AP2-type transcription factors determine stem cell identity in the moss Physcomitrella patens

Stem cells are formed at particular times and positions during the development of multicellular organisms. Whereas flowering plants form stem cells only in the sporophyte generation, non-seed plants form stem cells in both the sporophyte and gametophyte generations. Although the molecular mechanisms underlying stem cell formation in the sporophyte generation have been extensively studied, only a few transcription factors involved in the regulation of gametophyte stem cell formation have been reported. The moss Physcomitrella patens forms a hypha-like body (protonema) and a shoot-like body (gametophore) from a protonema apical cell and a gametophore apical cell, respectively. These apical cells have stem cell characteristics and are formed as side branches of differentiated protonema cells. Here, we show that four AP2-type transcription factors orthologous to Arabidopsis thaliana AINTEGUMENTA, PLETHORA and BABY BOOM (APB) are indispensable for the formation of gametophore apical cells from protonema cells. Quadruple disruption of all APB genes blocked gametophore formation, even in the presence of cytokinin, which enhances gametophore apical cell formation in the wild type. All APB genes were expressed in emerging gametophore apical cells, but not in protonema apical cells. Heat-shock induction of an APB4 transgene driven by a heat-shock promoter increased the number of gametophores. Expression of all APB genes was induced by auxin but not by cytokinin. Thus, the APB genes function synergistically with cytokinin signaling to determine the identity of the two types of stem cells.