Contrasting epigenetic control of transgenes and endogenous genes promotes post-transcriptional transgene silencing in Arabidopsis

Abstract

Transgenes that are stably expressed in plant genomes over many generations could be assumed to behave epigenetically the same as endogenous genes. Here, we report that whereas the histone H3K9me2 demethylase IBM1, but not the histone H3K4me3 demethylase JMJ14, counteracts DNA methylation of Arabidopsis endogenous genes, JMJ14, but not IBM1, counteracts DNA methylation of expressed transgenes. Additionally, JMJ14-mediated specific attenuation of transgene DNA methylation enhances the production of aberrant RNAs that readily induce systemic post-transcriptional transgene silencing (PTGS). Thus, the JMJ14 chromatin modifying complex maintains expressed transgenes in a probationary state of susceptibility to PTGS, suggesting that the host plant genome does not immediately accept expressed transgenes as being epigenetically the same as endogenous genes.

Epigenetic management of self and non-self: lessons from 40 years of transgenic plants

“…This step also is poorly understood, mostly because it requires performing reciprocal grafting experiments (wild-type recipient tissues grafted onto mutant emitting tissues vs mutant recipient tissues grafted onto wildtype emitting tissues) to determine at which step a given mutation impairs systemic PTGS. As expected, mutations in AGO1, DCL2/DCL4, RDR6 and SGS3 in either emitting or recipient tissues impair PTGS because these are core PTGS proteins required in every cell to execute PTGS [63][64][65][66]. So far, Figure 4.…”

“…It comprises genes encoding part of the TGS machinery: Po-lIV, RDR2, DCL3, AGO4 [63,65], and two genes, JMJ14 and NAC52, encoding a histone H3K4me3 demethylase and a NAC-domain transcription factor. JMJ14 and NAC52 are not required for PTGS execution nor for the production of the RNA silencing signal, but are required for transgene PTGS to become systemic [64,67]. Remarkably, DNA methylation increases at transgenic sequences in the absence of JMJ14 or NAC52, but not at any endogenous sequences [64].…”

“…JMJ14 and NAC52 are not required for PTGS execution nor for the production of the RNA silencing signal, but are required for transgene PTGS to become systemic [64,67]. Remarkably, DNA methylation increases at transgenic sequences in the absence of JMJ14 or NAC52, but not at any endogenous sequences [64]. Likely, it is not the absence of JMJ14 or NAC52 but transgene hypermethylation that prevents the reinitiation of PTGS.…”

“…Likely, it is not the absence of JMJ14 or NAC52 but transgene hypermethylation that prevents the reinitiation of PTGS. Indeed, PTGS reinitiation occurs normally in a jmj14 mutant background when hypermethylation is prevented by mutating simultaneously the DRM2 and CMT3 DNA methylases [64]. How JMJ14 specifically attenuates transgene DNA methylation and promotes systemic transgene PTGS is still not understood.…”

“…Indeed, whereas DNA methylation of endogenous PCGs is limited by the histone H3K9me2 demethylase IBM1, DNA methylation of transgenes driven by the 35S promoter is limited by the histone H3K4me3 demethylase JMJ14 but not IBM1. JMJ14 is not required for PTGS execution nor for the production of the RNA silencing signal, but is required for transgene PTGS to become systemic [64]. How JMJ14 specifically attenuates transgene DNA methylation and promotes systemic transgene PTGS is still not understood.…”