1. Department of Botany, University of Wisconsin, 430 Lincoln Drive, Madison, Wisconsin 53706, USA
2. Department of Horticulture and Landscape Architecture, 625 Agriculture Mall Drive, Purdue University, West Lafayette, Indiana 47907, USA
3. These authors contributed equally to this paper

Correspondence to: Edgar P. Spalding¹ Correspondence and requests for materials should be addressed to E.P.S. (Email: spalding@wisc.edu).

Abstract

Many aspects of plant growth and development are dependent on the flow of the hormone auxin down the plant from the growing shoot tip where it is synthesized^{1, 2}. The direction of auxin transport in stems is believed to result from the basal localization within cells of the PIN1 membrane protein, which controls the efflux of the auxin anion³. Mutations in two genes homologous to those encoding the P-glycoprotein ABC transporters that are especially abundant in multidrug-resistant tumour cells in animals⁴ were recently shown to block polar auxin transport in the hypocotyls of Arabidopsis seedlings⁵. Here we show that the mdr mutants display faster and greater gravitropism and enhanced phototropism instead of the impaired curvature development expected in mutants lacking polar auxin transport. We find that these phenotypes result from a disruption of the normal accumulation of PIN1 protein along the basal end of hypocotyl cells associated with basipetal auxin flow. Lateral auxin conductance becomes relatively larger as a result, enhancing the growth differentials responsible for tropic responses.