Auxin regulates most aspects of flowering-plant growth and development, including key developmental innovations that evolved within the vascular plant lineage after diverging from a bryophyte-like ancestor nearly 500 million years ago. Recent studies in Arabidopsis indicate that auxin acts by directly binding the TIR1 subunit of the SCF(TIR1) ubiquitin ligase; this binding results in degradation of the Aux/IAA transcriptional repressors and de-repression of auxin-responsive genes. Little is known, however, about the mechanism of auxin action in other plants. To characterize auxin signaling in a nonflowering plant, we utilized the genetically tractable moss Physcomitrella patens. We used a candidate-gene approach to show that previously identified auxin-resistant mutants of P. patens harbor mutations in Aux/IAA genes. Furthermore, we show that the moss Aux/IAA proteins interact with Arabidopsis TIR1 moss homologs called PpAFB and that a reduction in PpAFB levels results in a phenotype similar to that of the auxin-resistant mutants. Our results indicate that the molecular mechanism of auxin perception is conserved in land plants despite vast differences in the role auxin plays in different plant lineages.
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