EDM2 is required for RPP7‐dependent disease resistance in Arabidopsis and affects RPP7 transcript levels

Abstract

SummarySpecific disease resistance of Arabidopsis thaliana against the Hyaloperonospora parasitica isolate Hiks1 (HpHiks1) is mediated by RPP7. Although this disease resistance gene encodes a typical nucleotide binding site leucine-rich repeat (NB-LRR) disease resistance protein, its function is independent of the defense hormone salicylic acid and most known genes required for plant immune responses. We identified EDM2 (enhanced downy mildew 2) in a genetic screen for RPP7 suppressors. Mutations of EDM2 phenocopy RPP7 mutations, but do not affect other tested disease resistance genes. We isolated EDM2 by map-based cloning. The predicted EDM2 protein is structurally unrelated to previously identified components of the plant immune system, bears typical features of transcriptional regulators, including plant homeodomain (PHD)-finger-like domains, and defines a plant-specific protein family. In edm2 mutants both constitutive and HpHiks1-induced RPP7 transcript levels are reduced, suggesting that EDM2 is either a direct or an indirect regulator of RPP7 expression. Microarray analyses defined a set of defense-associated genes, the expression of which is suppressed during successful HpHiks1 colonization of either rpp7 or edm2 plants. This transcriptional phenotype is counteracted by an EDM2/RPP7-dependent mechanism.

An alternative polyadenylation mechanism coopted to the Arabidopsis RPP7 gene through intronic retrotransposon domestication

“…Such pathogen recognition-induced changes of the R7/ECL ratio, H3K9me2 levels at the APAS for ECL, or RPP7-coding transcript levels were not observed in edm2-2 ( Fig. 6 B and C) (23). Multiple mechanisms are likely to adjust levels of RPP7 coding transcripts after HpaHiks1 infection.…”

“…(E) Typical trypan blue-stained cotyledons of 2-wk-old Col-0, edm2-2, and suvh456 seedlings 7 d postinfection with 5 × 10 4 spores of the H. arabidopsidis (Hpa) isolate Hiks1 mL −1 . This Hpa isolate is highly specifically recognized by RPP7 (23,41).Trypan blue stains Hpa structures as well as plant hypersensitive response (HR) sites dark blue. HR, HR site; Hy, Hpa hyphae; Sp, Hpa sporangiophores.…”

“…We previously reported on the Arabidopsis gene EDM2 (enhanced downy mildew 2), which has a promoting effect on transcript levels of the NLR gene resistance to Peronospora parasitica (RPP7) (23). Although we also showed EDM2 to contribute to transcriptional TE silencing by modulating levels of the repressive PHM H3K9me2 (24), it has been unclear how EDM2 affects RPP7.…”

“…HR, HR site; Hy, Hpa hyphae; Sp, Hpa sporangiophores. HR is a programmed cell death response closely associated with immunity against Hpa whereas Hy and Sp occur when immunity fails and the plant is susceptible to Hpa (23,30).…”

Role of alternative polyadenylation in epigenetic silencing and antisilencing

“…The significance of the work on EDM2 and ASI1 is that they provide direct genetic evidence demonstrating an involvement of epigenetic marks in the selection of APA sites. EDM2, identified as an enhancer of fungal disease resistance (17), specifically recognizes intronic heterochromatin in its target genes presumably through its composite plant homeodomain (PHD) domain (4). This binding of EDM2 somehow blocks the poly(A) site choice of the transcript in the vicinity of the binding location.…”