Inside or outside? Localization of the receptor relevant to auxin-induced growth

The localization of the auxin receptor relevant to the control of elongation growth is still a matter of controversy. Auxin‐induced elongation of maize coleoptile segments was measured by means of a high resolution auxanometer. When indole‐3‐acetic acid (IAA) was removed from the bathing solution, a rapid cessation of auxin‐induced elongation was detected. This decline was delayed when the auxin efflux carrier was blocked by the phytotropins naphthylphthalamic acid (NPA) and pyrenoylbenzoic acid (PBA) or by triiodobenzoic acid (TIBA). The IAA concentration in NPA‐pretreated segments was 2–3 times higher than in NPA‐free controls 35 min after the removal of IAA in the bathing medium. A similar rapid drop of growth after removal of auxin was observed for the rapidly‐transported synthetic auxin, naphthaleneacetic acid (NAA). When the auxin efflux was blocked, growth induced by NAA was sustained much longer than IAA‐stimulated elongation. In comparison with NAA, the synthetic auxin 2,4‐dichlorophenoxyacetic acid (2,4‐D) is known to be excreted very slowly by the efflux carrier. 2,4‐D‐induced growth remained at a stimulated level when the auxin was washed off, even in the absence of any auxin efflux inhibitor. We conclude from these results that the presence of intracellular auxin is a necessary and sufficient condition for sustained auxin‐induced elongation growth, at least for the phases during the 2 h after its application. Consequently, we postulate the existence of an intracellular auxin receptor relevant to the control of growth.