Plant researchers at the University of California have shown that Phytophthora, a plant pathogen, produces proteins that suppress key host mechanisms of immunity. Phytophthora is a group of eukaryotes that includes P. infestans, the pathogen responsible for the devastating Irish potato famine of the 19th century, and P. sojae, which infects soya bean plants. Even today, P. infestans infection of potato crops around the world causes annual losses of $6 billion, with a further $2 billion lost due to P. sojae.
RNA silencing is a common defence against infection, particularly by viruses, in both plants and animals. Infection induces production of siRNAs, which bind to foreign RNAs and target them for cleavage by the enzyme Dicer. The idea is that this prevents translation of foreign RNAs; however, many pathogens have found ways to get around it by making proteins that interfere with siRNA function.
Many such molecules feature a conserved N-terminal RXLR motif, which was used to screen the Phytophthora genome and identify genes that could encode suppressors of RNA silencing. Wenbo Ma and colleagues transfected another plant, Nicotiana benthamiana, with these genes and co-expressed them with green fluorescent protein (GFP). Nicotiana benthamiana can be made to constitutively express GFP, which accumulates and induces production of siRNAs that prevent GFP production. Therefore, strong fluorescence will be observed if a Phytophthora protein that inhibits the silencing of GFP is co-expressed in the cell. Using this technique, the group identified two proteins that suppressed FRP silencing, which they named Phytophthora supressors of RNA silencing (PSR) 1 and 2.
The study, published this week in Nature Genetics, is the first to show that eukaryotic pathogens can also produce molecules that dampen the host RNA silencing defence. These results may have implications for improving crop resistance to these destructive pathogens in the future.
