The phragmoplast is a plant cell specific structure that forms during late cytokinesis. It serves as a scaffold for cell plate assembly and subsequent formation of a new cell wall separating the two daughter cells. The phragmoplast can only be observed in Phragmoplastophyta, a clade that includes the Coleochaetophyceae, Zygnematophyceae, Mesotaeniaceae, and Embryophyta (land plants). Some algae use another type of microtubule array, a phycoplast, during cytokinesis. [1] [2]
The phragmoplast is a complex assembly of microtubules (MTs), microfilaments (MFs), and endoplasmic reticulum (ER) elements, that assemble in two opposing sets perpendicular to the plane of the future cell plate during anaphase and telophase. It is initially barrel-shaped and forms from the mitotic spindle between the two daughter nuclei while nuclear envelopes reassemble around them. The cell plate initially forms as a disc between the two halves of the phragmoplast structure. While new cell plate material is added to the edges of the growing plate, the phragmoplast microtubules disappear in the center and regenerate at the edges of the growing cell plate. The two structures grow outwards until they reach the outer wall of the dividing cell. If a phragmosome was present in the cell, the phragmoplast and cell plate will grow through the space occupied by the phragmosome. They will reach the parent cell wall exactly at the position formerly occupied by the preprophase band.
The microtubules and actin filaments within the phragmoplast serve to guide vesicles with cell wall material to the growing cell plate. Actin filaments are also possibly involved in guiding the phragmoplast to the site of the former preprophase band location at the parent cell wall. While the cell plate is growing, segments of smooth endoplasmic reticulum are trapped within it, later forming the plasmodesmata connecting the two daughter cells.
The phragmoplast can be differentiated topographically into two areas, the midline that includes the central plane where some of the plus-ends of both anti-parallel sets of microtubules (MTs) interdigitate (as in the midbody matrix), and the distal regions at both sides of the midline. [3]
After anaphase, the phragmoplast emerges from the remnant spindle MTs in between the daughter nuclei. MT plus ends overlap the equator of phragmoplast at the site where the cell plate will form. The formation of the cell plate depends on localized secretory vesicle fusion to deliver membrane and cell-wall components. [4] Excess membrane lipid and cell-wall components are recycled by clathrin/dynamin-dependent retrograde membrane traffic. [5] Once the initial cell plate forms at its center, the phragmoplast begins to expand outward to reach the cell edges. Actin filaments also localize to phragmoplast and accumulate greatly at late telophase. Evidence suggests that actin filaments serve phragmoplast expansion more than initial organization, given that disorganization of actin filaments via drug treatments lead to the delay of cell-plate expansion. [6]
Many microtubule-associated proteins (MAPs) have been localized to the phragmoplast, including both constitutively expressed ones (such as MOR1, [7] katanin, CLASP, SPR2, and γ-tubulin complex proteins) and those expressed specifically during M-phase, such as EB1c, [8] TANGLED1 [9] and augmin complex proteins. [10] The functions of these proteins in the phragmoplast are presumably similar to their functions elsewhere in the cell. [4] Most research into phragmoplast MAPs have been focused on the midline because it is, first, where most of the membrane fusion takes place and, second, where the two sets of anti-parallel MTs are held together. The discovery of an important variety of molecules that localize to the phragmoplast midline is shedding light on the complex processes operating in this phragmoplast region. [3]
Two proteins that have critical functions for antiparallel MT bundling at the phragmoplast midline are MAP65-3 and kinesin-5. [11] [12] The kinesin-7 family proteins, HINKEL/AtNACK1 and AtNACK2/TES, recruit a mitogen-activated protein kinase (MAPK) cascade to the midline and induce MAP65 phosphorylation. [13] [14] [15] [16] Phosphorylated MAP65-1 also accumulates at the midline and reduces MT-bundling activities for cell-plate expansion. [17] The essential mechanism of MAPK cascade for phragmoplast expansion is suppressed by cyclin dependent kinase (CDK) activity before telophase. [18]
Certain phragmoplast midline-accumulating MAPs are essential proteins for cytokinesis. The kinesin-12 members, PAKRP1 and PAKRP1L, accumulate at the midline [19] and double loss-of-function mutants have defective cytokinesis during male gametogenesis. [20] PAKRP2 accumulates at midline and also in puncta throughout the phragmoplast, which implies that PAKRP2 participates in Golgi-derived vesicle transport. [21] Moss homologs of PAKRP2, KINID1a, and KINID1b localize to the phragmoplast midline and are essential for phragmoplast organization. [22] RUNKEL, which is a HEAT repeat-containing MAP, also accumulates at the midline and cytokinesis is aberrant in lines with the loss-of-function mutations in this protein. [23] [24] Another midline-localized protein, “two-in-on” (TIO), is a putative kinase and is also required for cytokinesis as shown by defects in a mutant. [25] TIO interacts with PAKRP1, PAKRP1L (kinesin-12), and NACK2/TES (kinesin-7) according to the yeast two hybrid assays. [26] [27] Finally, TPLATE, an adaptin-like protein, accumulates at the cell plate and is essential for cytokinesis. [28] [29]
Cytokinesis is the part of the cell division process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and meiosis. During cytokinesis the spindle apparatus partitions and transports duplicated chromatids into the cytoplasm of the separating daughter cells. It thereby ensures that chromosome number and complement are maintained from one generation to the next and that, except in special cases, the daughter cells will be functional copies of the parent cell. After the completion of the telophase and cytokinesis, each daughter cell enters the interphase of the cell cycle.
In cell biology, the spindle apparatus is the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a process that produces genetically identical daughter cells, or the meiotic spindle during meiosis, a process that produces gametes with half the number of chromosomes of the parent cell.
A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells from the pollen grain—either from the stigma to the ovules at the base of the pistil or directly through ovule tissue in some gymnosperms. In maize, this single cell can grow longer than 12 inches (30 cm) to traverse the length of the pistil.
In cell biology, the cleavage furrow is the indentation of the cell's surface that begins the progression of cleavage, by which animal and some algal cells undergo cytokinesis, the final splitting of the membrane, in the process of cell division. The same proteins responsible for muscle contraction, actin and myosin, begin the process of forming the cleavage furrow, creating an actomyosin ring. Other cytoskeletal proteins and actin binding proteins are involved in the procedure.
Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence.
Motor proteins are a class of molecular motors that can move along the cytoplasm of cells. They convert chemical energy into mechanical work by the hydrolysis of ATP. Flagellar rotation, however, is powered by a proton pump.
Aurora kinase B is a protein that functions in the attachment of the mitotic spindle to the centromere.
Rac GTPase-activating protein 1 is an enzyme that in humans is encoded by the RACGAP1 gene.
Kinesin-like protein KIF23 is a protein that in humans is encoded by the KIF23 gene.
Centromere-associated protein E is a protein that in humans is encoded by the CENPE gene.
Kinesin-like protein KIF3A is a protein that in humans is encoded by the KIF3A gene.
Citron Rho-interacting kinase is an enzyme that in humans is encoded by the CIT gene.
Anillin is a conserved protein implicated in cytoskeletal dynamics during cellularization and cytokinesis. The ANLN gene in humans and the scraps gene in Drosophila encode Anillin. In 1989, anillin was first isolated in embryos of Drosophila melanogaster. It was identified as an F-actin binding protein. Six years later, the anillin gene was cloned from cDNA originating from a Drosophila ovary. Staining with anti-anillin antibody showed the anillin localizes to the nucleus during interphase and to the contractile ring during cytokinesis. These observations agree with further research that found anillin in high concentrations near the cleavage furrow coinciding with RhoA, a key regulator of contractile ring formation.
Protein Regulator of cytokinesis 1 (PRC1) is a protein that in humans is encoded by the PRC1 gene and is involved in cytokinesis.
Midline-2 is a protein that in humans is encoded by the MID2 gene.
Kinesin family member 15 is a protein that in humans is encoded by the KIF15 gene.
Centralspindlin is a motor complex implicated in cell division. It contributes to virtually every step in cytokinesis, It is highly conserved in animal cells as a component of the spindle midzone and midbody. Centralspindlin is required for the assembly of the mitotic spindle as well as for microtubule bundling and anchoring of midbody microtubules to the plasma membrane. This complex is also implicated in tethering the spindle apparatus to the plasma membrane during cytokinesis This interaction permits cleavage furrow ingression. In addition, centralspindlin's interaction with the ESCRT III allows for abscission to occur.
Alice Barkan is an American molecular biologist and a professor of biology at the University of Oregon. She is known for her work on chloroplast gene regulation and protein synthesis.
Plant nucleus movement is the movement of the cell nucleus in plants by the cytoskeleton.
Lucas Andrew Staehelin was a retired Swiss-American cell biologist. He was professor emeritus at the University of Colorado Boulder.
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