Project description:Cellular differentiation is associated with changes in transcript populations. Accurate quantification of transcriptomes during development can thus provide global insights into differentiation processes including the fundamental specification and differentiation events operating during plant embryogenesis. However, multiple technical challenges have limited the ability to obtain high quality early embryonic transcriptomes, namely the low amount of RNA obtainable and contamination from surrounding endosperm and seed-coat tissues. We compared the performance of three low-input mRNA sequencing (mRNA-seq) library preparation kits on 0.1 to 5 nanograms (ng) of total RNA isolated from Arabidopsis thaliana (Arabidopsis) embryos and identified a low-cost method with superior performance. This mRNA-seq method was then used to profile the transcriptomes of Arabidopsis embryos across eight developmental stages. By comprehensively comparing embryonic and post-embryonic transcriptomes, we found that embryonic transcriptomes do not resemble any other plant tissue we analyzed. Moreover, transcriptome clustering analyses revealed the presence of four distinct phases of embryogenesis which are enriched in specific biological processes. We also compared zygotic embryo transcriptomes with publicly available somatic embryo transcriptomes. Strikingly, we found little resemblance between zygotic embryos and somatic embryos derived from late-staged zygotic embryos suggesting that somatic and zygotic embryo transcriptomes are distinct from each other. In addition to the biological insights gained from our systematic characterization of the Arabidopsis embryonic transcriptome, we provide a data-rich resource for the community to explore.
Project description:The piRNA pathway controls transposon expression in animal germ cells, thereby ensuring genome stability over generations. piRNAs are maternally deposited and required for proper transposon silencing in adult offspring. However, a long-standing question in the field is the precise function of maternally deposited piRNAs and its associated factors during embryogenesis. Here, we probe the spatio-temporal expression patterns of several piRNA pathway components during early stages of development. Amongst those, factors required for transcriptional gene silencing (TGS) showed ubiquitous abundance in somatic and pole cells throughout the first half of embryogenesis. We further analysed the transcriptomes of various embryo stages and correlated these with the presence of selected chromatin marks. We found that a number of transposon families show bursts of transcription during early embryonic stages. Transposons heavily targeted by maternally deposited piRNAs accumulated repressive chromatin marks following their spike in expression. Furthermore, depletion of maternally deposited Piwi protein in early embryos resulted in increased expression of transposons targeted by inherited piRNAs and was accompanied by a strong loss of repressive chromatin marks at coding sequences. Overall, our data suggests a pivotal role for the piRNA pathway in transposon defence during Drosophila embryogenesis in somatic cells.