An Evolutionarily Conserved Plant RKD Factor Controls Germ Cell Differentiation

Abstract

In contrast to animals, in which the germ cell lineage is established during embryogenesis, plant germ cells are generated in reproductive organs via reprogramming of somatic cells. The factors that control germ cell differentiation and reprogramming in plants are poorly understood. Members of the RKD subfamily of plant-specific RWP-RK transcription factors have been implicated in egg cell formation in Arabidopsis based on their expression patterns and ability to cause an egg-like transcriptome upon ectopic expression [1]; however, genetic evidence of their involvement is lacking, due to possible genetic redundancy, haploid lethality, and the technical difficulty of analyzing egg cell differentiation in angiosperms. Here we analyzed the factors that govern germ cell formation in the liverwort Marchantia polymorpha. This recently revived model bryophyte has several characteristics that make it ideal for studies of germ cell formation, such as low levels of genetic redundancy, readily accessible germ cells, and the ability to propagate asexually via gemma formation [2, 3]. Our analyses revealed that MpRKD, a single RWP-RK factor closely related to angiosperm RKDs, is preferentially expressed in developing eggs and sperm precursors in M. polymorpha. Targeted disruption of MpRKD had no effect on the gross morphology of the vegetative and reproductive organs but led to striking defects in egg and sperm cell differentiation, demonstrating that MpRKD is an essential regulator of germ cell differentiation. Together with previous findings [1, 4-6], our results suggest that RKD factors are evolutionarily conserved regulators of germ cell differentiation in land plants.

Marchantia MpRKD Regulates the Gametophyte-Sporophyte Transition by Keeping Egg Cells Quiescent in the Absence of Fertilization

“…After fertilization more neck cells expressed trpVNS ( Figure S3D), suggesting a possible function of MpRKD in spermatocyte interaction or attraction. Indeed, such a function is supported by an independent study, reporting that sperm attraction to archegonia depends on MpRKD [21].…”

“…Thus, MpRKD may be required for the maintenance of egg cell differentiation and/or quiescence, which may be tightly linked to each other, as they are in A. thaliana pollen development, where DUO1 and DUO3 play an analogous role to that of RKD-type genes in terms of proliferation and differentiation [33,34]. On the other hand, an involvement of MpRKD in cell fate specification is substantiated by the observations that MpRKD is required for sperm proliferation and differentiation during M. polymorpha male gametogenesis [21] and that the basal cell in Atrkd4 mutant embryos in A. thaliana differentiates normally and expresses cell-specific markers, but subsequently loses its fate and basal cell fate markers disappear [35]. The small protrusions around the rim of the cavity containing the gemmae in amiRNA-transformed M. polymorpha plants could represent an analogous example for initiation of a cell fate that is later aborted due to the lack of MpRKD function.…”

“…The accession numbers for MpRKD (Mapoly0022s0128), MpNIN (MpNLP in [21], Mapoly0083s0040), MpRWP (MpRWP1 in [21], Mapoly0049s0118), and MpMID (MpRWP2 in [21], Mapoly0014s0044) reported in this paper are GenBank: KU987910, KU987909, KU987911, and KU987912. …”

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.…”

Expression Analyses of Embryogenesis-Associated Genes during Somatic Embryogenesis of Adiantum capillus-veneris L. In vitro: New Insights into the Evolution of Reproductive Organs in Land Plants

“…In addition, the WUS clade was the latest derived lineage in the phylogenetic tree of WOX family genes (Figure 4D), which is consistent with previous findings in Arabidopsis as well as other plants (Lian et al, 2014). The RKD family of plant RWP-RK factors is expressed in reproductive cells of land plants, and has a single origin (Koi et al, 2016). In the gene trees (Figures 4B,C), AcRKD resided in the RKD clade corresponding to the RKD subfamily designated by Chardin et al (2014); AcLBD is located in the Class IB LBD gene lineage, which was reported to be involved in root development (Coudert et al, 2012; Chanderbali et al, 2015).…”