A New Class of Synthetic Auxin Transport Inhibitors

Auxin transport inhibition by a new class of synthetic plant growth regulants, the 2-(3-aryl-5-pyrazolyl)benzoic acids, was examined in bean (Phaseolus vulgaris L.) using the donor-receiver agar cylinder technique. These compounds can be prepared by the dehydrogenation and ring cleavage of compounds like DPX-1840 (2-(4-methoxyphenyl)-3,3adihydro-8H-pyrazolo[5,1-a] isoindol-8-one) which was previously reported (Plant Physiol. 1972. 50: 322-327) to be a potent inhibitor of auxin transport. These new growth regulators inhibit auxin transport more than DPX-1840 does as evidenced by their consistently greater reduction of basipetal auxin transport capacity in bean when incorporated into the receiver agar cylinder or applied foliarly to intact plants. Direct comparisons of the effect of DPX-1840, its dehydrogenation product (2-(4-methoxyphenyl)-8H-pyrazolo [5,1-a]isoindol-8-one), and its open-ring form (2-(3-(4-methoxyphenyl)-5-pyrazolyl) benzoic acid) on auxin transport indicated the following order of activity: ring-open > dehydrogenated form > DPX-1840. DPX-1840-(14)C, applied at 0.5 mg/l to etiolated bean hypocotyl hooks followed by extraction and thin layer chromatography, indicated the biological conversion of DPX-1840 to its open-ring form. Collectively, these results suggest that the biologically active forms of DPX-1840-type compounds are the open-ring (2-(3-aryl-5-pyrazolyl) benzoic acids.