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Relationship of Glyphosate Application and Foliar Amendment on IAA-Producing Bacteria and Microbial Activity in the Rhizosphere of Glyphosate-Resistant Soybean. = 1 µm. Su-Jung Kim Department of Soil, Environmental, and Atmospheric Sciences. Outline. Introduction

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Relationship of Glyphosate Application and Foliar Amendment on IAA-Producing Bacteria and Microbial Activity in the Rhizosphere of Glyphosate-Resistant Soybean



Su-Jung Kim

Department of Soil, Environmental, and Atmospheric Sciences

  • Introduction

- Characteristics of Herbicide Glyphosate

- Foliar Amendments

- Deleterious Rhizobacteria (DRB)

- Indole-3-acetic acid (IAA) Effects of DRB on Weed Growth

  • Hypotheses and Objectives
  • Methods and Materials (Experimental Design)
  • Results
  • Conclusions
glyphosate or n phosphonomethyl glycine
Glyphosate or N-(phosphonomethyl)glycine
  • A broad spectrum, non-selective postemergence herbicide of grasses and broadleaf weeds
  • The mode of action of glyphosate

- inhibition of 5-enolpyruvylshikimic acid-3-phosphate (EPSP) synthase

  • Roundup®is the most widely used herbicide (glyphosate is the active ingredient), produced by Monsanto corporation.
glyphosate in plants
Glyphosate in Plants
  • Glyphosate is not readily metabolized by plants; it is translocated and may accumulate in meristematic regions including roots and nodules (Duke, 1988; Hernandez et al., 1999, Reddy et al., 2000).
  • Glyphosate that accumulates in the roots of treated susceptible plants is eventually released into the rhizosphere (Coupland and Casely, 1979; Rodrigues et al., 1982).
effects of glyphosate on soil microorganisms
Effects of Glyphosate on Soil Microorganisms
  • Glyphosate increases soil bacteria and fungi populations (Wardel and Parkinson, 1992; Abdel-Mallek et al., 1994; Lévesque et al., 1987; Haney et al., 2000; Busse et al., 2001).
  • Glyphosate may be toxic to some bacteria and fungi possibly due to inhibition of microbial metabolic pathways (Mekwatanakarn and Sivasithamparam, 1987; Kawate et al., 1992; Abdel-Mallek et al., 1994; Busse et al., 2001).
foliar amendments
Foliar Amendments
  • Biostimulants increase plant growth through various mechanisms

- Establishment of selected microorganisms in soil

- Enhancement of soil microbial activity

- Promotion or augmentation of the activities of critical soil enzymes, providing chelating substances and plant growth hormones, or supplementation of micronutrients (Kinnersley, 1993).

foliar amendments7
Foliar Amendments
  • Benefits to soil fertility

- Directly soil organic fraction by microorganisms

- Indirectly microbially synthesized metabolites such as phytohormones (Yamada and Xu, 2000).

  • Furthermore, increase water stress resistance in plants (Yamada et al., 1997; Huilian et al., 1998).
foliar amendments8
Foliar Amendments
  • PT-21® (AgSpectrum, DeWitt, Iowa)

- A nutritional supplement (21.0% total nitrogen in the form of urea)

- Designed for foliar application to increase crop yield.

  • Grozyme® (AgSpectrum, DeWitt, Iowa)

- A biostimulant (boric acid, cobalt sulfate, copper sulfate, ferric nitrate, manganese nitrate, sodium molybdate, and zinc nitrate)

- Effects on rates of organic matter decomposition, soil microbial activity and mineralization.

importance of soil microorganisms
Importance of Soil Microorganisms
  • In nutrient cycling, decomposition, and plant growth.

- Management practices influencing soil microorganisms can affect crop yields and soil and environmental quality.

  • However, foliar application of Roundup®, Grozyme® and PT-21® and the potential impact on rhizosphere bacteria have not been studied.
deleterious rhizobacteria drb
Deleterious Rhizobacteria (DRB)
  • Naturally-occurring soil bacteria inhabiting rhizosphere
  • Toxigenic but not parasitic for plants
  • Species specific
  • Suppress weed growth; not affect crop growth Biological weed control

(Aldrich and Kremer, 1997; Kremer, 1987)



(Sylvia et al., 1998)

biological weed control
Biological Weed Control
  • Intentional use of living organisms for control of weeds (Quimby and Birdsall, 1995)
  • Biotic agents: foliar or stem fungal pathogens, foliar bacterial pathogens and non-pathogenic soil-borne fungi and bacteria (Kremer, 2002)

- Production of antibiotics, siderophores, and volatile compounds

- Parasitism, competition for nutrients and ecological niche

- Production of plant growth-promoting compounds, such as IAA, gibberellin, and cytokinin-like substances

an auxin indole 3 acetic acid iaa produced by bacteria
An auxin, Indole-3-acetic acid (IAA), produced by Bacteria
  • Indole-3-acetic acid

- Compounds that stimulate plant growth (coleoptile tissue) in lower concentrations.

- In contrast, if the concentration becomes higher, the effect reverses and elongation of root and shoot is inhibited.

  • Bacteria synthesize IAA (IAA-Producing Bacteria)

- Inhibit root growth in sugarbeet, blackcurrant, and morningglory

iaa producing bacteria in relationship of iaa with herbicides
IAA-Producing Bacteriain Relationship of IAA with herbicides
  • Natural auxins have modes of action similar to many herbicides that interfere with plant growth such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) (Patten and Glick, 1996)
  • The degree of growth inhibition in glyphosate-treated plants correlated with an increase in IAA metabolism (Lee, 1984; Lee and Dumas, 1985).
iaa producing bacteria in relationship of iaa with rhizobacteria
IAA-Producing Bacteria in Relationship of IAA with Rhizobacteria
  • Up to 80% of rhizobacteria can produce IAA (Loper and Schroth, 1986).
  • The inhibitory effect of some DRB (e.g., Enterobacter taylorae, a transgenic rhizosphere pseudomonad, and Pseudomonas putida) has been related to excretion of high amounts of IAA (Dubeikovsky etal, 1993; Sarwar and Kremer, 1995; Xie et al., 1996)

LSD (P≤0.05)

IAA Production of B. japonicum isolate GD3 and B. megaterium isolate GP4 and Suppressive Effect of isolate GD3 on Morningglory Growth

(Kim and Kremer, 2005 in Press)

  • Glyphosate released from glyphosate-resistant soybean may be toxic for rhizosphere microorganisms including IAA-producing bacteria.
  • Accordingly, microbial activity may be suppressed in the rhizosphere.
  • Foliar amendments may change in microbial activity in combination with glyphosate application.
  • Total C and N, soil respiration (CO2 efflux), and soil inorganic N mineralized may be influenced.
  • To describe changes in IAA-producing bacteria populations in the rhizosphere of glyphosate-resistant soybean (Glycine max, ‘Roundup Ready’).
  • To determine microbial activity through assessing C and N mineralization in the rhizosphere of glyphosate-resistant soybean.
methods and materials field
Methods and Materials - Field
  • At Bradford Agronomy Center of the University of Missouri-Columbia
  • Soil classified as a Mexico silt loam (fine, smectitic, mesic, aeric, Vertic Epiaqualf)
  • Roundup Ready soybean (Pioneer 94B01, RR soybean) planted, Roundup (RU) applied at prebloom stage, and 2 foliar amendments (Urea Solution and Biostimulant) applied at 10 days after RU application
experimental design field
Experimental Design - Field
  • Spilt-split block design arranged in completely randomized blocks with 4 replications
  • Soil and plant samples taken prior to glyphosate application and 10, 20 and 30 days after glyphosate application
methods and materials
Methods and Materials
  • Culture conditions for IAA-Producing Bacteria

- Rhizobacteria from RR soybean cultured on half strength King’s B medium for 24-h and colonies counted.

- All counted plates were screened for IAA production using an in situ membrane assay (Bric et al., 1991).

methods and materials22
Methods and Materials

No Glyphosate


methods and materials23
Methods and Materials
  • Total C and N were measured with a Truspec® C and N Determinator.
  • Soil Respiration (CO2 efflux)

- The incubation was conducted for 7 days with 5g of soil sample added 1ml of 5% glucose solution.

- CO2 efflux was measured with a Buck Scientific Model 910 gas chromatography via thermal conductivity detector (TCD).

methods and materials24
Methods and Materials
  • Soil inorganic N mineralized (NO3- and NH4+) was measured with Lachat ion analyzer (Zellweger Analytics, 1992, 1993).
  • Urease activity was estimated (Kandeler and Gerber, 1988).
methods and materials25
Methods and Materials

Truspec® C/N Determinator

Buck Scientific Model 910 gas chromatogr-aphy

Lachat Quikchem Automated ion analyzer

  • Selected chemical characteristics of Mexico silt loam at the field site
cumulative co 2 efflux for 7 d






Cumulative CO2 efflux for 7-d

Vertical bars indicate LSD (P≤0.05).

  • Glyphosate application followed by urea solution decreased soil CO2 efflux; however, glyphosate only application increased soil CO2 efflux from 20 to 30 days.
  • Urea solution without glyphosate application was higher than any other treatments from day 20 to 30; however, not significantly different from no glyphosate treatment.
  • Total rhizobacteria and IAA-producing bacteria populations were generally inhibited by application of glyphosate and significantly inhibited at day 20.
  • Biostimulant application increased total rhizobacteria and IAA-producing bacteria populations from day 20 to 30, irrespective of glyphosate application.
  • The lowest levels of total N and inorganic N were in soils treated with glyphosate.
  • Glyphosate and urea application considerably decreased total N; however, soil inorganic N was higher than any other applications.
  • Urease activity of soils treated with glyphosate was considerably lower than non-treated glyphosate soils.
  • Glyphosate-resistant soybean may modify the bacterial composition and activity in the rhizosphere to a limited extent.
  • These changes may impact crop productivity and soil biological processes.
  • Dr. Robert J. Kremer
  • Dr. Mark Ellersieck
  • Neal Bailey
  • USDA Special Grant-SCN