Genetic and biochemical characterization of corn inbred lines tolerant to the sulfonylurea herbicide primisulfuron

Involvement of cytochrome P-450 enzyme activity in the selectivity and safening action of pyrazosulfuron-ethyl

To investigate the selectivity and safening action of the sulfonylurea herbicide pyrazosulfuron-ethyl (PSE), pyrazosulfuron-ethyl O-demethylase (PSEOD) activity involving oxidative metabolism by cytochrome P-450 was studied in rice (Oryza sativa L cv Nipponbare) and Cyperus serotinus Rottb. Cytochrome P-450-dependent activity was demonstrated by the use of the inducers 1,8-naphthalic anhydride and ethanol, the herbicides PSE, bensulfuron-methyl, dimepiperate and dymron, or the inhibitor piperonyl butoxide (PBO). Growth inhibition in C serotinus seedlings was more severe than that in rice seedlings. O-Dealkylation activities of PSE were induced differently in rice and in C serotinus, with distinctly higher activity in rice seedlings. The induced PSEOD activities were slightly inhibited by PBO in rice seedlings, whereas they were strongly inhibited in C serotinus seedlings. Dimepiperate and dymron were effective safeners of rice against PSE treatment. Treatments with herbicide alone resulted in less induction of PSEOD activity compared with combined treatments of the herbicide and safener. PSEOD activity in rice seedlings induced with herbicide alone was strongly inhibited by PBO, whereas it was weakly inhibited in rice seedlings induced with combinations of PSE and two safeners. These results suggest that O-demethylation by cytochrome P-450 enzymes may be involved in the metabolism of PSE and may contribute to its selectivity and safening action. Furthermore, these results suggest the existence of a multiple form of cytochrome P-450 in plants.

Comparison of flash-induced light-scattering transients and proton efflux from auxinic-herbicide resistant and susceptible wild mustard protoplasts: a possible role for calcium in mediating auxinic herbicide resistance

Comparison of flash-induced light-scattering signals from protoplasts of auxinic herbicide-resistant ('R') and -susceptible ('S') wild mustard biotypes suggest that ATP-dependent activity manifested as a conformational change differs between the biotypes. The signal obtained from R protoplasts is unaffected by 50 microM picloram, an auxin analogue. In comparison, the signal amplitude from the S protoplasts decreases 40% in 20 microM picloram and is abolished in 50 microM picloram. Signal amplitude from the R protoplasts can be decreased by simultaneous incubation with 50 microM picloram and 50 microM verapamil, a calcium channel blocker. Incubation of the S protoplasts with the calcium ionophore A23187 in the presence of 0.5 mM calcium reduced the inhibitory effect of picloram. Absorbance spectrophotometry using acridine orange as a pH indicator suggests that picloram affects the proton flux in the S biotype only. Based on these results, we hypothesize that picloram affects the calcium and hydrogen dynamics in the S biotype. Because the scattering characteristics of S protoplasts incubated with picloram can be nearly reproduced in the R protoplasts by simultaneous incubation with picloram and verapamil, calcium may have a significant role in modulating auxinic herbicide resistance. To our knowledge, this is the first report to implicate such a role for calcium.

Herbicide resistance due to amplification of a mutant acetohydroxyacid synthase gene

We have selected a tobacco cell line, SU-27D5, that is highly resistant to sulfonylurea and imidazolinone herbicides. This line was developed by selection first on a lethal concentration of cinosulfuron and then on increasing concentrations of primisulfuron, both sulfonylurea herbicides. SU-27D5 was tested against five sulfonylureas and one imidazolinone herbicide and was shown, in every case, to be two to three orders of magnitude more resistant than wild-type cells. The acetohydroxyacid synthase (AHAS) of SU-27D5 was 50- to 780-fold less sensitive than that of wild-type cells to herbicide inhibition. The specific activity of AHAS in the SU-27D5 cell lysate was 6 to 7 times greater than that in wild-type cells. Using Southern analysis, we showed that cell line SU-27D5 had amplified its SuRB AHAS gene about 20-fold while maintaining a normal diploid complement of the SuRA AHAS gene. Genomic clones of both AHAS genes were isolated and used to transform wild-type tobacco protoplasts. SuRB clones gave rise to herbicide-resistant transformants, whereas SuRA clones did not. DNA sequencing showed that all SuRB clones contained a point mutation at nucleotide 588 that converted amino acid 196 of AHAS from proline to serine. In contrast, no mutations were found in the SuRA clones. The stability of SuRB gene amplification was variable in the absence of selection. In one experiment, the withdrawal of selection reduced the copy number of the amplified SuRB gene to the normal level within 30 days. In another experiment, amplification remained stable after extended cultivation on herbicide-free medium.(ABSTRACT TRUNCATED AT 250 WORDS)