Priming of plant immunity: dissecting the epigenetic basis of defense priming in Norway spruce

The spruce bark beetle (Ips typographus) with its blue-stain fungi has caused great economic and ecological losses in Norwegian spruce forests. Warming temperatures are predicted to cause an increase in the frequency of bark beetle attacks. Recent work has shown that treating spruce trees with a naturally produced tree hormone, methyl jasmonate, helps the trees defend themselves against spruce bark beetle attacks. This treatment is similar to a person getting a vaccination. Methyl jasmonate treatment produces new molecular "memories" for the tree. These "memories" can give the tree heightened defenses against bark beetles and other pathogens for months or even years after methyl jasmonate treatment. This ability to rapidly increase defenses against unwanted invaders is called «defense priming». In this project, we explored, at the cellular and molecular level, how these "memories" are made after methyl jasmonate treatment. We have sequenced messenger RNA and small regulating RNA from trees treated with methyl jasmonate and found that enzymes that protect spruce trees from fungal pathogens show a "primed" response. We also compared jasmonate "primed" responses of spruce and Arabidopsis to determine similarity and differences in "primed" response between a long-lived gymnosperm and a short-lived angiosperm. Finally, we analyzing epigenetic modifications to DNA and histone in methyl jasmonate treated spruce to determine what and where these changes occur.

We showed that treatment of Norway spruce with methyl jasmonate provides long-term protection to bark beetle attack. This demonstrates that Norway spruce defenses can be primed or sensitized using a naturally occurring phytohormone. These results can contribute to new forest management practices and the reduction in pesticide usage. We also published This work provides the bases for mechanistic defense priming studies in long-lived, non-model species such as Norway spruce. This work also provide a better understand of the complex defense response system of Norway spruce.

Priming of plant immunity and its molecular underpinning is a research topic of great basic and applied interest that until now only has been explored in angiosperm model plants. Over the last decade a primed defense response has been described in the gymnosperm Norway spruce with increased resistance to pests and pathogens, but absolutely nothing is known about the molecular or epigenetic mechanisms that govern this response. Norway spruce is attacked and killed by the spruce bark beetle Ips typographus and its symbiotic phytopathogenic bluestain fungi during intermittent outbreaks. An important factor maintaining the beetle-fungus complex at low, endemic levels most of the time is the tree's natural defenses. Spraying spruce trees with the plant hormone methyl jasmonate enables trees to respond more quickly and strongly when challenged several weeks after treatment, although initial changes in defense molecule accumulation are not observed. This type of response, known as defense priming, does not directly increase plant defenses against pests, but mediates a faster and/or stronger induction of defenses when an attack occurs. The primary objective of this project is to dissect the molecular and epigenetic basis of defense priming in Norway spruce. We will do this by determining changes in gene expression, metabolites, and DNA modification that contribute to defense priming, working together with internationally leading researchers on defense priming in Arabidopsis and taking advantage of massive RNA-Seq data generated in an ongoing FRIMEDBIO project. Methods to be used include DNA methylation, histone modification, and small regulatory RNA analysis. Gaining a deeper understanding of how defense priming works in Norway spruce at a molecular level will help inform how priming may be used in tree breeding and forest management practices, as well as provide the first knowledge of priming mechanisms in a gymnosperm/conifer.