- Preprophase
Preprophase is an additional phase during
mitosis inplant cell s that does not occur in othereukaryote s such asanimal s or fungi. It precedesprophase and is characterized by two distinct events:
*The formation of thepreprophase band , a densemicrotubule ring underneath theplasma membrane .
*The initiation ofmicrotubule nucleation at thenuclear envelope .Function of preprophase in the cell cycle
Plant cells are fixed with regards to their neighbor cells within the tissues they are growing in. In contrast to animals where certain cells can migrate within the
embryo to form new tissues, the seedlings of higher plants grow entirely based on the orientation ofcell division and subsequent elongation and differentiation of cells within theircell wall s. Therefore, the accurate control of cell division planes and placement of the future cell wall in plant cells is crucial for the correct architecture of plant tissues and organs.The preprophase stage of
somatic plant cell mitosis serves to establish the precise location of the division plane and future cell wall before the cell entersprophase . This is achieved through the formation of a transient microtubule structure, the preprophase band, and a so far unknown mechanism by which the cell is able to "memorize" the position of the preprophase band to guide the new cell wall growing duringcytokinesis to the correct location. Ingametophyte tissues during the reproductive phase of the plant life cycle, cell division planes may be established without the use of a preprophase band.cite journal|author= Otegui M, Staehelin LA | title = Cytokinesis in flowering plants: more than one way to divide a cell | journal = Current Opinion in Plant Biology | date = 2000 | volume = 3 | pages = 493–502 | doi = 10.1016/S1369-5266(00)00119-9 | unused_data = |PMID 11074381]In highly vacuolated plant cells, preprophase may be preceded by the formation of a
phragmosome . The function of the phragmosome is to suspend thecell nucleus in the center of the cell in preparation for mitosis. If a phragmosome is visible, the preprophase band will appear at its outer edge.Preprophase band formation
At the beginning of preprophase, the cortical microtubules of a plant cell disappear and aggregate into a dense ring underneath the plasma membrane. This preprophase band runs around the equatorial plane of the future
mitotic spindle and marks the plane of cell division and future fusion site for thecell plate . It consists of microtubules andmicrofilament s (actin ) and persists into prophase. Spindle formation occurs during prophase with the axis perpendicular to the plane surrounded by the preprophase band.Microtubule nucleation
In contrast to animal cells, plant cells do not possess
centrosome s to organize their mitotic spindles. Instead, the nuclear envelope acts as amicrotubule organizing center (MTOC) for spindle formation during preprophase. The first sign is a clear, actin-free zone appearing around the nuclear envelope. This zone fills with microtubules nucleating on the surface of the nucleus. The preprophase spindle forms by self-assembly of these microtubules in thecytoplasm surrounding the nuclear envelope. It is reinforced throughchromosome (kinetochore )-mediated spindle assembly after the nuclear envelope breaks down at the end of prophase.cite journal|author= Smirnova EA, Bajer AS | title = Early stages of spindle formation and independence of chromosome and microtubule cycles in "Haemantus" endosperm | journal = Cell Motil. Cytoskeleton | date = 1998 | volume = 40 | pages = 22–37 | pmid = 9605969 | doi = 10.1002/(SICI)1097-0169(1998)40:1<22::AID-CM3>3.0.CO;2-H | doilabel = 10.1002/(SICI)1097-0169(1998)40:122::AID-CM33.0.CO;2-H]Transition into prophase
During progression from preprophase into prophase, the randomly oriented microtubules align parallel along the nuclear surface according to the spindle axis. This structure is called the "prophase spindle". Triggered by nuclear envelope breakdown at the end of prophase, the preprophase band disappearscite journal|author= Dixit R, Cyr RJ | title = Spatio-temporal relationship between nuclear-envelope breakdown and preprophase band disappearance in cultured tobacco cells | journal = Protoplasma | date = 2002 | volume = 219 | pages = 116–121 | pmid = 11926062 | doi = 10.1007/s007090200012] and the prophase spindle matures into the
metaphase spindle occupying the space of the former nucleus. Experiments with drugs destroying microfilaments indicate that actin may play a role in keeping the cellular "memory" of the position of the division plane after the preprophase band breaks down to direct cytokinesis intelophase .cite journal|author= Hoshino H, Yoneda A, Kumagai F, Hasezawa S| title = Roles of actin-depleted zone and preprophase band in determining the division site of higher-plant cells, a tobacco BY-2 cell line expressing GFP-tubulin | journal = Protoplasma | date = 2003 | volume = 222 | pages = 157–165 | pmid = 14714204| doi = 10.1007/s00709-003-0012-8]Bibliography
*P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): "Biology of Plants", 7th Edition, W.H. Freeman and Company Publishers, New York, ISBN 0-7167-1007-2
Notes and references
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