Global warming threatens many aspects of human life, for example, by reducing crop yields. Breeding heat-tolerant crops using genes conferring thermotolerance is a fundamental way to help deal with this challenge. Here we identify a major quantitative trait locus (QTL) for thermotolerance in African rice (Oryza glaberrima), Thermo-tolerance 1 (TT1), which encodes an α2 subunit of the 26S proteasome involved in the degradation of ubiquitinated proteins. Ubiquitylome analysis indicated that OgTT1 protects cells from heat stress through more efficient elimination of cytotoxic denatured proteins and more effective maintenance of heat-response processes than achieved with OsTT1. Variation in TT1 has been selected for on the basis of climatic temperature and has had an important role in local adaptation during rice evolution. In addition, we found that overexpression of OgTT1 was associated with markedly enhanced thermotolerance in rice, Arabidopsis and Festuca elata. This discovery may lead to an increase in crop security in the face of the ongoing threat of global warming.

SUMMARYCucumber mosaic virus suppressor 2b (CMV2b) is a nuclear viral suppressor that interferes with local and systemic silencing and inhibits AGO1 slicer activity. CMV2b-mediated transgene hypomethylation and its localization in Cajal bodies suggests a role of CMV2b in RNA-directed DNA methylation (RdDM). However, its direct involvement in RdDM, or its binding with small RNAs (sRNAs) in vivo is not yet established. Here, we show that CMV2b binds both microRNAs (miRNAs) and small interfering RNAs (siRNAs) in vivo. sRNA sequencing data from the CMV2b immunocomplex revealed its preferential binding with 24-nt repeatassociated siRNAs. We provide evidence that CMV2b also has direct interaction with the AGO4 protein by recognizing its PAZ and PIWI domains. Subsequent analysis of AGO4 functions revealed that CMV2b reduced AGO4 slicer activity and the methylation of several loci, accompanied by the augmented accumulation of 24-nt siRNAs in Arabidopsis inflorescences. Intriguingly, CMV2b also regulated an AGO4-related epiallele independently of its catalytic potential, which further reinforces the repressive effects of CMV2b on AGO4 activity. Collectively, our results demonstrate that CMV2b can counteract AGO4-related functions. We propose that by adopting novel counter-host defense strategies against AGO1 and AGO4 proteins, CMV creates a favorable cellular niche for its proliferation.