Fusarium Resistance Via Biotechnology
By Linda Harris, Eastern Cereal & Oilseed Research Centre, Central Experimental Farm, Agriculture and Agri-Food
Canada
Plants are continually assaulted by fungal microorganisms. You could
say they are sitting
ducks. So, they have developed sophisticated defense strategies. These include strengthening cell walls and producing
a variety of enzymes (proteins) which can directly or indirectly attack any menacing microorganism. In the case
of gibberella, the invading fungi is able to somehow overcome the plant’s defense but most of the mechanisms used
by the fungus are still a mystery.
Figure 1
Trangenic corn embryogenic cultures (equivalent starting weights) after three weeks of growth on 10 ppm DON. The
top plate contains tissue with the unmodified RPL3 gene while the bottom plate contains tissue with the modified
DON-tolerant RPL3 gene.
We do know that the fungus produces several mycotoxins, including one nicknamed DON (also known as vomitoxin).
We know from our previous work that DON is not crucial in the initial infection process. But DON does help the
Fusarium fungus
spread through the plant tissue.
DON accomplishes this by interfering with the formation of proteins in cells by binding to a particular protein,
called RPL3, in the protein synthesis machinery. This means the corn plant may not be able to make its usual array
of defense proteins and is much more vulnerable to being overcome by the fungus.
One approach to increasing plant resistance to Gibberella ear rot (Fusarium) is to prevent the binding of DON to RPL3 protein in the cell. To accomplish
this, we have slightly modified the gene coding for RPL3 so that it now makes a protein for which DON has decreased
or negligible binding affinity.
This modified RPL3 protein has only one altered amino acid in its long chain of 389 amino acids. The modified RPL3
gene has been re-introduced into the corn plant and we have a number of different clones developing into plants
in the greenhouse. Once these plants have been propagated, we will be able to test for increased resistance to
Fusarium ear
rot. We are excited and encouraged by the observation that, in tissue culture, transgenic corn containing the modified
RPL3 gene was much more tolerant of the mycotoxin DON than transgenic corn with the original unmodified form of
the gene, as illustrated in Figure 1.
As part of the strategy to increase the plant’s overall disease resistance, a field trial is being conducted this
summer with transgenic corn plants containing a wheat gene (oxalate oxidase) that has been implicated in several
different plant defense pathways. Oxalate oxidase increases the production of hydrogen peroxide which acts to increase
the activity of plant defence genes. Hydrogen peroxide also induces cross-linking of cell wall components to provide
a stronger barrier against invading pathogens. Several populations of oxalate oxidase transgenics have been produced
and these plants will be screened for resistance to Gibberella (Fusarium) ear rot, stalk rot, and ear smut.
Another aspect of our research is increasing our knowledge of the infection process at the molecular level. We
have isolated more than 20 genes that appear to be turned on in the plant or the fungus during the first few days
of fungal infection of corn. The characterization of these genes will increase our understanding of what mechanisms
Fusarium requires
or exploits in ear rot development and how a susceptible plant responds to the attack, compared to a more tolerant
one. An understanding of the natural course of events in an infection can lead to the design of additional strategies
to control the Fusarium
problem.
Fusarium graminearum
(also known as Gibberella zeae) is a resourceful enemy. The scarcity and complexity of naturally occurring resistance
identified in corn and wheat reinforces this point. This research should provide alternate resistance strategies
which can be combined with resistance obtained through breeding to yield a superior Fusarium-resistant corn plant.
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