From the somatic cell to the germ cell

In contrast to humans and animals, plants do not set aside a specialized cell lineage (germline) for the future production of gametes during early embryogenesis. Instead, the germ cells of plants are established de novo from somatic cells in the floral reproductive organs, the stamens and carpels. To this end, the selected cells switch their cell division mode from mitosis, cell proliferation maintaining the chromosome number, to meiosis, the division that reduces the number of chromosomes and where genetic recombination occurs. Plants have therefore evolved strategies to enable somatic cells to switch to germline fate and to do so in the right place and at the right time.

Laux and colleagues have identified multiple genes in the model organism Arabidopsis thaliana that give the start signal for switching from mitose to meiose. The starting point for the findings presented in Science are mutants that create multiple germ cells instead of a singular one in each ovule. Key of the newly discovered pathway is the limitation of activity of the transcription factor WUSCHEL, which Laux's team had identified several years ago as an important regulator of pluripotent stem cells that are able to develop into every cell type in the organism. The involvement of WUSCHEL in creating germ cells is a discovery that provides molecular evidence for the longstanding hypothesis derived from paleobotanical studies that the reproductive ovules and the shoot meristem have evolved from the same precursor organ in ancient plants. The newly discovered regulatory mechanism shows how plants are able to limit switching to the germ cell program so that only a single germ cell emerges, while the surrounding cells take on other tasks.