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Strigolactone and KAI2-Ligand signaling in land plants: what does the moss Physcomitrium patens tell us?
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International audience. Specialized metabolites as strigolactones (SLs) may have been instrumental for land conquest by the common ancestor to bryophytes and vascular plants. Bryophytes include hornworts, liverworts and mosses, to which belongs the model Physcomitrium patens. SLs in P. patens are both hormones and allelopathic compounds: they regulate moss plant development, and signal the presence of the plant to its neighbors by inhibiting filament growth. Through reverse genetics tools, including CRISPR/Cas9 gene editing, we have targeted candidate orthologs to genes involved in SL signaling. In particular, we obtained mutants for the F-box protein MAX2, which in angiosperms is involved in two signaling pathways: the SL pathway and the KAI2-Ligand (KL) pathway. The KL pathway was named according to the KAI2 (KARRIKIN INSENSITIVE2) protein, that is found in all land plants, and is a paralogue of the SL receptor. Strikingly, P. patens genome shows 13 PpKAI2Like genes, for which we obtained multiple mutants. Mutant analysis and use of SL and KL mimics allowed us to highlight that the two pathways also exist in this moss. However, the SL pathway of P. patens is PpMAX2-independent, unlike the KAI2-Ligand pathway, which remains PpMAX2-dependent. Both SL and KL pathways regulate moss development in apparent opposite ways, requiring different subclades of PpKAI2Like genes. Recently, we obtained mutants for the downstream actors of SL and KL signaling pathways, the SMXL genes. Our results indicate that moss PpSMXL proteins are not repressors of the SL pathway, in contrast to some of their angiosperm homologs. PpSMXL are likely repressors of the moss KL pathway, but mutant analysis suggests PpSMXL proteins are involved in a crosstalk with SL signaling.
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