USING RNAi TO PROTECT CROPS

Field experiments testing the use of genetically engineered RNA interference against eukaryotic pathogens, an approach called host-induced gene silencing (HIGS), are just now embarking on the long road to commercialization.

But even if HIGS is successful at warding off disease-causing insects or fungi, cost may be an insurmountable hurdle. John Pitkin, Global Disease Management Lead at Monsanto, says the commercialization of a transgenic crop costs on the order of $130 million to $140 million. “Finding one single disease that reaches that bar for a HIGS approach is a pretty daunting task,” he says. In other words, can the financial burden of a pathogen justify the mammoth expense of getting a transgenic, RNAi-protected crop on the market?


Then there’s the public’s discomfort with genetically modified crops, particularly in Europe, notes Karl-Heinz Kogel, a plant biologist who uses HIGS at Justus Liebig University in Germany. To get around genetic manipulation, he and others have tested the possibility of an RNA spray. Rather than introducing small RNAs via transgenes in the plant, it may be possible to just apply the interfering molecules directly to the crop, an approach called spray-induced gene silencing (SIGS).


 Several months ago, Kogel’s team reported on its experiments spraying barley plants with a long noncoding double-stranded RNA—a precursor to the small RNAs used in RNAi—targeting the same genes critical for Fusarium graminearum survival that he attacked using HIGS in 2013. It worked: the plants suffered far less disease (PLOS Pathog, 12: e1005901, 2016).


Interestingly, the researchers found that the RNA was taken up by the plant and transferred into the fungus—results that add to another finding that RNAs could directly enter the pathogen as well. “Not only does the fungus take it up from the surface and is then killed, but the plant takes it up, transports it through the plant body, and then the fungus takes it up again,” says Kogel. He adds that direct RNA intake by the fungus is much less efficient, and thus the uptake and transfer by plants may be necessary to elicit the protective effect of the spray.