GENETIC-ANALYSIS OF THE EFFECTS OF POLAR AUXIN TRANSPORT INHIBITORS ON ROOT-GROWTH IN ARABIDOPSIS-THALIANA

Polar auxin transport inhibitors, including N-1-naphthylphthalamic aci d (NPA) and 2,3,5-triiodobenzoic acid (TIBA), have various effects on physiological and developmental events, such as the elongation and tro pism of roots and stems, in higher plants. We isolated NPA-resistant m utants of Arabidopsis thaliana, with mutations designated pir1 and pir 2, that were also resistant to TIBA. The mutations specifically affect ed the root-elongation process, and they were shown ultimately to be a llelic to aux1 and ein2, respectively, which are known as mutations th at affect responses to phytohormones. The mechanism of action of auxin transport inhibitors was investigated with these mutants, in relation to the effects of ethylene, auxin, and the polar transport of auxin. With respect to the inhibition of root elongation in A. thaliana, we d emonstrated that (I) the background level of ethylene intensifies the effects of auxin transport inhibitors, (2) auxin transport inhibitors might act also via an inhibitory pathway that does not involve ethylen e, auxin, or the polar transport of auxin, (3) the hypothesis that the inhibitory effect of NPA on root elongation is due to high-level accu mulation of auxin as a result of blockage of auxin transport is not ap plicable to A. thaliana, and (4) in contrast to NPA, TIBA itself has a weak auxin-like inhibitory effect.