Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] inhibition of gibberellin precursor biosynthesis

[2-14C]Mevalonic acid incorporation into gibberellic acid precursors was measured in cell-free extracts from sorghum [Sorghum bicolor (L.) Moench var. G-522 DR] coleoptiles. 14C incorporation into ent-kaur-16-ene was inhibited ca. 90% by 10−7 to 10−4 M metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide]. [14C]Geranylgeraniol (GG) content increased. [14C]Farnesol content was not altered and [14C]geraniol content decreased. Total 14C incorporation was decreased by metolachlor. In the safener [α-(cyanomethoximino)benacetonitrile]-treated sorghum seed coleoptile cell-free system, total 14C incorporation increased, [14C]kaurene and relative kaurence content increased 4× up to 105 M metolachlor, and [14C]farnesol, and [14C]GG contents increased while relative farnesol and relative GG contents were not influenced by metolachlor. Thus, the inhibition of kaurene synthesis by metolachlor was reversed by the safener. Since the biosynthetic processes are mevalonic acid → geraniol → farnesol → GG → copalylol → kaurene, these data corroborate a proposed gibberellic acid biosynthesis inhibition between GG and kaurene as well as a partial blockage between mevalonic acid and geraniol. Thus, a portion of metolachlor-induced growth inhibitions of sorghum could be explicable on the basis of gibberellic acid biosynthesis inhibitions.