title: Marchantia MpRKD Regulates the Gametophyte-Sporophyte Transition by Keeping Egg Cells Quiescent in the Absence of Fertilization.
abstract: Unlike in animals, the life cycle of land plants alternates between two multicellular generations, the haploid gametophyte and the diploid sporophyte [1]. Gamete differentiation initiates the transition from the gametophyte to the sporophyte generation and, upon maturation, the egg cell establishes a quiescent state that is maintained until fertilization. This quiescence represents a hallmark of the gametophyte-sporophyte transition. The underlying molecular mechanisms are complex and best characterized in the flowering plant Arabidopsis thaliana [2-4]. However, only few genes with egg cell-specific expression or defects have been identified [5-10]. Intriguingly, ectopic expression of members of a clade of RWP-RK domain (RKD)-containing transcription factors, which are absent from animal genomes [11-13], can induce an egg cell-like transcriptome in sporophytic cells of A. thaliana. Yet, to date, loss-of-function experiments have not produced phenotypes affecting the egg cell, likely due to genetic redundancy and/or cross-regulation among the five RKD genes of A. thaliana [10]. To reduce genetic complexity, we explored the genome of Marchantia polymorpha, a liverwort belonging to the basal lineage of extant land plants [14-17]. Based on sequence homology, we identified a single M. polymorpha RKD gene, MpRKD, which is orthologous to all five A. thaliana RKD genes. Analysis of the MpRKD expression pattern and characterization of lines with reduced MpRKD activity indicate that it functions as a regulator of gametophyte development and the gametophyte-sporophyte transition. In particular, MpRKD is required to establish and/or maintain the quiescent state of the egg cell in the absence of fertilization.
authors.affiliation: Department of Plant and Microbial Biology and Zurich-Basel Plant Science Center, University of Zurich, 8008 Zürich, Switzerland.
authors.forename: Moritz
authors.initials: M
authors.lastname: Rövekamp
authors.affiliation: School of Biological Sciences, Monash University, Clayton, Melbourne, VIC 3800, Australia; Department of Plant Biology, University of California, Davis, Davis, CA 95616, USA.
authors.forename: John L
authors.initials: JL
authors.lastname: Bowman
authors.affiliation: Department of Plant and Microbial Biology and Zurich-Basel Plant Science Center, University of Zurich, 8008 Zürich, Switzerland. Electronic address: grossnik@botinst.uzh.ch.
authors.forename: Ueli
authors.initials: U
authors.lastname: Grossniklaus
mesh_terms: D002454:Cell Differentiation; D005306:Fertilization; D018506:Gene Expression Regulation, Plant; D055993:Germ Cells, Plant / Q000502:physiology*; D047008:Marchantia / Q000235:genetics / Q000502:physiology*; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism
chemical_list: D010940:Plant Proteins; D014157:Transcription Factors