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.

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. 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.

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When the iron-molybdenum cofactor (FeMoco) was extracted from the MoFe protein of nitrogenase from a nifV mutant of Klebsiella pneumoniae and combined with the FeMoco-deficient MoFe protein from a nifB mutant, the resultant MoFe protein exhibited the NifV phenotype, i.e. in combination with wild-type Fe protein it exhibited poor N2-fixation activity and its H2-evolution activity was inhibited by CO. These data provide strong evidence that FeMoco contains the active site of nitrogenase. The metal contents and e.p.r. properties of FeMoco from wild-type and nifV mutants of K. pneumoniae are very similar.

Structure–activity profiles for the phytohormone auxin have been collected for over 70 years, and a number of synthetic auxins are used in agriculture. Auxin classification schemes and binding models followed from understanding auxin structures. However, all of the data came from whole plant bioassays, meaning the output was the integral of many different processes. The discovery of Transport Inhibitor-Response 1 (TIR1) and the Auxin F-Box (AFB) proteins as sites of auxin perception and the role of auxin as molecular glue in the assembly of co-receptor complexes has allowed the development of a definitive quantitative structure–activity relationship for TIR1 and AFB5. Factorial analysis of binding activities offered two uncorrelated factors associated with binding efficiency and binding selectivity. The six maximum-likelihood estimators of Efficiency are changes in the overlap matrixes, inferring that Efficiency is related to the volume of the electronic system. Using the subset of compounds that bound strongly, chemometric analyses based on quantum chemical calculations and similarity and self-similarity indices yielded three classes of Specificity that relate to differential binding. Specificity may not be defined by any one specific atom or position and is influenced by coulomb matrixes, suggesting that it is driven by electrostatic forces. These analyses give the first receptor-specific classification of auxins and indicate that AFB5 is the preferred site for a number of auxinic herbicides by allowing interactions with analogues having van der Waals surfaces larger than that of indole-3-acetic acid. The quality factors are also examined in terms of long-standing models for the mechanism of auxin binding.

The aim of this brief review is to draw information from studies of the mechanism of evolved resistance in weeds, together with information from laboratory studies of paraquat tolerance in model plants. Plants having mutations that limit paraquat uptake into cytoplasm, that confer various stress tolerances or that have transgenes that co-express two or more of the chloroplast Halliwell-Asada cycle enzymes can all exhibit enhanced tolerance to paraquat. However, none of these mechanisms correspond to the high-level resistances that have evolved naturally in weeds. Most, but not all, of the evidence from studies of paraquat-resistant biotypes of weeds can reasonably be reconciled with the proposal of a single major gene mechanism that sequesters paraquat away from chloroplasts and into the vacuole. However, the molecular details of this putative mechanism remain ill-defined.