Unraveling the epigenetic basis of transgenerational immune priming

López, Ana and Luna, Estrella and Roberts, Mike and Vera, Pablo and Ton, Jurriaan (2013) Unraveling the epigenetic basis of transgenerational immune priming. In: IOBC-WPRS Bulletin. UNSPECIFIED, pp. 33-35. ISBN 9789290672678

Abstract

Plants can prime their innate immune system after perception of certain environmental signals (Conrath et al. 2006). This defence state allows plants to respond faster and stronger to future attacks by microbes or insects. Priming is a long-lasting defence mechanism in plants, which was recently found to be transmittable to following plant generations (Slaughter et al. 2012, Luna et al. 2012, Rasmann et al. 2012). Long-lasting defence priming can, therefore, be regarded as a form of immunological memory in plants. Previously, we demonstrated that disease-exposed Arabidopsis produce progeny with enhanced resistance against biotrophic pathogens (Luna et al. 2012). This transgenerational resistance is based on priming of SA-inducible defence genes and an enrichment of chromatin marks associated with a permissive state of transcription at the promoter regions of these primed genes. Although histone modifications can have a long-lasting impact on gene expression (Vaillant & Paszkowski 2007), there is no convincing evidence that they can be transmitted through meiosis directly (Pecinka & Scheid 2012). In contrast, there is ample evidence for meiotic transmission of DNA methylation patterns, which is closely related to histone modifications (Law & Jacobsen 2010). Recently, we identified an allele of the NRPD2 gene in a forward mutant screen designed to reveal factors regulating immunity in Arabidopsis (Lopez et al. 2011). NRPD2 encodes the second largest subunit of the plant-specific RNA Polymerases IV and V, which are crucial for the RNA-directed DNA methylation (RdDM) pathway (Onodera et al. 2005). We discovered that RdDM defective mutants displayed enhanced disease resistance towards biotrophic pathogens, which was based on priming of expression of SA-dependent genes and an enrichment of positive chromatin marks at the promoter regions of these genes. It has been reported previously that infection by P. syringae causes large-scale DNA hypomethylation in Arabidopsis (Pavetet al. 2006, Dowen et al. 2012). We therefore hypothesized that a proportion of this hypo-methylated DNA may be transmitted to the next generation, where it guides trans-acting chromatin remodelers to relax the chromatin associated with priming-responsive defence genes. Indeed, experiments with the various RdDM mutants revealed similar defence priming phenotypes as those expressed by the progenies from P. syringae-infected wild-type plants (Luna et al., 2012; Luna & Ton, 2012). Together, these results strongly suggest that transgenerational defence priming is transmitted through hypomethylated regulatory genes. Further experiments are needed to elucidate the exact role of DNA methylation in transgenerational priming. These experiments aim to (i) address the trans-acting mechanisms by which SA-dependent defence genes become transcriptionally primed in progenies from diseased plants and (ii) identify the stable epialleles responsible for transmission and maintenance of transgenerational priming in following generations. For example, we will subject progenies to methylation-sensitive AFLP analysis in order to obtain a global impression of the extent of differentially methylated DNA regions (DMRs). Moreover, we will take advantage of publically available methylome data from recently published studies (Dowen et al. 2012, Stroud et al. 2013) to identify candidate DMRs controlling transgenerational priming. To verify the role of these regions, we will design methylation-sensitive, quantitative PCR markers to measure the degree and transgenerational consistency of differential methylation in each candidate region. These markers will also be used to study within-generation responses in systemic plant parts following localized pathogen inoculation. Finally, we will create an epigenetic mapping population of Arabidopsis progeny lines, in order to confirm causal relationships between transgenerational priming phenotypes and meta-stable DMRs. Priming boosts the responsiveness of plant innate immunity system in a cost-efficient manner because it prepares the plant to respond more efficiently to future attacks without a direct activation of costly defense mechanisms (Van Hulten et al. 2006). For these reasons, priming creates opportunities to improve integrated pest and disease management schemes for agricultural crops. Moreover, the project will address the fundamental question of how plants can acquire long-term adaptation of their immune system to hostile environmental conditions.