Molecular dissection of an adaptive epigenetic memory mechanism in norway spruce

Abstract

In Norway spruce, environmental conditions during the reproduction can greatly influence progeny performance. We found that the temperature during post meiotic megagametogenesis (zygotic embryogenesis) and seed maturation shift the growth cycle program of the embryos in the seeds, resulting in significant and long lasting phenotypic changes in the progeny. Traits that are affected include the timing of dehardening and bud burst in the spring; leader shoot growth cessation in the summer, and bud set and cold acclimation in the autumn. All processes are advanced or delayed in corre-spondence with the temperature during female repro-duction. Colder reproductive environment advance bud set and cold acclimation during autumn and deharden-ing and bud burst during spring in their progenies. Temperature dependent difference in timing of terminal bud formation in identical clones was equivalent to a 4– 6° latitudinal ecotypic difference. The progeny " remem-ber " the temperatures and photoperiod prevailing during zygotic embryogenesis and seed maturation and this memory, affecting the climatic adaptation in this species, is an epigenetic phenomenon. This phenomenon is not only of evolutionary signifi-cance but has clear practical implications. This memory can help the conifer to cope with the anticipated rapid change in climatic conditions. It will have importance for the deployment of seedlings produced in seed orch-ards containing clones that are translocated to warmer sites, and it may be used to produce seedlings that have specific adaptive properties. So, it is possible to produce distinct phenotypes (epitypes) in Norway spruce, how-ever this type of long lasting effects is not well docu-mented in other organisms so far. The molecular mechanism behind this striking epige-netic memory phenomenon is not yet unraveled but transcriptional changes are clearly involved. In epigen-etically different progenies, transcriptional analysis revealed that seedlings from full-sib families produced at different embryogenesis temperature under long and short day conditions differed. Suppressive subtracted cDNA libraries revealed significant differences in their transcriptomes. Using qRT-PCR, microRNA pathways genes PaDCL1 and 2 and PaSGS3 as well as transposons related genes are differential expressed in the epigeneti-cally different progenies with phenotypic differences in bud burst and bud set. MicroRNAs (miRNAs) are endogenous small RNAs that can exert epigenetic gene regulatory impacts. We have examined the possible role of miRNA in the epige-netic phenomena, and found that Norway spruce con-tains a set of conserved miRNAs as well as a large proportion of novel non-conserved miRNAs. From con-catemerized small RNA libraries from seedlings from the same parents, originated from seeds developed in a cold and warm environment from a family with distinct epigenetic effects, contrasted to one from a family with little response, miRNAs potentially involved in this epi-genetic memory was identified. Most of the miRNAs target unknown genes or genes with no known function. The expression of seven conserved and nine novel miR-NAs showed significant differences in transcript levels in progenies with distinct epigenetic difference in bud set, but not in the progenies from a non-responding family, making them excellent candidate miRNAs. The differentially expression of specific miRNAs in geneti-cally identical but epigenetically different progeny indi-cate their putative participation in the epigenetic regulation.