Developmental gene regulation by chromatin modification factor MpE(z) and its target genes KNOX and BELL in the basal land plant Marchantia polymorpha

2017-03-01T23:54:20Z (GMT) by Dierschke, Tom
Marchantia polymorpha has become a model system to study land plant evolution because it is one of the earliest diverging land plants, possesses limited genetic redundancy and tools for functional genomics have become readily available. All land plants evolved from an algal ancestor with a haplobiontic life cycle with zygotic meiosis, meaning a multicellular gametophytic (n) body with the only diploid (2n) phase being the fertilized egg (zygote) that undergoes meiosis to release haploid spores. In contrast, land plants, or embryophytes, exhibit an alternation of generations − the gametophyte produces male and female gametes and upon fertilization the zygote undergoes mitotic divisions to form the sporophyte or embryo. Recent studies suggest that chromatin modification factors, including the Polycomb Repressive Complex 2 (PRC2), play a major role in phase transitions by repressing sporophyte specific genes, for example KNOX genes. KNOX and BELL proteins belong to the TALE class homeodomain superclass of proteins that are present in animals, plants and fungi. KNOX-BELL TALE proteins have been proposed to play a key role in the evolution of alternation of generations. In green algae, two different mating types of gametes fuse to produce the zygote, each containing either a KNOX- or a BELL-like protein. Both proteins are cytosolic until fertilisation, when the two proteins heterodimerise and translocate to the nucleus where they activate zygote gene expression and development. In land plants, a duplication event resulted in two distinct classes of KNOX genes, KNOX1 and KNOX2. The Marchantia genome encodes two KNOX1, one KNOX2 and two BELL genes. In this study I demonstrate that inducible disruption of Marchantia PRC2 function by expressing an artificial microRNA targeting MpE(z) in the gametophytic stage of the life cycle causes de-repression of sporophyte specific MpKNOX2 and MpBELLB genes that result in developmental arrest. Plants co-expressing either MpBELLA or MpBELLB and MpKNOX2 genes phenocopy this lethal phenotype of knock-down MpE(z) lines. While transcriptome data suggests antheridia specific expression of MpBELLA, MpKNX2 is expressed in the egg cell and sporophyte. In theory, these proteins come into physical contact upon fertilization. Protein-protein interactions of MpBELL and MpKNOX proteins with subsequent intracellular trans-localisation to the nucleus was validated. To elucidate additional target genes of MpE(z) and KNOX/BELL heterodimers, RNAseq on inducible knock-down MpE(z) lines and co-expression of either MpBELL gene with MpKNOX2 showed de-repression of various sporophyte specific genes, making them good candidates for further studies. Thus, I have revealed that the Marchantia PRC2 is necessary for sporophyte specific MpKNOX and MpBELL repression. The interaction of both proteins could be demonstrated, and their expression in the gametophyte causes developmental arrest perhaps via activation/repression of downstream targets that probably reflect the determinate nature of the M. polymorpha sporophyte.