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Germ cell differentiation in plants

Germ cells possess unique characteristics, such as pluripotency to form multicellular bodies and the ability to fuse with a germ cell of opposite sex. How do germ cells acquire such unique abilities?

In contrast to animals where the germ cell lineage separates from the somatic lineage during early embryogenesis, plant germ cells differentiate in reproductive organs that are formed in later phases of plant life cycles. Therefore, formation of plant germ cells requires de novo reprogramming of somatic cells and acquisition of fertilization capacities.

Our knowledge on the mechanisms regulating germ cell formation in plants, however, is very limited. This is mainly because in widely used model plant species, such as Arabidopsis and rice, germ cell formation takes place deep inside the reproductive organs, and also because mutants of genes essential for germ cell formation are not maintainable and hence their functions are difficult to analyze genetically.

We realized the liverwort (Marchantia polymorpha) is an ideal species to study plant germ cell formation. Liverworts are dioecious species (male and female individuals are separated), and their germ cells, i.e. sperm and eggs, are relatively easily accessible. Moreover, liverworts can propagate asexually via gemma formation, which makes it possible to maintain mutants defective in germ cell formation.

 

The genome of M. polymorpha possesses a single-copy RKD gene, and we named it MpRKD. MpRKD is a homolog of Arabidopsis RKD4 that functions in embryo patterning. MpRKD knock-out resulted in plants with normal vegetative growth but with striking defects in the formation of both sperm and egg cells. The Arabidopsis genome contains a total of five RKD genes, some of which are expressed in the egg cell. Thus, RKD genes appear to be evolutionarily conserved regulators of germ cell formation in plants. We aim to elucidate regulatory network governing plant germ cell formation through functional analyses of RKD and their target genes.

 

Reference:
Koi, et al. (2016). " An evolutionarily conserved plant RKD factor controls germ cell differentiation." Curr. Biol. 26, 1775-1781.PubMed Publisher Press