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Frog Skin Antimicrobial Peptide Gene Confers Broad Spectrum
Disease Resistance |
Magainan 2 is an antimicrobial
peptide secreted from the skin of the African clawed frog
(Xenopus laevis). In this issue, DeGray et al.
(pp. 852-862) report that the expression of MSI-99, an analog of
magainan 2, in the chloroplast genome of tobacco (Nicotiana
tabacum), confers up to 95% resistance to the pathogens
Pseudomonas syringa pv tabaci, Aspergillus
flavus, Fusarium monoliforme Verticillium dahliae, and
Colletotrichum destructivum. The accumulation of MSI-99 in
transgenic chloroplasts did not affect normal growth and development.
The authors speculate that because the lipids of chloroplast membranes
are principally neutral, and because the lytic mode of action of MSI-99
requires phospholipids, the chloroplast may be a safe compartment for
containing MSI-99 in healthy plants. Only after cell damage occurs is
the MSI-99 released in sufficient amounts to induce the rapid necrosis of the infected area. Genetically engineering crop plants for disease
resistance via the chloroplast genome instead of the nuclear genome is
also desirable insofar as the high levels of expression needed for
action are achieved and transgene escape via pollen is prevented.
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Dark-Induced Leaf Senescence Is Cell Autonomous |
As in many species, leaf senescence in Arabidopsis can be
induced within 2 d by detaching leaves and placing them in
darkness. When entire Arabidopsis plants are placed into darkness,
however, their leaves do not senesce even after 5 d. In this
issue, Weaver and Amasino (pp. 876-886) report that
when leaves of intact Arabidopsis are made dark by covering with
"mittens", they also shown signs of senescence within 2 d. The
phenomenon is leaf age-dependent, occurring more rapidly and strongly
in older leaves. When, however, a hole is punched in the mittens, the
light-exposed portions remain viable (Fig.
1): This suggests that dark-induced leaf
senescence in intact Arabidopsis plants is cell-autonomous.
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Figure 1.
A demonstration of cell-autonomous
leaf senescence in intact Arabidopsis plants. Note that the portions
exposed to light through the hole punched through the black mittens (A)
remain viable 5 d later (B).
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Seed Coat Mucilage Mutants |
Fertilization of the angiosperm ovule not only results in the
development of the embryo and endosperm, but also initiates differentiation of the ovule integuments to form the seed coat. In the
Brassicaceae, the epidermal cells of the seed coat produce large
quantities of pectic polysaccharide (mucilage), which facilitate seed
hydration and possibly dispersal. Differentiation of the outer
integument epidermal cells to form the seed mucilage cells involves a
highly regulated series of events, including growth, morphogenesis,
mucilage biosynthesis and secretion, and secondary cell wall
production. In this issue, Western et al. (pp. 998-1011) report on their isolation of a variety of seed mucilage mutants in
Arabidopsis using a screen that employs Ruthenium red to stain the
mucilage (Fig. 2). The authors report on
their identification of five gene mutations (mum1-5) that
effect mucilage-modified phenotypes. These genes
include ones that regulate seed coat initiation, influence cell
morphogenesis, and are required for normal mucilage synthesis.
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Figure 2.
Ruthenium red staining of the seed coat mucilage
of developing Arabidopsis seeds provides a useful screen for
identifying mutants in seed coat form and function. Here, a
wild-type Arabidopsis (Col-2 ecotype) has been stained 10 d after
pollination.
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Auxin Metabolism and a Peroxisome Transport Mutant |
Peroxisomes are important organelles in plant
metabolism, containing all of the enzymes required for fatty acid
-oxidation. More than 20 proteins are required for peroxisomal
biogenesis and maintenance. Peroxisomes must import the cofactors and
substrates, such as fatty acids, required for the biochemical processes
they perform. Yeast (Saccharomyces cerevisiae)
and humans apparently transport long-chain fatty acids into peroxisomes
via ATP-binding cassette-containing ATPases in the peroxisomal
membrane. Using a map-based positional approach, Zolman et al.
(pp. 1266-1278) report on their discovery of a putative
peroxisomal ATP-binding cassette transporter that is 42%
identical to the human adrenoleukodystrophy protein, which is defective in patients with the demyelinating disorder
called X-linked adrenoleukodystrophy. The isolation of this putative
peroxisomal ATP-binding cassette transporter arose from a screen for
Arabidopsis mutants that were resistant to the natural auxin
indole-3-butyric acid (IBA). Because IBA is converted to the more
abundant indole-3-acetic acid, in a mechanism that parallels
-oxidation (i.e. the removal of two carbons from the IBA
side chain), the mutant (pxa1) may be IBA resistant because it cannot convert IBA to indole-3-acetic acid. The authors propose that
PXA1 may function in planta in the import of fatty acids and IBA into
the peroxisome for -oxidation. Another phenotypical attribute of the
pxa1 mutant that is consistent with it being defective in
-oxidation is its inability to grow without the presence of
supplemental Suc.
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Insights into the Synthesis of Phyto-sulfokines, a
Class of Peptide Growth Factors |
There is evidence accumulating on many fronts in plant biology
that various short peptides regulate a diversity of functions in
plants. Although peptide growth factors are well known to contribute to
cell cycle control in animal cells, only one class of mitogenic oligopeptides, the phytosulfokines (PSKs), has been identified in
plants so far. PSKs were originally isolated from conditioned medium of
asparagus (Asparagus officinalis) cultures. One type of
PSK, notably PSK-
, has been implicated in a variety of processes in
plants, including chlorophyll synthesis, growth, tracheary differentiation, somatic embryogenesis, and adventitious root and bud
formation. In this issue, Yang et al. (pp. 842-851) report
on their identification of two genes in Arabidopsis, AtPSK2 and AtPSK3, that encode for precursors of PSK. The
overexpression of either gene allowed the transgenic cells to divide 2 times faster than controls. Transgenic cells that expressed either of the antisense cDNAs did not dramatically decrease mitogenic activity, suggesting that these two genes may act redundantly. A comparison of
the amino acid sequences of the respective PSK precursors of Arabidopsis and rice (Oryza sativa) reveals that they do not
share significant similarity throughout, although the PSK- sequence and the three amino acids preceding it are perfectly conserved.
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Non-Coding RNAs in Arabidopsis |
Some genes encode RNAs rather than proteins as their final
products. Recently, it has become clear that in addition to tRNA and
rRNA, other non-coding RNAs (ncRNAs) exist that exert their action
mainly or exclusively at the RNA level. Only a few ncRNAs have been
reported in plants, but accumulating evidence suggests that their
expression may be regulated by a variety of stress or developmental
conditions, including cytokinins and phosphate starvation. In addition,
evidence from non-plant systems suggests a possible role for ncRNAs in
chromosomal silencing and transcriptional regulation. Current
strategies for genome annotation, although efficient in the
identification of protein-coding genes, rarely detect these ncRNA genes
due to the lack of significant open reading frames. MacIntosh et
al. (pp. 765-776) have begun a systematic sequence analysis for
ncRNAs in Arabidopsis as a first step toward elucidating their
functions. They examined Arabidopsis for the presence of ncRNAs
found in other kingdoms and collected and reanalyzed potential Arabidopsis ncRNAs reported previously. As a further screen,
they used computational tools that filtered out protein-coding genes
from the genomic sequences of 20,000 Arabidopsis expressed sequence
tags, thereby enabling them to identify sequences that exhibit
characteristics of ncRNAs. Their results indicate that there is a
significant number of ncRNAs in Arabidopsis, perhaps as much as 2% or
3% of all genes, as is apparently the case in S. cerevisiae. None of the specific clones that were identified were
found to have homologs outside the plant kingdom, indicating that most
Arabidopsis ncRNAs are likely to be plant specific.
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Frog Skin Antimicrobial Peptide...
Dark-Induced Leaf Senescence Is...