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Plant Physiol, March 2001, Vol. 125, pp. 1162-1163
Peter V.
Minorsky
Department of Biology
Vassar College
Poughkeepsie, NY 12604
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INTRODUCTION |
Because cereal grains account for
more than one-half of all agricultural crop production, they have
always been at the forefront of applied and basic plant research. A
citation analysis of the literature confirms the general theme of this
Special Issue: The cereal plants are, in their own right, an
experimental model system of great merit. In the analyses presented
here and in the inaugural version of The Hot and the
Classic (December 2000 issue), reviews and technique papers,
although of obvious importance to the progress of science, were
excluded because they tend to be cited disproportionately more often
than do experimental papers. Also excluded from consideration in the
present case were those papers in which a grass species was studied
only secondarily. Presented below are summaries of the most cited
cereal grain articles for each year of the 1990s.
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1990: Maize opaque-2 (o2) Gene Is a Transcription Factor |
The o2 mutation of maize (Zea mays)
effects a phenotypic change from hard glassy kernels to seeds with soft
powdery endosperm that are opaque to light. This change is correlated
with a reduction in the transcription and quantity of zein
storage proteins. Schmidt et al. (1990) report that the O2 protein
shares sequence similarity with the Leu zipper DNA-binding domain
characteristic of mammalian oncogenes and fungal transcription
activation factors. The Leu zipper domain of O2 apparently interacts
directly with one or more zein promoter elements.
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1991: Maize viviparous-1 (vp1) Is a Transcriptional
Activator |
Viviparous mutants of maize do not enter dormancy during
seed maturation. Most viviparous mutants are caused by dysfunctions in
the synthesis of abscisic acid or its precursors, but the
vp1 mutant is exceptional. McCarty et al. (1991)
demonstrate that the maize vp1 gene encodes for a
protein with no detectable homology to any known proteins but which
contains an acidic transcriptional activation sequence. These results
indicated that VP1 is a novel transcription factor possibly involved in
the potentiation of a seed-specific abscisic acid response.
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1992: Quantitative Trait Loci (QTL) in Maize |
To explore heterosis (hybrid vigor) and genotype-by-environment
interaction in maize, Stuber et al. (1992) mapped QTLs associated with
seven major traits (including grain yield) in a cross between two maize
inbred lines. Whenever a QTL for grain yield was detected, the
heterozygote had a higher phenotype than the respective homozygote (with only one exception), suggesting not only overdominance but also
that these detected QTLs play a significant role in heterosis. This
conclusion was reinforced by a high correlation between grain yield and
proportion of heterozygous markers. Although plant materials were grown
and measured in six diverse environments, there was little evidence for
genotype-by-environment interaction for most QTLs.
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1993: Bt Toxin Genetically Engineered into Maize |
Koziel et al. (1993) introduced a synthetic gene encoding
a truncated version of the CryIA(b) protein derived from
Bacillus thuringiensis (Bt) into immature
maize embryos. Plants expressing high levels of the insecticidal
Bt protein exhibited excellent resistance to heavy
infestations of European corn borer (Ostrinia nubialis)
under field conditions.
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1994: KNOTTED1-Related Homeobox Genes Predict Patterns of
Morphogenesis |
Mutations of the KNOTTED1 gene of maize perturb
specific aspects of maize leaf development. Jackson et al. (1994)
describe the expression patterns of a family of homeobox genes related to KNOTTED1 in maize. Four members of this gene family
are expressed in shoot meristems and the developing stem, but not in
determinate lateral organs such as leaves or floral organs. The genes
show distinct expression patterns in the vegetative shoot apical
meristem that together predict the site of leaf initiation and the
basal limit of the vegetative "phytomer" or segmentation unit of
the shoot.
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1995: Cloning of a Disease Resistance Gene in Rice |
Song et al. (1995), in the first successful use of positional
cloning in cereals, isolate the rice (Oryza sativa)
Xa21 gene, which confers resistance to
Xanthomonas oryzae. Fifty transgenic rice plants
carrying the cloned Xa21 gene displayed high levels of
resistance to the pathogen. The sequence of the predicted protein, which carries both a Leu-rich repeat motif and a Ser-Thr kinase-like domain, suggest that it may play a role in the recognition of a
pathogen-derived ligand at the cell surface and the subsequent activation of an intracellular defense response.
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1996: Nested Retrotransposons in Maize |
Plant genomes consist of repetitive DNA sequences
intermixed with genes. The maize genome, for example, contains about
60% to 80% repetitive DNA. Diagnostic sequencing by SanMiguel et al. (1996) indicated that a 280-kb region containing the maize
Adh1-F and u22 genes is composed primarily of
retrotransposons inserted within each other. Ten retroelement families
were discovered, with reiteration frequencies ranging from 10 to 30,000 copies per haploid genome, These retrotransposons accounted for more than 60% of the Adh1-F region and at least one-half of the
nDNA of maize. These elements were largely intact and are dispersed throughout the gene-containing regions of the maize genome.
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1997: Barley mlo Disease Resistance Gene Isolated |
Barley (Hordeum vulgare) that is homozygous recessive
at the Mlo locus exhibits spontaneous formation of leaf
lesions as well as broad-spectrum resistance to the fungal pathogen
Erysiphe graminis. Buschges et al. (1997) isolated the
mlo gene using positional cloning. The deduced protein
was homologous to other functionally unidentified amino acid sequences
in plants, but showed no homology to any known proteins outside the
kingdom Planta. The mlo protein appears to be membrane anchored by at
least six membrane-spanning helices. Functional mlo proteins may play a
role in slowing or preventing leaf cell death, and their absence may
prime the defense systems of the plant to respond more quickly to
pathogen attack.
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1998: Rapid Reorganization of R-Gene Homologs |
Plant resistance to particular pathogens involves recognition
events that are race specific and triggered by corresponding resistance
(R) genes in the host and avirulence
(Avr) genes in the pathogen. Leister et al. (1998) took
advantage of conserved domains in the major class of dicot resistance
R genes to isolate related gene fragments via PCR from
rice and barley. Interspecific analyses of R-like genes
frequently revealed non-syntenic map locations between the cereal
species rice, barley, and foxtail millet (Setaria italica),
although tight collinear gene order is a hallmark of monocot
genomes. These data suggest a dramatic rearrangement of
R-gene loci between related species and imply a different
mechanism for nucleotide binding site plus Leu-rich repeat gene
evolution compared with the rest of the monocot genome.
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1999: Molecular Biology of Maize Domestication |
Maize and teosinte (Z. mays subsp.
meticana) display profound morphological differences, the
major one being that teosinte has long branches with tassels at the
tip, whereas maize has short branches tipped by an ear. The
teosinte branched-1 (tb1) gene is thought
to be a major determinant of these morphological differences. Wang et
al. (1999) examined nucleotide polymorphism in tb1 in a
wide variety of accessions of teosinte and maize, and determined that
the effects of selection were limited to the gene's regulatory region
and could not be detected in the protein-coding region. These results
help to explain why maize is such a variable crop, and confirm previous
evidence that maize was domesticated from teosinte indigenous to
southwestern Mexico.
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LITERATURE CITED |
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Buschges R, Hollricher K, Panstruga R, Simons G, Wolter M, Frijters A, vanDaelen R, vanderLee T, Diergaarde P, Groenendijk J
(1997)
The barley mlo gene: a novel control element of plant pathogen resistance.
Cell
88: 695-705
[CrossRef][ISI][Medline]
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Jackson D, Veit B, Hake S
(1994)
Expression of maize knotted1 related homeobox genes in the shoot apical meristem predicts patterns of morphogenesis in the vegetative shoot.
Development
120: 405-413
[Abstract]
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Koziel MG, Beland GL, Bowman C, Carozzi NB, Crenshaw R, Crossland L, Dawson J, Desai N, Hill M, Kadwell S
(1993)
Field performance of elite transgenic maize plants expressing an insecticidal protein derived from Bacillus thuringiensis.
Bio-Technology
11: 194-200
[CrossRef]
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Leister D, Kurth J, Laurie DA, Yano M, Sasaki T, Devos K, Graner A, Schulze-Lefert P
(1998)
Rapid reorganization of resistance gene homologues in cereal genomes.
Proc Natl Acad Sci USA
95: 370-375
[Abstract/Free Full Text]
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McCarty DR, Hattori T, Carson CB, Vasil V, Lazar M, Vasil IK
(1991)
The viviparous-1 developmental gene of maize encodes a novel transcriptional activator.
Cell
66: 895-905
[CrossRef][ISI][Medline]
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SanMiguel P, Tikhonov A, Jin YK, Motchoulskaia N, Zakharov D, Melake Berhan A, Springer PS, Edwards KJ, Lee M, Avramova Z
(1996)
Nested retrotransposons in the intergenic regions of the maize genome.
Science
274: 765-768
[Abstract/Free Full Text]
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Schmidt RJ, Burr FA, Aukerman MJ, Burr B
(1990)
Maize regulatory gene opaque-2 encodes a protein with a "leucine-zipper" motif that binds to zein DNA.
Proc Natl Acad Sci USA
87: 46-50
[Abstract/Free Full Text]
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Song WY, Wang GL, Chen LL, Kim HS, Pi LY, Holsten T, Gardner J, Wang B, Zhai WX, Zhu LH
(1995)
Receptor kinase-like protein encoded by the rice disease resistance gene, Xa21.
Science
270: 1804-1806
[Abstract/Free Full Text]
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Stuber CW, Lincoln SE, Wolff DW, Helentjaris T, Lander ES
(1992)
Identification of genetic factors contributing to heterosis in a hybrid from 2 elite maize inbred lines using molecular markers.
Genetics
132: 823-839
[Abstract]
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Wang RL, Stec A, Hey J, Lukens L, Doebley J
(1999)
The limits of selection during maize domestication.
Nature
398: 236-239
[CrossRef][Medline]
© 2001 American Society of Plant Physiologists
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INTRODUCTION
1990: Maize opaque-2 (o2)...