Davis, California
June 30, 2009
A group of plant proteins that
"shut the door" on bacteria that would otherwise infect the
plant's leaves has been identified for the first time by a team
of researchers in Denmark, at the
University of California, Davis,
and at UC Berkeley.
Findings from the study, which appears in the June 29 issue of
the online journal Public
Library of Science Biology, provide a better understanding
of plants' immune systems and will likely find application in
better protecting agricultural crops and horticultural plants
against diseases.
"The ability of a plant's immune system to recognize
disease-causing microorganisms is critical to the plant's
survival and productivity," said Gitta Coaker, a UC Davis plant
pathologist and lead author on the study.
"In this study, we identified a complex of proteins in the
common research plant Arabidopsis that appear to play important
roles in the biochemical mechanisms that enable plants to
recognize and block out invading bacteria," Coaker said.
She noted that, over the last 20 years, scientists have
identified a number of proteins that are important for
regulating the plant immune system but still do not have a good
sense of what protein complexes these proteins belong to and how
they signal to confer disease resistance.
"Our ability to purify an immune protein complex will serve as a
starting point to understand how these proteins signal in the
plant," Coaker said. "A greater understanding of how these
proteins function is fundamental knowledge that can be applied
to prevent plant disease."
Plant immunity
Plants are continually exposed to bacteria, viruses and other
microorganisms, many of which have the ability to infect the
plant and cause disease.
Animals have what are known as innate, or preformed, immune
systems as well as adaptive immune systems that learn to
recognize and defend against disease-causing microbes. Plants,
however, only have innate immune systems. Rather than developing
immunity as they are exposed to various microbes, plants make
use of certain built-in cells and genetically programmed systems
to protect themselves against microbial invasion and related
diseases.
This type of innate immune system has two branches: one makes
use of receptor proteins outside the cell to recognize specific
molecular features of an invading microbe, while the other
branch uses similar proteins within the cell to recognize an
invading microbe during the infection process.
Up until now, scientists had identified only one protein, known
as RIN4, which is able to regulate these two branches of the
plant immune system in Arabidopsis. The protein is found in the
permeable plasma membrane that encases the cell on the inside of
the cell wall.
It has been unclear exactly how the protein and the two branches
of the immune system interact to trigger an immune response in
the plant.
The new findings
In studying the RIN4 protein, Coaker and her colleagues
identified six previously uncharacterized proteins that can
associate with RIN4 inside plant cells. One protein, called
AHA1, was characterized in-depth and found to be key to the
immune response in Arabidopsis plants.
AHA1 can act to regulate the opening and closing of tiny holes
called stomata, found on the underside of the leaf. The stomata
allow gases and water to pass in and out of the leaf. This is
the same opening that allows bacteria and other invading
microbes to gain entrance to the plant.
The stomata are each flanked by two guard cells, which control
these vitally important portals to the leaf. When the guard
cells swell, the stomata close. Conversely, when the water
content of the guard cells decreases, the stomata open.
The six proteins identified in this study were found to be
intricately involved with the biochemical processes that enable
the plant to recognize and block out invading bacteria.
The researchers found that RIN4 can act to regulate AHA1 and
that both proteins work together to control stomatal openings in
response to a disease-causing microorganism.
"These findings highlight how important regulation of the
stomata is in Arabidopsis immunity," Coaker said. "Further
research is needed to determine if RIN4 and its associated
proteins play the same role in other plant species."
Funding for the study was provided by the National Institutes of
Health and the National Science Foundation.
Collaborators on this study were Coaker, Jun Liu and James M.
Elmore, all of UC Davis; Anja T. Fuglsang and Michael G.
Palmgren, both of the Danish National Research Foundation and
the University of Copenhagen, Denmark; and Brian J. Staskawicz
of UC Berkeley.
For 100 years, UC Davis has engaged in teaching, research and
public service that matter to California and transform the
world. Located close to the state capital, UC Davis has 31,000
students, an annual research budget that exceeds $500 million, a
comprehensive health system and 13 specialized research centers.
The university offers interdisciplinary graduate study and more
than 100 undergraduate majors in four colleges -- Agricultural
and Environmental Sciences, Biological Sciences, Engineering,
and Letters and Science -- and advanced degrees from six
professional schools -- Education, Law, Management, Medicine,
Veterinary Medicine and the Betty Irene Moore School of Nursing. |
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