Do genetically modified plants impact arbuscular mycorrhizal fungi?

Comparison of bacterial diversity, root exudates and soil enzymatic activities in the rhizosphere of AVP1-transgenic and non-transgenic wheat (Triticum aestivum L.)

AIMS

Soil microbial communities are among the most diverse communities that might be affected due to transgenic crops. Therefore, risk assessment studies on transgenes are essentially required as any adverse effects may depend not only on the specific gene and crop involved but also on soil conditions.

METHODS AND RESULTS

The present study deals with the comparison of bacterial populations, root exudates, and activities of soil enzymes in non-transgenic and AVP1-transgenic wheat rhizosphere, overexpressing vacuolar H+pyrophosphatase for salinity and drought stress tolerance. Amounts of organic acids and sugars produced as root exudates and activities of dehydrogenase, phosphatase, and protease enzymes in soil solution showed no significant differences in AVP1-transgenic and non-transgenic wheat rhizosphere, except for urease and phenol oxidase activities. The higher copy number of nifH gene showed the abundance of nitrogen-fixing bacteria in the rhizosphere of AVP1-transgenic wheat compared with non-transgenic wheat. nifH gene sequence analysis indicated the common diazotrophic genera Azospirillum, Bradyrhizobium, Rhizobium, and Pseudomonas in AVP1-transgenic and non-transgenic wheat except for Zoogloea detected only in non-transgenic wheat. Using 454-pyrosequencing of 16S rRNA gene from soil DNA, a total of 156, 282 sequences of 18 phyla were obtained, which represented bacterial (128,006), Archeal (7,928), and unclassified (21,568) sequences. Proteobacteria, Crenarchaeota, and Firmicutes were the most abundant phyla in transgenic and non-transgenic wheat rhizosphere. Further comparison of different taxonomic units at the genus level showed similar distribution in transgenic and non-transgenic wheat rhizosphere.

CONCLUSION

We conclude that AVP1 gene in transgenic wheat has no apparent adverse effects on the soil environment and different bacterial communities. However, bacterial community depends on several other factors not only genetic composition of the host plants.

SIGNIFICANCE OF THE STUDY

The present research supports introduction and cultivation of transgenic plants in agricultural systems without any adverse effects on indigenous bacterial communities and soil ecosystem.

Root colonization and arbuscular mycorrhizal fungal community composition in a genetically modified maize, its non-modified isoline, and a landrace

The use of genetically modified (GM) plants has increased in recent decades, but there are uncertainties about their effects on soil microbial communities. Aiming to quantify root colonization and characterize arbuscular mycorrhizal fungi (AMF) communities associated with roots and rhizosphere soil of different maize genotypes, a field trial was carried out in Southern Brazil with three maize genotypes as follows: a GM hybrid (DKB 240 VTPRO), its non-modified isoline (DKB 240), and a landrace (Pixurum). Soil samples were collected to evaluate the occurrence of AMF during the growth of corn genotypes at sowing and V3 (vegetative), R1 (flowering), and R3 (grain formation) stages of the crop. The occurrence of AMF was determined by the morphological identification of spores, and by analyzing AMF community composition in soil and roots of maize, using PCR-DGGE. The GM genotype of maize promoted lower mycorrhizal colonization in the vegetative stage and had lower sporulation at grain development than the conventional hybrid and the landrace maize. Twenty AMF morphotypes were identified and 13 were associated with all maize genotypes. The genera Acaulospora, Glomus, and Dentiscutata had the largest numbers of species. There were no differences in AMF community composition due to maize genotypes or genetic modification, but crop phenological stages affected AMF communities associated with maize roots.

Antifungal genes expressed in transgenic pea (Pisum sativum L.) do not affect root colonization of arbuscular mycorrhizae fungi

Genetically modified crops have raised concerns about unintended consequences on non-target organisms including beneficial soil associates. Pea transformed with four antifungal genes 1-3 β glucanase, endochitinase, polygalacturonase-inhibiting proteins, and stilbene synthase is currently under field-testing for efficacy against fungal diseases in Canada. Transgenes had lower expression in the roots than leaves in greenhouse experiment. To determine the impact of disease-tolerant pea or gene products on colonization by non-target arbuscular mycorrhizae and nodulation by rhizobium, a field trial was established. Transgene insertion, as single gene or stacked genes, did not alter root colonization by arbuscular mycorrhiza fungus (AMF) or root nodulation by rhizobium inoculation in the field. We found no effect of transgenes on the plant growth and performance although, having a dual inoculant with both AMF and rhizobium yielded higher fresh weight shoot-to-root ratio in all the lines tested. This initial risk assessment of transgenic peas expressing antifungal genes showed no deleterious effect on non-target organisms.

Reprint of "Fast and sensitive in vivo studies under controlled environmental conditions to substitute long-term field trials with genetically modified plants"

We introduce an easy, fast and effective method to analyze the influence of genetically modified (GM) plants on soil and model organisms in the laboratory to substitute laborious and time consuming field trials. For the studies described here we focused on two GM plants of the so-called 3rd generation: GM plants producing pharmaceuticals (PMP) and plant made industrials (PMI). Cyanophycin synthetase (cphA) was chosen as model for PMI and Choleratoxin B (CTB) as model for PMP. The model genes are expressed in transgenic roots of composite Vicia hirsuta plants grown in petri dishes for semi-sterile growth or small containers filled with non-sterile soil. No significant influence of the model gene expression on root induction, growth, biomass, interaction with symbionts such as rhizobia (number, size and functionality of nodules, selection of nodulating strains) or arbuscular mycorrhizal fungi could be detected. In vitro, but not in situ under field conditions, structural diversity of the bulk soil microbial community between transgenic and non-transgenic cultivars was determined by PLFA pattern-derived ratios of bacteria: fungi and of gram⁺: gram^- bacteria. Significant differences in PLFA ratios were associated with dissimilarities in the quantity and molecular composition of rhizodeposits as revealed by Py-FIMS analyses. Contrary to field trials, where small effects based on the transgene expression might be hidden by the immense influence of various environmental factors, our in vitro system can detect even minor effects and correlates them to transgene expression with less space, time and labour.

Fast and sensitive in vivo studies under controlled environmental conditions to substitute long-term field trials with genetically modified plants

We introduce an easy, fast and effective method to analyze the influence of genetically modified (GM) plants on soil and model organisms in the laboratory to substitute laborious and time consuming field trials. For the studies described here we focused on two GM plants of the so-called 3rd generation: GM plants producing pharmaceuticals (PMP) and plant made industrials (PMI). Cyanophycin synthetase (cphA) was chosen as model for PMI and Choleratoxin B (CTB) as model for PMP. The model genes are expressed in transgenic roots of composite Vicia hirsuta plants grown in petri dishes for semi-sterile growth or small containers filled with non-sterile soil. No significant influence of the model gene expression on root induction, growth, biomass, interaction with symbionts such as rhizobia (number, size and functionality of nodules, selection of nodulating strains) or arbuscular mycorrhizal fungi could be detected. In vitro, but not in situ under field conditions, structural diversity of the bulk soil microbial community between transgenic and non-transgenic cultivars was determined by PLFA pattern-derived ratios of bacteria: fungi and of gram⁺: gram^- bacteria. Significant differences in PLFA ratios were associated with dissimilarities in the quantity and molecular composition of rhizodeposits as revealed by Py-FIMS analyses. Contrary to field trials, where small effects based on the transgene expression might be hidden by the immense influence of various environmental factors, our in vitro system can detect even minor effects and correlates them to transgene expression with less space, time and labour.

Arbuscular Mycorrhizal Fungal Association in Genetically Modified Drought-Tolerant Corn

The commercial use of genetically modified (GM) plants has significantly increased worldwide. The interactions between GM plants and arbuscular mycorrhizal (AM) fungi are of considerable importance given the agricultural and ecological role of AM and the lack of knowledge regarding potential effects of drought-tolerant GM corn ( L.) on AM fungal symbiosis. This work studied AM fungal colonization in five corn lines growing under two different irrigation regimes (30 and 100% of soil field capacity [SFC]). Four of the lines were GM corn, and two of these were drought tolerant. The experiment was conducted for 60 d in a growth chamber under constant irrigation, after which mycorrhization, corn biomass, and days to plant senescence (DTS) were evaluated. Arbuscular mycorrhizal fungal species of the order were predominant in the soil inocula. At the end of the experiment, all plants showed AM colonization. Mycorrhization was higher at 30% SFC than at 100% SFC. Within the same corn line, the AM fungi produced more vesicles in plant roots under drought stress. Among treatments, DTS varied significantly, and drought-tolerant GM corn lines survived longer than the wild-type corn when maintained at 100% SFC. Corn biomass did not vary among treatments, and no correlations were found between DTS or biomass and mycorrhization. We conclude that overexpression of the gene in corn plants under the experimental conditions of this study did not affect AM fungal infectivity and improved the tolerance of the corn to drought stress.

Difference in leaf water use efficiency/photosynthetic nitrogen use efficiency of Bt-cotton and its conventional peer

This study is to test the effects of Bt gene introduction on the foliar water/nitrogen use efficiency in cotton. We measured leaf stomatal conductance, photosynthetic rate, and transpiration rate under light saturation condition at different stages of a conventional cultivar (zhongmian no. 16) and its counterpart Bt cultivar (zhongmian no. 30) that were cultured on three levels of fertilization, based on which leaf instantaneous water use efficiency was derived. Leaf nitrogen concentration was measured to calculate leaf photosynthetic nitrogen use efficiency, and leaf δ¹³C was used to characterize long term water use efficiency. Bt cultivar was found to have lower stomatal conductance, net photosynthetic rates and transpiration rates, but higher instantaneous and long time water use efficiency. In addition, foliar nitrogen concentration was found to be higher but net photosynthetic rate was lower in the mature leaves of Bt cultivar, which led to lower photosynthetic nitrogen use efficiency. This might result from the significant decrease of photosynthetic rate due to the decrease of stomatal conductance. In conclusion, our findings show that the introduction of Bt gene should significantly increase foliar water use efficiency but decrease leaf nitrogen use efficiency in cotton under no selective pressure.

Risk assessment of Bt crops on the non-target plant-associated insects and soil organisms

Transgenic plants containing Bacillus thuringiensis (Bt) genes are being cultivated worldwide to express toxic insecticidal proteins. However, the commercial utilisation of Bt crops greatly highlights biosafety issues worldwide. Therefore, assessing the risks caused by genetically modified crops prior to their commercial cultivation is a critical issue to be addressed. In agricultural biotechnology, the goal of safety assessment is not just to identify the safety of a genetically modified (GM) plant, rather to demonstrate its impact on the ecosystem. Various experimental studies have been made worldwide during the last 20 years to investigate the risks and fears associated with non-target organisms (NTOs). The NTOs include beneficial insects, natural pest controllers, rhizobacteria, growth promoting microbes, pollinators, soil dwellers, aquatic and terrestrial vertebrates, mammals and humans. To highlight all the possible risks associated with different GM events, information has been gathered from a total of 76 articles, regarding non-target plant and soil inhabiting organisms, and summarised in the form of the current review article. No significant harmful impact has been reported in any case study related to approved GM events, although critical risk assessments are still needed before commercialisation of these crops. © 2016 Society of Chemical Industry.

Community Structure of Arbuscular Mycorrhizal Fungi in Rhizospheric Soil of a Transgenic High-Methionine Soybean and a Near Isogenic Variety

The use of transgenic plants in agriculture provides many economic benefits, but it also raises concerns over the potential impact of transgenic plants on the environment. We here examined the impact of transgenic high-methionine soybean ZD91 on the arbuscular mycorrhizal (AM) fungal community structure in rhizosphere soil. Our investigations based on clone libraries were conducted in field trials at four growth stages of the crops each year from 2012 to 2013. A total of 155 operational taxonomic units (OTUs) of AM fungi were identified based on the sequences of small subunit ribosomal RNA (SSU rRNA) genes. There were no significant differences found in AM fungal diversity in rhizosphere soil during the same growth stage between transgenic soybean ZD91 and its non-transgenic parental soybean ZD. In addition, plant growth stage and year had the strongest effect on the AM fungal community structure while the genetically modified (GM) trait studied was the least explanatory factor. In conclusion, we found no indication that transgenic soybean ZD91 cultivation poses a risk for AM fungal communities in agricultural soils.

Effect of PGPR Serratia marcescens BC-3 and AMF Glomus intraradices on phytoremediation of petroleum contaminated soil

Soil contamination caused by petroleum hydrocarbons has become a worldwide environmental problem. Microorganism combined with phytoremediation appears to be more effective for removal and/or degradation of petroleum hydrocarbons from impacted soils. The current study investigated the effect of inoculated with PGPR Serratia marcescens BC-3 alone or in combination with AMF Glomus intraradices on the phytoremediation of petroleum-contaminated soil. Pot experiments were conducted to analyze the effect on plant and soil for 90 days in greenhouse. The inoculation treatments showed higher plant biomass and antioxidant enzyme activities than the non inoculation control. Inoculation treatments also improved rhizosphere microbial populations in petroleum contaminated soil. The degradation rate of total petroleum hydrocarbons with PGPR and AMP co-inoculation treatment was up to 72.24 %. The results indicated that plant combined with microorganisms for remediation of petroleum hydrocarbons would be a feasible method.

Biosafety assessment of GFP transplastomic tobacco to rhizosphere microbial community

Green fluorescent protein (GFP) is one of the most widely studied and exploited proteins in biochemistry, and has many applications as a marker, especially in plant transformation system. Although a number of studies have been conducted to assess the toxify of this protein to specific organisms, little is known about GFP on rhizosphere microbial community, which is regarded as good indicator for environmental risk assessment. Chloroplast genetic engineering has shown superiority over traditional nuclear genetic engineering, and has been used in many aspects of plant genetic engineering. High levels of chloroplast-based protein accumulation make this technology as an ideal strategy to evaluate biosafety of transgenes. In the present study, the effects of field-released GFP transplastomic tobacco (Nicotiana tabacum) on rhizosphere microbes over a whole growth cycle were investigated by using both culture-dependent and culture-independent methods. Compared to wild-type control, transplastomic tobacco had no significant influence on the microbial population at the seedling, vegetative, flowering and senescing stages. However, developmental stages had more influence than ecotypes (GFP-transformed and wild-type). This was confirmed by colony forming unit, Biolog Eco(TM) and PCR-DGGE analysis. Thus, these results suggest chloroplast transformation with a GFP reporter gene has no significant influence on rhizosphere microbial community, and will be potential platform for plant biotechnology in future.

Tracking fungal community responses to maize plants by DNA- and RNA-based pyrosequencing

We assessed soil fungal diversity and community structure at two sampling times (t1 = 47 days and t2 = 104 days of plant age) in pots associated with four maize cultivars, including two genetically modified (GM) cultivars by high-throughput pyrosequencing of the 18S rRNA gene using DNA and RNA templates. We detected no significant differences in soil fungal diversity and community structure associated with different plant cultivars. However, DNA-based analyses yielded lower fungal OTU richness as compared to RNA-based analyses. Clear differences in fungal community structure were also observed in relation to sampling time and the nucleic acid pool targeted (DNA versus RNA). The most abundant soil fungi, as recovered by DNA-based methods, did not necessary represent the most “active” fungi (as recovered via RNA). Interestingly, RNA-derived community compositions at t1 were highly similar to DNA-derived communities at t2, based on presence/absence measures of OTUs. We recovered large proportions of fungal sequences belonging to arbuscular mycorrhizal fungi and Basidiomycota, especially at the RNA level, suggesting that these important and potentially beneficial fungi are not affected by the plant cultivars nor by GM traits (Bt toxin production). Our results suggest that even though DNA- and RNA-derived soil fungal communities can be very different at a given time, RNA composition may have a predictive power of fungal community development through time.

Does wheat genetically modified for disease resistance affect root-colonizing pseudomonads and arbuscular mycorrhizal fungi?

This study aimed to evaluate the impact of genetically modified (GM) wheat with introduced pm3b mildew resistance transgene, on two types of root-colonizing microorganisms, namely pseudomonads and arbuscular mycorrhizal fungi (AMF). Our investigations were carried out in field trials over three field seasons and at two locations. Serial dilution in selective King's B medium and microscopy were used to assess the abundance of cultivable pseudomonads and AMF, respectively. We developed a denaturing gradient gel electrophoresis (DGGE) method to characterize the diversity of the pqqC gene, which is involved in Pseudomonas phosphate solubilization. A major result was that in the first field season Pseudomonas abundances and diversity on roots of GM pm3b lines, but also on non-GM sister lines were different from those of the parental lines and conventional wheat cultivars. This indicates a strong effect of the procedures by which these plants were created, as GM and sister lines were generated via tissue cultures and propagated in the greenhouse. Moreover, Pseudomonas population sizes and DGGE profiles varied considerably between individual GM lines with different genomic locations of the pm3b transgene. At individual time points, differences in Pseudomonas and AMF accumulation between GM and control lines were detected, but they were not consistent and much less pronounced than differences detected between young and old plants, different conventional wheat cultivars or at different locations and field seasons. Thus, we conclude that impacts of GM wheat on plant-beneficial root-colonizing microorganisms are minor and not of ecological importance. The cultivation-independent pqqC-DGGE approach proved to be a useful tool for monitoring the dynamics of Pseudomonas populations in a wheat field and even sensitive enough for detecting population responses to altered plant physiology.

Effects of genetic modifications to flax (Linum usitatissimum) on arbuscular mycorrhiza and plant performance

Although arbuscular mycorrhizal fungi (AMF) are known for their positive effect on flax growth, the impact of genetic manipulation in this crop on arbuscular mycorrhiza and plant performance was assessed for the first time. Five types of transgenic flax that were generated to improve fiber quality and resistance to pathogens, through increased levels of either phenylpropanoids (W92.40), glycosyltransferase (GT4, GT5), or PR2 beta-1,3-glucanase (B14) or produce polyhydroxybutyrate (M50), were used. Introduced genetic modifications did not change the degree of mycorrhizal colonization as compared to parent cultivars Linola and Nike. Arbuscules were well developed in each tested transgenic type (except M50). In two lines (W92.40 and B14), a higher abundance of arbuscules was observed when compared to control, untransformed flax plants. However, in some cases (W92.40, GT4, GT5, and B14 Md), the mycorrhizal dependency for biomass production of transgenic plants was slightly lower when compared to the original cultivars. No significant influence of mycorrhiza on the photosynthetic activity of transformed lines was found, but in most cases P concentration in mycorrhizal plants remained higher than in nonmycorrhizal ones. The transformed flax lines meet the demands for better quality of fiber and higher resistance to pathogens, without significantly influencing the interaction with AMF.

Testing potential effects of maize expressing the Bacillus thuringiensis Cry1Ab endotoxin (Bt maize) on mycorrhizal fungal communities via DNA- and RNA-based pyrosequencing and molecular fingerprinting

The cultivation of genetically modified (GM) crops has increased significantly over the last decades. However, concerns have been raised that some GM traits may negatively affect beneficial soil biota, such as arbuscular mycorrhizal fungi (AMF), potentially leading to alterations in soil functioning. Here, we test two maize varieties expressing the Bacillus thuringiensis Cry1Ab endotoxin (Bt maize) for their effects on soil AM fungal communities. We target both fungal DNA and RNA, which is new for AM fungi, and we use two strategies as an inclusive and robust way of detecting community differences: (i) 454 pyrosequencing using general fungal rRNA gene-directed primers and (ii) terminal restriction fragment length polymorphism (T-RFLP) profiling using AM fungus-specific markers. Potential GM-induced effects were compared to the normal natural variation of AM fungal communities across 15 different agricultural fields. AM fungi were found to be abundant in the experiment, accounting for 8% and 21% of total recovered DNA- and RNA-derived fungal sequences, respectively, after 104 days of plant growth. RNA- and DNA-based sequence analyses yielded most of the same AM fungal lineages. Our research yielded three major conclusions. First, no consistent differences were detected between AM fungal communities associated with GM plants and non-GM plants. Second, temporal variation in AMF community composition (between two measured time points) was bigger than GM trait-induced variation. Third, natural variation of AMF communities across 15 agricultural fields in The Netherlands, as well as within-field temporal variation, was much higher than GM-induced variation. In conclusion, we found no indication that Bt maize cultivation poses a risk for AMF.

Activation of basal defense mechanisms of rice plants by Glomus intraradices does not affect the arbuscular mycorrhizal symbiosis

• Arbuscular mycorrhizal (AM) fungi establish symbiotic associations with a wide range of plant species. AM fungi must then have the ability to suppress, neutralize or evade the plant defense response. We investigated the physiological and molecular responses of rice to inoculation with the AM fungus Glomus intraradices, focusing on the relevance of the plant defense response during the symbiotic mycorrhizal interaction. • Defense gene expression analysis and proteomic approaches were used. The impact of defense gene expression on the mycorrhizal process was analyzed using transgenic PRms (Pathogenesis-Related maize seed) rice plants, the PRms plants exhibiting constitutive expression of endogenous defense genes. • Inoculation with G. intraradices stimulated growth and biomass production in wild-type and PRms plants. Evidence is presented on the activation of the basal defense response in mycorrhizal rice roots. Analysis of the symbiotic proteome confirmed the accumulation of stress-related proteins in mycorrhizal roots, including PR proteins and antioxidant enzymes. Although constitutive expression of defense genes occurred in the roots of PRms plants, the symbiotic efficiency of G. intraradices in these plants was not affected. • These results suggest that AM fungi have evolved the capacity to circumvent defense mechanisms that are controlled by the plant's immune system.