Priming For Enhancedargonaute2activation Accompanies Induced Resistance To Cucumber Mosaic Virus Inarabidopsis Thaliana

Systemic acquired resistance (SAR) is a broad-spectrum disease resistance response that can be induced upon infection from pathogens or by chemical treatment, such as with benzo-(1,2,3)-thiadiazole-7-carbothioic acidS-methyl ester (BTH). SAR involves priming for more robust activation of defence genes upon pathogen attack. Whether priming for SAR would involve components of RNA silencing remained unknown. Here, we show that upon leaf infiltration of water, BTH-primedArabidopsis thalianaplants accumulate higher amounts of mRNA ofARGONAUTE (AGO)2andAGO3, key components of RNA silencing. The enhancedAGO2expression is associated with prior-to-activation trimethylation of lysine 4 in histone H3 and acetylation of histone H3 in theAGO2promoter and with induced resistance to the yellow strain of cucumber mosaic virus (CMV[Y]). The results suggest that primingA. thalianafor enhanced defence involves modification of histones in theAGO2promoter that conditionAGO2for enhanced activation, associated with resistance to CMV(Y). Consistently, the fold-reduction in CMV(Y) coat protein accumulation by BTH pretreatment was lower inago2than in wild type, pointing to reduced capacity ofago2to activate BTH-induced CMV(Y) resistance. A role ofAGO2in pathogen-induced SAR is suggested by the enhanced activation ofAGO2after infiltrating systemic leaves of plants expressing a localized hypersensitive response upon CMV(Y) infection. In addition, local inoculation of SAR-inducingPseudomonas syringaepv.maculicolacauses systemic priming for enhancedAGO2expression. Together our results indicate that defence priming targets theAGO2component of RNA silencing whose enhanced expression is likely to contribute to SAR.