Abstract
Key messageThis review compares the molecular mechanisms of stem cell control in the shoot apical meristems of mosses and angiosperms and reveals the conserved features and evolution of plant stem cells.The establishment and maintenance of pluripotent stem cells in the shoot apical meristem (SAM) are key developmental processes in land plants including the most basal, bryophytes. Bryophytes, such as Physcomitrium (Physcomitrella) patens and Marchantia polymorpha, are emerging as attractive model species to study the conserved features and evolutionary processes in the mechanisms controlling stem cells. Recent studies using these model bryophyte species have started to uncover the similarities and differences in stem cell regulation between bryophytes and angiosperms. In this review, we summarize findings on stem cell function and its regulation focusing on different aspects including hormonal, genetic, and epigenetic control. Stem cell regulation through auxin, cytokinin, CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) signaling and chromatin modification by Polycomb Repressive Complex 2 (PRC2) and PRC1 is well conserved. Several transcription factors crucial for SAM regulation in angiosperms are not involved in the regulation of the SAM in mosses, but similarities also exist. These findings provide insights into the evolutionary trajectory of the SAM and the fundamental mechanisms involved in stem cell regulation that are conserved across land plants.
Highlights
The establishment of the basic architecture in the shoot system depends on the activity of the shoot apical meristem (SAM) (Sussex and Kerk 2001; Shi and Vernoux 2019)
Bryophytes, the most basal group in the land plant lineage, spend most of their life cycle as gametophytes and it is in this phase that the SAM is formed
Seed plants spend most of their lifetime as sporophytes, SAM formation occurs in the sporophytic phase (Harrison 2017)
Summary
The establishment of the basic architecture in the shoot system depends on the activity of the shoot apical meristem (SAM) (Sussex and Kerk 2001; Shi and Vernoux 2019). In the P. patens SAM, the auxin signaling level at the shoot apical cell is low, whereas it is high in differentiated tissues such as developed leaves and the stem (Thelander et al 2019). These defects indicate that CLE signaling may be involved in repression of stem cell identity and determination of the cell division plane orientation in the P. patens SAM.
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