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Weed control failures due to herbicide resistance are an increasing and worldwide problem significantly impacting crop yields. Herbicide resistance due to increased herbicide metabolism in weeds is not well characterized at the genetic level. An RNA-Seq transcriptome analysis was used to identify genes conferring metabolism-based herbicide resistance (MBHR) in a population (R) of a major global weed (Lolium rigidum), in which resistance to the herbicide diclofop-methyl was experimentally evolved through recurrent selection from a susceptible (S) progenitor population. A reference transcriptome of 19,623 contigs was assembled using 454 sequencing technology on a cDNA library and annotated using UniProt and Pfam databases. Transcriptomic-level gene expression was measured using Illumina 100 bp reads from untreated control, mock, and diclofop-methyl treatments of R and S. Due to the established importance of cytochrome P450 (CytP450), glutathione-S-transferase (GST), and glucosyltransferase (GT) genes in MBHR, 11 contigs with these annotations and higher constitutive expression in untreated R than in untreated S were selected as candidate genes for hypothesis testing, along with 17 additional differentially expressed contigs with annotations related to metabolism or signal transduction. In a forward genetics validation experiment, higher constitutive expression of nine contigs co-segregated with the resistance phenotype in an F2 population, including 3 CytP450, 3 GST, and 1 GT. At least nine genes with heritable increased constitutive expression are associated with MBHR trait. In a physiological validation experiment where 2, 4-D pre-treatment induced diclofop-methyl protection in S individuals due to increased metabolism, seven of the nine genetically-validated contigs were significantly induced. These data help explain accumulation of resistance-endowing genes and rapid evolution of MBHR, and provide the opportunity to improve diagnostics of MBHR using molecular tools such as transcriptional markers.

BACKGROUND: A population of Echinochloa colona infesting agricultural fields in the northern region of Western Australia evolved glyphosate resistance after 10 years of glyphosate selection. This study identified two phenotypic (susceptible S versus resistant R) lines from within a segregating glyphosate-resistant population. Estimation of survival, growth and reproductive rates of the phenotypes in response to glyphosate selection helped to characterise the level of resistance, fitness and the selection intensity for glyphosate in this species. RESULTS: Estimations of LD50 (lethal dose) and GR50 (growth rate) showed an eightfold glyphosate resistance in this population. The resistant index based on the estimation of seed number (SYn50) showed a 13-fold resistance. As a result of linear combination of plant survival and fecundity rates, plant fitness values of 0.2 and 0.8 were estimated for the S and R phenotypes when exposed to the low dose of 270 g glyphosate ha−1. At the recommended dose of 540 g glyphosate ha−1, fitness significantly decreased (fivefold) in S plants but remained markedly similar (0.7) in plants of the R phenotype. Thus, the calculated selection intensity (SI) at 540 g glyphosate ha−1 was much greater (SI = 17) than at 270 g glyphosate ha−1 (SI = 4). CONCLUSIONS: The assessment of plant survival and fecundity in response to glyphosate selection in the S and R phenotypes allowed a greater accuracy in the estimation of population fitness of both phenotypes and thus of glyphosate selection intensity in E. colona. The estimation of seed number or mass of phenotypes under herbicide selection is a true ecological measure of resistance with implications for herbicide resistance evolution. Fil: Sheng Goh, Sou. University of Western Australia; Australia Fil: Vila Aiub, Martin Miguel. University of Western Australia; Australia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Busi, Roberto. University of Western Australia; Australia Fil: Powles, Stephen B.. University of Western Australia; Australia

The rate of herbicide resistance evolution in plants depends on fitness traits endowed by alleles in both the presence and absence (resistance cost) of herbicide selection. The effect of two Lolium rigidum spontaneous homozygous target-site resistance-endowing mutations (Ile-1781-Leu, Asp-2078-Gly) on both ACCase activity and various plant growth traits have been investigated here. Relative growth rate (RGR) and components (net assimilation rate, leaf area ratio), resource allocation to different organs, and growth responses in competition with a wheat crop were assessed. Unlike plants carrying the Ile-1781-Leu resistance mutation, plants homozygous for the Asp-2078-Gly mutation exhibited a significantly lower RGR (30%), which translated into lower allocation of biomass to roots, shoots, and leaves,and poor responses to plant competition. Both the negligible and significant growth reductions associated, respectively, with the Ile-1781-Leu and Asp-2078-Gly resistance mutations correlated with their impact on ACCase activity. Whereas the Ile-1781-Leu mutation showed no pleiotropic effects on ACCase kinetics, the Asp-2078-Gly mutation ledto a significant reduction in ACCase activity. The impaired growth traits are discussed in the context of resistance costs and the effects of each resistance allele on ACCase activity. Similar effects of these two particular ACCase mutations on the ACCase activity of Alopecurus myosuroides were also confirmed. Fil: Vila Aiub, Martin Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. University of Western Australia. School of Plant Biology. Australian Herbicide Resistance Initiative; Australia Fil: Yu, Quin. University of Western Australia. School of Plant Biology. Australian Herbicide Resistance Initiative; Australia Fil: Han, Heping. University of Western Australia. School of Plant Biology. Australian Herbicide Resistance Initiative; Australia Fil: Powles, Stephen B.. University of Western Australia. School of Plant Biology. Australian Herbicide Resistance Initiative; Australia

Background Gene mutations that endow herbicide resistance may cause pleiotropic effects on plant ecology and physiology. This paper reports on the effect of a number of known and novel target-site resistance mutations of the ALS gene (Ala-122-Tyr, Pro-197-Ser, Asp-376-Glu or Trp-574-Leu) on vegetative growth traits of the weed Raphanus raphanistrum. Results The results from a series of experiments have indicated that none of these ALS resistance mutations imposes negative pleiotropic effects on relative growth rate (RGR), photosynthesis and resource-competitive ability in R. raphanistrum plants. The absence of pleiotropic effects on plant growth occurs in spite of increased (Ala-122-Tyr, Pro-197-Ser, Asp-376-Glu) and decreased (Trp-574-Leu) extractable ALS activity. Conclusion The absence of detrimental pleiotropic effects on plant growth associated with the ALS target-site resistance mutations reported here is a contributing factor in resistance alleles being at relatively high frequencies in ALS-herbicide-unselected R. raphanistrum populations. Fil: Li, Mei. University of Western Australia. School of Plant Biology. Australian Herbicide Resistance Initiative; Australia. Shandong Academy of Agricultural Sciences. Institute of Plant Protection; China Fil: Yu, Qin. University of Western Australia. School of Plant Biology. Australian Herbicide Resistance Initiative; Australia Fil: Han, Heping. University of Western Australia. School of Plant Biology. Australian Herbicide Resistance Initiative; Australia Fil: Vila Aiub, Martin Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. University of Western Australia. School of Plant Biology. Australian Herbicide Resistance Initiative; Australia Fil: Powles, Stephen B.. University of Western Australia. School of Plant Biology. Australian Herbicide Resistance Initiative; Australia

International audience; Modern herbicides greatly contribute to world agricultural production but their sustainability is threatened by the widespread evolution of herbicide resistant weedy plant populations. Despite the commercial and scientific importance of resistance, there has not been an experimental model system for pro-actively evaluating the potential for herbicide resistance evolution. Here, utilizing the rapidly growing, unicellular photosynthetic microalgae Chlamydomonas reinhardtii (Dangeard), a ratchet protocol has been developed that solves the problem of maintaining both large populations and strong herbicide selection. The ratchet protocol is a progressive set of cycles, each cycle commencing with a population of approximately one million individuals apportioned amongst three herbicide doses for 14 days. Whenever the evolving population demonstrates growth across the three herbicide selection intensities, then the population ratchets to the next cycle of higher herbicide dose. Therefore, by always maintaining large populations under selection pressure, this system offers the opportunity for beneficial mutations to arise and be enriched. Using the well-characterized atrazine herbicide, the ratchet protocol resulted in rapid evolution of populations with different levels of resistance. This robust laboratory based Chlamydomonas system is proposed for application in establishing the respective propensity for resistance evolution to herbicides or other selecting agents.

Unlike in the pesticide and antibiotic resistance literature, potential social costs and externalities associated with herbicide resistance have not generally been considered by economists. The economics of managing herbicide resistance in weeds has focused on cost-effective responses by growers to the development of resistance at the individual farm and field level. Economic analyses of optimal herbicide use have focused on optimising farmer returns in the long run. Weeds have been considered less mobile, compared to insects and diseases, suggesting that externalities resulting from resistance spread will be minimal and any consequent social costs low. Glyphosate is the world's most widely used broad-spectrum non-selective herbicide. Declining glyphosate prices, the adoption of no-till and minimum-till systems and the adoption of glyphosate-tolerant crops, have combined to cause a rapid increase in the use of glyphosate, and resistance is now appearing. In this paper we argue that the increasing possibility of widespread glyphosate resistance, exacerbated in some situations by spread through resistance mobility, presents a case where social costs associated with glyphosate resistance need to be considered when assessing optimal use of this herbicide resource at the farm level. Possible social costs associated with the loss of glyphosate efficacy include potential failure of herbicide-resistant crop systems, reduced use of conservation tillage techniques, and potentially more reliance on herbicides with greater environmental and health risks.

Unlike in the pesticide and antibiotic resistance literature, potential social costs and externalities associated with herbicide resistance have not generally been considered by economists. The economics of managing herbicide resistance in weeds has focused on cost-effective responses by growers to the development of resistance at the individual farm and field level. Economic analyses of optimal herbicide use have focused on optimising farmer returns in the long run. Weeds have been considered less mobile, compared to insects and diseases, suggesting that externalities resulting from resistance spread will be minimal and any consequent social costs low. Glyphosate is the world's most widely used broad-spectrum non-selective herbicide. Declining glyphosate prices, the adoption of no-till and minimum-till systems and the adoption of glyphosate-tolerant crops, have combined to cause a rapid increase in the use of glyphosate, and resistance is now appearing. In this paper we argue that the increasing possibility of widespread glyphosate resistance, exacerbated in some situations by spread through resistance mobility, presents a case where social costs associated with glyphosate resistance need to be considered when assessing optimal use of this herbicide resource at the farm level. Possible social costs associated with the loss of glyphosate efficacy include potential failure of herbicide-resistant crop systems, reduced use of conservation tillage techniques, and potentially more reliance on herbicides with greater environmental and health risks.

In grasses, residues homologous to residues Ile-1,781 and Ile-2,041 in the carboxyl-transferase (CT) domain of the chloroplastic acetyl-coenzyme A (CoA) carboxylase (ACCase) from the grass weed black-grass (Alopecurus myosuroides [Huds.]) are critical determinants for sensitivity to two classes of ACCase inhibitors, aryloxyphenoxypropionates (APPs) and cyclohexanediones. Using natural mutants of black-grass, we demonstrated through a molecular, biological, and biochemical approach that residues Trp-2,027, Asp-2,078, and Gly-2,096 are also involved in sensitivity to ACCase inhibitors. In addition, residues Trp-2,027 and Asp-2,078 are very likely involved in CT activity. Using three-dimensional modeling, we found that the side chains of the five residues are adjacent, located at the surface of the inside of the cavity of the CT active site, in the vicinity of the binding site for APPs. Residues 1,781 and 2,078 are involved in sensitivity to both APPs and cyclohexanediones, whereas residues 2,027, 2,041, and 2,096 are involved in sensitivity to APPs only. This suggests that the binding sites for these two classes of compounds are overlapping, although distinct. Comparison of three-dimensional models for black-grass wild-type and mutant CTs and for CTs from organisms with contrasted sensitivity to ACCase inhibitors suggested that inhibitors fitting into the cavity of the CT active site of the chloroplastic ACCase from grasses to reach their active sites may be tight. The three-dimensional shape of this cavity is thus likely of high importance for the efficacy of ACCase inhibitors.

Perceptions are known to play an important role in the innovation adoption decision. Once influential perceptions have been identified, there is the potential for information to influence adoption by changing these perceptions. In this paper, the influence of an extension workshop targeting grain growers’ perceptions known to be associated with the adoption of integrated weed management and herbicide resistance management has been measured using regression analysis. Consistent with a Bayesian learning framework, the greatest influence on grower perceptions and intended adoption behaviour was observed where information could be delivered with a high degree of certainty and validity.

Perceptions are known to play an important role in the innovation adoption decision. Once influential perceptions have been identified, there is the potential for information to influence adoption by changing these perceptions. In this paper, the influence of an extension workshop targeting grain growers’ perceptions known to be associated with the adoption of integrated weed management and herbicide resistance management has been measured using regression analysis. Consistent with a Bayesian learning framework, the greatest influence on grower perceptions and intended adoption behaviour was observed where information could be delivered with a high degree of certainty and validity.

The Multi-species RIM (Resistance and Integrated Management) model is used in this analysis to investigate the value of Roundup-Ready® canola in the simultaneous management of annual ryegrass (Lolium rigidum) and wild radish (Raphanus raphanistrum). It is likely that the transgenic canola variety resistant to the non-selective herbicides glyphosate will soon be introduced in Australian agriculture. The perceived advantage of growing these crops is the potential to control post-emergent weeds with excellent broad-spectrum herbicides, and without the yield penalty evident in triazine-resistant canola (grown widely in WA). This may also help prolong the life of selective herbicides, to which ryegrass and radish can be highly resistant. Therefore, the introduction of genetically modified glyphosate-resistant canola will, other factors being equal, not only increase the options for weed control, but increase the yield of the vast canola crops grown in WA. Conversely, increased usage of the herbicide to which the new crop is resistant can result in the evolution of resistance to that herbicide in weeds. These trade-offs are discussed here.

Extension programs in Australia are encouraging farmers to adopt integrated weed management (IWM) practices, in order to delay the development of herbicide resistance in weeds infesting cropping land. Logistic and Tobit regression models were developed and used to analyse, survey data from Western Australian grain growers. Factors shown to influence IWM adoption included perceptions of IWM practices and expectations about the future of the herbicide resource. The results were consistent with the prior that farmers regard IWM as an information-intensive package of techniques, and that extension may have the potential to increase the rate of adoption.

The Multi-species RIM (Resistance and Integrated Management) model is used in this analysis to investigate the value of Roundup-Ready® canola in the simultaneous management of annual ryegrass (Lolium rigidum) and wild radish (Raphanus raphanistrum). It is likely that the transgenic canola variety resistant to the non-selective herbicides glyphosate will soon be introduced in Australian agriculture. The perceived advantage of growing these crops is the potential to control post-emergent weeds with excellent broad-spectrum herbicides, and without the yield penalty evident in triazine-resistant canola (grown widely in WA). This may also help prolong the life of selective herbicides, to which ryegrass and radish can be highly resistant. Therefore, the introduction of genetically modified glyphosate-resistant canola will, other factors being equal, not only increase the options for weed control, but increase the yield of the vast canola crops grown in WA. Conversely, increased usage of the herbicide to which the new crop is resistant can result in the evolution of resistance to that herbicide in weeds. These trade-offs are discussed here.

Extension programs in Australia are encouraging farmers to adopt integrated weed management (IWM) practices, in order to delay the development of herbicide resistance in weeds infesting cropping land. Logistic and Tobit regression models were developed and used to analyse, survey data from Western Australian grain growers. Factors shown to influence IWM adoption included perceptions of IWM practices and expectations about the future of the herbicide resource. The results were consistent with the prior that farmers regard IWM as an information-intensive package of techniques, and that extension may have the potential to increase the rate of adoption.

A multi-species version of the bio-economic RIM (Resistance and Integrated Management) model has been developed to deal with the complexities involved in the long-term integrated management of annual ryegrass (Lolium rigidum Gaud.) and wild radish (Raphanus raphanistrum L.), which dominate and co-exist in southern Australia. In this paper, we present a review of the existing options on how to model multi-species competition in order to select the best approach for incorporation in the RIM framework. Furthermore, we show how we have extended the original single-species ryegrass RIM model to include other aspects of the wild radish biology as well as a set of extra weed management practices used to control this weed species. We also demonstrate how the Multi-species RIM model can be used to evaluate weed management scenarios of co-existing herbicide resistant species. This is done through investigating the implications of using Roundup Ready® canola in the system.

A multi-species version of the bio-economic RIM (Resistance and Integrated Management) model has been developed to deal with the complexities involved in the long-term integrated management of annual ryegrass (Lolium rigidum Gaud.) and wild radish (Raphanus raphanistrum L.), which dominate and co-exist in southern Australia. In this paper, we present a review of the existing options on how to model multi-species competition in order to select the best approach for incorporation in the RIM framework. Furthermore, we show how we have extended the original single-species ryegrass RIM model to include other aspects of the wild radish biology as well as a set of extra weed management practices used to control this weed species. We also demonstrate how the Multi-species RIM model can be used to evaluate weed management scenarios of co-existing herbicide resistant species. This is done through investigating the implications of using Roundup Ready® canola in the system.

Herbicide resistance has become a major problem in Australian dryland agriculture. This situation has resulted from the repeated use of herbicides in place of the traditional weed control provided by cultivation and grazing. Farmers have addressed the problem of herbicide resistance by adopting a system of integrated weed management that allows weed control with a range of different techniques and herbicides. One of the non-chemical methods being considered by farmers is “green manuring”, which involves ploughing a healthy growing crop or pasture into the soil in order to prevent weed seed production and provide other benefits. In this study, the trade-offs between the effective weed control and biological benefits provided by green manuring and the large short-term economic losses associated with this practice are investigated for various rotations and patterns of herbicide use. This analysis is conducted using RIM, a bio-economic management model for ryegrass (Lolium rigidum).

Herbicide resistance has become a major problem in dryland agriculture. In Australia this particularly applies to annual ryegrass (Lolium rigidum) which has developed multiple resistance to a wide range of commonly used selective herbicides. Although herbicides are a very cost-effective means of reducing weed density, major changes to their use are required if sustainable weed management is to be achieved. In this study a model of ryegrass population dynamics was used to identify the best integrated weed management strategies and to evaluate changes in the economic payoff when choosing to conserve herbicides rather than exploit them rapidly. A situation of evolving herbicide resistance was simulated for a continuous wheat-lupin rotation and two cropping sequences including one and two pasture phases. Conservation of the last four shots of the selective herbicide Hoegrass was found to be less profitable than their exploitation. Nevertheless, conservation provided a better long-term weed control. Benefits from conservation were lower in a situation of reduced level of weed control, but appeared to increase with the inclusion of more pasture in the rotation.

Herbicide resistance has become a major problem in dryland agriculture. In Australia this particularly applies to annual ryegrass (Lolium rigidum) which has developed multiple resistance to a wide range of commonly used selective herbicides. Although herbicides are a very cost-effective means of reducing weed density, major changes to their use are required if sustainable weed management is to be achieved. In this study a model of ryegrass population dynamics was used to identify the best integrated weed management strategies and to evaluate changes in the economic payoff when choosing to conserve herbicides rather than exploit them rapidly. A situation of evolving herbicide resistance was simulated for a continuous wheat-lupin rotation and two cropping sequences including one and two pasture phases. Conservation of the last four shots of the selective herbicide Hoegrass was found to be less profitable than their exploitation. Nevertheless, conservation provided a better long-term weed control. Benefits from conservation were lower in a situation of reduced level of weed control, but appeared to increase with the inclusion of more pasture in the rotation.

Annual ryegrass (Lolium rigidum) biotype SLR 31 is resistant to the postemergent graminicide methyl-2-[4-(2,4-dichlorophenoxy)phenoxy]-propanoate (diclofop-methyl). Uptake of [(14)C](U-phenyl)diclofop-methyl and root/shoot distribution of radioactivity in susceptible and resistant plants were similar. In both biotypes, diclofop-methyl was rapidly demethylated to the biocidal metabolite diclofop acid which, in turn, was metabolized to ester and aryl-O-sugar conjugates. Susceptible plants accumulated 5 to 15% more radioactivity in dicloflop acid than did resistant plants. Resistant plants had a slightly greater capacity to form nonbiocidal sugar conjugates. Despite these differences, resistant plants retained 20% of (14)C in the biocidal metabolite diclofop acid 192 hours after treatment, whereas susceptible plants, which were close to death, retained 30% in diclofop acid. The small differences in the pool sizes of the active and inactive metabolites are by themselves unlikely to account for a 30-fold difference in sensitivity to the herbicide at the whole plant level. Similar high-pressure liquid chromatography elution patterns of conjugates from both susceptible and resistant biotypes indicated that the mechanisms and the products of catabolism in the biotypes are similar. It is suggested that metabolism of diclofop-methyl by the resistant biotype does not alone explain resistance observed at the whole-plant level. Diclofop acid reduced the electrochemical potential of membranes in etiolated coleoptiles of both biotypes; 50% depolarization required 1 to 4 mum diclofop acid. After removal of diclofop acid, membranes from the resistant biotype recovered polarity, whereas membranes from the susceptible biotype did not. Internal concentrations of diclofop acid 4 h after exposing plants to herbicide were estimated to be 36 to 39 micromolar in a membrane fraction and 16 to 17 micromolar in a soluble fraction. Such concentrations should be sufficient to fully depolarize membranes. It is postulated that differences in the ability of membranes to recover from depolarization are correlated with the resistance response of biotype SLR 31.

Photoinhibition was examined in paraquat-resistant and paraquat-susceptible biotypes of Hordeum glaucum Steud., Hordeum leporinum Link., Arctotheca calendula (L.) Levyns., and Conyza bonariensis (L.) Cronq. Plants were photoinhibited at low temperature, and the extent of photoinhibition determined by O(2) evolution and 77 K fluorescence. No difference in the degree of photoinhibition was detected between paraquat-resistant and paraquat-susceptible biotypes for any of the species examined. C. bonariensis plants were also photoinhibited by treatment without CO(2) at either 21% (volume/volume) O(2) or 4% (volume/volume) O(2), and again no difference was observed between the paraquat-resistant and paraquat-susceptible biotypes in reduction of the ratio of variable fluorescence to maximal fluorescence. This is in contrast to a recent report (MAK Jansen, Y Shaaltiel, D Kazzes, O Canaani, S Malkin, J Gressel, [1989] Plant Physiol 91: 1174-1178 in which it was claimed that a paraquat-resistant biotype of C. bonariensis was more tolerant of photoinhibition than a paraquat-susceptible biotype. We conclude that paraquat-resistant biotypes of these plant species are not more tolerant of photoinhibition when compared with the paraquat-susceptible biotypes.

Lolium rigidum Gaud. biotype SLR31 is resistant to the herbicide diclofop-methyl and cross-resistant to several sulfonylurea herbicides. Wheat and the cross-resistant ryegrass exhibit similar patterns of resistance to sulfonylurea herbicides, suggesting that the mechanism of resistance may be similar. Cross-resistant ryegrass is also resistant to the wheat-selective imidazolinone herbicide imazamethabenz. The cross-resistant biotype SLR31 metabolized [phenyl-U-(14)C]chlorsulfuron at a faster rate than a biotype which is susceptible to both diclofop-methyl and chlorsulfuron. A third biotype which is resistant to diclofop-methyl but not to chlorsulfuron metabolized chlorsulfuron at the same rate as the susceptible biotype. The increased metabolism of chlorsulfuron observed in the cross-resistant biotype is, therefore, correlated with the patterns of resistance observed in these L. rigidum biotypes. During high performance liquid chromatography analysis the major metabolite of chlorsulfuron in both susceptible and cross-resistant ryegrass coeluted with the major metabolite produced in wheat. The major product is clearly different from the major product in the tolerant dicot species, flax (Linium usitatissimum). The elution pattern of metabolites of chlorsulfuron was the same for both the susceptible and cross-resistant ryegrass but the cross-resistant ryegrass metabolized chlorsulfuron more rapidly. The investigation of the dose response to sulfonylurea herbicides at the whole plant level and the study of the metabolism of chlorsulfuron provide two independent sets of data which both suggest that the resistance to chlorsulfuron in cross-resistant ryegrass biotype SLR31 involves a wheat-like detoxification system.

Made available in DSpace on 2015-09-17T15:26:26Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-05-25. Added 1 bitstream(s) on 2015-09-17T15:45:39Z : No. of bitstreams: 1 000847006.pdf: 350277 bytes, checksum: db6c508303c5e5af00e03576f372683c (MD5) Se aplicados no estágio e na dose correta, os herbicidas são altamente tóxicos para as plantas. As populações de plantas daninhas suscetíveis a um determinado herbicida são totalmente controladas quando a dose recomendada é corretamente aplicada. No entanto, há uma evolução contínua na sobrevivência e crescimento das plantas em resposta a baixas doses dos herbicidas como resultado da presença de características que podem causar um baixo nível de resistência a estes produtos. Estudos de seleção recorrente podem revelar se a variação fenotípica em resposta a uma dose baixa do herbicida é hereditária e pode conduzir à resistência ao herbicida. Estudos de seleção recorrente de plantas alógamas de Lolium rigidum sobreviventes baixas doses de herbicidas mostraram rápida evolução da resistência a herbicidas em apenas três gerações por intensificação progressiva de características de resistência a herbicidas inicialmente pouco relevantes. Em um estudo de campo, populações autógamas de Avena fatua foram submetidas a um processo de seleção recorrente resultante da aplicação de baixas doses do herbicida diclofop-methyl (inibidor da ACCase) ou o glyphosate (inibidor da EPSPS). As aplicações sucessivas de diclofop-methyl induziram o desenvolvimento de progênies distintas da população original em termos de resistência a este herbicida, com o aumento de duas vezes na LD50. As progênies selecionadas pelo uso de diclofop-methyl também mostraram-se mais resistentes a herbicidas inibidores da ALS indicando sugerindo a ocorrência de resistência cruzada. As aplicações sucessivas de glyphosate não induziram a formação de progênies mais resistentes. A capacidade da A. fatua para responder à pressão de seleção imposta por baixas doses de herbicidas é limitada e muito mais baixa ... Herbicides when applied at the correct stage and dose are highly toxic to plants. Herbicide-susceptible weed populations are completely controlled at the recommended herbicide label dose. However, there is continuous variation in plant survival and growth in response to herbicide low doses (below-label) because specific plant traits can confer some low level resistance. Recurrent (directional) selection studies can reveal whether such a phenotypic variation in response to a herbicide low dose is heritable and can lead to herbicide resistance. Recurrent selection studies with allogamous cross-pollinated Lolium rigidum plants surviving at a low herbicide dose showed rapid evolution of herbicide resistance in three generations by progressive enrichment of minor herbicide resistance traits. In a common experimental garden study, we have subjected to low-dose recurrent selection a susceptible population of autogamous self-pollinated Avena fatua with the ACCase-inhibiting herbicide diclofop-methyl or the EPSPS-inhibiting glyphosate. Significant differences in response to selection between the selected progenies and parents were evident in the lines selected by low-dose diclofop-methyl, whereas glyphosate selection did not results in significant shifts. A two-fold diclofop resistance was quantified by comparison of estimated LD50 values and a significant increase in survival to ALS-inhibiting herbicides suggests cross-resistance. Importantly, the capacity of A. fatua to respond to low-dose herbicide selection is marginal and much lower than in L. rigidum. The dynamics of selection and the factors driving the evolution of resistance to herbicides in plants are discussed with particular emphasis on the implications of low-dose herbicide use weed management of autogamous self-pollinated vs allogamous cross ...