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The Segregation of Golgi during Cytokinesis |
How is the elaborate choreography of mitosis and cytokinesis
achieved, and how do the various organelles manage to wind up where
they do? Most studies have concerned the division of the genetic
material, but the cytoplasm and its organelles must be divided
equitably too. In mammalian cells, for example, the single perinuclear
Golgi apparatus breaks down at the onset of mitosis and reforms in both
daughter cells after cytokinesis. In contrast to mammalian cells, the
Golgi apparatus of plant cells consists of many independent stacks that
continue to produce secretory products during all stages of mitosis and
cytokinesis, especially during cell plate formation. In this issue,
Nebenführ et al. (pp. 135-151) track the
three-dimensional redistribution of Golgi stacks during mitosis and
cytokinesis in tobacco suspension cells by means of an in vivo
Golgi marker consisting of soybean (-1,2 mannosidase tagged with green
fluorescent protein [GFP]). In addition, they employed an ER-targeted
GFP construct and fluorescent Mitotracker dye, which labels
mitochondria and plastids, to localize these organelles during the cell
cycle. Their observations suggest that there is a striking
segregation of organelles into different cytoplasmic
domains throughout the cell cycle (Fig.
1). Golgi stacks, at least, are sorted to
specific areas of the cytoplasm that appear to be related to their
sites of action. Cytoskeleton-disrupting drugs revealed that the
maintenance of this distinct organellar segregation and localization
does not depend on the integrity of microfilaments or microtubules.
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Figure 1.
Equal division of chromosomes (blue), microtubules
(red), and Golgi (green) in tobacco telophase cell.
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How Onion Cells Stretch |
The diverse repertoire of rheological properties of
the plant cell wall is attributable to its biochemical complexity. Cell walls consist of a microfibrillar cellulose phase as well as a matrix
phase that contains a variety of polymers including hemicellulose, pectin, proteins, and phenolics such as lignin. In this issue, Wilson et al. (pp. 397-405) use Fourier-transform infrared (FT-IR) microspectroscopy to determine the orientation of
macromolecules in hydrated cell walls of onion epidermis under normal
and mechanically stressed conditions. The IR spectra of onion epidermis
are dominated by absorption bands of cellulose and pectin, while
minor constituents such as protein, ferulic acid, lignin,
and hemicelluoses can also be detected. With polarized light the
orientation of particular functional groups can be determined.
Cellulose and pectin exhibit little preferential orientation in
non-mechanically stressed cells. When, however, a weight is applied to
one end of a narrow strip of epidermis, both cellulose and pectin
molecules become more axially arranged. To investigate
whether the cellulose and pectin phases act independently
in mechanically stressed epidermal strips, the authors took advantage
of a new technique called two-dimensional IR spectroscopy. They report
that although the cellulose network is the main
stress-bearing element, the kinetics of its re-orientation are slower
than that of the more mobile pectin matrix. This suggests that
cellulose and pectin molecules do not interact directly during cell extension.
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Transgenic Soybean Oil |
Meadowfoam (Limnanthes alba) is a specialty crop
cultivated on limited acreage in the U.S. Pacific Northwest. An
attractive feature of meadowfoam is that 60% of the fatty acids in its
seed oil is composed of the uncommon fatty acid 
5-eicosenoic
(20:15). 5-Eicosenoic acid is more stable to oxidation than is
oleic acid (18:19), the primary mono-unsaturated fatty acid in most
seed oils. This feature makes meadowfoam seed oil
especially desirable for use in cosmetics, surfactants and lubricants.
In this issue, Cahoon et al. (pp. 243-251) report on their
initial success in producing meadowfoam-type seed oil in transgenic
soybean. Two steps are necessary to go from an 18:19 to a 20:15
fatty acid
a change in the position of the double bond and
an increase the length of the fatty acid chain. This is the basic
approach used by Cahoon and his co-workers. First, they found that the
seed-specific expression of a cDNA for Limnanthes
acyl-coenzyme A (acyl-CoA) desaturase in somatic soybean embryos
resulted in the accumulation of 16:15-hexadecenoic acid. Second,
they found that expression of a cDNA for Limnanthes fatty
acid elongase led to the accumulation of 20:0 eicosanoic acid. The
co-expression of the cDNAs for Limnanthes acyl-CoA
desaturase and fatty acid elongase yielded transgenic soybean embryos
in which 20:15 and 5-docosenoic acid (22:15) composed up to
12% of the fatty acids. Potential strategies for raising this
percentage even more are discussed.
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A Step toward C4 Rice |
Rice, a C3 species, is commonly cultivated in
warm, tropical areas where C4 grasses are generally more efficient at
photosynthesis. Few feats of genetic engineering would help feed the
world as much as would the introduction of C4 photosynthesis into rice. In this issue, Suzuki et al. (pp. 163-172) report on
their initial attempt to increase the carboxylase
efficiency of Rubisco in the chloroplasts of rice by the transgenic
introduction of a CO2-generating C4
photosynthetic enzyme, phosphoenolpyruvate carboxykinase
(PCK) from Urochloa panicoides. In PCK-type C4 plants, PCK
reversibly catalyzes the decarboxylation of oxaloacetate to phosphoenolpyruvate (PEP). Thus, in addition to providing
Rubisco with a higher CO2 environment, it was
hoped that the PEP generated by PKC would also serve as a
substrate for the non-C4 phosphoenolpyruvate carboxylase
(PEPC) that occurs naturally in the cytosol of rice mesophyll cells. In
the excised leaves of the transgenic rice expressing PKC, up to 20% of
the 14CO2 label was
incorporated into 4C compounds, as compared to 1% in the excised
leaves of controls. However, no significant differences in
the net photosynthetic rate or the CO2 compensation point were observed
between the transgenic lines and controls, probably due to the low
levels of both PCK expression and endogenous
phosphenolpyruvate carboxylase (PEPC) activity. The authors
speculate that if a C4-type PEPC and PKC can be co-expressed in C3
mesophyll at high levels in the cytosol and chloroplasts,
respectively, it might endow C3 plants with beneficial C4-like attributes.
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Oligandrin: A New Elicitin-Like Protein |
Due to the evolution of heightened pesticide resistance in
various strains of plant pathogens, the replacement of chemical pesticides by alternative agents or products has become the focus of intensive research effort. Pythium oligandrum is a
fungus that might prove to be a promising biocontrol agent of several
soil-borne plant pathogens in tomato. In addition to antagonizing a
wide range of fungal pathogens, P. oligandrum is able to
penetrate the tomato root system without inducing extensive cell
damage. In this issue, Picard et al. (pp. 379-395) report
that the active factor from P. oligandrum is a
low-Mr, water-soluble protein called
oligandrin. When applied to decapitated
tomato plants, oligandrin induces plant defense reactions that restrict stem cell invasion by the parasitic oomycete Phytophthora
parasitica. (Fig. 2) Unlike true elicitins, however, oligandrin
does not elicit a hypersensitive response. An ultrastructural
investigation of the infected tomato stems from non-treated plants
revealed a rapid and massive colonization of all tissues concomitant
with marked host cell disorganization. In contrast, fungal growth was
limited to the outermost tissues in oligandrin-treated plants and the invading hyphae appeared to be extensively disrupted. Host reactions included the plugging of intercellular spaces and the
occasional formation of wall appositions. Pathogen entrance in the
epidermis was associated with the deposition of an electron-dense
material in the intercellular spaces. Oligandrin probably does
not act directly on the invading fungus, but appears to elicit the
plant to produce fungitoxic compounds.
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The Segregation of Golgi...
How Onion Cells Stretch
