FRAGMOPLASTS: CHARACTERISTICS, FUNCTIONS, COMPOSITION - BIOLOGY - 2025

The phragmoplasts are structures formed mainly by a microtubule assembly or microfibrils are arranged in a barrel within the plant cell division and are formed during anaphase (third phase of mitosis) or late telophase (fourth and final phase of the mitosis) early.

Cytokinesis is the final stage of the cell cycle and consists of the separation and segmentation of the cytoplasm. This process takes place during the last phase of mitosis and is different in plants, fungi and animals. In plants it usually involves the formation of fragmoplasts, the cell plate and the cell wall. The role of fragmoplasts is essential during cytokinesis in plants.

Fragmoplast. Taken and edited from

biologia.fciencias.unam.mx/plantasvasulares/GlosarioPlantas/AnatomiaVegetal/index.html

Preliminary considerations

Plants, fungi, as well as some algae, bacteria and archaea have their cells protected by a cell wall, which is a resistant, sometimes rigid layer that is located on the outside of the plasma membrane.

The functions of the cell wall are to protect the contents of the cell, to give it rigidity, as well as to act as a mediator in all the relations of the cell with the environment and as a cell compartment.

Cytokinesis is more complex in plant cells than in animal cells, because the latter lack a rigid external cell wall. The presence of cytoskeletal structures such as the preprophase band (PPB) and fragmoplasts can be considered a proof of the difficulties that the cell wall imposes on the process of cell division.

These two structures, unique to plant cells, are necessary to ensure proper positioning and assembly of a new cell wall to separate two sister nuclei.

Fragmoplasts bear only small and distant structural similarities to the midbody of animal cytokinetic cells.

General characteristics of fragmoplasts

Fragmoplasts are structures unique to plant cells of land plants and some groups of algae.

They are cylindrical in shape, composed of two opposing disks of microtubules (from mitotic use), membranes, vesicles (from the Golgi complex), and actin filaments.

On the other hand, it should be noted that its formation originates in the area previously occupied by the equatorial plate.

Features

Fragmoplasts have an important variety of functions, but the most relevant are:

-Essentially, it begins the formation of the cell plate

-Deposits wall material containing vesicles from the Golgi apparatus, which is then used to build a new closed transverse membrane wall (cell plate).

-Forms a kind of middle lamellae, which are necessary for the assembly of the cell wall.

-The communication between the cytoplasmic fragmoplast and the cortical remains of a cytoplasmic structure called the preprophase microtubule band, is what allows control over symmetric and asymmetric cell divisions.

Formation of the cell wall thanks to the activity of the fragmoplast. Taken and edited from

Composition

The fragmoplast is made up of elements of the endoplasmic reticulum, cellular structures made up of protein polymers called microtubules, microfilaments of a globular protein called actin, and a multitude of other unknown proteins.

Myosin has also been found in fragmoplasts and its role is believed to assist in the transport of vesicles from the Golgi apparatus to the cell plate.

How are they formed?

Because the plant cell has a cell wall, plant cytokinesis is quite different from animal cell cytokinesis. During this process of cell division, plant cells build a cell plate in the center of the cell.

Fragmoplasts are mainly composed of two protein cell structures. These are the training processes:

Microtubules

During the process of cell plate formation, the fragmoplast is formed. This is assembled from the remains of the mitotic spindle and is made up of a series of polar microtubules that appear to arise from the remains of the mitotic spindle apparatus and are organized in an antiparallel matrix.

These microtubules are aligned perpendicular to the plane of division with their "+" ends located at or near the site of cell division, and their negative ends face the two daughter nuclei.

The so-called “+” ends are the fast growing ends and are the place where the microtubules bond. Therefore, it is important to note that these "+" ends are immersed in an electrodense material located in the central area.

In the later phase of anaphase, the slightly extended microtubules in the intermediate zone unite laterally in a cylindrical structure, the fragmoplast itself.

This structure subsequently shortens in length and expands laterally until it finally reaches the side wall. During this stage of fragmoplast expansion, a change occurs in the organization of the microtubules.

Whereas the initial fragmoplast cylinder originates from pre-existing microtubules, new microtubules must form in later stages of centrifugal growth.

Actin microfilaments

Actin microfilaments are also an important cytoskeletal component of fragmoplasts. Their alignment, like that of microtubules, is perpendicular to the plane of the cell plate, with the “+” ends directed proximally.

Unlike microtubules, they are organized into two opposing assemblages that do not overlap or join directly. With the positive proximal ends, the actin microfilaments are also organized in a way that would facilitate the transport of vesicles to the plane of the plate.

How does it participate in the formation of the cell wall?

The site where cell division will occur is established from a rearrangement of the microtubules that form the preprophase band, the mitotic spindle, and the fragmoplast. When mitosis begins, the microtubules depolymerize and rearrange, forming the preprophase band around the nucleus.

Subsequently, the vesicles directed from the trans Golgi network (network of cellular structures and cisterns of the Golgi apparatus) towards the fragmoplast fuse and give rise to the cell plate. Then, the bipolar organization of the microtubules allows the directional transport of the vesicles to the site of cell division.

Finally, the microtubules, actin filaments from the fragmoplast, and the cell plate expand centrifugally towards the periphery of the cell as cytokinesis progresses, where the cell plate then attaches to the cell wall of the stem cell to complete the process of cytokinesis.

References

1. A. Salazar and A. Gamboa (2013). Importance of pectins in cell wall dynamics during plant development. Journal of Biochemical Education.
2. CM Kimmy, T. Hotta, F. Guo, RW Roberson, YR Julie, and B. Liua (2011). Interaction of Antiparallel Microtubules in the Phragmoplast Is Mediated by the Microtubule-Associated Protein MAP65-3 in Arabidopsis. The Plant Cell.
3. D. Van Damme, FY. Bouget †, K. Van Poucke, D. Inze´ and D. Geelen (2004). Molecular dissection of plant cytokinesis and phragmoplast structure: a survey of GFP-tagged proteins. The Plant Journal.
4. Phragmoplast function? Lifeeasy Biology. Recovered from biology.lifeeasy.org.
5. LA Staehelin and PK Hepler (1996). Cytokinesis in Higher The Plants Cell.
6. The cell. Cellular cycle. Phase M. Mitosis and cytokinesis (2018) Atlas of plant and animal histology. University of Vigo. Recovered from mmegias.webs.uvigo.es.
7. Taiz and E. Zeiger. (2006). Plant Physiology 3rd Edição. ARMED Editora SA 719 pp.
8. L. Taiz and E. Zeiger. (2006). Plant physiology Vol 2. Costelló de la Plana: Publicacions de la Universitat Jaume I. 656 pp.
9. MS Otegui, KJ Verbrugghe and AR Skop (2005) Midbodies and phragmoplasts: analogous structures involved in cytokinesis. Trends in Cell Bioloy.
10. J. de Keijzer, BM Mulder and E. Marcel (2014). Microtubule networks for plant cell division. Systems and Synthetic Biology.
11. O. Marisa and LA Staehelin (2000) Cytokinesis in flowering plants: more than one way to divide a cell. Cerrent Opinion in Plant Biology.
12. LA Staehelin and PK Hepler (1996) Cytokinesis in Higher Plants. Cell.
13. D. Van Damme, FY. Bouget, K. Van Poucke, D. Inzé and Danny Geelen (2004) Molecular dissection of plant cytokinesis and phragmoplast structure: a survey of GFP-tagged proteins. The Plant Journal.