Host-induced gene silencing of the amino acid biosynthesis gene acetolactate synthase of Phytophthora infestans caused strong enhanced late blight resistance of potato in the field.

Late blight caused by Phytophthora infestans is the most serious disease of potatoes. Here we present the effectiveness of the host-induced gene silencing (HIGS) technology against an amino acid biosynthesis gene of the pathogen to increase the resistance against the plant-infecting oomycete in the field. A RNAi hairpin construct directed against the acetolactate synthase (ALS) gene of Phytophthora infestans was transferred into potato. HIGS-ALS potato lines displayed efficient target gene silencing revealed by a luciferase reporter gene assay. Plant-derived siRNAs targeting the oomycete's ALS gene were detected by small RNA sequencing. ALS gene expression of P. infestans was reduced during the early infection stages of HIGS-ALS potatoes, as shown by qRT-PCR. HIGS-ALS plants revealed an enhanced late blight resistance in detached leaf assays. ALS gene silencing also conferred strong enhanced late blight resistance to the HIGS lines in trials under near-field conditions in Europe and in field trials in the USA against European and US P. infestans isolates, respectively. These results demonstrated the value of the HIGS technology for the development of a new quantitative resistance source for potato against Phytophthora infestans.