Abstract

Stem cells are characterized by their ability to self-renew and give rise to differentiated cells. Several types of stem cells to produce different types of differentiated cells are formed during development. Their appropriate regulation is necessary for development of multicellular organisms.

The moss Physcomitrella patens is a good model organism to research how different types of stem cell identity are determined. In P. patens, two different types of stem cells, gametophore apical stem cells and protonema apical stem cells, are derived from side branch initial cells. This stem cell formation process is controlled by two phytohormones, auxin and cytokinin. However, molecular mechanisms for such a stem cell formation has been largely unknown.

Here we show that four AP2 -type transcription factors are indispensable for the formation of gametophore apical stem cells from side branch initial cells. We named the four P. patens genes APB1 , APB2, APB3 , and APB4 . Quadruple disruption of all APB genes disrupted gametophore apical stem cell formation, even in the presence of cytokinin, which induces gametophore apical stem cell in the wild type. Time-lapse observation showed that side branch initial cells do not acquire gametophore apical stem cell identity and directly differentiate into protonema apical stem cells based on their morphology. Signals of APB -YFP fusion proteins were continuously detected during gametophore apical stem cell formation, whereas the signals disappeared during protonema apical stem cell formation. We conclude that the APB genes function as a molecular switch to regulate the identity of stem cells in P. patens. In order to reveal how APB genes determine the gametophore stem cell identity, the transcriptome analysis was performed. Overexpression of APB4 gene induced different types of gene sets depending on the presence of cytokinin. These data suggest that the possibility that APB genes interact with cytokinin signaling and control specific type of gene sets necessary for gametophore stem cell formation.