Impact of the ahas transgene and of herbicides associated with the soybean crop on soil microbial communities

Abstract

Although Brazil has recently reached the position as the second largest producer of genetically modified soybean [Glycine max (L.) Merr.], there are few reports on the effects of transgenic crops and the associated use of specific herbicides on soil microbial communities, both under the edaphoclimatic conditions in Brazil, and in other producer regions in the southern hemisphere. The aim of this study was to evaluate the effects of transgenic soybean containing the ahas gene conferring resistance to herbicides of the imidazolinone group, and of the herbicides associated with transgenic soybeans on the soil microbial community. Twenty field experiments were carried out during three growing seasons (summer of 2006/2007, short-season of 2007 and summer of 2007/2008), in nine municipalities located in six Brazilian states and in the Federal District. The experiments were conducted using a completely randomized block design with four replicates and three treatments: (1) conventional (non-transgenic) soybean cultivar Conquista with conventional herbicides (bentazone + acifluorfen-sodium and other herbicides, depending on the level of infestation in each region); (2) near-isogenic transgenic Cultivance (CV127) containing the ahas gene, with conventional herbicides; (3) transgenic Cultivance with specific herbicide of the imidazolinone group (imazapyr). As the objective of the study was to verify impacts of the transgene and herbicides on the soil microbial community of the whole area and not only a punctual rhizospheric effects, samples were taken at the 0-10 cm layer prior to cropping and at R2 soybean growth stage, between plant rows. Quantitative (microbial biomass C and N, MB-C and MB-N) and qualitative (DGGE of the 16S rDNA region) parameters of soil microbial community were evaluated. No qualitative or quantitative differences were found that could be attributed to the transgene ahas. A comparison of Cultivance soybean with conventional and imidazolinone-group herbicides applications also failed to reveal differences that could be attributed to the specific use of imazapyr, even after three consecutive croppings at the same site. Finally, no differences were detected between conventional (Conquista and conventional herbicides) and transgenic soybean managements (Cultivance and imazapyr). However, marked differences were observed in MB-C and MB-N between the different sites and times of year and, for the 16S rDNA-DGGE profiles, between different sites. In conclusion, microbial community evaluations were found to be sensitive and viable for monitoring different technologies and agricultural management methods, but no differences could be attributed to the ahas transgene for three consecutive cropping seasons.

Residual effect of transgenic soybean in soil microbiota

“…Genetic modified (GM) soybean was approved for trading firstly in USA in 1996. In 1997, a cultivar was developed by introducing the ahas transgene from Arabidopsis thaliana, resistant to herbicides of the imidazolinone group (Souza et al, 2013). It is known that the DNA of transgenic plants obtained from exudates, senescence, or degradation of plants or pollen are persistent in the soil (Lunch et al, 2004).…”

“…The BRS245 transgenic soybean, tolerant to herbicides of the imidazolinone group, received the csr1-2 transgene in its constitution, which codes for the Acetohydroxyacid Synthase (AHAS) (EC 4.1.3.18), the first enzyme leading to the biosynthesis of valine, leucine, and isoleucine (first step of the synthesis of branched-chain amino acids) in plants and microorganisms (Souza et al, 2013). The inhibition of ahas activity leads to cell death by disabling cells to produce amino acids essential for protein synthesis and other key metabolic pathways (Souza et al, 2013).…”

“…Knowing the impact of transgenic crops and technologies associated with them on environment is an obligation of scientists, government and society. Most of studies on GM plants have assessed their potential only above soil; studies investigating their effects into soil are scarce (O'Callaghan and Glare, 2001;Bruinsma et al, 2003;Kowalchuk et al, 2003;Dunfield and Germida, 2004;Souza et al, 2013). GM plants can change soil microbiota by releasing several substances (exudates), However, the review by Bruinsma et al (2003) does not address significant effects of transgenic plants on the bacterial and fungal population and microbial respiration of the soil.…”

“…The contents of carbon (C), nitrogen (N) and phosphorus (P) on the soil microbial biomass as well as the activity of microorganisms are very important to understand the nutrient flow in natural ecosystems (Kaschuk et al, 2010(Kaschuk et al, , 2011. In addition, plant roots release exudates that can affect the soil microbiota (Koranda et al, 2011), and such changes may occur due to exudation of proteins by transgenic plants (Dunfield and Germida, 2004;Souza et al, 2013).…”

“…In recent years, there has largely been cultivation of transgenic soybean in Brazil and in other countries of the southern hemisphere. However, little is known about the impact of GM crops on soil microbial populations and on related parameters, especially under cropping conditions (Souza et al, 2013).…”