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 Hyaloperonospora arabidopsidis, 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 H. arabidopsidis sRNA (HpasRNA) functionality in ck-RNAi, we designed a novel CRISPR endoribonuclease Csy4/GUS reporter that enabled in situ visualization of HpasRNA-induced target suppression in Arabidopsis. The significant role of HpasRNAs together with AtAGO1 in virulence was revealed in plant atago1 mutants and by transgenic Arabidopsis expressing a short-tandem-target-mimic to block HpasRNAs, that both exhibited enhanced resistance. HpasRNA-targeted plant genes contributed to host immunity, as Arabidopsis gene knockout mutants displayed quantitatively enhanced susceptibility.

An Arabidopsis downy mildew non-RxLR effector suppresses induced plant cell death to promote biotroph infection

“…Ha CR1 ( HpaG806256 ) and Ha A1E ( HpaG814621 ) are putative pathogenicity factors that are highly expressed in H. arabidopsidis during Arabidopsis infection ( Asai et al , 2014 ). Ha DCL1 ( HpaG808216 ) is likely involved in biogenesis of H. arabidopsidis small RNAs, which we recently found to play an important role in suppressing plant genes for host infection ( Dunker et al , 2020 ). The fungal plant pathogen Botrytis cinerea uses small RNAs for Arabidopsis plant infection, too ( Weiberg et al , 2013 ), and HIGS against Botrytis DCL s indeed conferred disease resistance ( Wang et al , 2016 ).…”

“…S4B ). Why HaDCL1 RNAi plants did not reveal higher resistance despite the important role of pathogen small RNAs during infection ( Dunker et al , 2020 ) remains to be investigated. One possible explanation might be functional redundancy of the two Ha DCLs identified in the genome of H. arabidopsidis ( Bollmann et al , 2016 ).…”

Extracellular vesicles from Heligmosomoides bakeri and Trichuris muris contain distinct microRNA families and small RNAs that could underpin different functions in the host

“…S3 ). There is emerging data suggesting sRNAs from repetitive regions and transposable elements can in fact act as virulence factors in diverse parasite-host models including plant-parasite interactions ( Cai et al, 2019 , Dunker et al, 2020 , Hudzik et al, 2020 ). These classes of extracellular sRNAs that are found across diverse nematodes and other helminths therefore merit further attention.…”

The Evolutionary Significance of RNAi in the Fungal Kingdom

“…Lastly, in the oomycete biotrophic pathogen Hyaloperonospora arabidopsidis, 34 sRNAs with the capacity to translocate into plant cells and suppress host target genes were found. However, Arabidopsis siRNA biogenesis mutant strains displayed increased H. arabidopsidis growth, demonstrating the important role of siRNAs in plant immunity [ 69 ]. Therefore, the host–pathogen interaction is constantly evolving, and RNAi has become an advantage for both the host and the pathogen.…”