Mesotrione is a new herbicide being developed for the selective pre- and post-emergence control of a wide range of broad-leaved and grass weeds in maize (Zea mays). It is a member of the benzoylcyclohexane-1,3-dione family of herbicides, which are chemically derived from a natural phytotoxin obtained from the Californian bottlebrush plant, Callistemon citrinus. The compound acts by competitive inhibition of the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD), a component of the biochemical pathway that converts tyrosine to plastoquinone and alpha-tocopherol. Mesotrione is an extremely potent inhibitor of HPPD from Arabidopsis thaliana, with a Ki value of c 6-18 pM. It is rapidly taken up by weed species following foliar application, and is distributed within the plants by both acropetal and basipetal movement. Maize is tolerant to mesotrione as a consequence of selective metabolism by the crop plant. Slower uptake of mesotrione, relative to susceptible weed species, may also contribute to its utility as a selective herbicide for use in maize.

Previous research reported the first case of resistance to mesotrione and other 4-hydroxyphenylpyruvate dioxygenase (HPPD) herbicides in a waterhemp (Amaranthus tuberculatus) population designated MCR (for McLean County mesotrione-and atrazineresistant). Herein, experiments were conducted to determine if target site or nontarget site mechanisms confer mesotrione resistance in MCR. Additionally, the basis for atrazine resistance was investigated in MCR and an atrazine-resistant but mesotrione-sensitive population (ACR for Adams County mesotrione-sensitive but atrazine-resistant). A standard sensitive population (WCS for Wayne County herbicide-sensitive) was also used for comparison. Mesotrione resistance was not due to an alteration in HPPD sequence, HPPD expression, or reduced herbicide absorption. Metabolism studies using whole plants and excised leaves revealed that the time for 50% of absorbed mesotrione to degrade in MCR was significantly shorter than in ACR and WCS, which correlated with previous phenotypic responses to mesotrione and the quantity of the metabolite 4-hydroxy-mesotrione in excised leaves. The cytochrome P450 monooxygenase inhibitors malathion and tetcyclacis significantly reduced mesotrione metabolism in MCR and corn (Zea mays) excised leaves but not in ACR. Furthermore, malathion increased mesotrione activity in MCR seedlings in greenhouse studies. These results indicate that enhanced oxidative metabolism contributes significantly to mesotrione resistance in MCR. Sequence analysis of atrazine-resistant (MCR and ACR) and atrazine-sensitive (WCS) waterhemp populations detected no differences in the psbA gene. The times for 50% of absorbed atrazine to degrade in corn, MCR, and ACR leaves were shorter than in WCS, and a polar metabolite of atrazine was detected in corn, MCR, and ACR that cochromatographed with a synthetic atrazineglutathione conjugate. Thus, elevated rates of metabolism via distinct detoxification mechanisms contribute to mesotrione and atrazine resistance within the MCR population.

No abstract

. 11. Complementary DNAs encoding the short forms of G␣ s WT and G␣ s * (9) were subcloned into pUAST (7) and used to transform flies [A. Spradling, in Drosophila: A Practical Approach, D. Roberts, Ed. (IRL Press, Oxford, 1986), pp. 75 -197]. The UAS-G␣ s lines were verified by Msc I digestion of polymerase chain reaction-amplified G␣ s transgenes to detect the presence of a site found in G␣ s WT but lacking as a result of the Q215L mutation. 12. 238Y and 201Y are described in (17) and C232 in (18). C747 shows identical expression to that of C772 (17) (24). C309 is described by J. D. Armstrong [thesis, University of Glasgow, Scotland (1995)]. The O'Kane laboratory screened 500 P-GAL4 lines to identify OK66, OK86, OK62, OK107, OK348, and OK415. 13. T. Tully and W. Quinn, J. Comp. Physiol. 157, 263 (1985); T. Tully, T. Preat, S. Boynton, M. Del Vecchio, Cell 79, 35 (1994). 14. In total, we analyzed behaviorally 16 P-GAL4 insertions that expressed to some degree in MBs. Flies with four insertions (OK66, OK86, KL65, and KL107) showed reduced learning in the absence of expression of constitutively activated G␣ s *, most likely because of genetic background differences. These were not tested further. Flies with eight insertions showed defects in olfactory acuity (30Y, C35, OK62, OK107, C302, OK415, and C532) or shock reactivity (C772), as transheterozygotes with UAS-G␣ s *. These were eliminated from further behavioral testing. Flies with the remaining four P-GAL4 insertions (201Y, 238Y, C309, and C747) showed disrupted learning but normal olfactory acuity and shock reactivity, as transheterozygotes with at least one UAS-G␣ s * insertion. We characterized a total of two P-GAL4 insertions (C232 and OK348) that expressed to some degree in the CC. 15. We suggest that olfactory learning results from G sdependent convergence of the conditional stimulus (CS) and unconditional stimulus (US) in the MBs. This belief is reinforced by the observation that expression of G␣ s * in the CC does not affect learning.

No abstract