After the onset of defence priming has been established, the resistance can be maintained throughout the life cycle of the plant. Since epigenetic mechanisms, such as chromatin modifications and DNA hypomethylation, can cause long-lasting adjustments in the sensitivity of defence genes, we investigated the possibility that maintenance of defence priming is under epigenetic control. We discovered that progeny from severely diseased Arabidopsis plants are primed to express defence genes (1). Isogenic Arabidopsis plants were exposed to fitness-reducing levels of bacterial speck disease by P. syringae, upon which their progenies were collected and tested for disease resistance. Compared to progeny from mock-inoculated plants, progeny from diseased plants were more resistant to (hemi-)biotrophic pathogens, such as downy mildew. This trans-generational resistance is maintained over at least two stress-free generation, indicating a relatively stable epigenetic basis of the resistance. Chromatin-immunoprecipitation analysis revealed that promoters of defence genes in primed progeny from diseased plants are enriched with chromatin marks that facilitate transcription induction (1).
To further examine the mechanisms by which trans-generational priming is transmitted to following generations, we carried out experiments with mutants in RNA-directed DNA methyation (RdDM), a signalling process that drives two forms of DNA methyation that are particularly abundant in plants (1,2). This analysis suggested that transgenerational priming is transmitted through a reduction in non-CpG DNA methylation. Future research will focus on identifying the hypo-methylated epi-alleles that maintain transgenerational priming over subsequent generations.
The discovery of epigenetic inheritance of defence priming not only changes the conventional view on how plant communities adapt to long-lasting biotic stress, it also provides concrete opportunities for novel strategies of sustainable crop crop protection. For instance, crop seed stocks are commonly derived from healthy plants. An efficient induction of epigenetic defence priming in parental plants would generate more resistant crop lines that require fewer chemicals to control disease.
1. Luna E et al. & Ton J (2012) Plant Physiol. 158, 844-853 2. Luna E & Ton J (2012) Plant Signal Behav 7, 615-618.