In Arabidopsis, the MYC2 transcription factor on the one hand and the AP2/ERF transcription factors ORA59 and ERF1 on the other hand regulate distinct branches of the jasmonic acid (JA) signaling pathway in an antagonistic fashion, co-regulated by abscisic acid (ABA) and ethylene, respectively. Feeding by larvae of the specialist herbivorous insect Pieris rapae (small cabbage white butterfly) results in activation of the MYC-branch and concomitant suppression of the ERF-branch in insect-damaged leaves. Here we investigated differential JA signaling activation in undamaged systemic leaves of P. rapae-infested plants. We found that the MYC2 transcription factor gene was induced both in the local insect-damaged leaves and the systemic undamaged leaves of P. rapae-infested Arabidopsis plants. However, in contrast to the insect-damaged leaves, the undamaged tissue did not show activation of the MYC-branch marker gene VSP1. Comparison of the hormone signal signature revealed that the levels of JA and (+)-7-iso-jasmonoyl-L-isoleucine raised to similar extents in locally damaged and systemically undamaged leaves, but the production of ABA and the JA precursor 12-oxo-phytodienoic acid was enhanced only in the local herbivore-damaged leaves, and not in the distal undamaged leaves. Challenge of undamaged leaves of pre-infested plants with either P. rapae larvae or exogenously applied ABA led to potentiated expression levels of MYC2 and VSP1, with the latter reaching extremely high expression levels. Moreover, P. rapae-induced resistance, as measured by reduction of caterpillar growth on pre-infested plants, was blocked in the ABA biosynthesis mutant aba2-1, that was also impaired in P. rapae-induced expression of VSP1. Together, these results suggest that ABA is a crucial regulator of herbivore-induced resistance by activating primed JA-regulated defense responses upon secondary herbivore attack in Arabidopsis.

The cucumber mosaic virus (CMV) 2b viral suppressor of RNA silencing (VSR) is a potent counter-defense and pathogenicity factor that inhibits antiviral silencing by titration of short double-stranded RNAs. It also disrupts microRNA-mediated regulation of host gene expression by binding ARGONAUTE 1 (AGO1). But in Arabidopsis thaliana complete inhibition of AGO1 is counterproductive to CMV since this triggers another layer of antiviral silencing mediated by AGO2, de-represses strong resistance against aphids (the insect vectors of CMV), and exacerbates symptoms. Using confocal laser scanning microscopy, bimolecular fluorescence complementation, and co-immunoprecipitation assays we found that the CMV 1a protein, a component of the viral replicase complex, regulates the 2b-AGO1 interaction. By binding 2b protein molecules and sequestering them in P-bodies, the 1a protein limits the proportion of 2b protein molecules available to bind AGO1, which ameliorates 2b-induced disease symptoms, and moderates induction of resistance to CMV and to its aphid vector. However, the 1a protein-2b protein interaction does not inhibit the ability of the 2b protein to inhibit silencing of reporter gene expression in agroinfiltration assays. The interaction between the CMV 1a and 2b proteins represents a novel regulatory system in which specific functions of a VSR are selectively modulated by another viral protein. The finding also provides a mechanism that explains how CMV, and possibly other viruses, modulates symptom induction and manipulates host-vector interactions.

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19Jasmonic acid (JA) is an important plant hormone in the regulation of defenses 20 against chewing herbivores and necrotrophic pathogens. In Arabidopsis thaliana, the 21 JA response pathway consists of two antagonistic branches that are regulated by 22 MYC-and ERF-type transcription factors, respectively. The role of abscisic acid 23 (ABA) and ethylene (ET) in the molecular regulation of the MYC/ERF antagonism 24 during plant-insect interactions is still unclear. Here, we show that production of ABA 25 induced in response to leaf-chewing Pieris rapae caterpillars is required for both the 26 activation of the MYC-branch and the suppression of the ERF-branch during 27 herbivory. Exogenous application of ABA suppressed ectopic ERF-mediated PDF1.2 28 expression in 35S::ORA59 plants. Moreover, the GCC-box promoter motif, which is 29 required for JA/ET-induced activation of the ERF-branch genes ORA59 and PDF1.2, 30 was targeted by ABA. Application of gaseous ET counteracted activation of the 31 MYC-branch and repression of the ERF-branch by P. rapae, but infection with the 32 ET-inducing necrotrophic pathogen Botrytis cinerea did not. Accordingly, P. rapae 33 performed equally well on B. cinerea-infected and control plants, whereas activation 34 of the MYC-branch resulted in reduced caterpillar performance. Together, these data 35 indicate that upon feeding by P. rapae, ABA is essential for activating the MYC-36 branch and suppressing the ERF-branch of the JA pathway, which maximizes 37 defense against caterpillars.38 65 proteins are degraded thereby releasing transcription factors that can activate JA-66 regulated genes.67 Within the JA pathway, two distinct, antagonistic branches of transcriptional 68 regulation are recognized; the MYC-branch and the ERF-branch. Feeding by 69 chewing herbivores activates the MYC-branch (Verhage et al., 2011; Vos et al., 70 2013b). This branch is controlled by the basic helix-loop-helix leucine zipper 71 transcription factors MYC2, MYC3 and MYC4 leading to transcription of hundreds of 72 4 JA-responsive MYC-branch regulated genes, including VSP1 and VSP2 (Anderson 73 et al. , 2004; Lorenzo et al., 2004; Fernández-Calvo et al., 2011; Niu et al., 2011). 74 Furthermore, previous studies have indicated that ABA plays a co-regulating role in 75 the activation of the MYC-branch (Anderson et al., 2004; Bodenhausen and 76 Reymond, 2007; Sánchez-Vallet et al., 2012; Vos et al., 2013b). For example, in the 77 ABA-deficient mutant aba2-1, expression of the JA-responsive gene VSP1 was 78 reduced upon feeding by caterpillars of Pieris rapae (small cabbage white) compared 79 to wild-type Col-0 plants (Vos et al., 2013b). In contrast to the herbivore-induced 80 MYC-branch, the ERF-branch is activated upon infection with necrotrophic 81 pathogens. The transcription factors EIN3 and EIL1 and the ERF transcription 82 factors ERF1 and ORA59 activate a large set of JA-responsive ERF-branch 83 regulated genes, including PDF1.2 (Caarls et al., 2015). The expression of ERF1, 84 ORA59 and PDF1.2 is im...

Cucumber mosaic virus (CMV) is an insect-transmitted virus that modifies interactions between its infected host plants and its aphid vectors in ways that increase the probability of transmission over various ranges and timescales (Donnelly et al., 2019; Carr et al., 2020). CMV does not infect its aphid vectors but influences their behaviour by altering the biochemistry of infected host plants. The paradigmatic example of this phenomenon is the interaction of the aphids Aphis gossypii and Myzus persicae with Cucurbita pepo plants infected with the Fny strain of CMV (Fny-CMV) (Mauck et al., 2010). CMV infection causes infected cucurbits to emit a mix of volatile organic compounds that attract aphids, but infection also induces accumulation of antixenotic, that is, feeding-deterrent, compounds in the leaves that ensure that aphids feed for only a brief time before moving to another plant (Mauck et al., 2010; Carmo-Souza et al., 2014). Because CMV is a nonpersistently transmitted virus (virus particles acquired from an infected plant are attached loosely to an aphid's stylet mouthparts), this short feed is sufficient to render the aphids competent to transmit infection to neighbouring hosts (Krenz et al., 2015).