Performance Story: Evaluation of exogenous dsRNA application for species-specific control of Fusarium Head Blight
Shawn Clark, National Research Council of Canada, Saskatoon
Over the past decade, Fusarium head blight (FHB) has become one of the primary issues facing wheat producers in Western Canada. Resistance to the disease in bread wheat has improved but is still not strong enough to withstand major outbreaks, while resistance in durum wheat remains limited at this time. Triazole fungicides are typically applied to wheat crops to protect against infection; however, these treatments can be ineffective and Fusarium resistance to this fungicide has been reported. The application of dsRNA to control various plant pests and pathogens is an emerging field that could provide RNA-mediated control without the need for transgenic plants. This approach was first demonstrated for the control of plant viruses and is also viable for the control of insect feeding. Following the discovery that fungal pathogens can be controlled through RNA transfer, various groups have begun to test the ability of dsRNA applied to the plant to control fungal disease. Recent publications have demonstrated the ability of exogenous dsRNA application to reduce disease symptoms for subsequent Botrytis cinerea, Fusarium graminearum, Puccinia emaculata and Sclerotinia sclerotiorum infections. In the case of Fusarium, this proof of concept was done with the infection of detached barley leaves rather than the infection of wheat heads.
This project tested exogenously applied dsRNA as an alternative approach to reduce FHB symptoms. The ability of dsRNA applications to reduce FHB symptoms in wheat heads was also studied. Two RNA application methods were tested: root uptake through a soil drench treatment and foliar spray treatment of wheat heads during anthesis. Both soil drench and spray treatments caused a significant reduction of FHB symptoms compared to control plants. A concentration of 1 mg/ml of dsRNA was optimal for spray treatment and found that dsRNA produced in bacterial cultures could be applied to wheat heads without complete purification. Initial testing for exogenous dsRNA control of FHB symptoms was conducted with a gene target previously shown to provide strong resistance to Fusarium in transgenic wheat. Within this study researchers also tested five new F. graminearum gene targets to expand the list of viable targets for controlling this pathogen and, one of the five new targets consistently caused a significant reduction in FHB symptoms while three of the targets significantly reduced symptoms in at least one experiment. The final gene target had no impact on FHB symptoms. They also tested the impact of targeting multiple genes at once to control FHB symptoms and found that targeting multiple genes simultaneously did not appear to improve disease reduction compared to single gene-targeting treatments.
The dsRNA treatments used in this study did not have a great impact on FHB symptoms as indicated by previous publications using transgenic plants; however, exogenous dsRNA treatments reduced symptoms by 33-50 % suggesting that it is a promising method to control FHB. Further work is needed to determine how dsRNA treatments will perform in a field setting. Also, more understanding of the dsRNA uptake into fungi is needed to understand how durable this approach will be to resistance and, to make the treatments more effective.