GSE185133
Arabidopsis Apoplastic Fluid Contains sRNA- and Circular RNA-Protein Complexes that Are Located Outside Extracellular Vesicles
The extracellular space within plant leaves is called the apoplast, and functions as a key battlefield between plants and pathogens. Previously, we have shown that apoplastic wash fluid purified from Arabidopsis leaves contains small RNAs (sRNAs). To investigate whether these RNAs are encapsulated inside extracellular vesicles (EVs), we treated EVs isolated from Arabidopsis leaves with the protease trypsin and RNase A, which should degrade RNAs located outside of EVs, but not those located inside. These analyses revealed that apoplastic RNAs are mostly located outside of EVs and are associated with proteins. Additional analyses of these extracellular RNAs (exRNAs) revealed that they are made up both sRNAs and long non-coding RNAs (lncRNAs), including circular RNAs (circRNAs). We also found that exRNAs are highly enriched in the post-transcriptional modification N6-methyladenine (m6A). Consistent with this, we identified a putative m6A-binding protein in apoplastic wash fluids, GLYCINE-RICH RNA BINDING PROTEIN 7 (GRP7), as wells as the small RNA-binding protein ARGONAUTE2 (AGO2).  These two proteins co-immunoprecipitated with each other, and with lncRNAs, including circRNAs. Mutation of GRP7 or AGO2 caused changes in both the sRNA and lncRNA content of apoplastic wash fluid, suggesting that these proteins contribute to the secretion and/or stabilization of exRNAs. Overall design: circular RNA profiling of extracellular RNA treated in ago2 and grp7 mutants.