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Winter Embolism and Alpine Tree Line Formation |
It is tough being a conifer
growing at the alpine timberline (Fig.
1). During winter, the freezing of the
ground and parts of the stem stops water influx completely. At the same
time, evaporative stresses become stronger because of intense radiation
and high wind speeds. The two dominant species of the European Central Alps timberline are Norway spruce (Picea abies L. Karst)
and stone pine (Pinus cembra). Stone pine, however,
generally reaches higher altitudes than Norway spruce, and in this
issue Mayr et al. (780-792) seek to answer the question,
"Why?". They report that xylem embolisms in winter were
observed only at the timberline and only in Norway spruce. Stone pine
managed to avoid achieving the critical water potentials that cause
cavitation by two adaptations. First, the cuticular conductance of
stone pine is 3.5-fold lower than that of Norway spruce. Second, the
angles between the needles and axes of stone pine decrease in
dehydrating branches. Both of these adaptations would be expected to
reduce winter water loss. The data presented support the idea that
winter embolisms are a major factor in determining tree line formation
for certain species.
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Figure 1.
The "treeline" of mountains may be a
reflection of threshold conditions for irreversible drought
stress.
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An Arabidopsis Hydrotropism Mutant |
The mechanisms underlying positive root hydrotropism (the
bending of roots toward increased moisture) are poorly understood. To
gain insight into this phenomenon, Eapen et al. (pp.
536-546) developed a screen for isolating Arabidopsis mutants
whose roots respond abnormally to a hydrotropic stimulation. This
system consists of a vertically oriented Petri dish that contains a
nutrient medium in the upper part, and the same nutrient medium
supplemented with glycerol and alginic acid in the lower part. In this
issue, Eapen et al. report on their isolation of a no hydrotropic
response (nhr) mutant of Arabidopsis. The roots of
wild-type Arabidopsis tend to bend away from the supplemented medium
that has the more negative water potential. In contrast, the roots of
the nrh mutant continue to grow downward seemingly unfazed
by the increasing negativity of the water potential gradient they
encounter. Heterozygous nhr1 seedlings were also
distinguishable from wild type by their faster and wavier root growth.
The finding that heterozygous nhr1 roots developed
significantly faster gravitropic responses emphasizes the point that
the perception of the hydrotropic stimulus is impaired in
nrh1, not the ability to bend tropically. Seedlings of the nhr1 mutant had abnormal root cap morphogenesis and reduced
root growth sensitivity to abscisic acid (ABA) and the polar auxin transport inhibitor N-(1-naphthyl)phtalamic acid
(NPA). Because the root caps of heterozygous
nhr1 roots had severe anomalies, the
authors suggest that NHR1 may control pattern formation in the root cap
in addition to its role in the perception of water potential gradients.
These results show that hydrotropism is amenable to genetic
analysis and that an ABA-signaling pathway participates in the
root cap's ability to sense water potential gradients.
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Phytoremediation of Methylmercury |
Methylmercury is an extremely toxic environmental pollutant
that tends to become biomagnified by several orders of magnitude in
long, aquatic food chains. It is also the primary source of human
mercury poisoning from the consumption of fish. In an effort to
detoxify methylmercury by phytoremediation, plants have been engineered
that express the bacterial mercury resistance enzymes organomercurial lyase MerB and mercuric ion reductase MerA.
MerB transforms methylmercury to ionic mercury Hg(II), and MerA
electrochemically reduces Hg(II) to the least toxic metallic mercury of
all, Hg(0). When the MerB and MerA enzymes are co-expressed in
transgenic plants, the coupled reaction has been found to transform
methylmercury to Hg(0). The rate at which transgenic plants perform
this process, however, is limited by the MerB-catalyzed reaction, even
though the cytoplasmic expression of MerB is high.
Conceivably, one cause of this problem may be that the hydrophobicity
of the organomercurial substrates prevents them from diffusing
efficiently to the cytoplasmically expressed MerB enzyme. In this
issue, Bizily et al. (pp. 463-471) examine the impact of
subcellular protein targeting on the efficacy of this phytoremediation
strategy. To optimize the reaction kinetics for organic mercury
compounds, the merB gene was engineered to target MerB for
accumulation in the endoplasmic reticulum (ER) and for secretion into
the cell wall. They report that plants expressing the targeted MerB
proteins and cytoplasmic MerA are highly resistant to organic mercury
and degrade organic mercury at rates 10 to 70 times higher than plants
with the cytoplasmically distributed wild-type MerB enzyme. MerB
protein in ER-targeted plants appears to accumulate in large vesicular
structures that can be visualized in immunolabeled plant cells. The
genetic engineering of wetland trees and aquatic plants expressing
bacterial enzymes that efficiently degraded methylmercury to Hg(II) or
Hg(0) could someday help lower the entrance of methylmercury into
aquatic food chains.
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A Maize (Zea mays) Inositol Phosphate Kinase
Mutant |
Phytic acid, myo-inositol
1,2,3,4,5,6-hexakisphosphate, is an abundant component of
plant seeds and is deposited in protein bodies as a mixed salt of
mineral cations. Typically, 50% to 80% of the P in seeds is found in
this compound. Because monogastric animals digest phytic acid poorly,
animal feed often has to be supplemented with inorganic phosphate (Pi).
Moreover, undigested phytic acid is eliminated and is a leading source
of P pollution. Low-phytic acid grain and legume in feed could reduce
both the amount of P supplementation required in animal feeds and the
amount of P pollution in the environment. In maize kernels, nearly 90% of the phytic acid is accumulated in embryos and about 10% in aleurone
layers. Low-phytic acid mutants have been used in genetic breeding, but
it is not known what genes are responsible for the low-phytic acid
phenotype. Shi et al. (pp. 507-515) report in this issue
that the maize low-phytic acid lpa2 mutant is caused by
mutation in an inositol phosphate kinase gene. The maize inositol phosphate kinase (ZmIpk) gene was identified through
sequence comparison with human and Arabidopsis Ins(1, 3, 4)
P3 5/6-kinase genes. The purified
recombinant ZmIpk protein catalyzes the phosphorylation of several
inositol polyphosphates, including Ins(1, 3, 4)
P3, Ins(3, 5, 6) P3, Ins(3,
4, 5, 6) P4, and Ins(1, 2, 5, 6)
P4. As expected, the ZmIpk mRNA is
expressed in the embryo. In the ZmIpk Mutator insertion
mutants, seed phytic acid content is reduced approximately 30%, and
inorganic phosphate is increased about 3-fold. The mutants also
accumulate myo-inositol and inositol phosphates. These
results provide evidence that ZmIpk is one of the kinases responsible
for phytic acid biosynthesis in developing maize seeds.
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Gene Expression in Autumn Leaves |
Deciduous trees that shed their leaves too early have lower
than optimal productivity, whereas trees that initiate the senescence process too late have insufficient time to recapture nutrients and
complete the requisite hardening process prior to the onset of winter.
Thus, from both ecological and biotechnological perspectives, understanding the factors that control the onset and course of leaf
senescence in the autumn is important. Bhalerao et al. (pp.
430-442) have embarked on a project to elucidate the genetic
basis of autumn senescence in aspen (Populus tremula × tremuloides) leaves. In this issue, they describe their
initial progress. Two cDNA libraries were prepared, one from leaves of a field-grown aspen tree, harvested just before any visible sign of
leaf senescence in the autumn, and one from young but fully expanded leaves of greenhouse-grown aspen. The patterns of gene expression suggest that even before there are any visible signs of leaf
senescence, there is a 10-fold decrease in plastid protein synthesis,
and that mitochondria apparently take over the chloroplast's role as
energy-generating organelles during this period. A strikingly high
fraction of expressed sequence tags in the autumn leaf library showed
no significant homology to any known protein in public databases,
although this could simply be a consequence of the fact that young,
green leaves have been more extensively studied than senescent leaves.
Overall, most of the metabolic characteristics previously reported for
senescing leaves (down-regulation of photosynthesis and up-regulation
of genes involved in protein, lipid, pigment degradation, and
respiration, as well as stress-related genes) were also found in aspen
autumn leaves. This confirms that the general pattern of metabolism is
the same in autumn leaves as in senescing leaves of annual plants.
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Engineering Vitamin E Content in Arabidopsis |
Tocopherols (vitamin E) are a class of lipid-soluble
antioxidants synthesized exclusively by photosynthetic organisms. By their antioxidant activities, dietary tocopherols improve immune function and limit the incidence and progression of several
degenerative human diseases, including certain types of cancer,
cataracts, neurological disorders, and cardiovascular disease. Despite
its health benefits, 
-toco-pherol is limited in the average
American diet. Homogentisate phytyltransferase (HPT), which catalyzes
the committed step of tocopherol biosynthesis in plants, has recently been cloned and characterized. In this issue, Collakova and DellaPenna (pp. 632-642) report on the effects of the
overexpression of a gene coding for HPT (HPT1) in different
tissues of Arabidopsis on the titer of total tocopherols. In leaves,
HPT1 overexpression resulted in a 4.4-fold increase in total
tocopherol content relative to wild type. In seeds, HPT1
overexpression resulted in a total seed tocopherol content that was
40% higher than wild type, primarily because of an increase in
-tocopherol content. This enlarged pool of -tocopherol was almost
entirely converted to -tocopherol by crossing HPT1
overexpressing plants with lines constitutively overexpressing
-tocopherol methyltransferase. Seeds of the resulting double
overexpressing lines had a 12-fold increase in vitamin E activity
relative to wild type. These results indicate that HPT activity is
limiting in various Arabidopsis tissues and that total tocopherol
levels and vitamin E activity can be elevated in leaves and seeds by
the combined overexpression of the HPT1 and
-tocopherol methyltransferase genes.
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Winter Embolism and Alpine...
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