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INTRODUCTION |
Science citations are a bit like secondary sexual
characteristics: Those who carry large endowments tend to think they
are important, whereas those who do not tend to dismiss the whole business as infinitely silly. There are numerous reasons why papers may
be under-cited, and all of us can think of many profound and important
contributions, particularly our own, that have, for a multitude of
reasons, not received anywhere close to their fair share of citations.
It is clear that if a paper is under-cited, it does not necessarily
follow that the paper is unimportant. It is, however, difficult to
refute the argument that a highly cited paper can be anything but a
major contribution to the literature. It was with this thought in mind
that a search of the literature was made for the most cited Arabidopsis
articles of the 1990s (excluding reviews and technique papers). It is
important to recognize that these papers, one from each year in the
1990's, represent the most-cited Arabidopsis papers as of October
2000. It is quite possible that these 10 papers represent the hares
that will later be passed by the tortoises. Moreover, some of the
runner-ups in a given year have been slighted, for they have been cited
far more than some of the winners in other years. However, it is
inarguable that all of the following papers have been enormously
important. Therefore, let us raise our micropipettors on high and
salute the authors of these trailblazing researchers in Arabidopsis biology.
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1990: Homeotic Flower Gene/Transcription Factor |
Certain genes result in homeotic transformations of floral
organs. Mutations in the agamous gene result in the overall
phenotype of a flower within a flower and the absence of stamens or
carpels. Yanofsky et al. (1990)
used a T-DNA insertion mutation to
clone and characterize the Arabidopsis homeotic agamous
gene. The protein encoded by the agamous gene resembles
transcription factors from humans and yeast, and a homeotic gene from snapdragon.
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1991: Control of Body Patterns |
Mayer et al. (1991) reported on their systematic search for
mutations that disrupt the spatial organization of Arabidopsis seedlings by altering embryogenesis. Mutations in nine genes were found
to affect three different aspects of the body organization: apical-basal pattern, radial pattern, and shape. Their analysis supports the view that the relative positions of cells in the early
embryo determine their developmental fates. This suggests that pattern
formation in plant embryos may depend on positional information as it
does in animal embryos.
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1992: LEAFY Controls Floral Meristem Identity |
Weigel et al. (1992) analyzed how mutant alleles of the
Arabidopsis gene LEAFY affect the generation of the
floral meristem. They found that LEAFY interacts with
another floral control gene, APETALA1, to promote the
transition from inflorescence to floral meristem. They cloned the
LEAFY gene, and as expected from the mutant phenotype,
found that LEAFY RNA is expressed strongly in young
flower primordia. LEAFY expression precedes expression
of the homeotic genes AGAMOUS and
APETALA3, which specify organ identity within the
flower. Furthermore, LEAFY was found to be the
Arabidopsis homolog of the FLORICAULA gene, which
controls floral meristem identity in snapdragon.
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1993: Ethylene Receptors Are Two-Component Sensors |
Ethylene insensitivity is conferred by dominant mutations in the
ETR1 gene early in the ethylene signal transduction
pathway of Arabidopsis. Chang et al. (1993) cloned the
ETR1 gene by chromosome walking. Each of the four known
etr1 mutant alleles was found to contain a missense mutation
near the amino terminus. Although the sequence of the amino-terminal
half of the deduced ETR1 protein is novel, the carboxyl-terminal half
is similar in sequence to both components of the prokaryotic family of
two-component signal transducers. Thus, an early step in ethylene
signal transduction in plants may involve transfer of phosphate as in
prokaryotic two-component systems.
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1994: Plant Disease Resistance Genes |
Plants recognize pathogens through the action of disease
resistance (R) genes, which confer resistance to
pathogens expressing unique corresponding avirulence
(avr) genes. Bent et al. (1994) used a positional cloning
strategy to isolate RPS2, an R gene of
Arabidopsis that confers resistance to Pseudomonas
syringae bacteria expressing the avirulence gene
avrRpt2. The derived amino acid sequence of RPS2 was found
to contain Leu-rich repeat, membrane-spanning, Leu zipper, and P-loop
domains. Based on its amino acid sequence, the authors postulated that
the role of the RPS2 gene product in defense signal
transduction involves nucleotide triphosphate-binding, protein-protein interactions, and possibly the reception of an elicitor
produced by the avirulent pathogen.
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1995: Dual Specificity of a Disease Resistance Gene |
The molecular basis of gene-for-gene specificity is poorly
understood. Grant et al. (1995) discovered that the Arabidopsis RPM1 gene renders a plant resistant to pathogens
expressing either of two unrelated P. syringae avr
genes. Despite this dual specificity, RPM1 encodes
for protein-sharing molecular features with
R-gene-encoded proteins that confer resistance to only
one pathogen strain. The authors speculated that the evolution of
R genes that determine multiple specificities may be one
way of reducing the absolute number of R genes required
for plant defense.
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1996: Molecular Basis of the KNOTTED Phenotype |
The KNOTTED class of plant genes encodes
for homeodomain proteins, and their expression patterns suggest they
play an important role in shoot meristem function. Long et al. (1996)
presented evidence that the Arabidopsis
SHOOTMERISTEMLESS (STM) gene, required for shoot apical meristem formation during embryogenesis, encodes a
class-I KNOTTED-like protein. They also described the expression pattern of this gene in the wild-type plant. STM is the
first gene shown to mark a specific pattern element in the developing plant embryo both phenotypically and molecularly.
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1997: The Control of Meristem Size |
Cell proliferation and cell differentiation normally are
balanced in the shoot, so that the structure and size of the shoot meristem is maintained. However, Arabidopsis plants homozygous for
mutations at the CLAVATA1 (CLV1) locus accumulate
excess undifferentiated cells. Clark et al. (1997) described the
molecular cloning and expression pattern of the CLV1 gene.
The extracellular domain of CLV1 is composed of 21 tandem Leu-rich
repeats that resemble the Leu-rich repeats found in many animal hormone
receptors. Its resemblance to a receptor kinase suggests a role in
signal transduction. The authors provide evidence that CLV1
expression in the inflorescence is specifically associated with
meristematic activity.
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1998: Nitric Oxide in Disease Resistance |
Recognition of an avirulent pathogen triggers the rapid
production of the reactive oxygen intermediates. This oxidative burst drives cross-linking of the cell wall, induces several plant genes involved in cellular protection and defense, and is necessary for the
initiation of host cell death in the hypersensitive disease resistance
response. Delledonne et al. (1998) showed that nitric oxide,
which acts as a signal in the immune, nervous, and vascular systems,
potentiates the induction of hypersensitive cell death in soybean cells
by reactive oxygen intermediates and functions independently of such
intermediates to induce genes for the synthesis of protective natural
products. Moreover, inhibitors of nitric oxide synthesis compromise the
hypersensitive disease resistance response of Arabidopsis leaves to
P. syringae, promoting disease and
bacterial growth.
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1999: Phytochrome Kinase |
Fankhauser et al. (1999) identified a phytochrome-binding
protein, phytochrome kinase substrate 1, that is a substrate for light-regulated phytochrome kinase activity in vitro. In
vivo experiments suggested that phytochrome kinase substrate 1 is phosphorylated in a phytochrome-dependent manner and
negatively regulates phytochrome signaling. The data suggested that
phytochrome signal transduction involves Ser-Thr phosphorylation.
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INTRODUCTION
1990: Homeotic Flower...