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Herbicide Resistant Weeds

   

Added on  2020-02-24

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ABSTRACTHerbicide resistance is the heritable ability of weeds to survive and reproduce in the presence ofherbicide doses that are lethal to the wild type of the species. One mechanism of such resistanceis non-target site reduced translocation of the herbicide, in which vacuolar sequestration preventsthe chemical from spreading around the plant. Resistance of Erigeron canadensis to glyphosateis thought to involve this mechanism and it is believed that EPSPS and the ABC transportergenes M10 and M11 may be responsible for this resistance. This study therefore aims atdetermining through quantitative PCR and RNA-Seq if these genes provide the mechanism forglyphosate resistance in Erigeron bonariensis, a close relative of Erigeron canadensis, or if thereare other genes responsible for the resistance observed in this species. RNA will be extractedfrom the leaves of glyphosate treated and untreated individuals, collected from 10 sites in theCentral Valley and two control populations of Erigeron bonariensis, for quantitative real timePCR and RNA-Seq analysis. Total RNA for RNA-Seq analysis will be used in cDNA librarysynthesis and sequenced via Illumina HiSeq. Sequenced reads will be assembled de novo usingthe software Trinity, assigned to respective genes with the pipeline HTSeq, tested for differentialexpression by DESeq and functionally annotated using the NCBI nonredundant protein (Nr)database.1
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INTRODUCTIONMoss (2002) defines herbicide resistance as the heritable ability of weeds to survive andreproduce in the presence of herbicide doses that are lethal to the wild type of the species.Herbicide resistance was first observed by an ornamental nursery owner in 1968 (Jasieniuk at al.1996). The first of resistance to herbicide was recorded inSenecio vulgaris; at which seeds fromthe resistant biotypes were found to resist the chemicals simazine and atrazine (Pieterse 2010). In1974, resistance to glyphosate herbicide became a problem for corn growers (Gressel et al.1982). Since then, more than 187 species of weeds throughout the globe have developedresistance against various herbicides which have targeted a broad range of biochemical processes(Pieterse 2010). The first case of herbicide resistance was reported in 1981 at UC Riverside,California (Holt at al. 1981) and recently, more species which are have also evolved resistance tovarious other herbicide chemicals employed by farmers in California (Malone 2014).Weed species resistant to herbicide in CaliforniaHerbicidesSpeciesTriazineSenecio vulgarisSulfonylureaLolium perenne, Sagittaria montevidensis,Scirpus mucronatus, Salsola tragus,Ammania auriculata,Scirpus mucronatus,ThiocarbamateEchinochloa phyllopogonAryloxyphenixy propionic acidEchinochloa phyllopogonDinitroanilineEchinochloa crus-galli,Pyrazolium saltAvena fatua,Ubstituted amino acidLolium rigidumGlyphosate herbicide (marketed by Monsanto as RoundUp®) contains N-phosphonomethyl glycine that acts against plants by hindering aromatic amino acid synthesis(Bridges 2003). Upon application on leaves, the plant takes in glyphosate through pores and is2
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transported through the phloem alongside with other products of photosynthesis to all parts of theplant. In susceptible plants, glyphosate hinders the role of the plastidine enzyme 5-enolpiruvilshikimate-3-phosphate synthase (EPSPS) important in the prechorismate step of theshikimate pathway. In the absence of glyphosate, this enzyme works by condensing shikimate-3-phosphate and phosphoenolpyruvate into 5-enolpiruvilshikimate-3-phosphate (EPSP) withinorganic phosphate, initiating the anabolism of aromatic amino acids (Ferreira 2008).Disruption of the synthesis of aromatic amino acids (phenylalanine, tryptophan, and tyrosine)eventually kills the plant (Herman and Weaver 1999). Glyphosate has become the world’s most commonly used herbicide since its marketintroduction in 1974 (Baylis 2000). Its use around the globe is contributed by several factors tomake it the most utilized herbicide. The ideal characteristics that it has includethe potencyagainst an extensive variety of species (monocots and dicots), less harmful activity againstanimals than other herbicides (the enzyme EPSPS which is not found in animals), rapidmicrobial degeneration and low cost (Duke and Powles 2008). Also, it has been commonly usedin recent years as a part of reduced tillage frameworks with a dependence on herbicides tocontrolweeds that have numerous environmental benefits and economic importance (Owen2008; Powles 2008; Shaner 2000). With the adoption of genetically modified crops withglyphosate resistance in 1996, the already high levels of glyphosate application increased(Powles & Preston 2006). The combined effects of glyphosate ignorant over-usage in glyphosateresistant hairy fleabane and completely reduced tillage has created an agricultural environmentthat has an increased risk ofevolution of glyphosate resistance (Neve at al. 2003). Herbicideresistance is stimulated by the re-current use of herbicides with the same active chemicalingredients (LeBaron 1991), and so inevitably, glyphosate resistance has evolved in many weeds3
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(Powles et al. 1998). Because of its ability to control numerous weed species adaptability to lowtillage systems, and low animal toxicity result in increased glyphosate remaining to be a keyfactor in modern systems because growers are unwilling to return to the greater tillage systems orolder, more toxic herbicides (Beckie 2012).Two mechanisms that confer resistance have contributed to glyphosate resistance inweeds are target-site and non-target site resistance (REF). Target site-based resistance is acondition where resistance evolves due to a gene mutation resulting in a structural or chemicalchange to a target site enzyme so that the herbicide fails to effectively inhibit the normal enzymefunction (Powles and Preston 2006). The missense mutation may involve a specific nucleotidesubstitution in the coding region producing a different amino acid that results in structural,hydrophobicity or charge change in site of the enzyme that the herbicide targets, making it lesssensitive to inhibition by the herbicide. A few weeds such as goosegrass (Eleusine indica (L.)Gaertn.), have evolved weak target site mutagenesis (Lee and Ngim, 2000; Dinelli et al. 2006) ofthe enzyme EPSPS, via a substitution mutation that replaced the amino acid proline with serineat position 106 (Pro106-Ser). Ng et al. (2004 & 2005) demonstrated that substitution of prolineby threonine (Pro106-Thr) confers resistance glyphosate resistance to goosegrass The mutatedEPSPS enzyme has a low affinity for glyphosate but almost normal affinity for phosphoenolpyruvate (the enzyme’s usual substrate); permitting the shikimate pathway can proceed normallyeven in the presence of glyphosate (Gaines et al. 2010).Non-target site mutations including sequestration and detoxification in addition totranslocation function by limiting the translocation of glyphosate (Wakelin et al. 2004) to targetsites. According to Claus and Brehrens’ (1976) study-rapid and widespread glyphosatetranslocation is necessary to achieve high herbicide efficacy. There is therefore a possibility that4
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changes in its translocation may confer resistance. The study that unraveled this phenomenonwas carried out in rigid ryegrass (Lolium rigidum Gaud.; Lorraine-Colwill et al. 2002), andindicated that resistance in at least one biotype was not due to EPSPS enzyme target mutagenesisor degradation. In the same study, it was shown that there was no significant difference betweenglyphosate resistance and susceptible species in EPSPS sensitivity or expression level or inglyphosate absorption. However, the patters of glyphosate translocation differed. Theresearchers observed accumulation of glyphosate at lower parts of the plant through diffusionand to some extend in the roots in susceptible plants whereas in resistant plants, it accumulatedin the tip of the leaves through transpiration with a negligible amount transported to the roots(Lorraine-Colwill et al. 2002). (Welkelin et al. 2004) found the same pattern of reducedglyphosate translocation when working with only four glyphosate resistant ryegrass populationsin Australia.Researchers investigating mechanisms of glyphosate resistance in all Lolium rigidumspecies have not found large differences in glyphosate translocation. Perez et al. (2004) found nosignificant difference in glyphosate absorption and translocation between susceptible andresistant Chilean Lolium plants. In an investigation in glyphosate resistance in CalifornianLolium, Simarmata et al. (2003) found significantly higher glyphosate concentration in treatedleaves of glyphosate resistant plants 2-3 days after treatment. Interestingly, these authorsobserved no other significant differences in glyphosate absorption or translocation betweenresistant and susceptible plants (Lorraine-Colwill et al. 2002). These varying results suggestpotentially different non-target resistance mechanisms to glyphosate in different Loliumpopulations. In general, most studies observed no differences in glyphosate absorption, but withphloem translocation reduced greatly (Feng et al. 2004; Koger and Reddy 2005) in resistant5
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species. Failure of glyphosate translocation from leaves to the roots seems to be importantmechanisms that lead to resistance in certain Lolium biotypes (Preston 2002). Erigeron species are annual or short-lived perennial plants native to the Americas thathave in the recent past become cosmopolitan and invasive weeds of many crops and arable lands(Prieur-Richard et al. 2000). The genus is in the sunflower family (Asteraceae) and the weedspecies were formerly placed in the genus Conyza before taxonomic revision (Baldwin, 2012).Erigeron spp. are prolific seed producers: a single plant is capable of producing thousands ofviable wind dispersed seeds (Weaver 2001; Shields et al. 2006).Erigeronspp. have become very common and problematic weedy plants in agronomiccrops around the world (Weaver 2001). This is probably because they are capable of adapting toplain soils where they establish in the absence of tillage (Buhler 1992). The opportunistic natureof Erigeron in undisturbed areas makes them well-suited for becoming established inagricultural fields and surrounding areas; particularly in no-tillage or where tillage is minimal(Bruce and Kells 1990). Considering the fact that over the last two decades the amount of crophectares in conservation tillage has increased in is=n CA (Young 2006), weeds such as Erigeronspp. are becoming a great concern. The two main species of Erigeron in California are hairy fleabane (Erigeron bonariensisL.) and horseweed (Erigeron canadensis L.), both summer annuals. Unlike horseweed, which isweedy across the U.S., fleabane is an agricultural problem specific to California (Shrestha 2008).The optimal temperature for germination of hairy fleabane ranges between 2.03 and 2.34 degreescentigrade and the seeds usually germinate under moderate water availability conditions(Karlsson and Milberg, 2007). Based on these characteristics, conditions are ideal for fleabanegermination in the cooler seasons of California’s Central Valley, October-March. Hairy fleabane6
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