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. Constitutively expressed in roots, rosette leaves, inflorescence stems, and flowers. Present in inflorescence meristems, floral meristems and vascular tissues. Altered expression of light-regulated genes; overexpression of CAB (photosystem II type I chlorophyll a/b-binding proteins) genes in the dark (e.g. CAB1 and CAB3). Reduced auxin transport. Under long-day growth conditions (18 hr light/6 hr dark), reduced apical dominance in mature plants. Reduced plant growth under both short-day (9 hr light/15 hr dark) and long-day conditions resulting in shorter plants. Strong deficiency in lateral root production. Reduced cell elongation in siliques, pedicels, roots, and the inflorescence leading to a compact rosette, more secondary corymb-like inflorescence, and small seeds. Delayed flowering. Reduced sensitivity to 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). Organ-specific defect in response to cytokinins, ethylene and gibberellic acid (GA). Increased expression of auxin transporters PIN1 and PIN6. Reduced growth-promotion and lateral root development stimulation mediated by the fungus T.virens. May be or regulate a N-1-naphthylphthalamic acid (NPA) binding protein (NBP), an auxin transport inhibitor. Belongs to the UBR4 family.