A Novel Application of Host-Induced Gene Silencing Methods

Background

Fusarium Head Blight (FHB) is an important disease of wheat (Triticum aestivum) and other small grain cereals caused by the fungus Fusarium graminearum. Destroying $2.491 billion dollars of wheat crop in the United States alone during the period of 1993-2001 (Nganje et al. 89), FHB has been responsible for causing vast economic loss for farmers across North America. Following infection, affected wheat spikelets first show symptoms of premature bleaching (chlorosis) after infection. These spikelets begin to host fungal mycotoxins that are dangerous for human consumption, while the fungal infection causes the developing seed in each spikelet to shrivel and wrinkle in shape, drastically reducing yield, if not cutting yield altogether. F. graminearum can also infect leaves of the model plant Arabidopsis thaliana through infiltration through a leaf’s vascular network. After infection, the infected leaf will begin to display signs of bleaching as a result of infection. Host-induced gene silencing (HIGS) is a specialized form of gene silencing with the potential to provide plants a new barrier against infection. Whereas regular gene silencing occurs within the organism itself and acts as a method of self regulation in response to certain inducing factors, host-induced gene silencing facilitates the production of short interfering RNA constructs that do not interfere with regular transcriptional activity, but rather directly activate in their role when a pathogen or pest, whether it be a virus, bacterium, insect, fungus, or nematode, is encountered.

Rationale

The rationale for this project is to test the efficacy of host-induced gene silencing methods (HIGS) in preventing the spread of FHB in bread wheat, done by engineering plants that employ HIGS methods – allowing the plant to express recombinant constructs to produce small interfering RNA molecules that can silence expression of fungal genes required for pathogenicity when taken up by the fungus. The inability of the fungus to accumulate the mRNA transcripts of these pathogenicity genes prevents the fungus from establishing itself on the host, curbing disease progression. Prior research has shown that there exists a lipase secreted by the fungus required for pathogenicity. Similarly, an oxidase encoded by the fungus that potentially metabolizes defense-related phenolics is suggested to be required by the fungus for limiting the activation of plant defenses. In this study, in addition to wheat, the model plant Arabidopsis thaliana, which is also colonized by F. graminearum, will be the plant that will be targeted for HIGS-mediated silencing of these specific fungal lipase and oxidase genes. By using Arabidopsis, the intent is to utilize data collected from the model organism as a preliminary understanding of the tested RNAi constructs before primary testing occurs on wheat. Additionally, utilizing a short hairpin structure rather than a transient virus-siRNA transmission method allows for a stable knockdown of critical fungal pathogenicity islands.

What's next for A Novel Application of Host-Induced Gene Silencing Methods

Further research along the trend of this project would mainly involve testing other fungal RNAi constructs directly against F. graminearum infections. Other possibilities would involve testing the stability of these specific constructs’ theoretical applicability across most, if not all, small grain cereals and testing this construct designed for wheat on maize, barley, oat, etc. Finally, the most readily available method to continue this research is to test these constructs’ efficacy against other species of genus Fusarium to understand if other geographical areas, such as Asia, could benefit within their grain industries if they utilized these constructs.