Davis, California
January 22, 2008
Using tomatoes as a research
plant, scientists at the
University of California, Davis, have discovered that two
plant enzymes that occur in the plant's cell walls cooperate
with each other to make ripe fruit more susceptible to a
disease-causing fungus.
"Identifying the role that these cell-wall enzymes play in
making the harvested fruit more vulnerable to disease is
important for designing strategies for limiting fruit losses to
disease during storage, handling and distribution," said Ann
Powell, a plant scientist who led the research team on this
project.
The study findings are reported in the Jan. 22 issue of the
Proceedings of the National Academy of Sciences.
One of the hallmarks of plant cells is their tough exterior cell
wall. As the fruit ripens, the cell walls break down, and the
fruit becomes softer and more flavorful. At the same time, the
fruit also becomes more susceptible to diseases caused by fungi
and bacteria.
Researchers have known for some time that two enzymes, known as
polygalacturonase and expansin, contribute individually to the
ripening-related breakdown of the cell walls. (Enzymes are
proteins that trigger and control chemical reactions.) The UC
Davis research team wondered if these two cell wall enzymes
might also be responsible for the increased
disease-susceptibility of ripening fruit. These diseases are
responsible for substantial losses of high-quality harvested
fruit during storage, shipping, marketing and consumer handling.
To test this notion, they selected two genetically modified
tomato plant varieties. One of the varieties had been altered so
that it did not produce polygalacturonase, and the other did not
produce expansin. Powell had crossed these two varieties,
resulting in a variety that produced neither of these enzymes.
The researchers, including plant biology graduate student Dario
Cantu and postdoctoral fellow Ariel Vicente, inoculated tomatoes
from each of the genetically modified varieties, as well as the
variety resulting from the cross, with Botrytis cinerea, a
common fungus that causes rotting on many fruits and vegetables.
Tomatoes from a control group, whose enzyme production had not
been altered, were also inoculated with the fungus.
The research team found that tomatoes from the plants that were
genetically modified to suppress production of only one of the
cell-wall enzymes were not any less susceptible to the fungus.
However, when both of the enzymes were lacking in the crossed
variety, the cell walls of the fruit did not readily break down,
and the fruit was dramatically less susceptible to infection by
the Botrytis cinerea fungus.
"It appears that these two enzymes work cooperatively in a way
that breaks down the cell wall and leaves the fruit more
vulnerable to pathogens like the gray mold caused by this
fungus," Powell said.
"Interestingly, this process occurs at a stage in the plant's
development that allows both the plant and the pathogen to
successfully reproduce," she noted. "This convenient and
mutually beneficial timing may well have resulted from the
co-evolution of the fruit and its respective pathogens."
Collaborating on this study with Powell, Cantu and Vicente were
professional researcher Molly Dewey, formerly of UC Davis and
Oxford University; researcher Carl Greve and plant science
professors John Labavitch and Alan Bennett.
The study was funded by a National Science Foundation grant.
Cantu was supported by the UC Davis Department of Plant
Sciences, and Vicente was supported by the Fulbright Commission
and the Consejo Nacional de Investigaciones Cientificas y
Tecnicas of Argentina. |
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