PLARNASENS · Sensing of cytoplasmic double-stranded RNA in plants

Elimination of foreign nucleic acids is key to host survival and reproduction. Long double-stranded RNA (dsRNA) in the cytoplasm is used to distinguish viral from host genetic material, and several metazoan dsRNA receptors elicit innate immune responses upon binding to dsRNA. Plants are thought to rely exclusively on RNA interference (RNAi) to eliminate RNA with double-stranded features. Two DICER-LIKE (DCL) enzymes, DCL4 and DCL2, act redundantly to cleave dsRNA into small interfering RNAs that in turn program RNA Induced Silencing Complexes to silence viral RNA using base pairing as specificity determinant. Our recent work shows that this view of plant innate antiviral immunity is incomplete. DCL4 has a dedicated role in RNAi, while DCL2 also mediates dsRNA sensing when DCL4-mediated RNAi has been defeated by viral anti-RNAi factors. dsRNA cleavage by DCL2 leads to activation of at least two nucleotide-binding leucine-rich repeat (NLR) intracellular immune receptors that act in series, not in parallel, to initiate signalling that ultimately establishes an immune state. This proposal launches unbiased screening approaches and tests specific hypotheses to answer four questions on this recently discovered plant dsRNA sensing pathway. (1) What is the nature of the molecular entity produced by DCL2 that is recognized by the sensor NLR? (2) How is activation of the sensor NLR linked to activation of the downstream signaling NLR? (3) Does DCL4-mediated RNAi and DCL2-mediated sensing occur in the same or in distinct cells, and where in the cell does DCL2-mediated dsRNA sensing occur? (4) How do viruses circumvent innate immune activation via DCL2-mediated dsRNA sensing? Answers to these questions represent a leap forward in our understanding of plant-virus interactions, with substantial impact in agrotechnology. The answers also deliver broad conceptual advances on how RNAi components are linked to immune signalling, and how NLR signalling operates molecularly.