Required for auxin efflux and polar auxin transport (PAT) influencing auxin-mediated developmental responses (e.g. cell elongation, apical dominance, lateral root production, inflorescence architecture, general growth and development). Controls the elongation of the pedicels and stem internodes through auxin action. Involved in the expression modulation of light-regulated genes. Represses CAB1 and CAB3 genes expression in etiolated seedlings. Confers sensitivity to the auxin transport inhibitors N-1-naphthylphthalamic acid (NPA), 2-carboxyphenyl-3-phenylpropane-l,2-dione (CPD), and methyl-2-chloro-9-hydroxyfluorene-9-carboxylate (CFM). Influences the polarized subcellular distribution of the auxin transporter PIN1 in response to auxin transport inhibitors. Plays a role in the regulation of responses to phytohormones such as auxin, cytokinins, ethylene and gibberellic acid (GA), particularly during light-mediated stimuli (e.g. shade ovoidance, etiolation). Required for pericycle cell activation to form lateral root primordia (LRP) in both high and low phosphate P conditions. Necessary for the plant-growth promotion and lateral root development mediated by the fungus Trichoderma virens