Developing Weed Resistance : - Hurdle The Notability Of "Roundup Ready"
Authors: *D.R. Sapkal, V.R. Sapkal, M.Y. Ladole and P.U.Ingle
*Dr.D.R. Sapkal, Assistant Professor, Lovely Professional University, Jalandhar, State : Punjab (INDIA) Email: deepaksapkal7878@rediffmail.com


Glyphosate (Roundup Ready) is a familiar and trusted herbicide to most of the farmers because it was used continuously for nearly two decades to control weeds in non crop areas before introduction of glyphosate resistance. The herbicide glyphosate has so many characteristics that make it an ideal choice for use in resistant crop. Its non selective, broad-spectrum herbicide that is highly effective against of annual and perennial grasses and broad leaved weeds. Glyphosate also has eco-friendly and safety characteristics such as rapid soil binding, that help to reduced leaching, rapid biodegradation as well as extremely low toxicity to mammals birds and fishes (Malik et al 1989). For this reasons the herbicide glyphosate became widely used in the United States and other parts of the world after the commercialization of glyphosate-resistant crops. Glyphosate, (the active ingredient in Roundup) kills plants by interfering with the Shikimate pathway in plants, which is essential for the synthesis of the aromatic amino acid phenylalanine, tyrosine and tryptophan. More specifically, glyphosate inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). Increased use of glyphosate over multiple years imposes selective genetic pressure on weed populations. It was investigated recently discovered glyphosate-resistant Amaranthus palmeri populations but now a day's some other weed species also showing resistance (Todd A. Gaines et al 2010).

EPSPS enzyme activity from resistant and susceptible plants was equally inhibited by glyphosate, Todd A. Gaines et al 2010 carried out research on Gene amplification confers glyphosate resistance in Amaranthus palmeri and concluded that genomes of resistant plants contained from 5-fold to more than 160-fold more copies of the EPSPS gene than did genomes of susceptible plants. Quantitative RT-PCR on cDNA revealed that EPSPS expression was positively correlated with genomic EPSPS relative copy number. Immunoblot analyses showed that increased EPSPS protein level also correlated with EPSPS genomic copy number. EPSPS gene amplification was heritable, correlated with resistance in pseudo-F2 populations, and is proposed to be the molecular basis of glyphosate resistance. FISH revealed that EPSPS genes were present on every chromosome and, therefore, gene amplification was likely not caused by unequal chromosome crossing over. This occurrence of gene amplification as an herbicide resistance mechanism in a naturally occurring weed population is particularly significant because it could threaten the sustainable use of glyphosate-resistant crop technology.

Now it was proved that glyphosate resistance is developed by gene amplification. Massive overproduction of EPSPS produced by the additional EPSPS gene copies allows the plant to cope despite the presence of glyphosate (earlier EPSPS resistance gene was present on single chromosome then it were multiply on others chromosome so it increased EPSPS, and the normal glyphosate rate or dose cannot inhibit all of the available EPSPS).it is essentially acting as a molecular sponge to soak up the herbicide and allowing normal metabolic functions to continue. Even more fascinating is that this gene amplification may have occurred via a mobile genetic element (Transposable genetic elements). The EPSPS enzyme targeted by glyphosate is in the shikimate pathway, linking carbohydrate metabolism to the synthesis of aromatic compounds in microorganisms and plants. (Herrmann KM and Weaver LM 1999).

In this way weed plant survive and showed resistance against glyphosate. Since the first reports of glyphosate resistant weeds in 1996, there is now a total of 5 resistant weed species that have been documented. With this rate of new species developing resistance it is sure that new species with this trait will be continually seen. Scientists studying this problem do not see this as a detrimental issue currently. Their best suggestions to growers is that if they create and implement a long term plan to help decrease the amount of resistant weeds, the issue will continue to be a lesser one with small costs to the farmer. This long term plan would be best if growers avoided growing Roundup Ready crops predominantly and rotating herbicides used on their fields annually. It is hoped that this plan can help keep the inexpensive use of glyphosate available to farmers. However, farmers must be willing to add some minor costs of rotating herbicides in order the help ensure the likelihood of avoiding more weeds with resistance.

References
1. Gaines TA, et al. Gene amplification confers glyphosate resistance in Amaranthus palmeri. Proc Natl Acad Sci USA. 2010;107:1029-1034.

2. Herrmann KM, Weaver LM. The shikimate pathway. Annu Rev Plant Physiol Plant Mol Biol. 1999;50:473-503.

3. Malik J, Barry G, Kishore G. The herbicide glyphosate. Biofactors. 1989;2:17-25.

About Author / Additional Info: