Oomycete small RNAs bind to the plant RNA-induced silencing complex for virulence.

Abstract

The exchange of small RNAs (sRNAs) between hosts and pathogens can lead to gene silencing in the recipient organism, a mechanism termed cross-kingdom RNAi (ck-RNAi). While fungal sRNAs promoting virulence are established, the significance of ck-RNAi in distinct plant pathogens is not clear. Here, we describe that sRNAs of the pathogen <i>Hyaloperonospora arabidopsidis</i>, which represents the kingdom of oomycetes and is phylogenetically distant from fungi, employ the host plant's Argonaute (AGO)/RNA-induced silencing complex for virulence. To demonstrate <i>H. arabidopsidis</i> sRNA (<i>Hpa</i>sRNA) functionality in ck-RNAi, we designed a novel CRISPR endoribonuclease Csy4/GUS reporter that enabled in situ visualization of <i>Hpa</i>sRNA-induced target suppression in Arabidopsis. The significant role of <i>Hpa</i>sRNAs together with <i>At</i>AGO1 in virulence was revealed in plant <i>atago1</i> mutants and by transgenic Arabidopsis expressing a short-tandem-target-mimic to block <i>Hpa</i>sRNAs, that both exhibited enhanced resistance. <i>Hpa</i>sRNA-targeted plant genes contributed to host immunity, as Arabidopsis gene knockout mutants displayed quantitatively enhanced susceptibility.