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AP2-type transcription factors control the gametophore stem cell identity

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Stem cells are characterized by their ability to self-renew and give rise to differentiated cells. Formation of several types of stem cells to produce different types of differentiated cells is properly regulated during 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, a single protonema apical stem cell is formed from a spore. The protonema apical stem cell exhibits apical growth and cell division to produce differentiated protonema cells. These differentiated protonema cells often perform a few additional times of cell division to produce a side branch initial cell. Then the side branch initial cell directly changes its identity to either a protonema apical stem cell or a gametophore apical stem cell. Differentiation of two different types of stem cells from side branch initial cells 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 orthologous to Arabidopsis thaliana eight genes 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 , based on the initials of their orthologous genes ANINTEGUMENTA , PLETHORA, and BABY BOOM in A. thaliana. Quadruple disruption of all APB genes blocked gametophore apical stem cell formation, even in the presence of cytokinin, which enhances gametophore apical stem cell formation 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 even in the conditions of gametophore apical stem cell induction, at least based on their morphology. Signals of APB -YFP fusion proteins were detected in emerging gametophore apical stem cells and continuously detected during gametophore apical stem cell formation, whereas the signals disappeared during protonema apical stem cell formation. We conclude that the AP2 -type transcription factors function as a molecular switch to promote the development of different types of stem cells in P. patens. In addition, 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 the possibility that APB genes interact with cytokinin signaling and control specific type of gene sets necessary for gametophore stem cell formation. In addition, I also would like to talk about recent our approaches to visualize auxin and its transport. In P. patens, differentiated leaf cells are easily reprogrammed to protonema apical stem cells by the excision of a leaf. This reprogramming process is controlled by auxin. Both application of anti-auxin and overexpression of a stable form of PpIAA1A inhibited the reprogramming. Excision of a leaf induced the expression of ARF11 via the down regulation of miR1219, miR390, and TAS3 ta-si RNA . Then ARF11 induced the expression of STEMIN gene, which induce the reprogramming. Although it is demonstrated that auxin is important for these reprogramming processes, it is still unclear when auxin is accumulated, where auxin maxima exist, and how auxin is distributed among cells via auxin polar transport. Then we started to reveal these problems by using new auxin sensor and visualizing auxin transporter (PIN and AUX ). I also would like to discuss about these recent preliminary data.

This talk is part of the Plant Sciences Talks series.

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