The invention of WUS-like stem cell-promoting functions in plants predates leptosporangiate ferns
The growth of land plants depends on stem cell-containing meristems which show major differences in their architecture from basal to higher plant species. In Arabidopsis, the stem cell niches in the shoot and root meristems are promoted by WUSCHEL (WUS) and WOX5, respectively. Both genes are members of a non-ancestral clade of the WUS-related homeobox (WOX) gene family, which is absent in extant bryophytes and lycophytes. Our analyses of five fern species suggest that a single WUS orthologue was present in the last common ancestor (LCA) of leptosporangiate ferns and seed plants. In the extant fern Ceratopteris richardii, the WUS pro-orthologue marks the pluripotent cell fate of immediate descendants of the root apical initial, so-called merophytes, which undergo a series of stereotypic cell divisions and give rise to all cell types of the root except the root cap. The invention of a WUS-like function within the WOX gene family in an ancestor of leptosporangiate ferns and seed plants and its amplification and sub-functionalisation to different stem cell niches might relate to the success of seed plants, especially angiosperms.
KeywordsWOX phylogeny Plant evolution Meristem Ceratopteris WUS orthologue
We thank Dr. J. Chandler and Pascal Reisewitz for suggestions, stimulating discussions and critically reading the manuscript, Heike Shahbodaghi-Rückert for excellent technical assistance. We especially wish to thank Leslie Hickok for providing Ceratopteris richardii spores. This work was supported by the Deutsche Forschungsgemeinschaft through SFB 680 and grant WE 1262/7.
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