Inhibited Mitotic Entry is the Cause of Growth Inhibition by Cinmethylin

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Abstract
The herbicide cinmethylin {exo-1-methyl-4-(1-methylethyl)-2-[(2-methylphenyl)methoxy]-7-oxabicyclo [2.2.1] heptane} inhibited oat (Avena sativa L. ‘Porter’) root growth during the first 6 h of treatment at a concentration of 6.7 × 10-8 M. A concentration of 1 × 10-8 M cinmethylin inhibited root growth within 12 to 18 h. Inhibition of shoot growth was less sensitive, but was inhibited by 36 to 48 h after treatment with 1 × 10-7 M and by 12 to 24 h after treatment with 1 × 10-5 M cinmethylin. Cinmethylin concentrations of 1 × 10-5 M and lower did not inhibit cell elongation in isolated oat coleoptiles during a 24-h exposure. Mitotic frequency in oat root tips was reduced after 12 h of treatment with 1 × 10-7 M cinmethylin. The frequency of all stages of mitosis (prophase, metaphase, and anaphase + telophase) was reduced. Concentrations of 1 × 10-6 M cinmethylin resulted in nearly complete arrest (87% inhibition) of mitosis. These data suggest cinmethylin inhibits growth by inhibiting entry of cells into mitosis. The cause of mitotic arrest is unknown; however, the mechanism appears to be different from other herbicides known to inhibit mitosis.