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Variable NaCl Tolerance in Arabidopsis Accessions |
With the aim of identifying loci
that control natural variations in salt tolerance during germination
and vegetative growth, Quesada et al. (pp. 951-963)
analyzed the natural variations in NaCl tolerance of 102 Arabidopsis
accessions. They found that the ability to germinate under saline
conditions varied widely between accessions. The most salt-tolerant
accessions were crossed with the most salt-sensitive ones to ascertain
whether their salt tolerance at germination is a monogenic trait.
Genetic analysis suggested that the salt tolerance during germination
was under polygenic control. The responses of various accessions to
salt stress during germination and vegetative growth, respectively, indicated that the most tolerant accessions to NaCl at germination were
the most sensitive to this salt during vegetative growth. This suggests
that the genetic controls underlying NaCl tolerance in Arabidopsis are
different at various points in development. Genomic regions involved in
the responses to NaCl at germination and during vegetative growth were
also identified. They detected 11 quantitative trait loci (QTL)
harboring naturally occurring alleles that contribute to natural
variation in NaCl tolerance in Arabidopsis, six at the germination and
five at the vegetative growth stages, respectively. At least five of
these QTL are likely to represent new loci not yet described by their
relationships with salt tolerance. The map positions of the QTL
detected for germination were not coincident with those obtained for
the QTL involved in salt response during vegetative growth, consistent with idea that the mechanisms controlling salt tolerance at both stages
are different.
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Endoplasmic Microtubules in Legume Root Hairs |
In addition to the cortical microtubules (CMTs) that the root
hairs of all species have, legume root hairs also possess endoplasmic microtubules (EMTs). To investigate the configuration and function of
EMTs and CMTs in leguminous root hairs, Sieberer et al. (977-988) made use of green fluorescent protein (GFP) technology to visualize MTs during the development of living root hairs of Medicago truncatula (Fig. 1).
CMTs were present in all stages of hair development in stage-specific
configurations, but EMTs were only present in the subapical region of
vigorously growing root hairs. The MT-depolymerizing drug oryzalin (1 µM) slowed the growth rate of the root hairs
and caused a dramatic change in the cytoarchitecture of the subapex:
EMTs but not CMTs depolymerized, the subapical cytoplasm itself became
shorter, and the nucleus lagged behind the advancing tip. Taxol (1 µM) affected neither the cytoarchitecture, nor
the position of the nucleus, nor the configuration of EMTs, but did
reduce the growth rate of the root hairs by 60%. Conceivably, the
presence of EMTs in legume root hairs may be related to the symbiotic
relationships that legumes form with rhizobial bacteria. The authors
speculate that EMTs may be important for the curling of root hairs
around rhizobia and for infection thread formation.
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Figure 1.
Legume root hairs have unique endoplasmic
microtubules, most prevalent in the subapical cytoplasm, in addition to
cortical microtubules such as those found in the roots hairs of other
species.
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Activation of Phospholipases C and D during Cold
Exposure |
Ruelland et al. (pp. 999-1007) provide evidence
that a drop in temperature induces the simultaneous activation of
phospholipases C and D in Arabidopsis suspension cells. When the
temperature was lowered to 0°C, the quantity of phosphatidic acid
(PA), a minor phospholipid in non-treated cells (less than 1% of the
total lipids), rose up to reach about 9% of total phospholipids after 10 min, then decreased slowly and still represented about 6% of total
phospholipids after 140 min. PA could be synthesized by two pathways:
either directly by the action of a phospholipase D (PLD) or by the
combined action of a phospholipase C (PLC) that produces
diacylglycerol, followed by the action of a diacylglycerol kinase. An
investigation of which one of these pathways was activated by cold
revealed that PLC activity accounts for 80% of the formation of
cold-induced PA. In cells subjected to 0°C, the maximum inositol trisphosphate level was quickly attained in less than 2 min, then diminished progressively. Inhibitors of phosphoinositide signaling prevented this activation. PLD activation was also a fast phenomenon, taking place immediately after the temperature drop. The addition of
chemical reagents modifying Ca2+ availability
(EGTA or La3+) inhibited the formation of PA,
showing that the cold-induced activation of both phospholipase pathways
was dependent on Ca2+ entry. The fatty-acid
composition of the cold-generated PA was also determined and found to
be predominantly saturated, consistent with the majority of its
production being related to phosphoinositide turnover.
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Transcription Profiling of the Early Gravitropic
Response |
Moseyko et al. (pp. 720-728) examined the
expression of 8,300 genes during early stages of the gravitropic
response of Arabidopsis using high-density oligonucleotide probe
microarrays. Approximately 1.7% of the genes represented on the
array exhibited significant expression changes within the first 30 min of gravity stimulation. A general trend that emerged was that the
majority of the gravitropism-affected genes were down-regulated
15 min after treatment, whereas a majority of them were up-regulated after 30 min. Most of the gravity-regulated genes belonged to the
following functional categories: oxidative stress/plant defense, metabolism, transcription, cell wall/plasma membrane, signal
transduction, heat shock proteins, ethylene-responsive
element-binding factors, and calcium-binding proteins. At
present, 28% of the identified genes have no functional assignment.
Surprisingly, the oxidative burst/plant defense group was the largest
functional category of the gravity-regulated genes. Several
ethylene-responsive element-binding factors significantly changed
their expression levels after gravistimulation, supporting the idea
that ethylene is involved in the early gravitropic response. The
authors also identified several potential cis-regulatory elements of
the gravity-induced genes using a computational approach. They found
that 40 genes, which were up-regulated 30 min after gravistimulation
and which belonged to the same expression pattern cluster, have common
promoter motifs. About 39% of the gravity-regulated genes were also
regulated by a gentle mechanical perturbation, emphasizing the
interplay between the gravitropic and mechanical responses, and the
extreme sensitivity of plants to even very gentle mechanical perturbations.
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Dissecting Intron-Mediated Enhancement |
Most plant genes contain intervening sequences (introns) that
are transcribed into pre-mRNA and later removed by splicing. Introns,
however, are more than just "genomic chaff": Some, but not all,
introns enhance the expression of specific protein-coding sequences
when placed in the 5' region of the transcription unit. Such
intron-mediated enhancement is particularly pronounced in monocots
where it can lead to as much as a 100-fold increase in transcription
rate (compared with 2- to 10-fold for dicots). One of the most
effective plant introns in stimulating gene expression is the 1,028-bp
first intron of the Sh1 gene that encodes maize (Zea mays) Suc synthase. To address the mechanisms of
intron-mediated enhancement, Clancy and Hannah (pp.
918-929) used reporter gene fusions to identify features of the
Sh1 first intron required for enhancement in cultured maize
cells. A 145-bp derivative conferred approximately the same 20- to
50-fold stimulation typical for the full-length intron in this
transient expression system. A 35-bp motif contained within the intron
is required for maximum levels of enhancement but not for efficient
transcript splicing. The important feature of this redundant 35-bp
motif is T-richness rather than the specific sequence. When transcript
splicing was abolished by mutations at the intron borders, enhancement
was reduced to about 2-fold. The requirement of splicing for
enhancement was not due to upstream translation initiation codons
contained in unspliced transcripts. Based on these findings, the
authors propose that the splicing of the Sh1 intron is
integral to enhancement, and they hypothesize that transcript
modifications triggered by the T-rich motif and splicing may increase
transport, maturation, stability, and/or translatability of the mRNA.
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A Dicer Homolog in Plant Development |
The importance of maternal cells in controlling early
embryogenesis in plants is poorly understood. Previously, the maternal activity of the SIN1 (SHORT INTEGUMENTS1)
gene of Arabidopsis was shown to be essential for embryo pattern
formation and viability, and its postembryonic activity for several
processes in reproductive development, including flowering time control
and ovule morphogenesis. Golden et al. (pp. 808-822) report
the cloning of SIN1 and demonstrate its identity to the
CAF (CARPEL FACTORY) gene important for normal
flower morphogenesis and to the SUS1 (SUSPENSOR1) gene essential for embryogenesis.
SIN1/SUS1/CAF has sequence similarity to the Drosophila melanogaster gene Dicer, which
encodes a multidomain ribonuclease specific for double-stranded RNA.
The Dicer protein is essential for temporal control of development in
animals, through the processing of small RNA hairpins that in turn
inhibit the translation of target mRNAs. Structural modeling of the
wild-type and sin1 mutant proteins indicates that the RNA
helicase domain of SIN1/SUS1/CAF is functionally important. The
mRNA was detected in floral meristems, ovules, and early embryos,
consistent with the mutant phenotypes. A 3.3-kb region at the 5' end of
the SIN1/SUS1/CAF gene shows
asymmetric parent-of-origin activity in the embryo: It confers
transcriptional activation of a reporter gene in early embryos only
when transmitted through the maternal gamete. Recent studies have
implicated dsRNA molecules in transcriptional repression of transgenic
promoter sequences in plants. Plants may use small dsRNA hairpins (or
their cleaved products) as developmental regulators over long distances
in much the same way that dsRNA fragments of RNA viral genomes induce
systemic signaling for defense against viral pathogens. Movement of a
target RNA from the maternal sporophyte into the developing embryo
could explain the role of the sporophyte in embryogenesis.
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