Inflorescences of a dominant mutant of Arabidopsis Aux/IAA7, axr2, showed negative phototropism with a similar fluence response curve to the positive phototropism of wild-type stems. Application of a synthetic auxin, NAA, and an inhibitor of polar auxin transport, NPA, increased and decreased respectively the magnitude of the phototropic response in the wild type, while in axr2 application of NAA reduced the negative phototropic response and NPA had no effect. Decapitation of the apex induced a small negative phototropism in wild-type stems, and had no effect in axr2 plants. Inflorescences of the double mutants of auxin transporters, pgp1 pgp19, showed no phototropic response, while decapitation resulted in a negative phototropic response. These results suggest that negative phototropism can occur when the level of auxin or of auxin signaling is reduced to a minimal level, and that in plant axial organs the default phototropic response to unilateral blue light may be negative. Expression of axr2 protein by an endodermis-specific promoter resulted in agravitropism of inflorescences in a similar way to that of axr2, but phototropism was normal, confirming that the endodermis does not play a critical role in phototropism.
Keywords: Arabidopsis thaliana; Aux/IAA, auxin/inodole-3-acetic acid; NAA, 1-naphtha-leneacetic acid; NPA, 1-N-naphthylpthalamic acid; SAUR, small auxin up RNA; SCR, scarecrow; TIR1, transport inhibitor 1; axr2, auxin resistant 2; dominant Aux/IAA mutants; gravitropism; hypocotyl; inflorescence; pgp, p-glycoprotein; phot, phototropin; phototropism.