Phytophthora infestans Ago1-associated miRNA promotes potato late blight disease.

Phytophthora spp. incite serious plant damages by exploiting a large number of effector proteins and small RNAs (sRNAs). Several reports are describing modulation of host RNA biogenesis and defence gene expression. Here, we analysed Phytophthora infestans Argonaute (Ago) 1 associated small RNAs during potato leaf infection. sRNAs were co-immunoprecipitated, deep sequenced and analysed against the P. infestans and potato genomes, followed by transcript analyses and transgenic assays on a predicted target. Extensive targeting of potato and pathogen-derived sRNAs to a range of mRNAs was observed, including 638 sequences coding for resistance (R) proteins in the host genome. The single miRNA encoded by P. infestans (miR8788) was found to target a potato alpha/beta hydrolase-type encoding gene (StABH1), a protein localized to the plasma membrane. Analyses of stable transgenic potato lines harbouring overexpressed StABH1 or artificial miRNA gene constructs demonstrated the importance of StABH1 during infection by P. infestans. miR8788 knock-down strains showed reduced growth on potato. In analogy, elevated StABH1 expression levels were observed when inoculated with the two knock-down strains compared to the wild-type strain 88069. The data suggest that sRNA encoded by P. infestans can affect potato mRNA, and thereby expand its already known multifaceted strategies to facilitate infection.