Site‐specific analysis reveals candidate cross‐kingdom small RNAs, tRNA and rRNA fragments, and signs of fungal RNA phasing in the barley–powdery mildew interaction

The establishment of host–microbe interactions requires molecular communication between both partners, which may involve the mutual transfer of noncoding small RNAs. Previous evidence suggests that this is also true for powdery mildew disease in barley, which is caused by the fungal pathogen Blumeria hordei. However, previous studies lacked spatial resolution regarding the accumulation of small RNAs upon host infection by B. hordei. Here, we analysed site‐specific small RNA repertoires in the context of the barley–B. hordei interaction. To this end, we dissected infected leaves into separate fractions representing different sites that are key to the pathogenic process: epiphytic fungal mycelium, infected plant epidermis, isolated haustoria, a vesicle‐enriched fraction from infected epidermis, and extracellular vesicles. Unexpectedly, we discovered enrichment of specific 31–33‐base 5′‐terminal fragments of barley 5.8S ribosomal RNA in extracellular vesicles and infected epidermis, as well as particular B. hordei transfer RNA fragments in haustoria. We describe canonical small RNAs from both the plant host and the fungal pathogen that may confer cross‐kingdom RNA interference activity. Interestingly, we found first evidence of phased small interfering RNAs in B. hordei, a feature usually attributed to plants, which may be associated with the posttranscriptional control of fungal coding genes, pseudogenes, and transposable elements. Our data suggest a key and possibly site‐specific role for cross‐kingdom RNA interference and noncoding RNA fragments in the host–pathogen communication between B. hordei and its host barley.