College STation, Texas
April 25, 2005
Texas A&M
University scientists are looking for ways to deal with a
plant pathogen that destroys enough rice every year to feed 60
million people. The pathogen, rice blast, is so clever in it's
its genetic design that it can mutate faster than breeders can
develop resistant varieties.
Rice farming is the largest single use of cultivatable land for
producing food, with more than 300 million acres worldwide. It
is the staple food for the largest number of people on earth and
accounts for 23 percent of the total calories consumed,
according to the International Rice Research Institute.
Understanding rice blast, whose scientific name is Magnaporthe
grisea, has been a priority for researchers over the past
several decades. Recently, Texas A&M professor Dr. Daniel Ebbole
collaborated on an international project that led to the
sequencing of the rice blast genome.
On April 21, the researchers presented their findings in the
journal Nature, and already the article is receiving worldwide
attention from research teams working on blast resistant rice
varieties.
"This is a very important step in understanding how to control
rice blast disease," Ebbole said, "especially since there are
many different races of rice blast fungus out there. The race
defines which cultivars are resistant or susceptible to the
pathogen, and the sequence reveals the genes of the fungus that
can define race."
In sequencing M. grisea, scientists learned it contains a unique
family of G-protein-coupled receptors. These receptors are the
‘eyes' of the fungus, and relay basic information, such as when
it has found a suitable host plant, to more complex reports on
what defenses the host plant may have. Once the protein
assimilates the information, a signal within the cell determines
what action should be taken to initiate the infection.
"There are several avenues that lead to infection, but we
believe the primary mechanism is the use of a special cell,
called the appressorium, which uses turgor pressure to punch
through the leaf surface," Ebbole said. These cells can be
thought of as tiny jackhammers, which produce a pressure equal
to 750 feet below sea level.
"In addition, the fungus contains an arsenal of proteins that
secrete enzymes that degrade the waxy polymer of the leaf
cuticle. This further paves the way for infection of the host
plant," said Ebbole.
The host plant does have its own defenses, though, for example,
an enzyme called chitinase, which can degrade the cell wall of
the fungus. However, the M. grisea has chitin-binding proteins
that neutralize this threat, according to the article in Nature.
Researchers have found that the blast fungus also has a high
degree of genetic variability, with extensive copies of
repetitive DNA. This leads to novel pathogenic variants capable
of infecting formerly resistant host plants.
Because blast resistant varieties don't always stay resistant
under field conditions, protecting against them has challenged
rice breeders.
"Our research should help with this problem," Ebbole said, "as
the genome sequencing of the rice blast fungus will help
scientists better understand the mechanisms a plant needs to
resist infection."
In addition to better breeding lines, the research will also
help in the development of new fungicides that can block avenues
of infection, creating a protective layer around the host plant,
namely rice, said Ebbole.
Texas A&M has a diverse group of scientists who study fungi such
as rice blast as part of the Program for the Biology of
Filamentous Fungi. In addition to Ebbole, assistant professor
Michael Thon, research assistant Elena Kolomiets, and graduate
student Slavica Djonovic participated in studying the rice blast
genome sequence at Texas A&M.
Related
release:
Genetic sequence
of rice blast fungus provides critical piece of disease puzzle |