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

Granbarkbillen (Ips typograhus) med sin blåved sopp er ett av de få insektene i Norge som kan drepe friske grantrær og denne billen har gjort stor økonomisk skade i norske granskoger. Med global oppvarming er det forventet hyppigere angrep fra granbarkbillen i Norge. Nyere forskning har vist at behandling av grantrær med det naturlige plantehormonet metyl-jasmonat øker trærnes naturlige evne til å forsvare seg mot barkbilleangrep. Denne behandlingen minner om vaksinasjon. Behandling med metyl-jasmonat gjør også at grantreet produserer nye molekylære «minner». Disse «minnene» kan gi treet økt forsvarsevne mot barkbiller og andre patogener i flere måneder eller til og med år fremover. Denne muligheten til å øke forsvaret mot uønskede inntrengere raskt kalles "forsvarspriming". I dette prosjektet vi hadde lyst å forstå, på et cellulært og molekylært nivå, de forandringene som gjør treet i stand til å respondere raskt og hvordan disse «minnene» blir dannet etter metyl-jasmonat behandling. Vi bruket sekvensering av budbringer RNAs og små regulerende RNAs og lært at «forsvarspriming» av enzymer som beskytter grantrær mot sopp patogener er en av de viktige minnene lagret etter mety-ljasmonat behandling. Vi også sammenlignet jasmonate "minnene" fra gran og Arabidopsis for å bestemme likhet og forskjeller mellom en langvarig gymnosperm og en kortvarig angiosperm. Til slutt analyserer vi epigenetiske modifikasjoner av DNA og histon i metyljasmonatbehandlet gran for å bestemme hva og hvor disse endringene skjer.

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.