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FRIMEDBIO-Fri prosj.st. med.,helse,biol

Dissection of an epigenetic memory mechanism modulating adaptive phenological traits in trees

Alternative title: Undersøkelse av en epigenetisk hukommelsesmekanisme som påvirker klimatilpasningen hos trær

Awarded: NOK 9.0 mill.

For trees to survive through the winter, the growth-dormancy cycle must be closely synchronized with the climatic conditions. In Norway spruce, such climatic adaptation characteristics were shown to be affected by temperature during seed development. Studies of genetically identical seeds (i.e. embryos), generated by tissue culture, have shown that these differences in climatic adaptation traits must be due to an epigenetic memory effect established during embryogenesis. The project aims to understand this epigenetic memory mechanism, and genes involved. From previous work we know that the timing of bud set in the autumn differs in Norway spruce plants from genetically identical seeds developed at different temperatures. This must accordingly be due to an epigenetic memory mechanism affecting gene activities. In this project we have shown that also the timing of de-hardening (reduced frost tolerance) and spring bud burst differ between such plants. Plants from a cooler embryogenesis climate deharden and show bud burst earlier than plants from warmer conditions during embryogenesis. Studies of embryos developed under different temperatures revealed differential expression of a range of genes associated with epigenetic regulation and differences in levels of small RNA molecules. This is consistent with action of micro-RNAs as an important epigenetic mechanism regulating the amount of specific mRNA molecules, and thus protein level. In addition, specific genes involved in response to temperature and stress conditions, also showed different activity depending on temperature during embryogenesis. In plants germinated from embryos developed under cooler and warmer climate, we have also found comprehensive differences in gene activities in late winter/spring. This was the case particularly for genes encoding transcription factors (proteins controlling activities of other genes), genes involved in protein synthesis, cell cycle control, hormone metabolism, stress-responses, regulation of growth and development as well as dehydrine genes involved in frost tolerance. We have also shown significant differences in gene expression between different parts of the shoot apical meristem. Particularly the stem cells (central mother cells) show differential expression of genes encoding transcription factors between plants from cooler and warmer embryogenesis climate. We have also observed significant differences in DNA methylation pattern in genes showing differential expression in embryos developed under cooler and warmer conditions and plants grown from these. Similar to different gene expression patterns between cells/tissues in the apical meristems, the DNA methylation patterns differed between cell/tissues. The largest degree of DNA methylation was observed under warm embryogenesis conditions, consistent with the later dehardening and spring bud burst in such plants. The project has contributed significantly to improved understanding of the background of the effect of temperature during embryogenesis on later climatic adaptation traits of the plants. In summary, the temperature under embryogenesis affects the epigenetic machinery and thus genes controlling the timing of bud burst and dehardening (important climatic adaptation traits). Differences in DNA methylation and micro RNAs due to cooler and warmer embryogenesis conditions are important epigenetic mechanisms involved.

Prosjektet har vært viktig for å bedre forståelsen av betydningen av epigenetikk i klimatilpasning hos trær. Spesielt har det bidratt til forståelse av mekanismene bak den epigenetiske hukommelsen av temperaturen under frøutviklingen. Slik kunnskap har betydelige praktiske implikasjoner for frøproduksjon under ulike klimaforhold og gir mulighet for å oppnå bedre klimatilpasset plantemateriale innen planteforedlingen. For prosjektets deltakere og omgivelser har prosjektet vært av stor betydning for å styrke kompetansen ndg. epigenetikk, klimatilpasning, plantemolekylærbiologi, -fysiologi og -genetikk. Prosjektet har styrket nasjonalt og internasjonalt forskningssamarbeid og bidratt vesentlig i utdanning av MSc-, PhD og postdoktor-kandidater så vel som i lavere-gradsutdanning og formidling. Den oppnådde kompetansen har stor overføringsverdi til andre planter enn trær og andre planteprosesser som f.eks. sykdomsresistens der epigenetikk er vist å spille en vesentlig rolle.

Trees are ecologically and economically extremely important, and climatic adaptation affecting their phenology (seasonal life cycle events) is crucial for their survival. It is still an open question to which extent climate change will compromise the mechanisms of the climatic adaptation in long-lived organisms with long generation intervals, such as trees. Surprisingly, traits such as the critical daylength for bud formation in the autumn and timing of bud burst in the spring, which are believed to have high heritability and to be under strong selection, were demonstrated to be greatly and persistently influenced by temperature during seed development in the conifer Norway spruce. The evidences strongly support that this is due to an epigenetic memory effect, but how it is initiated and maintained is unknown. There is also family variation in the ability to express the memory effect, indicating that there is genetic variation in the epigenetic machinery itself. In this project we aim to explore the basis of this epigenetic mechanism and to investigate the background of the variation between different families of Norway spruce in expressing the memory. Besides the obvious economic and ecological implications of epigenetically altered seed material, understanding overwintering mechanisms of trees has wide implications and will help to evaluate their abilities to adjust their growth-dormancy cycle to climatic conditions changing more rapidly than classical selection may keep up with. Understanding the epigenetic memory mechanism might also help to explain how clinal variation (continuous gradient) in daylength requirement for growth and bud formation in tree species, is established in nature.

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FRIMEDBIO-Fri prosj.st. med.,helse,biol

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