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On the Scent of a Flower |
The flowers of most
insect-pollinated species produce complex mixtures of volatiles that
give each species their unique fragrance. Floral fragrances belong to a
broad category of secondary metabolites comprised mainly of terpenoid,
benzenoid, and phenylpropanoid compounds. Despite their importance in
pollination biology, floral scents have received surprisingly scant
attention from plant physiologists and molecular biologists. In this
issue, Kolosova et al. (pp. 956-964) reveal that the forces
of selection that have crafted the exquisite macroscopic architecture
of flowers have been no less impressive at the
subcellular level. Their subject of study was
S-adenosyl-L-Met:benzoic acid
carboxyl methyl transferase (BAMT), the enzyme that yields
methylbenzoate, the benzenoid ester that is one of the most abundant
scent compounds in snapdragon (Antirrhinum majus). BAMT was
found to be immunolocalized predominantly in the conical cells of the
inner (adaxial) epidermis of snapdragon petal lobes (Fig.
1). The localization of scent production
to those flower surfaces upon which the pollinators alight is an architectural nuance that serves to increase pollination efficiency and
minimize the biosynthetic cost of advertising for pollinators.
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Figure 1.
Scanning electron micrograph of the conical,
scent-emitting cells of the adaxial epidermis of snapdragon
flowers.
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Calreticulin and Ca2+ Homeostasis |
Along with the vacuole, the endoplasmic reticulum (ER)
is one of the major intracellular storage sites for
Ca2+ ions in plant cells. It contains a variety
of Ca2+ binding proteins in its lumen,
including calreticulin. In other eukaryotes, calreticulin plays an
important role in regulating the activity of ER
Ca2+ fluxes. In this issue, Persson et al.
(pp. 1092-1104) report that the perturbation of calreticulin
levels affects ER Ca2+ in tobacco
(Nicotiana tabacum) suspension cells and Arabidopsis plants.
Calreticulin levels were selectively modulated by means of a heat
shock-inducible promoter coupled to maize (Zea mays) calreticulin cDNA. Heat shock-induced production of calreticulin led to
a significant increase in the accumulation of
Ca2+ in ER-enriched vesicles from tobacco
cells. Overexpression of calreticulin in intact Arabidopsis plants
was also found to enhance the tolerance of the plants to
Ca2+-deficient conditions. The evidence presented
supports the hypothesis that ERCa2+
is involved in maintaining Ca2+ homeostasis
in plant cells and that calreticulin plays a major role in the ability
of the ER to sequester Ca2+.
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Cholesterol Oxidase and the Control of Boll Weevils |
The cotton boll weevil (Anthomus grandis grandis) is
particularly difficult to control because its larvae feed and develop while safely sheltered from chemical sprays within the immature flower
buds or fruit (bolls) of cotton (Gossypium hirsutum).
Cholesterol oxidase is a bacterial enzyme that has potent insecticidal
activity against cotton boll weevils. Upon ingestion, this protein
causes developmental arrest and death of boll weevil larvae and a
decrease in the fecundity of adult female weevils. Unlike the
-endotoxin insecticidal proteins from Bacillus
thuringiensis, which have no known enzyme activity, cholesterol
oxidase is an insecticidal protein that has well-known enzymatic
properties. Because cholesterol oxidase can oxidize many plant sterols,
including sitosterol, stigmasterol, and campesterol, the production of
foreign cholesterol oxidase in plant cells could potentially result in
the oxidation of plant sterols and have detrimental effects on plant
growth and development. In this issue, Corbin et al. (pp.
1116-1128) transformed tobacco plants with the cholesterol
oxidase ChoM gene. Plants that were engineered to express ChoM
cytosolically demonstrated severe developmental abnormalities,
including a failure to produce floral buds. In contrast, plants that
expressed ChoM from a chloroplast-targeted vector were developmentally
normal (Fig. 2). In addition to its obvious applied interest, this
study also provides insights into the function
of phytosterols in plants.
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Figure 2.
Non-targeted expression of boll weevil-killing
cholesterol oxidase prevents flowering (left), whereas
chloroplast-targeted expression yields normal-looking plants (right;
control, middle).
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Double-Stranded RNA and RNA Silencing |
One of the major mechanisms that plants use to combat invading
viruses is RNA silencing. As a result of this process, the mRNAs of
invading viruses are recognized and degraded in a sequence-specific manner. A key step in RNA silencing is the formation of double-stranded RNA (dsRNA). In the case of most plant viruses, dsRNA is formed during
genome replication, and this may explain why viruses are often potent
inducers of RNA silencing. Unfortunately, plants also employ this same
defense in response to transfection by foreign RNA. This limits the
applicability of techniques such as the Agrobacterium tumefaciens delivery system for continuously expressing foreign DNA in plant cells. For example, under the control of the potent 35S
promoter, the gene for green fluorescent protein (GFP) is subject to
RNA silencing after an initial burst of gene expression in
Nicotiana benthamiana cells. In this issue, Johansen
and Carrington (pp. 930-938) report on the effects of an
artificial dsRNA construct on RNA silencing in N. benthamiana. The dsRNA construct was made from a 35S-GFP/antisense
GFP gene containing the full-length GFP coding sequence, an intron, and
a full-length GFP sequence in the inverted orientation. Transcription
of this gene and RNA processing yields an intron-spliced hairpin RNA
referred to as double-stranded GFP (dsGFP) RNA. When leaves of N. benthamiana were infiltrated with cultures of
Agrobacterium containing a vector harboring both the GFP and
dsGFP constructs, GFP fluorescence was abolished. Viruses, however,
have evolved mechanisms to circumvent RNA silencing. For example, the
tobacco etch virus RNA silencing suppressor P1/HC-Pro has been found to
suppress RNA silencing. In contrast to the lack of GFP fluorescence in
tissues injected with mixtures of Agrobacterium containing
GFP plus dsGFP genes, tissues infiltrated with the triple mixture
containing GFP, dsGFP, and P1/HC-Pro genes exhibited bright green
fluorescence. The authors propose that P1/HC-Pro may potentially
suppress the RNA silencing of other constructs and result in the
maintenance of high expression for extended periods. This will greatly
enhance the efficiency of the Agrobacterium delivery system
to introduce foreign genes into leaf tissue, and to interpret the
effects of their continued expression on metabolism.
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A Triple Mutant That Doesn't Flower Unless Vernalized |
Many mutations that delay the flowering of Arabidopsis have been
isolated, but none of them prevent flowering under all conditions. Genetic and physiological analyses have led to the idea that there are
three independent pathways that promote flowering
the long-day, autonomous, and gibberellin (GA)-dependent pathways. Mutations affecting the long-day pathway delay flowering under long but not short
days, whereas mutations affecting the autonomous pathway delay
flowering irrespective of photoperiod. Mutations affecting GA synthesis
delay flowering under long and short days, but have their strongest
effect under short days. In this issue, Reeves and Coupland (pp.
1085-1091) describe a triple mutant in which the activities of
the long-day, autonomous, and GA-dependent pathways are impaired. These
flowers do not flower under long or short days. Only after the plants
have been vernalized by extended exposure to low temperatures after
germination do they flower. This finding suggests that
vernalization-induced flowering probably employs a fourth pathway
different from those described above. The triple mutant can also be
used for comparison purposes in experiments comparing the time to
flowering in double mutants in which only one pathway is active.
Studies of such double mutants revealed that under long photoperiods,
the long-day pathway promoted flowering most effectively, whereas under
short periods the GA pathway had the strongest effect. The autonomous
pathway had a weak effect when acting alone under either photoperiod
but appeared to play a role in facilitating the promotion of flowering
by the other two pathways.
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Engineered Resistance to Sulfur Pollutants |
Pollution by sulfur-containing compounds, particularly gaseous
SO2, is a major environmental problem. In the
case of plants, the most common symptoms of SO2
toxicity are leaf chlorosis and necrosis. Although sulfur-containing
compounds such as SO2 are toxic to plants at high
concentrations, sulfur is also an essential nutrient for plants.
Inorganic sulfur is assimilated into plants by the Cys biosynthetic
pathway. In this issue, Noji et al. (pp. 973-980) explore
the possibility of genetically manipulating the Cys biosynthetic
pathway as a means to confer tolerance to sulfur-containing pollutants.
The final step in Cys biosynthesis is the joining of sulfide with
O-acetyl-L-Ser in a reaction catalyzed by Cys synthase, an enzyme found mostly in the cytoplasm and the chloroplasts. The authors constructed two transgenic tobacco lines with
enhanced expression of Cys synthase in the cytosol and the chloroplast,
respectively. As anticipated, the F1 progeny of
these two transgenic lines were highly tolerant to
SO2 and underwent an increase in Cys titer in
response to fumigation with SO2.
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On the Scent of...
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