Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2017
Publication Date: 5/30/2017
Publication URL: http://handle.nal.usda.gov/10113/5722697
Citation: Cooper, B., Campbell, K. 2017. Protection against common bean rust conferred by a gene silencing method. Phytopathology. 107(8):920-927. https://doi.org/10.1094/PHYTO-03-17-0095-R.
Interpretive Summary: Millions of indigent people in Central America and Africa who practice subsistence agriculture depend on dry beans as a primary source of protein. Their food security is threatened by Uromyces appendiculatus, an obligate biotrophic fungus that causes rust disease on the common, dry bean plant, Phaseolus vulgaris. To better understand which fungal proteins are important for infection, we decreased the amounts of those fungal proteins using a gene silencing mechanism. Gene silencing occurs through a biochemical process that specifically degrades RNA as it is made from a specific DNA gene. When the RNA amounts decrease, less protein for that gene is made. When the amounts for four fungal proteins were reduced, the fungus did not accumulate on the leaves and the leaves had less disease. In conclusion, our experiments have shown that four bean rust fungal genes promote fungal infection, and that the inhibition of these proteins can protect bean plants from rust. These results are most likely to influence scientists at universities, government agencies and companies who are designing new methods to fight rust diseases.
Technical Abstract: Rust disease of the dry bean plant, Phaseolus vulgaris, is caused by the fungus Uromyces appendiculatus. The fungus acquires its nutrients and energy from bean leaves using a specialized cell structure, the haustorium, through which it secretes effector proteins that contribute to pathogenicity by defeating the plant immune system. Candidate effectors have been identified by DNA sequencing and motif analysis, and some candidates have been observed in infected leaves by mass spectrometry. To assess their roles in pathogenicity, we have inserted small fragments of genes for five candidates into bean pod mottle virus (BPMV). Plants were infected with recombinant BPMV and then inoculated with U. appendiculatus. Virus-infected plants expressing gene fragments for four of five candidate effectors accumulated lower amounts of rust and had dramatically less rust disease. By contrast, controls that included a fungal gene fragment for a septin protein not expressed in the haustorium died from a synergistic reaction between the virus and the fungus. The results imply that RNA generated in the plant moved across the fungal haustorium to silence effector genes important to fungal pathogenicity. This study shows that four bean rust fungal genes encode effectors and that the expression of fungal RNA in the plant can be an effective method for protecting beans from rust.