Attenuation of Histone Methyltransferase KRYPTONITE-mediated transcriptional gene silencing by Geminivirus

Abstract

Although histone H3K9 methylation has been intensively studied in animals and a model plant Arabidopsis thaliana, little is known about the evolution of the histone methyltransferase and its roles in plant biotic stress response. Here we identified a Nicotiana benthamiana homolog of H3K9 histone methyltransferase KRYPTONITE (NbKYP) and demonstrated its fundamental roles on methylation of plant and virus, beside of leading to the suppression of endogenous gene expression and virus replication. NbKYP and another gene encoding DNA methyltransferase CHROMOMETHYLTRANSFERASE 3 (NbCMT3-1) were further identified as the key components of maintenance of transcriptional gene silencing, a DNA methylation involved anti-virus machinery. All three types of DNA methylations (asymmetric CHH and symmetric CHG/CG) were severely affected in NbKYP-silenced plants, but only severe reduction of CHG methylation found in NbCMT3-1-silenced plants. Attesting to the importance of plant histone H3K9 methylation immunity to virus, the virulence of geminiviruses requires virus-encoded trans-activator AC2 which inhibits the expression of KYP via activation of an EAR-motif-containing transcription repressor RAV2 (RELATED TO ABI3 and VP1). The reduction of KYP was correlated with virulence of various similar geminiviruses. These findings provide a novel mechanism of how virus trans-activates a plant endogenous anti-silencing machinery to gain high virulence.

Antagonistic Actions of FPA and IBM2 Regulate Transcript Processing from Genes Containing Heterochromatin

“…In the presence of Hpa Hiks1, the fine regulation of RPP7 transcripts mediated by both IBM2 and FPA favors accumulation of a long transcript to induce a rapid and specific immune response . Similarly, the importance of H3K9 methylation in resistance to viruses has been described (Sun et al, 2015), and the virulence of geminiviruses requires a viral protein that inhibits expression of the main plant H3K9 methyltransferase, KYP. To reinforce the action of KYP, rapid modulation of IBM1 gene expression, mediated by both FPA and IBM2, is likely essential.…”

Differential response of cassava genotypes to infection by cassava mosaic geminiviruses

“…Virus encoded suppressors of gene silencing (VSR) have been implicated in virus pathogenicity (Csorba et al, 2015, Derrien et al, 2012, Garcia-Ruiz et al, 2015). Although the suppressors encoded by CMGs have not been well characterized, it is suggested that ACMV-CM AC4 targets post-transcriptional gene silencing (PTGS) whereas EACMV and ICMV AC2 potentially target both PTGS and transcriptional gene silencing (TGS) (Sun et al, 2015, Vanitharani et al, 2004). ICMV isolate ICMV-SG was recently shown to accumulate higher virus titer and greatly represses TGS in N .…”

“…ICMV isolate ICMV-SG was recently shown to accumulate higher virus titer and greatly represses TGS in N . benthamiana compared to a less virulent isolate ICMV-Dha (Sun et al, 2015). Targeting PTGS and TGS by EACMV KE2 (K201) AC2 may contribute to the high pathogenicity of the former as compared to ACMV-CM reported here.…”

“…Similar results were reported in pepper plants recovering from infection by pepper golden mosaic virus (PepGMV), suggesting interruption of plant defense mechanisms downstream of vsRNAs production (Rodriguez-Negrete et al, 2009). Methylation of the geminivirus DNA genome has been reported as a dominant defense mechanism that impedes viral replication and transcription, resulting in host recovery from infection (Butterbach et al, 2014, Raja et al, 2014, Sun et al, 2015). It is important that further investigations focus on determining whether targeted methylation of the viral genome is a functional component of the CMD1, CMD2 and CMD3 resistance mechanisms in cassava.…”

Aphid feeding induces the relaxation of epigenetic control and the associated regulation of the defense response inArabidopsis

“…This result correlates with our observed reduction of sRNAs in centromeric and pericentromeric regions (rich in H3K9me2) and the observed changes in CHH and CHG methylation (tightly associated with H3K9me2). Interestingly KYP has been previously associated with the regulation of the defense against geminiviruses (Castillo-Gonzalez, Liu et al, 2015, Raja, Sanville et al, 2008, Sun, Tee et al, 2015 and the maintenance of baminobutyric acid (BABA)-induced priming of the salicylic acid (SA)-dependent defense response (Luna, Lopez et al, 2014). In summary, our proof-of-concept analysis indicates that, indeed, mutants in different layers of epigenetic regulation show enhance resistance against aphid settlement.…”