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.

An Evolutionarily Conserved Plant RKD Factor Controls Germ Cell Differentiation

“…Similar results are reported in the accompanying paper by Rö vekamp et al, in which MpRKD expression was repressed by artificial microRNAs [29]. The broad occurrence of RKD genes in land plants and the findings that at least some RKD genes are expressed in egg cells and activate eggspecific genes suggest that RKD proteins have an evolutionarily conserved role in germ cell differentiation [1,5].…”

A cis ‐acting bidirectional transcription switch controls sexual dimorphism in the liverwort

“…The liverwort Marchantia polymorpha, a recently revived model bryophyte, provides a unique opportunity to study sexual differentiation in gametophytes (Bowman, 2016;Shimamura, 2016;Bowman et al, 2017). Taking advantage of these attributes, recent studies utilizing M. polymorpha have revealed several key factors controlling critical steps of sexual plant reproduction (Koi et al, 2016;Rövekamp et al, 2016;Nakajima, 2017;Yamaoka et al, 2018). Taking advantage of these attributes, recent studies utilizing M. polymorpha have revealed several key factors controlling critical steps of sexual plant reproduction (Koi et al, 2016;Rövekamp et al, 2016;Nakajima, 2017;Yamaoka et al, 2018).…”

Patterning the Female Gametophyte of Flowering Plants

“…In fact, Marchantia encodes a single RKD copy only, advancing loss-of-function studies. Rövekamp et al (2016) and Koi et al (2016) generated RNAi and recombination-induced mutants, respectively. Both studies report defects in egg cell development.…”