Phytophthora infestans Ago1-bound 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 P. 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 transgenic and biochemical analyses on a predicted host target. Extensive targeting of potato and pathogen-derived sRNAs to a large number of mRNAs was observed, including 206 sequences coding for resistance (R) proteins in the host genome. The single miRNA encoded by P. infestans (miR8788) was found to target a potato lipase-like membrane protein-encoding gene (StLL1) localized to the tonoplast. Analyses of stable transgenic potato lines harbouring overexpressed StLL1 or artificial miRNA gene constructs demonstrated the importance of StLL1 during infection by P. infestans. Similarly, a miR8788 knock-down strain showed reduced growth on potato compared to the wild-type strain 88069. The data suggest that sRNA encoded by P. infestans can affect potato mRNA and thereby promote disease. Knowledge of the impact of pathogen small RNAs in plant defence mechanisms is of major significance to succeed in improved disease control management.