Bolstering Rice Against Blast
Lesions caused by the rice
blast fungus, Magnaporthe
grisea, on rice plant leaves.
In some parts of Asia, farmers are planting and growing record amounts
of rice. For the two-thirds of the world's people who rely on the staple
grain, this sounds like positive news. But intensive cultivation of
rice has also given rise to something elsenew, more virulent strains
of the rice blast fungus, Magnaporthe grisea. Blast, which weakens
and kills rice plants, can reduce yields by up to 75 percent in infested
Caught in a relentless tug-of-war, scientists, like those with the
Agricultural Research Service,
are continuously hunting for rice cultivars that resist the nimble fungus.
But even after just one growing season, the blast pathogen can often
find a way to bypass once-resistant rice plantsusually showing
up as spindle-shaped lesions on plant leaves.
According to ARS chemist Sally A. Leong, the pathogen has become a chronic problem in parts of Asia and Latin America. "In Asia, farmers are planting rice in almost any available spot, so they're seeing a rapid evolution of the Magnaporthe fungus," she says. Blast has also been inflicting damage on plants in the United States, alarming growers in California and several southern states and threatening to increase the price of U.S. rice.
Leaf from resistant rice
variety CO39 showing ability
to resist formation of
lesions when the Magnaporthe
grisea strain carries the
AVR1-CO39 avirulence gene.
The dark spots are areas
where plant cells have died
to abort rice blast infections.
Leong and colleagues in the ARS Cereal Crops Research Unit, Madison, Wisconsin, have been working to unlock rice's natural defenses against the ever-changing fungus. Making significant strides, they've identified new resistance genes in rice that are uniquely linked to a gene in the fungal pathogen. They are also developing a novel way to use these key genes that ensures rice plants even greater built-in protection.
Chromosomes That Click
Having evolved side-by-side in many parts of the world, M. grisea
and its rice plant host share a very specific relationship. In an interplay
known as gene-for-gene resistance, the rice plant will launch a vigorous
defense if it recognizes a particular gene producta molecule or
proteinin the invading fungus. The fungal gene is referred to
as an avirulence, or Avr, gene. The plant gene that starts the
defense response is called a resistance, or R, gene.
"There are thousands of genes in rice that mediate various aspects
of plant protection," says Leong. "But only a specific R
geneone that matches the fungus's Avr geneis capable
of setting off a strong resistance response in rice."
During this defense response, plant cells near the fungal-infected
leaf tissue are programmed to die. In this pronounced bid for survival,
the plant cuts off life support to the fungus, preventing further advance.
But this cell death is actually a secondary level of resistance. Other
cell studies suggest that the plant is likely responding to an even
earlier signal produced in its interaction with the blast fungus. What
is this first line of defense? Are there others in the plant's arsenal?
To begin to answer these questions, Leong and postdoctoral collaborator
Mark L. Farman, now a plant pathologist with the University of Kentucky
in Lexington, cloned and sequenced an Avr gene in M. grisea
that is recognized by the resistant rice plant. This gene, called AVR1-C039,
is the second Avr gene from the rice blast fungus to be cloned
by scientists. Leong and other colleagues identified, mapped, and cloned
the R gene in rice that corresponds to the fungus's AVR1-C039
To understand all the cues and signals that are being exchanged between plant and fungus, the researchers are watching how the newly defined genes interact on a molecular level as well as cellular one. "From the moment the fungal spore lands on the rice leaf," Leong says, "we want to know where, when, and how it communicates with its host."
A new rice plant is also arising from this gene work. The resistance
genes that Leong and her colleagues are studying can be inserted into
nonresistant rice plants to trigger their defenses.
But to provide even broader resistance, the scientists have gone a
step further. They're developing a system that applies the operative
genes from both the rice and its pathogen. This new strategy appears
different from past efforts, which have dodged the blast pathogen by
breeding new resistant rice plants through classical methods.
"We're not worrying about what the pathogen is doing or trying
to anticipate how it will change next," says Leong. "Instead,
we're focusing on the plant, hoping to provide rice plants with the
most generic resistance possible."
With this in mind, the scientists have designed a system that enables
the expression of both the rice gene and pathogen gene in the host plantresulting
in a situation known as "complete signaling." Still fine-tuning
their methods, Leong and researchers are working to better understand
when and where the Avr gene's molecules or proteins are first
perceived by the plant cell.
In another approach, the researchers are working to instigate the resistance
response by pretreating the transformed rice plantone already
possessing the desired resistance genewith the molecules or proteins
produced by the pathogenic Avr gene. "We introduce these
gene products through a benign bacterium or fungus," says Leong.
"Even better is using nitrogen-fixing bacteria as the vehicle,
an approach that results in dual benefits for the rice plant."
Rice isn't the only beneficiary of this work. That's because the Magnaporthe
fungus is known to attack other cereals and grasses, including wheat,
barley, maize, and even turfgrass. "We hope to be able to use our
gene pair in these grasses, to achieve the same kind of signaling,"
Leong and colleagues are also extending their research findings to
help save finger millet, an important source of nutrition for resource-poor
farmers in developing countries. It, too, is under attack by certain
strains of the blast fungus. The researchers are sharing their gene
discoveries with collaborators from the University of Bangalore, India.
The ARS rice blast research will have broad applications. "This
is one of the few systems in which both plant host and pathogen genes
are cloned," says Leong, "so it serves as an excellent model
for understanding the genetic and molecular bases of plant-fungus interactions."By
Erin K. Peabody,
Agricultural Research Service Information Staff.
This research is part of Plant Diseases, an ARS National Program
(#303) described on the World Wide Web at www.nps.ars.usda.gov.
"Bolstering Rice Against Blast" was published in the August 2004 issue of Agricultural Research magazine.