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Glyphosate effects and phytoalexin accumulation in rust- infected, glyphosate-resistant soybean


Soybean was introduced in Brazil in early years of the 20th Century as the main source of vegetable oil and protein with broad range of applications in the food and oleochemistry industry. The United State is the major soybean producer in world with 40% of production, followed by Brazil with 24% and Argentina with 17%. Lately, there have been lots of speculations and predictions on the Asian soybean rust’s impact caused by Phakopsora pachyrhizi, because this aggressive pathogen could cause 80% reduction on yield. In Brazil, rust has reached epidemic levels, and growers have used several fungicidal applications to assure their production with increasing production costs. In the United States, rust has reached the Middle West in limited areas, but the epidemic potential is great. Recently, cultivation of glyphosate-resistant GR) crops was approved by Brazil’s govern, while in the United States and Argentina, seeds of GRS crops are available to farmers for the past 10 years. Glyphosate inhibits the shikimate pathway by inhibiting 5-enolpyruvyl-shikimate-3-phosphate synthase (EPSPS), which results in reduced aromatic amino acids and deregulation of the shikimate pathway causing massive flow of carbon into the pathway, with accumulation of high levels of shikimic acid and its derivatives. Being resistant to glyphosate, the genetic modified soybean can receive three applications during the growing season for weed control. Previous studies have shown that glyphosate has a controlling effect on other rust-inducing pathogens. Thus, scientists of Brazil and United States have established an international collaboration agreement proposing to jointly examine the impact of Asian soybean rust on GRS crops growing under different conditions and the glyphosate effect on host-pathogen interaction. Our proposed study is based on the hypothesis that post-emergence glyphosate applications on GRS varieties will: 1- enhance the production of plant defense compounds such as peptides and phytoalexins. 2) glyphosate will control Phakopsora pachyrhizi by inhibiting fungal growth. 3) if the CP4 EPSPS gene, responsible for soybean resistance to glyphosate would not be fully expressed, this herbicide could have a direct impact on the shikimic acid pathway, blocking the enzymes involved with phytoalexin production, thus resulting in plants even more susceptible to the fungal attack. To accomplish such wide range of objectives, a team of researchers working in Brazilian institutions including Biology Institute, EMBRAPA, UNAERP and in the United States, the National Center for Natural Products Research (NCNPR) and USDA/ARS have presented this proposed study in the English version. Field experiments will be conducted in Brazil using GRS crops varieties treated with three glyphosate applications in combination with surfactant that will be the control treatment. After those applications, disease severity will be assessed during weekly visits and leaves will be harvested for shikimic and the phytoalexins analysis. The ultimate goal of our project is to know more about host-pathogen interaction as a means to manage Asian soybean rust, thus decreasing production costs in controlling this disease. (AU)