Disclosing ancient epigenetic regulators in one of the earliest diverging land plants - AfS, WCUB ENWW

A key mechanism of gene expression control in eukaryotes is post-transcriptional gene silencing. It operates based on the production of small non-coding RNAs that are 21-24 nucleotides long and complementary to very specific regions of mRNAs of protein coding genes. Those small RNAs guide an RNA slicer protein to mRNA targets, leading to mRNA cleavage or degradation. In plants, this process is mainly realized by the microRNA (miRNA) type of small non-coding RNAs. Although the miRNA biogenesis pathway has been studied extensively in the angiosperm Arabidopsis thaliana, many open questions remain to be tackled. This project employs a new model species for the purpose of shedding light on yet undiscovered aspects of miRNA expression and biogenesis. Namely, this is the liverwort Marchantia polymorpha, one of the earliest diverging embryophytes. As such, it provides some significant benefits over A. thaliana, which makes it an invaluable tool for the discovery of new genes. Thanks to conservation of many of its genes throughout the plant kingdom, findings in M. polymorpha could potentially be used in agriculturally relevant species. Indeed, as miRNAs are involved in controlling all aspects of plant development and environmental responses, fundamental research in miRNA biology is essential for applied crop science. Here, miRNA biogenesis will be studied in the light of rhizoid development. Rhizoids in liverworts are morphologically similar structures to root hairs in angiosperms. As shown recently, their differentiation in M. polymorpha is under control of the b-HLH transcription factor MpRSL1 and the miRNA MpFRH1. Indeed, loss of MpRSL1 and misexpression of MpFRH1 both result in abolished rhizoid initiation and other pleiotropic defects, underscoring their developmental significance. With this discovery, an exemplary model situation for studying miRNA regulation has been established. Interestingly, the identified frh1 mutant represents a gain-of-function mutant (FRH1GOF) that forms almost no rhizoids. This characteristic phenotype comes handy for investigating the biogenesis pathway of MpFRH1. The notion is that upon mutagenesis of an MpFRH1 biogenesis component, the overexpressing effect of the FRH1GOF mutant is lost, resulting in regained ability to grow rhizoids. The fact that mutants can be phenotypically selected by the presence of rhizoids, provides an advantage. It can be used for the performance of a high-throughput repressor mutagenesis screen in order to discover novel components of miRNA biogenesis.

Summary of addressed BBSRC priority areas
Studying key regulators of plant development allows understanding and addressing questions concerning plant growth and health. Following up on that, problems regarding sustainable enhancement of agricultural production can be tackled. Moreover, it lays a basis for finding approaches to increase the nutritious value of crop plants.