The mechanism of glyphosate resistance in horseweed was investigated. Eleven biotypes of putative sensitive (S) and resistant (R) horseweed were obtained from regions across the United States and examined for foliar retention, absorption, translocation, and metabolism of glyphosate. Initial studies used spray application of14C-glyphosate to simulate field application. When S and R biotypes were compared in the absence of toxicity at a sublethal dose, we observed comparable retention and absorption but reduced root translocation in the R biotypes. S and R biotypes from Delaware were further examined at field use rates and results confirmed similar retention and absorption but reduced root translocation in the R biotypes. Application of14C-glyphosate to a single leaf demonstrated reduced export out of the treated leaf and lower glyphosate import into other leaves, the roots, and the crown in R relative to S biotypes. Examination of the treated leaf by autoradiography showed that glyphosate loading into the apoplast and phloem was delayed and reduced in the R biotype. Our results consistently showed a strong correlation between impaired glyphosate translocation and resistance. Tissues from both S and R biotypes showed elevated levels of shikimate suggesting that 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) remained sensitive to glyphosate. Analysis of tissue shikimate levels demonstrated reduced efficiency in EPSPS inhibition in the R biotypes. Our results suggest that resistance is likely due to altered cellular distribution that impaired phloem loading and plastidic import of glyphosate resulting in reduced overall translocation as well as inhibition of EPSPS.

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Glyphosate is a broad-spectrum herbicide used for the control of weeds in glyphosate-resistant crops. Glyphosate inhibits 5-enolpyruvyl shikimate 3-phosphate synthase, a key enzyme in the synthesis of aromatic amino acids in plants, fungi, and bacteria. Studies with glyphosate-resistant wheat have shown that glyphosate provided both preventive and curative activities against Puccinia striiformis f. sp. tritici and Puccinia triticina, which cause stripe and leaf rusts, respectively, in wheat. Growth-chamber studies demonstrated wheat rust control at multiple plant growth stages with a glyphosate spray dose typically recommended for weed control. Rust control was absent in formulation controls without glyphosate, dependent on systemic glyphosate concentrations in leaf tissues, and not mediated through induction of four common systemic acquired resistance genes. A field test with endemic stripe rust inoculum confirmed the activities of glyphosate pre-and postinfestation. Preliminary greenhouse studies also demonstrated that application of glyphosate in glyphosate-resistant soybeans suppressed Asian soybean rust, caused by Phakopsora pachyrhizi.Phakopsora pachyrhizi ͉ Puccinia striiformis f. sp. tritici ͉ Puccinia triticina ͉ disease control

Previous association analyses showed that variation at major regulatory genes contributes to standing variation for complex traits in Balsas teosinte, the progenitor of maize. This study expands our previous association mapping effort in teosinte by testing 123 markers in 52 candidate genes for association with 31 traits in a population of 817 individuals. Thirty-three significant associations for markers from 15 candidate genes and 10 traits survive correction for multiple testing. Our analyses suggest several new putative causative relationships between specific genes and trait variation in teosinte. For example, two ramosa genes (ra1 and ra2) associate with ear structure, and the MADS-box gene, zagl1, associates with ear shattering. Since zagl1 was previously shown to be a target of selection during maize domestication, we suggest that this gene was under selection for its effect on the loss of ear shattering, a key domestication trait. All observed effects were relatively small in terms of the percentage of phenotypic variation explained (,10%). We also detected several epistatic interactions between markers in the same gene that associate with the same trait. Candidategene-based association mapping appears to be a promising method for investigating the inheritance of complex traits in teosinte.T HROUGH the characterization of major loss-offunction mutants, geneticists have determined the function of a vast number of genes. Despite a general knowledge of how these genes control developmental and physiological processes, very little is known about how (or if) they contribute to natural variation for complex traits. Association mapping with its high mapping resolution, its potential to sample multiple alleles, and its use of preexisting populations provides a powerful tool to investigate the role of these genes in the genetic architecture of complex traits ( Previously, we detected significant associations between polymorphisms in nine candidate genes and phenotypic variation in the maize ancestor, Balsas teosinte (Zea mays ssp. parviglumis) (Weber et al. 2007). Our study builds upon our prior analyses in several ways, including an increase in the numbers of individuals, candidate genes, and traits. We also selected our association mapping panel to decrease the amount of population structure as compared to our prior study. With this strategy, we detected 33 associations between complex traits in teosinte and our candidate genes that survive a correction for multiple testing. These include associations between indeterminate spikelet1 and inflorescence branching, ramosa1 and ramosa2 and ear structure, sugary1 and seed oil content, and terminal ear1 and ear length. We also observed an association between zea agamous-like1 (zagl1) and ear shattering. Since zagl1 was a target of selection during domestication, we propose that it was selected for its role in ear disarticulation. Several epistatic 1