An Evolutionarily Conserved Plant RKD Factor Controls Germ Cell Differentiation

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.