ABSTRACT
The role of auxin redistribution in the graviresponse of the leaf-sheath pulvinus of oat (Avena sativa L.) was assessed using 3H-indole-3-acetic acid (3H-IAA) preloaded into isolated pulvini. When pulvini were totally isolated from subtending nodal tissue as well as leaf-sheath and internode, gravistimulation failed to induce an asymmetric growth response. Presence of either the nodal tissue or the internode/leaf-sheath tissue was sufficient to restore a normal graviresponse. Gravistimulation of totally isolated pulvini inhibited basipetal export of label (i.e., 3H-IAA) without generating any asymmetry of label within the pulvinus. In contrast, gravistimulation of pulvini with nodes intact generated an asymmetric distribution of label (initiation by 1 h; final ratio, lower/upper = 1.5) as well as the upward bending response. The kinetics of formation of the asymmetry of label paralleled the kinetics of initiation of the asymmetric growth response. The addition of 0.1 M sucrose to all agar blocks shortened both the time to initiation of label redistribution and the time to initial upward bending. However, sucrose did not change the final magnitude of label asymmetry although it increased the final steady state bending rate four fold. The inhibitors of polar auxin transport N-1-naphthylphthalamaic acid (NPA), 2,3,5-triiodobenzoic acid (TIBA), morphactin, naringenin, kaempferol and myricetin all significantly decreased the bending response of oat pulvini, but this inhibition was less than 50%. In contrast, TIBA and naringenin (each at 100 micromoles), effectively eliminated the redistribution of label, but did not eliminate the bending response. These results indicate that the active basipetal export of auxin is inhibited by gravistimulation of the oat pulvinus, while active lateral transport is induced. It is concluded that, while lateral transport of auxin occurs following gravistimulation, it is not necessary for a graviresponse. Other processes, such as localized changes in tissue responsiveness or the conversion of conjugated hormone to free (active) hormone, may suffice to drive the graviresponse.
