Specificity patterns indicate that auxin exporters and receptors are the same proteins

A study of transport and action of synthetic auxin analogues can help to identify transporters and receptors of this plant hormone. Both aspects--transportability and action on growth--were tested with 2-naphthoxyacetic acid (2-NOA) and compared across several plant species. 2-NOA stimulates elongation effectively at low concentrations in petioles of the gymnosperm Ginkgo biloba L., in hypocotyls or internodes of the dicot legumes, mung bean (Vigna mungo L.) and pea (Pisum sativum L.), in cotyledons of onion (Allium cepa L.) and in leaf bases of chive (Allium schoenoprasum L.), the latter two of the monocot order Asparagales. In contrast, elongation of coleoptile segments of maize (Zea mays L.) is poorly responsive to 2-NOA. Significant auxin-like transport of 2-NOA was observed in segments of mung bean hypocotyls, pea internodes, and chive leaf bases, but not in segments of the grass coleoptiles. Thus, for the two assays, elongation and polar transportability, the same difference in ligand specificity was observed between the grass and all other species assayed. This finding supports the hypothesis that a common protein mediates auxin efflux as well as auxin action on elongation.