Mode of action of glyphosate in relation to metabolism of indole-3-acetic acid

Studies were conducted with radio-labeled indole-3-acetic acid (2-¹⁴C] IAA) and tobacco callus culture ( Nicotiana tabacum L. cv. White Gold) to investigate the mode of action of the herbicide glyphosate (N-phosphonomethylglycine). The tissue was first grown with or without glyphosate for 1 to 14 days and then incubated with 2-¹⁴C] IAA for 4 h. Metabolism of 2-¹⁴C] IAA in the tissue was studies by solvent fractionation, high performance liquid chromatography and liquid scintillation counting. The tissue grown with 0.2 m M glyphosate had low level of free 2-¹⁴C] IAA and high levels of other fractions containing metabolites and conjugates of the labeled IAA. After 1 day of glyphosate treatment the free 2-¹⁴C] IAA level in the tissue was reduced by 77% compared to that of the control; after 10 days of treatment the decrease was 96%. The decrease in the free 2-¹⁴C] IAA level was not due to inhibition of IAA uptake, but due to enhanced rates of oxidation and conjugate formation of IAA. The increased oxidation of IAA in the treated tissue was not due to a direct effect of glyphosate on IAA-oxidase since glyphosate was inactive on IAA oxidation in a cell-free system in vitro. The glyphosate-induced growth inhibition was partially overcome by addition of 1 μ M 2,4-dichlorophenoxyacetic acid to the medium. The results lead to the conclusion that glyphosate inhibits growth by depletion of free IAA through rapid acceleration of both conjugate formation and oxidative degradation of IAA.