West Lafayette, Indiana
November 12, 2008
A five-year study that could help
increase disease resistance, stress tolerance and plant yields
is under way at Purdue
University.
The $4 million project uses a new technique called
"mutant-assisted gene identification and characterization," or
MAGIC, to identify potentially useful gene combinations in crop
species.
"If we can understand these genes better, we could engineer
plants to be immune to most diseases," said principal
investigator Guri Johal, an associate professor of botany and
plant pathology.
First using the corn genome, the method will add to the
collection of useful alleles, or pairs of genes, that create
certain traits. This will improve crop gene diversity, a quality
that dwindles as crops are bred. Since natural selection has
preserved such alleles, they likely confer a selective advantage
that increases the ability of plants to survive, Johal said.
The MAGIC technique is described in a review article published
this month in the journal Crop
Science.
Maize contains more genetic diversity than any other model
organism, making it an ideal plant for gene exploration, Johal
said. In fact, two lines of corn are more different from one
another than humans are from chimpanzees, said study co-author
Cliff Weil, a professor of agronomy.
"Maize grows in places as different as northern Quebec, where it
is cold and growing seasons are short, and the Mexican
highlands, where it is very hot and dry," he said. "Natural
adaptation to different environments has come by combining just
the right sets of alleles in each variation."
MAGIC is a new tool needed to find genes, Johal said. Many
recent research methods used to this end involve mutagenesis,
with scientists deliberately causing a specific gene or genes to
malfunction in order to determine the gene's impact on the
plant.
"Mutagenesis has worked well, but we are reaching a period of
diminishing returns," Johal said. "We've identified most of the
genes that have effects on their own, but now we need to
understand how combinations of genes interact. We suggest going
back to nature to find additional genes involved in a wide range
of different processes."
Any genes discovered also could benefit other plants; all use
the same pathway to fight infection, Johal said.
"The same approach could be used in other organisms, such as in
animals," he said. "And insights could also apply to human
disease."
To map genes, scientists often cross mutant plants with crop
lines that have well-described genetics. In doing so, they
usually try to reduce or eliminate the impact of unknown natural
variants so the information they're looking for - typically
regarding the mutant gene - is not altered.
"To date most of us were taught in genetics class that when you
find a mutation, for example in corn, you cross it with corn
from different backgrounds, pick the background where the
mutant's appearance, or its phenotype, is the most dramatically
altered, and then find the genetic changes that cause the
phenotype," Weil said.
But Weil and Johal are instead looking for natural genes that
either enhance or diminish certain traits.
"We are basically 'mining' natural variation for genes of
interest," Weil said.
The research started when Johal crossed a mutant gene that
affects lesions to a couple of different inbred lines of corn.
In one cross it disappeared; in another it became toxic.
"We figured the natural variations in these two inbreds were
having a huge effect and decided to take advantage of a large,
existing set of mapping data for the two inbreds to find out
why," Weil said.
Another example is sweet corn, Johal said. The varieties most
people are familiar with derive from a specific mutation that
originally rendered sweet-tasting kernels small and shrunken.
But researchers bred it with various lines - effectively using
natural variation to their advantage - to increase kernel size.
Funding from the National Science Foundation began last month
for the study, which will also include educational components.
North Carolina State University researcher Peter Balint-Kurti is
a review co-author and study collaborator.
"The nice thing is knowing this idea is going to work," Weil
said. “"The alleles, the variation in expression and the data to
map them are already there. We will find a lot of things we
expected and a whole lot of things we never even imagined."
ABSTRACT
Mining and Harnessing Natural Variation: A Little MAGIC
Guri S. Johal, Peter Balint-Kurti, and Clifford F. Weil
The success of a breeding program depends on having adequate
diversity in the germplasm. However, as advanced breeding stocks
and materials are generated, one casualty is the diversity
itself. As a result, breeding programs in many crop species have
reached a point of diminishing returns and it is feared that
unless new diversity is infused into the breeding germplasm, we
face catastrophic reductions in crop productivity if the climate
turns adverse. Although some scientists favor transgenic
approaches, a "back to nature" approach to genetic diversity may
prove faster and more effective. Wild and exotic relatives of
crop plants hold a wealth of alleles that, if we can find them,
can help break yield barriers and enhance tolerance to stresses.
Many approaches, based largely on quantitative trait loci
genetics, have been proposed and used for this purpose, but most
are either highly laborious or discover relevant variation
inefficiently. Here, we propose a gene-centered approach, dubbed
MAGIC (mutant-assisted gene identification and
characterization), that uses Mendelian mutants or other genetic
variants in a trait of interest as reporters to identify novel
genes and variants for that trait. MAGIC is similar to
enhancer-suppressor screens, but rather than relying on
variation created in the laboratory, it reveals variation
created and refined by nature over millions of years of
evolution. This approach could be an effective tool for
exploring novel variation and a valuable means to harness
natural diversity and define genetic networks.
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