A spray containing double-stranded RNA (dsRNA) molecules can inhibit the growth of a notorious fungal pest, according to a new study of barley plants (PLOS Pathog. 2016, DOI: 10.1371/journal.ppat.1005901). Such RNA sprays may provide an alternative strategy to traditional chemical pesticides and genetically modified crops for combating agricultural pests.
Karl-Heinz Kogel of Justus Liebig University Giessen and colleagues created an RNA that turns off a gene in Fusarium graminearum, a pest of corn, wheat, and barley that reduces crop yields and produces toxins that can contaminate the food supply. Common weapons against this fungus include compounds that block CYP51 enzymes responsible for producing ergosterol, the essential cholesterol analog in fungus. Kogel’s team blocked ergosterol formation in fungi with a dsRNA that bound CYP51 gene transcripts.
Simply spritzing barley leaves with the dsRNA reduced expression of the CYP51 gene by about 50%. “We were expecting that the dsRNA would be immediately taken up by the fungus at the plant surface,” Kogel says. But when they examined the plants, they saw that the plant absorbed the dsRNA into its vasculature and transported it to the ends of the leaves. There the fungus grabbed the dsRNA from the leaf and used its own cellular machinery to process the long dsRNA into shorter RNA strands that then silenced the enzyme gene. “This is unexpected, and might make RNA sprays even more efficient than we thought,” because the plant vasculature could distribute the RNA throughout the plant’s tissues, Kogel says.
“Every time we turn around, we find a paper that shows that dsRNA goes outside the boundaries that we previously thought it had,” says geneticist Jack Heinemann of the University of Canterbury.
Kim Hammond-Kosack of Rothamsted Research, an agricultural research institute in England, says dsRNA fungicides may prove advantageous if they can simultaneously target multiple genes essential for fungal survival. But such sprays also have disadvantages, including a potentially higher cost and the inability to affect fungi that lack RNA-processing machinery. There is also a risk that mutations in the RNA-processing machinery of pathogens could lead to a wider resistance of dsRNA fungicides.