Auxin and Red Light in the Control of Hypocotyl Hook Opening in Beans

Evidence is presented to support the suggestion that endogenous auxinlike substances participate in controlling the unbending of the hypocotyl hook of Phaseolus vulgaris L. (cv. Black Valentine). An acidic indole was detected in hook diffusates by fluorometry; triiodobenzoic acid, an inhibitor of auxin transport, prevented red light-induced unbending, and indoleacetic acid can be substituted for tissue just above the elbow region as an inhibitor of opening. Indoleacetic acid also stimulated growth of shank cells, and red light increased the sensitivity of this tissue to the hormone. A small red light-induced stimulation of auxin transport through the inside half of the hypocotyl shank was observed and may be related to light-induced unbending of the hook.