Arabidopsis AGO1 N-terminal Poly-Q domain promotes phase separation and association with stress granules during heat stress

SUMMARY In Arabidopsis thaliana , ARGONAUTE1 (AGO1) plays a central role in microRNA (miRNA) and small interfering RNA (siRNA)-mediated silencing. Nuclear AGO1 is loaded with miRNAs and exported to the cytosol where it associates to the rough ER to conduct miRNA-mediated translational repression, mRNA cleavage and biogenesis of phased siRNAs. These latter, as well as other cytosolic siRNAs, are loaded into cytosolic AGO1, but in which compartment this happens is not known. Moreover, the effect of stress on AGO1 localization is still unclear. Here, we show that a 37°C heat stress (HS) promotes AGO1 protein accumulation in cytosolic condensates where it co-localizes with components of siRNA bodies and of stress granules (SGs). AGO1 contains a prion-like domain in its poorly characterized N-terminal Poly-Q domain, which, is sufficient to undergo phase separation, independent of the presence or absence of SGS3. HS only moderately affects the small RNA repertoire, the loading of AGO1 by miRNAs and the signatures of target cleavage, suggesting that its localization in condensates protects AGO1 rather than promotes or impairs its activity in reprograming gene expressing during stress. Collectively, our work shed new light on the impact of high temperature on a main effector of RNA silencing in plants.