We have hitherto reported that the differential flank growth that causes phototropic curvature, is regulated by a local gradient, induced at the site of illumination, of blue light-induced growth inhibitor(s) interfering with the action of the still evenly distributed auxin (Bruinsma-Hasegawa theory, 1990). In this study, we report a role of inhibitor in the phototropism of Arabidopsis thaliana hypocotyls. It is very difficult to dissect the extremely small Arabidopsis hypocotyls into illuminated and shaded side. Therefore, using the wild-type and the nph3-101 mutant that shows no phototropic response, we compared endogenous chemical substances in phototropically stimulated whole hypocotyls and the dark controls. From light-grown wild-type shoots an inhibitor was isolated and identified as indole-3-acetonitrile (IAN) from its ^1H NMR spectrum. The content of endogenous IAN in the hypocotyls of wild-type and mutant unilaterally exposed to blue light was determined using a physicochemical assay. The IAN concentration in the phototropically stimulated wild-type hypocotyls was about 3 times larger than in the dark-control. However its content in the mutant hypocotyls did not change. IAN inhibited the hypocotyls growth of the nph3-101 to the same extent as in the wild-type. These results suggest that IAN plays an important role in the phototropism of LArabidopsis thaliana hypocotyls.
