The plant hormone auxin serves as central regulator of growth and development. Auxin transporters in the plasma membrane are assumed to define tissue-level patterns of auxin distribution [1, 2]. However, auxin is small enough to diffuse through the plasmodesmata that connect neighboring cells , presenting an alternative pathway, whose contribution to auxin transport remained largely unexplored . Here, photoactivation microscopy [5, 6] was used to measure the capacity for small-molecule diffusion in the epidermis of Arabidopsis thaliana leaves. In the elongated epidermis cells covering the midrib and petiole, the plasmodesmata-mediated cell-wall permeability was found to be several times higher in the longitudinal than in the transverse direction. The physiological relevance of this asymmetry was tested through quantification of the shade-avoidance response, which depends on auxin transport from the leaf tip to the petiole in the abaxial side of the leaf , with the hypothesis that directionality of diffusion supplements transporter-mediated auxin movement . Triggering the response by auxin application at the tip led to stronger leaf movement in wild-type plants than in gsl8 mutants , which lack the callose synthase necessary to establish directionality. The results match the predictions of a mathematical model of auxin transport based on the permeabilities measured in wild-type and mutant plants. It is concluded that plasmodesmata permeability can be selectively modulated within a plant cell and that the conferred directionality in diffusion can influence the tissue-specific distribution patterns of small molecules, like auxin.