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Plasmodesmata: Diverse Forms and Functions |
Plasmodesmata are dynamic
structures with a diverse repertoire of forms and functions. For
example, the size of the plasmodesmatal annulus can undergo dramatic
changes in state, sometimes being completely shut or slightly open, and
at other times being so dilated as to allow the passage of large
macromolecules (e.g. proteins) and viruses. It is the movement of these
large macromolecules that is the focus of Crawford and
Zambryski's (pp. 1802-1812) contribution in this issue. These
authors distinguish between two types of symplasmic protein transport:
targeted and non-targeted. Non-targeted transport refers simply to the
diffusion of proteins through highly dilated plasmodesmata. In targeted
transport, however, the protein interacts directly with plasmodesmata
and facilitates its own movement through them. Viral movement proteins
are examples of proteins that undergo such targeted transport. The
authors used a quantitative low-pressure biolistic assay to examine the effect of leaf age and plant growth conditions on these two modes of
protein movement in tobacco (Nicotiana tabacum) leaves.
Green fluorescent protein (GFP) was used as a tracer of non-targeted protein movement. A construct of GFP and
P30 (GFP::P30), a viral movement protein of tobacco mosaic
virus, was used to study targeted movement (Fig. 1). Leaf age
dramatically reduced the ability of cells to exchange GFP, and
greenhouse-grown plants exhibited higher diffusion rates of GFP than
did culture-grown plants. In marked contrast, GFP::P30
movement was not noticeably affected by age or growth conditions. These
results underscore the fundamental differences between targeted and
non-targeted protein transport.
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Figure 1.
A complex of GFP and a viral movement protein of
tobacco mosaic virus moves easily between adjacent cells in the sink
tissue of tobacco leaves.
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Partial Expression of a Herbicide Resistance Gene Expressed in
Plastid DNA |
Perhaps the most valid concern raised by critics of plant
biotechnology is the possibility that crop-to-weed transgene escape might occur in the field. It is conceivable that the pollen of a
cultivar genetically engineered to have a herbicide-resistance gene
could land upon the stigma of a closely related weed and by
hybridization lead to the inadvertent creation of a "super-weed" that would be extremely difficult to control. A possible strategy to prevent transgene escape is to insert transgenes into the
plastid genome of crop species. Since plastid DNA is maternally
inherited, crop-to-weed transgene escape through pollen could not
occur. In this issue, Lutz et al. (pp. 1585-1590) report
upon their success in expressing a bacterial bar gene in
tobacco plastids that confers field-level tolerance to a commercial
herbicide containing phosphinothricin (Fig.
2). Crosses between the transplastomic plants and wild type confirmed that there was no pollen transmission of
plastid DNA. Horizontal gene transfer between transgenic plants and
soil or epiphytic microbes and enteric bacteria is also a topic of
concern. The authors, therefore, created a synthetic bar
gene that was optimal for plastid gene expression, but which had a high
frequency of those triplets (AGA and AGG) which are rare in
Escherichia coli and for which E. coli has few
tRNAs. As expected, the expression of the synthetic bar gene
in E. coli was greatly reduced compared to the expression of
the non-modified bar gene. These results exemplify how codon
modification can be used to reduce unwanted gene expression in
potential secondary hosts.
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Of Spectrin, Spitzenkörpers, and Space
Shuttles |
The tip-growing protonemata and rhizoids of mosses and characean
cells are becoming increasingly popular as model systems for
visualizing the cytological changes accompanying gravisensing and
gravitropism. In this issue, Braun (pp. 1611-1619) reports
upon the changes in spectrin distribution during gravitropic curvatures
in the rhizoids and protonemata of Chara globularis. Spectrin labeling is associated with a distinct actin-organized, endoplasmic reticulum membrane aggregate called the Spitzenkörper (apical body), a spherical clear zone occurring in the elongating rhizoids and protonemata of Chara. The position of the
spectrin aggregate was unchanged during the positive gravitropic
curvature that occurs in rhizoids, but was displaced to the upper flank during the negative gravitropic response of protonemata (Fig. 3). Braun proposes that spectrin and
Spitzenkörpers may play a role in controlling
Ca2+ homeostasis and regulating the oppositely
directed gravitropic tip growth in rhizoids and protonemata. Although
the cytological position of Spitzenkörpers and spectrin in
protonemata is altered by gravitropic stimulation, it is the amylopasts
that act as statoliths, and this is the topic of a second contribution
in this issue by Kern et al. (pp. 2085-2094). These authors
note that plastids generally do not sediment in response to
gravity in most cells, presumably because of attachments to the
cytoskeleton. Gravity-sensing cells are exceptional in this regard.
Kern et al. offer a detailed description of the complex plastid
zonation that occurs in dark-grown protonemata of the moss
Ceratodon purpureus. Only some amyloplasts sediment along
the length of the tip cell. If gravity is the main determinant of the
position of these amyloplasts in this gravisensing zone, then in
microgravity the amyloplasts of this area should be randomly
distributed. Instead, amyloplasts became clustered in a subapical
region in both clinostated cells and in those flown in the space
shuttle Columbia. These findings indicate the existence of
as yet unknown endogenous forces and mechanisms that influence amyloplast position and which are normally masked under conditions of
normal gravity.
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Isoprene: Why do Plants Pollute? |
When President Reagan remarked in 1981 that "trees
cause more pollution than automobiles do," he probably had isoprene
in mind. Many plants, especially trees, release isoprene in great amounts, as much as 500 Tg per year globally. Crop species, however, are generally poor emitters of isoprene and contribute very little to
this total. It has been hypothesized that isoprene production helps
plants cope with the deleterious effects of stressful high temperatures
on photosynthesis, a problem that is particularly acute for
broad-leaved trees that have their foliage high up in the canopy. Until
now, it was not possible to test the thermotolerance hypothesis
directly because researchers could not control endogenous isoprene
synthesis simply. In this issue, Sharkey et al. (pp. 2001-2006) take advantage of fosidomycin, a new and specific chemical inhibitor of isoprene production, to study the effects of
isoprene emission on thermotolerance. Consistent with their hypothesis,
fosidomycin blocked isoprene emission and lowered the
thermotolerance of kudzu (Pueraria lobata) and red
oak (Quercus rubra) leaves. The application of exogenous
isoprene to fosidomycin-treated leaves restored their
thermotolerance. Isoprene also increased thermotolerance in bean
(Phaseolus vulgaris), a plant that does not emit isoprene.
The authors propose several mechanisms by which the intercalation of
isoprene molecules into the bulk phase of thylakoid membranes may lead
to increases in thermotolerance.
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Mechanism of Free Amino Acid Accumulation in Maize
opaque-2 Mutants |
Maize (Zea mays) varieties are generally low
in the essential amino acid Lys. The opaque-2
(o2) mutation nearly doubles the Lys content in maize
endosperm. Despite much research, the mechanism by which the
o2 mutation increases the Lys content in maize endosperm is
only partially understood. Certainly, there is a large increase in the
ratio of Lys-rich non-zein storage proteins to Lys-poor zein storage
proteins, but this is only part of the story. The levels of free amino
acids, particularly those derived from the Asp pathway, are also
enhanced in o2 mutants. In this issue, Wang and
Larkins (pp. 1766-1777) report upon the results of a genetic analysis aimed at elucidating the biochemistry underlying the increase
in free amino acids in o2 mutants. Four quantitative trait
loci were identified that accounted for about half of the phenotypic
variance between different types of o2 mutants. One of these
loci was associated with an Asp kinase located on the long arm of
chromosome 2. In a companion paper, Wang et al. (pp.
1778-1787) demonstrate that in one type of o2 mutant that had especially high levels of free amino acids, this particular species of Asp kinase was less sensitive to feedback inhibition by Lys.
These results indicate that the O2 gene encodes for a transcription factor that not only regulates zein gene expression, but
also has diverse effects on carbon and amino acid metabolism.
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Plasmodesmata: Diverse Forms...
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