2003 Volume 28 Issue 3 Pages 324-329
Very-long-chain fatty acids (VLCFAs) having more than 18 C-atoms (like C20, 22, 24) are formed by a microsomal elongase system of the endoplasmic reticulum. This 4-step catalytic system is strongly inhibited by chloroacetamides with I50-values of 10 to 100 nM. Inhibition depends on the amide structure and on stereospecificity (for e.g. metolachlor). Also structures different from chloroacetamides like cafenstrole or fentrazamide (a tetrazolinone) exhibit strong inhibitory activity. A cell-free assay was developed using isolated microsomes from Allium porrum (leek) with labeled malonyl-CoA and C18, C20 or C22 acyl-CoA as primer substrates. All elongation steps were strongly inhibited by those phytotoxic compounds which also have been found active in the intact plant. While the plant contains in total about 1% VLCFAs, the plasma membrane (plasmalemma) is enriched. Isolated plasma membranes (from cucumber) have a small VLCFA content when treating the intact plant with the herbicides. There is evidence that such membrane looses its stability and function, and we conclude that herbicidal phytotoxicity is correlated with a non-balanced VLCFA pattern. Data with transformed yeast demonstrate that the key target for chloroacetamides is exclusively the condensing starter enzyme (=VLCFA synthase) of the elongase system. An irreversible binding complex of synthase and inhibitor is assumed. Reports in literature indicate that flavonoid and anthocyanin biosynthesis is affected by chloroacetamides. Since it was speculated that this inhibition may cause plant death chalcone synthase was assayed catalyzing the key condensing reaction between malonyl-CoA and coumaroyl-CoA. Also this enzyme was inhibited by chloroacetamides or cafenstrole. Our enzymological studies, however, could demonstrate that this inhibition does not cause the herbicidal effect. — Based on our recent findings a reasoning is presented why resistance of weeds against this class of herbicides is a rare event.
