Argonaute 15 regulates immunity of potato to filamentous pathogens through transposon-derived small RNAs
Abstract
Canonical ribonucleases such as Argonaute (AGO) participate in RNA-based gene regulatory processes. They are important for small RNA (sRNA) association and the formation of RNA-induced RNA silencing complexes. Potato (Solanum tuberosum) has 14 AGO-encoding genes, including the Solanum-specific StAGO15, which is activated upon infection by filamentous pathogens. The resistant phenotype conferred by ectopic expression of StAGO15 was abolished in plants in which a 21-nt sequence in the PAZ-encoding region of StAGO15 was targeted using an artificial miRNA. We then analyzed the sRNAs associated with StAGO15 in Phytophthora infestans leaf infection. Small RNAs were coimmunoprecipitated, subjected to deep sequencing, and analyzed against the P. infestans and potato genomes, in which cleavage sites were predicted by applying the R package, smartPARE. The most notable difference between pathogen-and water-inoculated StAGO15 samples was observed in the transposon (TE) category. Here, two categories of nonautonomous TE dominated: the miniature inverted-repeat transposable elements (MITEs) and short interspersed nuclear elements. The StAGO15-associated sRNAs bind and target complementary endogenous potato RNAs. One example is siRNA122, which acts on a cleavage site in the resistance gene PGSC0003DMG400015681 (StRES1). siRNA122 is derived from a MITE; upon deletion of this source sequence, resistance is restored. Our findings provide new insights into the plant immune surveillance network and demonstrate that deleting a 21 nt RNA sequence from a MITE transposon restored a defense response against two important filamentous pathogens. Significance Statement Potato is a major food crop in the world with increasing importance in the developing countries due to its high adaptive capability to variable climate conditions. The potato plant is known to be attacked by many pathogens and insects during its growing season. Here we demonstrate the importance of a small RNA affecting the defense level to the late and early blight diseases which provides new possibilities for crop improvement.