane.

17. Plastids may contain high concentrations of pigments or starch

18. Proplastids pass through distinct developmental stages to form specialized plastids.

19. In plastids and mitochondria, DNA replication and fission are regulated independently of nuclear division.

20. A three-dimensional network of microtubules and microfilaments organizes the cytosol.

21. Microtubules and microfilaments can assemble and disassemble.

22. Molecular motors associated with components of the cytoskeleton move organelles thorough the cytoplasm.

23. During cytoplasmic streaming, interaction of the F-actin with myosin provides for independent movement of organelles, including chloroplasts.

24. The cell cycle, during which cells replicate their DNA and reproduce themselves, consists of four phases.

25. Successful mitosis and cytokinesis require the participation of microtubules and the endomembrane system.

26. Tubular extensions of the plasma membrane traverse the cell wall and connect the

cytoplasm of clonally derived cells, allowing water and small molecules to move

between cells without crossing a membrane.

Review Questions

1. What are the typical plant cell organelles?

2. Which organelles belong to the endomembrane system in plant cells?

3. Which organelles are independently dividing and derived from the endomembrane system?

4. Which organelles are independently dividing and semiautonomous?

5. How is the plant cytoskeleton organized?

6. What is the factors of cell cycle regulation?

7. What are plasmodesmata in plant cells?

Discussion Questions

1. What are the differences in different types of plant cell walls?

2. What types of cell walls can be found in the plant body?

3. How do Golgi body and ER affect lipid and carbohydrate biosynthesis in plants?

4. Why are plastids so important in plant cells?

Additional Reading

1. Crang R, Lyons-Sobaski S, Wise R. Plant Anatomy: A Concept-Based Approach to the

Structure of Seed Plants [Internet]. Springer International Publishing; 2018 [cited 2019 Aug

30]. Available from: https://www.springer.com/gp/book/9783319772080

In document Molecular plant physiology (Pldal 28-32)