The herbicidally active compound N-2-(6-methyl-pyridyl)-aminomethylene bisphosphonic acid inhibits in vivo aromatic biosynthesis

The effect of N-2-(6-methyl-pyridyl)-aminomethylene bisphosphonic acid (M-p yr-AMBPA), a compound previously shown to exhibit herbicidal properties on whole plants and to inhibit in vitro activity of the first enzyme in the sh ikimate pathway, 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthas e, was investigated on Nicotiana plumbaginifolia suspension cultured cells and compared to that of the herbicide glyphosate. The addition of M-pyr-AMB PA from 10(-4) to 10(-3) M was found to cause a severe cell growth reductio n. Kinetic analysis of partially purified DAHP synthase accounted for non-c ompetitive inhibition type with respect to both phospho-enol-pyruvate and e rythrose-4-phosphate, with K-I values of 0.43 and 0.62 mM, respectively. Am ino acid pool measurements of cells grown in the presence of sublethal dose s of M-pyr-AMBPA pointed to an actual reduction of free aromatic amino acid s, showing that DAHP synthase inhibition takes place in vivo, and suggestin g that the interference of this aminophosphonate with plant aromatic biosyn thesis may account for a large part of its phytotoxicity. However, exogenou s supply of a mixture of phenylalanine, tyrosine and tryptophan failed to a chieve full reversal of cell growth inhibition, yet the occurrence of other target(s) cannot be ruled out.