Diminished Auxin Signaling Triggers Cellular Reprogramming by Inducing a Regeneration Factor in the Liverwort Marchantia polymorpha. (7th January 2022) - Ishida, Sakiko
Abstract: Regeneration in land plants is accompanied by the establishment of new stem cells, which often involves reactivation of the cell division potential in differentiated cells. The phytohormone auxin plays pivotal roles in this process. In bryophytes, regeneration is enhanced by the removal of the apex and repressed by exogenously applied auxin, which has long been proposed as a form of apical dominance. However, the molecular basis behind these observations remains unexplored. Here, we demonstrate that in the liverwort Marchantia polymorpha, the level of endogenous auxin is transiently decreased in the cut surface of decapitated explants, and identify by transcriptome analysis a key transcription factor gene, LOW- AUXIN RESPONSIVE (Mp LAXR ), which is induced upon auxin reduction. Loss of Mp LAXR function resulted in delayed cell cycle reactivation, and transient expression of Mp LAXR was sufficient to overcome the inhibition of regeneration by exogenously applied auxin. Furthermore, ectopic expression of Mp LAXR caused cell proliferation in normally quiescent tissues. Together, these data indicate that decapitation causes a reduction of auxin level at the cut surface, where, in response, Mp LAXR is up-regulated to trigger cellular reprogramming. Mp LAXR is an ortholog of Arabidopsis ENHANCER OF SHOOT REGENERATION 1/DORNR Ö SCHEN, which has dual functions as a shoot regeneration factor and a regulator of axillary meristem initiation, the latter of which requires a low auxin level. Thus, our findings provide insights into stem cell regulation as well as apical dominance establishment in land plants.