Glyphosate effects on plant mineral nutrition, crop rhizosphere microbiota, and plant disease in glyphosate-resistant crops

Abstract

Claims have been made recently that glyphosate-resistant (GR) crops sometimes have mineral deficiencies and increased plant disease. This review evaluates the literature that is germane to these claims. Our conclusions are: (1) although there is conflicting literature on the effects of glyphosate on mineral nutrition on GR crops, most of the literature indicates that mineral nutrition in GR crops is not affected by either the GR trait or by application of glyphosate; (2) most of the available data support the view that neither the GR transgenes nor glyphosate use in GR crops increases crop disease; and (3) yield data on GR crops do not support the hypotheses that there are substantive mineral nutrition or disease problems that are specific to GR crops.

Figure 1

U.S. adoption of the three most widely grown, herbicide-resistant (HR) crops in the United States. Almost all HR crops during this period were GR crops. Data for each crop category include varieties with both HR as a single and stacked trait with insect resistance. Sources: 1996–1999 data are from Fernandez-Cornejo and McBride. Data for 2000–2012 are available in the USDA, Economic Research Service data product, Adoption of Genetically Engineered Crops in the U.S., Tables 1–3 (http://www.ers.usda.gov/data-products/adoption-of-genetically-engineered-crops-in-the-us.aspx, accessed September 12, 2012). Note that adoption data for 1996–1999 include HR corn and soybeans obtained using traditional breeding methods (not transgenic). The more recent data (2000–2011) excluded these varieties.

Figure 2

Agricultural herbicide usage in the U.S. Closed circles = all herbicides minus glyphosate, open circles = glyphosate only. Data from the U.S. Department of Agriculture, National Agricultural Statistics Service Data and Statistics site http://www.nass.usda.gov/Data_and_Statistics/Quick_Stats/ (accessed September 12, 2012).

Figure 3

Catabolic degradation pathways of glyphosate.

Figure 4

Distribution of ¹⁴C after the (A) first or (B) fifth glyphosate application to silt-loam soil The sequential glyphosate applications were made at 2 week intervals. The extractable fraction includes glyphosate and its transformation products extracted with 0.1 M NaOH..

Figure 5

Example of “yellow flash” in GR soybeans sprayed with glyphosate in Illinois.

Figure 6

Effects of two, successive glyphosate treatments (0.86 kg ai h^–1 at both 3 and 6 weeks after planting) on the metal content of mature seeds of field-grown GR soybean plants. Bars respresent 1 SE. There were no differences among any of the paired mean values at the 95% confidence level..

Figure 7

Eubacterial phyla (16S-rDNA sequence abundance) recovered from GR-corn rhizosphere treated with glyphosate (G) or without glyphosate (C) in two fields (upper and lower panels). Sampling was 7 days after glyphosate application. Drawn from data from Barriuso et al.

Figure 8

Possible effects of glyphosate treatment of glyphosate-senstive plants on shikimic acid pathway metabolites considered to be important in defense. Products of the shikimic acid pathway involved in plant defenses are outlined by boxes. Metabolites and metabolic groups in red have been demonstrated or are hypothesized to be reduced in GS plants after glyphosate treatment. Of these, only isoflavonoid phytoalexins from bean and soybean and lignin deposition in bean have been examined for effects of glyphosate (and only in GS plants). Those in black are expected to increase after glyphosate treatment. Protocatechuic acid has been demonstrated to increase in GS plant tissue after glyphosate treatment. **EPSPS: 5-enolpyruvylshikimic acid-3-phosphate synthase, the site of glyphosate action.

Figure 9

U.S. yields of the three crops over the past 30 years that are now grown mostly as GR cultivars. The shaded area represent the years since the introduction of each GR crop. Data are from the USDA, National Agricultural Statistics Service Data and Statistics Web site: http://www.nass.usda.gov/Data_and_Statistics/Quick_Stats/ (accessed September 12, 2012). GR crop adoption rates can be seen in Figure 1.