Basipetal auxin transport is required for gravitropism in roots of arabidopsis

Auxin transport has been reported to occur in two distinct polarities, acro petally and basipetally, in two different root tissues. The goals of this s tudy were to determine whether both polarities of indole-3-acetic acid (IAA ) transport occur in roots of Arabidopsis and to determine which polarity c ontrols the gravity response. Global application of the auxin transport inh ibitor naphthylphthalamic acid (NPA) to roots blocked the gravity response, root waving, and root elongation. Immediately after the application of NPA , the root gravity response was completely blocked, as measured by an autom ated video digitizer. Basipetal [H-3]IAA transport in Arabidopsis roots was inhibited by NPA, whereas the movement of [C-14]benzoic acid was not affec ted. Inhibition of basipetal IAA transport by local application of NPA bloc ked the gravity response. Inhibition of acropetal IAA transport by applicat ion of NPA at the root-shoot junction only partially reduced the gravity re sponse at high NPA concentrations. Excised root tips, which do not receive auxin from the shoot, exhibited a normal response to gravity. The Arabidops is mutant eir1, which has agravitropic roots, exhibited reduced basipetal I AA transport but wild-type levels of acropetal IAA transport. These results support the hypothesis that basipetally transported IAA controls root grav itropism in Arabidopsis.