Effects of Haloxyfop and CGA-82725 on Cell Cycle and Cell Division of Oat (Avena sativa) Root Tips

Abstract

The mode of action of two experimental pyridinyloxyphenoxy propionate herbicides, haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy] phenoxy] propanoic acid} and CGA-82725 {2-propynyl-2-[4-[(3,5-dichloro-2-pyridinyl)oxy] phenoxy] propanoic acid} was studied. Oat (Avena sativaL.) roots showed a rate-dependent sensitivity to both herbicides. Consequently, a series of assays was performed to study cell division, cell cycle dynamics, and nucleic acid and protein synthesis in oat root tips in order to characterize growth inhibition. Both herbicides inhibited cell division, apparently by inhibiting protein synthesis in the G2 stage of interphase. Supporting evidence for this assumption comes from the following observations: a) There was almost complete absence of leucine incorporation within 1 h of exposure to 2.7 × 10−4M of either herbicide; b) the number of3H-labeled dividing cells was reduced to about 20% of the control within 4 h, and essentially to zero within 8 h, after exposure to 2.7 × 10−4M of herbicide; c) the number of unlabeled cells was reduced to less than 10% of the control within 8 h of exposure to 2.7 × 10−4M of herbicide; d) during this 8-h period, the number of3H-labeled cells in interphase which had been exposed to herbicide greatly exceeded the control; e) thymidine incorporation into DNA was inhibited but was not proportional to reduction in cell division; f) uridine incorporation was enhanced during the first 8 h of exposure before a reduction was evident at 12 h; and g) an oat coleoptile elongation bioassay showed that cell enlargement was not inhibited at herbicide concentrations less than 2.7 × 10−4M.