Public Release: March 2006
Mutation in a Single Gene Switches a Fungus-Grass Symbiosis from Mutualistic to Antagonistic
-- Finding Could Help Engineer Resistance to Crop Pathogens
In research reported this month in The Plant Cell, scientists highlight a novel
role for reactive oxygen species (ROS) in a symbiotic association between a
filamentous fungus (Epichloë festucae) and a grass (Lolium perenne).
They isolated the fungal gene responsible for the production of ROS and found
that disruption of this gene causes the fungus to become pathogenic rather than
beneficial. The authors propose that the function of ROS in this association
is to control the growth of the fungus within the plant. A surprising result
of this work is that the mutation of only one gene was required to switch a
symbiotic association from a beneficial to an antagonistic one.
Contact: Beatrice Grabowski
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American Society of Plant Biologists
THE PLANT CELL http://www.plantcell.org
in a Single Gene Switches a Fungus-Grass Symbiosis from Mutualistic to Antagonistic
Research Highlights a Novel Role for Reactive Oxygen Species in the Fungus-Grass
relationship is one in which two organisms of different species interact in
ways that profoundly affect their livelihoods and reproductive success. Such
interactions range from mutually beneficial to antagonistic and are considered
to be of major ecological and evolutionary importance in shaping plant and animal
communities. Examples of beneficial symbioses include the microbes that live
in the guts of herbivorous mammals like cows and help to digest cellulose, ants
that protect plants from herbivores, and the fig wasps that pollinate fig trees
by depositing their eggs in the fig flowers, which their larvae then feed on.
Plants participate in numerous symbiotic associations. Examples include the
nitrogen-fixing bacteria that live in plant roots, the fungus-alga association
that makes up lichens, and grasses and endophytic fungi (fungi that live inside
the leaves, stems, and other structures of the plant).
in the genus Epichloë form symbiotic associations with many grasses.
Studies have shown that Epichloë endophytes can result in enhanced biomass
production, seed production, and root growth of the grass plants as well as
improved recovery after drought compared to plants without endophytes. Like
other endophytes, the symbioses of grass species with Epichloë fungi
can be mutualistic or antagonistic or both. In the beneficial interactions,
Epichloë endophytes are strictly limited in their intercellular growth
throughout the plant. The growth of the endophyte is synchronized with that
of the grass; fungal hyphae grow actively in expanding leaves but cease to grow
as the leaf matures.
Daigo Takemotot and Barry Scott at the Centre for Functional Genomics at Massey
University in New Zealand; Michael Christensen at the Grasslands Research Centre,
also in New Zealand, and Pyoyun Park at the Graduate School of Science and Technology
at Kobe University, Japan, studied the interaction of the fungal endophyte Epichloë
festucae and its host, perennial ryegrass, Lolium perenne. As a
result, they discovered a novel role for reactive oxygen species (ROS) in regulating
the mutualistic interaction between E. festucae and its grass
Tanaka et al.
used a forward genetics approach to create mutants of the endophyte that would
be unable to establish or maintain a mutualistic relationship with perennial
ryegrass. They inserted foreign DNA randomly into the genome of Epichloë
festucae, resulting in a population of fungal strains having disruptions
in different genes throughout the fungal genome. From this collection they isolated
a mutant that is unable to synchronize its growth with that of the plant host.
with the mutant fungus showed stunted growth, premature senescence, and death,
whereas those infected with the wild-type fungus exhibited their usual growth
pattern. This was accompanied by a dramatic increase in fungal endophyte growth
within the plant compared with plants inoculated with wild-type fungus. The
fungal hyphae of the wild type fungus showed limited branching and were mostly
oriented parallel to the intercellular spaces of the leaf. On the other hand,
the hyphae of the mutant fungus showed extensive colonization of the leaf—similar
to a pathogenic infection. As a result, the biomass of the mutant fungus increased
significantly compared to wild type. Thus a mutualistic interaction became an
antagonistic one with the mutation of a single gene.
Tanaka et al.
then went on identify and sequence the fungal gene responsible for the mutant
phenotype. They determined that the foreign DNA had disrupted a fungal gene,
called noxA, which encodes an enzyme that catalyzes the conversion of
molecular oxygen to superoxide. The altered symbiotic phenotype is due to a
mutation (caused by the insertion of a segment of foreign DNA) in the E.
festucae noxA gene.
catalyzes the production of ROS or superoxides by transferring electrons from
NADPH (a ubiquitous electron donor in nature) to molecular oxygen, with secondary
generation of hydrogen peroxide. Superoxides are unstable and highly reactive
molecules that can be extremely destructive in biological systems and have been
implicated, for example, as causal agents in cancer formation. For this reason,
antioxidants, which destroy superoxides are recommended as cancer prevention
measures. However, ROS can be part of the arsenal that plants use to protect
themselves, as NADPH oxidase enzymes generate superoxides in response to pathogen
Tanaka et al.
looked at the production of the ROS hydrogen peroxide (H2O2)
in plants infected with wild type and mutant E. festucae by electron
microscopy. Cerium perhydroxides, which are formed by a reaction with H2O2,
were detected in actively growing tissue of plants with wild type fungus but
rarely in the same tissue of plants with mutant fungus. These results confirmed
that it is the fungus, not the plant, that is mainly responsible for ROS production.
proposed that ROS produced by the endophyte NoxA enzyme in the plant negatively
regulates the growth of the fungus, preventing excessive colonization of the
host. Thus, the ROS act as a brake on the growth of the fungus, preventing
it from becoming pathogenic and allowing it to maintain a beneficial, mutualistic
symbiosis with the plant. When this gene is disrupted, the growth of the fungus
is uncontrolled and the association becomes pathogenic. This study has highlighted
a previously unknown role for ROS in maintaining a mutualistic symbiosis between
endophytic fungi and plants and shown that the mutation of the fungal noxA
gene can switch the symbiosis from beneficial to antagonistic.
The authors of this study are Aiko Tanaka, Daigo Takemoto, and Barry Scott of
the Centre for Functional Genomics, Institute of Molecular BioSciences, Massey
University, New Zealand; Michael J. Christensen, AgResearch, Grasslands Research
Centre, Palmerston North, New Zealand; and Pyoyun Park, Graduate School of Science
and Technology, Kobe University, Japan.
The research paper cited in this report is available at the following link:
The Plant Cell (http://www.plantcell.org/)
is published by the American Society of Plant Biologists. For more information
about ASPB, please visit http://www.aspb.org/.