Temperature-dependent differential transcriptomes during formation of an epigenetic memory in Norway spruce embryogenesis

Embryogenesis is the initial stage of plant life, when the basics of body plan and the post-embryonic development are laid down. Epigenetic memory formed in the Norway spruce embryos permanently affect the timing of bud burst and bud set in progenies, vitally important adaptive traits in this long-lived forest species. The epigenetic memory marks are established in response to the temperature conditions prevailing during zygotic and somatic embryogenesis; the epitype is fixed by the time the embryo is fully developed and is mitotically propagated throughout the tree’s life span. Somatic embryogenesis closely mimics the natural zygotic embryo formation and results in epigenetically different plants in a predictable temperature-dependent manner with respect to altered phenology. Using Illumina-based Massive Analysis of cDNA Ends, the transcriptome changes were monitored in somatic embryos during morphogenesis stage under two different temperatures (18 vs. 30 °C). We found distinct differences in transcriptomes between the genetically identical embryogenic tissues grown under the two epitype-inducing temperatures suggesting temperature-dependent canalizing of gene expression during embryo formation, putatively based on chromatin modifications. From 448 transcripts of genes coding for proteins involved in epigenetic machinery, we found 35 of these to be differentially expressed at high level under the epitype-inducing conditions. Therefore, temperature conditions during embryogenesis significantly alter transcriptional profiles including numerous orthologs of transcriptional regulators, epigenetic-related genes, and large sets of unknown and uncharacterized transcripts.