The plant Spc98p homologue colocalizes with gamma-tubulin at microtubule nucleation sites and is required for microtubule nucleation.
(57/693)
The molecular basis of microtubule nucleation is still not known in higher plant cells. This process is better understood in yeast and animals cells. In the yeast spindle pole body and the centrosome in animal cells, gamma-tubulin small complexes and gamma-tubulin ring complexes, respectively, nucleate all microtubules. In addition to gamma-tubulin, Spc98p or its homologues plays an essential role. We report here the characterization of rice and Arabidopsis homologues of SPC98. Spc98p colocalizes with gamma-tubulin at the nuclear surface where microtubules are nucleated on isolated tobacco nuclei and in living cells. AtSpc98p-GFP also localizes at the cell cortex. Spc98p is not associated with gamma-tubulin along microtubules. These data suggest that multiple microtubule-nucleating sites are active in plant cells. Microtubule nucleation involving Spc98p-containing gamma-tubulin complexes could then be conserved among all eukaryotes, despite differences in structure and spatial distribution of microtubule organizing centers. (+info)
Cytokinesis in eukaryotes.
(58/693)
Cytokinesis is the final event of the cell division cycle, and its completion results in irreversible partition of a mother cell into two daughter cells. Cytokinesis was one of the first cell cycle events observed by simple cell biological techniques; however, molecular characterization of cytokinesis has been slowed by its particular resistance to in vitro biochemical approaches. In recent years, the use of genetic model organisms has greatly advanced our molecular understanding of cytokinesis. While the outcome of cytokinesis is conserved in all dividing organisms, the mechanism of division varies across the major eukaryotic kingdoms. Yeasts and animals, for instance, use a contractile ring that ingresses to the cell middle in order to divide, while plant cells build new cell wall outward to the cortex. As would be expected, there is considerable conservation of molecules involved in cytokinesis between yeast and animal cells, while at first glance, plant cells seem quite different. However, in recent years, it has become clear that some aspects of division are conserved between plant, yeast, and animal cells. In this review we discuss the major recent advances in defining cytokinesis, focusing on deciding where to divide, building the division apparatus, and dividing. In addition, we discuss the complex problem of coordinating the division cycle with the nuclear cycle, which has recently become an area of intense research. In conclusion, we discuss how certain cells have utilized cytokinesis to direct development. (+info)
Plant K+ transport: not just an uphill struggle.
(59/693)
Most plant cells can accumulate K+ to concentrations much higher than those in the external medium. Recent studies are providing new insights into the relative roles of channels and transporters in K+ accumulation. These studies are also pointing to specific roles for K+ channels and transporters in polarized development. (+info)
Plant D-type cyclins and the control of G1 progression.
(60/693)
The basic pattern of controls that operate during the G1 phase of the plant cell cycle shows much closer similarity to animals than to the yeasts and other fungi. The activity of D-type cyclin (CycD) kinases is induced in response to stimulatory signals, and these phosphorylate the plant homologue of the retinoblastoma tumour susceptibility (Rb) protein. It is likely that Rb phosphorylation results in the activation of genes under the control of E2F transcription factors, including those required for S phase entry. As the initial triggers of the cascade, attention has focused on the CycDs, and a family of 10 genes is present in Arabidopsis, divided into three major and three minor groups. Analysis to date suggests that these groups are functionally distinct. (+info)
Roles for kinesin and myosin during cytokinesis.
(61/693)
Cytokinesis in higher plants involves the phragmoplast, a complex cytoplasmic structure that consists of microtubules (MTs), microfilaments (MFs) and membrane elements. Both MTs and MFs are essential for cell plate formation, although it is not clear which motor proteins are involved. Some candidate processes for motor proteins include transport of Golgi vesicles to the plane of the cell plate and the spatiotemporal organization of the cytoskeletal elements in order to achieve proper deposition and alignment of the cell plate. We have focused on the kinesin-like calmodulin binding protein (KCBP) and, more broadly, on myosins. Using an antibody that constitutively activates KCBP, we find that this MT motor, which is minus-end directed, contributes to the organization of the spindle and phragmoplast MTs. It does not participate in vesicle transport; rather, because of the orientation of the phragmoplast MTs, it is supposed that plus-end kinesins fill this role. Myosins, on the other hand, based on their inhibition with 2,3-butanedione monoxime and 1-(5-iodonaphthalene-1-sulphonyl)-1H-hexahydro-1,4-diazepine (ML-7), are associated with the process of post-mitotic spindle/phragmoplast alignment and with late lateral expansion of the cell plate. They are also not the principal motors involved in vesicle transport. (+info)
Control of plant cytokinesis by an NPK1-mediated mitogen-activated protein kinase cascade.
(62/693)
Cytokinesis is the last essential step in the distribution of genetic information to daughter cells and partition of the cytoplasm. In plant cells, various proteins have been found in the phragmoplast, which corresponds to the cytokinetic apparatus, and in the cell plate, which corresponds to a new cross wall, but our understanding of the functions of these proteins in cytokinesis remains incomplete. Reverse genetic analysis of NPK1 MAPKKK (nucleus- and phragmoplast-localized protein kinase 1 mitogen-activated protein kinase kinase kinase) and investigations of factors that might be functionally related to NPK1 have helped to clarify new aspects of the mechanisms of cytokinesis in plant cells. In this review, we summarize the evidence for the involvement of NPK1 in cytokinesis. We also describe the characteristics of a kinesin-like protein and the homologue of a mitogen-activated protein kinase that we identified recently, and we discuss possible relationships among these proteins in cytokinesis. (+info)
Plants and sodium ions: keeping company with the enemy.
(63/693)
Plants face a dilemma about sodium metabolism. Uptake of ubiquitous sodium ions is desirable as a way to build osmotic potential, absorb water and sustain turgor, but excess sodium ions may be toxic. Information from a number of plant species about the proteins involved in sodium-ion uptake helps to explain how plants manage to take in just the right amount. (+info)
Plants in a cold climate.
(64/693)
Plants are able to survive prolonged exposure to sub-zero temperatures; this ability is enhanced by pre-exposure to low, but above-zero temperatures. This process, known as cold acclimation, is briefly reviewed from the perception of cold, through transduction of the low-temperature signal to functional analysis of cold-induced gene products. The stresses that freezing of apoplastic water imposes on plant cells is considered and what is understood about the mechanisms that plants use to combat those stresses discussed, with particular emphasis on the role of the extracellular matrix. (+info)