Physcomitrella patens Auxin-Resistant Mutants Affect Conserved Elements of an Auxin-Signaling Pathway

Summary 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 [1‐3]. 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 auxinresponsive genes [4‐9]. 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 auxinresistant 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.