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On the Inside
Sex Differences in Plastid
DNA Integration into
the Nucleus
The plastid (chloroplast) genome of
higher plants has been reduced to
approximately 130 genes, while its
cyanobacterial ancestor is estimated to
have contained more than 3,000 genes.
Thus, many of the ancestral endosymbiont genes were either lost or transferred to the nucleus during evolution.
Indeed, thousands of functional nuclear genes in Arabidopsis (Arabidopsis
thaliana) are derived from the endosymbiont genome. Sheppard et al.
(pp. 328–336) examine the question
of whether there are sex-specific differences in the rates of plastid DNA
relocation. Male gametes of most
angiosperm species undergo a programmed elimination of plastids during pollen development, a process that
underpins maternal inheritance of organellar genes. The authors have monitored the cellular location of a kanamycin
resistance gene tailored for nuclear expression (35SneoSTLS2) in the progeny of
reciprocal crosses of tobacco (Nicotiana
tabacum) in which, at the start of the
experiments, the reporter gene was
confined either to the male or the female
parental plastid genome. Among
146,000 progeny from crosses where
the transplastomic parent was male, 13
transposition events were identified,
whereas only one atypical transposition
was identified in a screen of 273,000
transplastomic ovules. Thus, the transfer of plastid DNA to the nucleus takes
place far more frequently in the male
germline than in the female germline.
Some plant biotechnologists have advocated the placement of transgenes in
the plastid genome to prevent transgene escape through pollen dispersal.
This study demonstrates that the frequency of plastid transgene relocation
to the nucleus in the male germline is at
least an order of magnitude higher than
in the female germline. Hence, plastid
transgenesis alone does not provide
complete transgene containment in tobacco, and additional safeguards may
be necessary to eliminate all possibility
of transgene escape.
www.plantphysiol.org/cgi/doi/
10.1104/pp.104.900270
Pathogen-specific defense is mediated by Resistance (R) proteins, which
recognize pathogenic effector molecules. The activation of R proteins
initiates signaling events that usually
culminate in programmed death of cells
at the site of infection and containment
of the invading pathogen (the hypersensitive response). Most known R
proteins belong to the NBS-LRR class,
with carboxyl-terminal Leu-rich repeats (LRRs) and a central nucleotide
binding site (NBS) domain. Although
many R genes have been cloned, the
signaling events downstream of
R-protein activation remain elusive.
The proper localization of defense signaling components and their interaction
with other proteins are imperative for
successful defense responses, and these
often depend on posttranslational modifications. To search for additional components required for R-protein signaling,
Goritschnig et al. (pp. 348–357) took
advantage of the plant autoimmune
model suppressor of npr1 constitutive1
(snc1), a gain-of-function allele of an
NBS-LRR R gene. In addition to having
constitutive resistance against virulent
bacterial and oomycete pathogens, the
snc1 mutant also displays increased
levels of endogenous salicylic acid and
constitutive expression of PR genes. In
a screen for modifiers of snc1 (MOS),
the authors have identified mos8, a
genetic suppressor of snc1. Interestingly,
mos8 turns out to be a novel allele of
ERA1 (for ENHANCED RESPONSE TO
ABSCISIC ACID1), which encodes the
protein farnesyltransferase b-subunit.
Protein farnesylation involves attachment
of C15-prenyl residues to the carboxyl
termini of specific target proteins, and has
been shown to be important in development and hormonal responses. mos8
affects basal resistance against virulent
pathogens as well as some R-proteinmediated resistance responses. Thus,
the farnesylation of proteins in response
to biotic stresses adds another layer of
complexity to the signaling network of
plant innate immunity.
nucleotides long!) infectious agents in
plants. The mechanism(s) by which
these pathogenic RNAs interact with
hosts to induce disease symptoms is
uncertain. The discovery of small RNAs
that regulate host and nonhost gene
expression in eukaryotes has led to a
new hypothesis of viroid pathogenesis
that is based on RNA silencing. RNA
silencing is a sequence-specific RNAinactivation mechanism in eukaryotes
that guides chromatin modification and
gene regulation. Many small RNA
pathways in plants have a requirement
for RDR6, an RNA-dependent RNA
polymerase. Gómez et al. (pp. 414–423)
reasoned that if the symptoms following viroid infection are mediated by
RNA silencing, then the following predictions should be fulfilled: (1) the
inhibition of the RNA silencing activity
should be associated with a decrease in
the severity of symptoms, and (2)
infected plants with deficiencies in the
RNA silencing pathway would either
be asymptomatic or show a reduction
in the severity of symptoms. In an
attempt to validate these predictions,
they used a symptomatic transgenic
line of tobacco (Nicotiana benthamiana)
that expresses and processes dimeric
forms of Hop stunt viroid (HSVd) into
the biologically active monomeric circular and linear forms. The HSVdexpressing tobacco plants were used as
stocks in grafting assays with an rdr6i
line, in which RDR6 is constitutively
silenced. The authors demonstrate that
the expression of symptoms in tobacco
plants is independent of HSVd accumulation levels, but dependent on an
active state of the viroid-specific RNA
silencing pathway. The scion of rdr6i
plants remained asymptomatic when
grafted onto symptomatic plants, despite an accumulation of a high level of
mature forms of HSVd, indicating the
requirement of RDR6 for the viroidinduced symptom production. Thus,
the symptoms induced by HSVd infection in tobacco are dependent on RDR6
activity, a key component of diverse RNA
silencing pathways, and their severity
is associated with efficient viroidspecific RNA silencing activity.
Viroids and RNA Silencing
Proteomics of Seed Aging
Self-replicating RNAs known as viroids are the smallest (only 246 to 401
Seed aging refers to the progressive
loss of germination vigor over time.
Protein Farnesylation and
Plant Innate Immunity
Plant Physiology, September 2008, Vol. 148, pp. 1–2, www.plantphysiol.org Ó 2008 American Society of Plant Biologists
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Seed aging is of both ecological and
agricultural interest, and is especially
troublesome to programs devoted to
the preservation of germplasm. Previous
genetic studies have revealed that seed
longevity is a multigenic trait. In this
issue, Rajjou et al. (pp. 620–641) relate
the results of proteomic studies aimed at
elucidating the molecular biology underlying seed aging in Arabidopsis. The
authors used a hydration/dehydration
protocol known as controlled deterioration that is widely believed to mimic
natural aging. Germination tests showed
a progressive decrease of germination
vigor depending on the duration of
controlled deterioration. Proteomic analyses revealed that seed deterioration
was accompanied by a massive increase
in carbonylated proteins. There were
also decreases in dehydrins and other
desiccation-associated proteins as well
as increases in certain glycolytic enzymes. The levels of b-mercaptopyruvate
sulfurtransferase, an enzyme that functions in cyanide detoxification, also
decreased. This finding suggests a possible role for sulfur metabolism in the
maintenance of seed vigor. The authors
conclude that that the essential mechanisms for seed vigor include translational capacity, the ability to mobilize
seed storage reserves in a controlled
manner, and detoxification efficiency.
Leaf Development in a
C4 Species Lacking
Kranz Anatomy
C4 photosynthesis was once thought
to be inextractably linked to Kranz
anatomy, a morphology in which leaf
chlorenchyma is separated into mesophyll
cells containing phosphoenolpyruvate
carboxylase (PEPC) and into bundle
sheath cells containing Rubisco. This
concept was turned on its head when
three succulent species in the family
Chenopodiaceae, including Bienertia
sinuspersici, were shown to have a
unique photosynthetic mechanism in
which C4 photosynthesis occurs within
individual photosynthetic cells. This
single-celled C4 photosynthesis is possible because B. sinuspersici develops
two cytoplasmic compartments in its
chlorenchyma cells: a large central cytoplasmic compartment packed with
chloroplasts and mitochondria, and a
peripheral layer of cytoplasm with
chloroplasts. The peripheral cytoplasm
is analogous to the mesophyll cells of
typical C4 plants. Atmospheric CO2,
upon entry into chlorenchyma cells,
is incorporated into the C4 acid oxaloacetate by PEPC in the peripheral
cytoplasm. The central cytoplasmic
compartment, on the other hand, is
analogous to the bundle sheath cell of
typical C4 plants. Rubisco, localized
exclusively in the central cytoplasmic
compartment, fixes the CO2 released by
the organic acids produced by PEPC.
The studies of Lara et al. (pp. 593–610)
shed light on the timing and pattern of
expression of photosynthetic genes in
B. sinuspersici as well as the stability of
its unusual cytoplasmic architecture.
They report that Rubisco subunits, and
enzymes of the glycolate pathway, accumulate more rapidly than enzymes
associated with the C4 cycle, suggesting
a progressive development of C4 photosynthesis. They also report that when
branches containing mature leaves
were enclosed in darkness for one month,
the cytoplasmic spatial domains were
maintained.
Insights into the Function
of an Outer Chloroplast
Membrane Protein
The Omp85 (Outer membrane protein, 85 kD) superfamily of b-barrel
proteins is found in chloroplasts, mitochondria, and Gram-negative bacteria.
The Omp85 proteins in bacteria and
mitochondria mediate biogenesis of
other b-barrel proteins, and are indispensable for viability. Chloroplasts
contain at least two distinct types
of Omp85 homologs, namely, Toc75
(Translocon at the outer envelope
membrane of chloroplasts, 75 kD) and
OEP80 (Outer Envelope Protein, 80
kD). Since both homologs exist in
extant cyanobacteria, these proteins
presumably arose from a common ancestor in the original endosymbiont.
Indeed, detailed phylogenetic analyses
suggested that Toc75 and OEP80 diverged early in the evolution of chloroplasts. Toc75 functions as a preprotein
translocation channel during chloroplast import, but the role of OEP80
remains elusive. Patel et al. (pp. 235–
245) demonstrate that AtOEP80 is essential for viability, and reveal that the
N-terminal part of the protein is not
required for its biogenesis or function.
Peter V. Minorsky
Division of Health Professions and Natural Sciences
Mercy College
Dobbs Ferry, New York 10522
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Plant Physiol. Vol. 148, 2008
Downloaded from on August 3, 2017 - Published by www.plantphysiol.org
Copyright © 2008 American Society of Plant Biologists. All rights reserved.