Glyphosate Resistance in Crops and Weeds E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 638

Veröffentlichungsjahr: 2010

Beschreibung

New technologies are becoming available for managing glyphosate resistant (GR) weeds and reducing their spread. GR crop technology has revolutionized crop production in the developed world and the benefits are gradually spilling over to the developing world. In order to sustain an effective, environmentally safe herbicide such as glyphosate and the GR crop technology well in to the future, it is imperative that the issue of GR weeds be comprehensively understood. This book provides such an essential, up-to-date source of information on glyphosate resistance for researchers, extension workers, land managers, government personnel, and other decision makers. 

• Provides comprehensive coverage of the intensely studied topic of glyphosate resistant (GR) in crops
• Details the development of glyphosate resistance and how to detect and manage the problem in crops
• Helps standardize global approaches to glyphosate resistance
• Encompasses interdisciplinary approaches in chemistry, weed science, biochemistry, plant physiology, plant biotechnology, genetics, ecology
• Includes a chapter on economic analysis of GR impact on crops

Leseprobe

Table of Contents

Cover

Table of Contents

Half title page

Title page

Copyright page

Dedication

PREFACE

ACKNOWLEDGMENTS

CONTRIBUTORS

1 GLYPHOSATE: DISCOVERY, DEVELOPMENT, APPLICATIONS, AND PROPERTIES

1.1 HISTORICAL PERSPECTIVE AND MODE OF ACTION

1.2 UPTAKE AND TRANSLOCATION OF GLYPHOSATE

1.3 GLYPHOSATE’S FUNGICIDAL ACTIVITIES

1.4 EFFECT OF GLYPHOSATE ON NONTARGET ORGANISMS

1.5 PHYSICAL AND ENVIRONMENTAL PROPERTIES OF GLYPHOSATE

1.6 GLYPHOSATE TOXICOLOGY AND APPLICATOR EXPOSURE

1.7 COMMERCIAL PROCESS CHEMISTRY FOR PREPARING GLYPHOSATE

1.8 GLYPHOSATE FORMULATION

1.9 CONCLUSION

2 HERBICIDE RESISTANCE: DEFINITIONS AND CONCEPTS

1.1 INTRODUCTION

2.2 TOLERANCE AND RESISTANCE

2.3 MECHANISMS OF RESISTANCE

2.4 DEFINITIONS USED IN HERBICIDE RESISTANCE LITERATURE

2.5 HISTORY OF GLYPHOSATE RESISTANCE DEVELOPMENT IN WEEDS

2.6 GLYPHOSATE RESISTANCE—AN EVOLUTIONARY TAKE

2.7 CONCLUSIONS

3 GLYPHOSATE-RESISTANT CROPS: DEVELOPING THE NEXT GENERATION PRODUCTS

3.1 INTRODUCTION

3.2 MECHANISMS OF ENGINEERING GLYPHOSATE RESISTANCE

3.3 DEVELOPMENT OF FIRST-GENERATION GR CROPS

3.4 IDENTIFICATION OF AT-RISK TISSUES FOR GLYPHOSATE INJURY

3.5 DEVELOPMENT OF SECOND-GENERATION GR CROPS

3.6 USE OF ALTERNATIVE HERBICIDES FOR WEED RESISTANCE MANAGEMENT

3.7 ENGINEERING CROP RESISTANCE TO THE GLUFOSINATE HERBICIDE

3.8 ENGINEERING CROP RESISTANCE TO THE DICAMBA HERBICIDE

3.9 STACKING OF HERBICIDE-RESISTANT TRAITS

ACKNOWLEDGMENTS

4 TRANSITIONING FROM SINGLE TO MULTIPLE HERBICIDE-RESISTANT CROPS

4.1 INTRODUCTION

4.2 THE FIRST HR CROPS

4.3 GR CROPS

4.4 OTHER MECHANISMS THAT CAN BE STACKED WITH GLYPHOSATE RESISTANCE

4.5 SUMMARY

5 TESTING METHODS FOR GLYPHOSATE RESISTANCE

5.1 INTRODUCTION

5.2 TESTING METHODS

6 BIOCHEMICAL MECHANISMS AND MOLECULAR BASIS OF EVOLVED GLYPHOSATE RESISTANCE IN WEED SPECIES

6.1 INTRODUCTION

6.2 MODE OF ACTION OF GLYPHOSATE

6.3 MECHANISMS OF GLYPHOSATE RESISTANCE IN WEEDS

6.4 SUMMARY

7 GLYPHOSATE RESISTANCE: GENETIC BASIS IN WEEDS

7.1 OVERVIEW

7.2 FREQUENCY AND FITNESS OF GLYPHOSATE RESISTANCE GENES AMONG WEEDS

7.3 INHERITANCE OF GLYPHOSATE RESISTANCE IN WEEDS

7.4 MOLECULAR GENETICS OF GLYPHOSATE RESISTANCE IN WEEDS

7.5 FUTURE OUTLOOK

ACKNOWLEDGMENTS

8 GENOMICS OF GLYPHOSATE RESISTANCE

8.1 INTRODUCTION

8.2 TARGET-SITE GENOMICS

8.3 NONTARGET-SITE GENOMICS

8.4 THE BIOCHEMISTRY AND PHYSIOLOGY CONNECTION

8.5 SCREENING CANDIDATE GENES FOR FUNCTIONALITY

8.6 CONCLUSIONS

ACKNOWLEDGMENTS

9 GLYPHOSATE-RESISTANT CROP PRODUCTION SYSTEMS: IMPACT ON WEED SPECIES SHIFTS

9.1 OVERVIEW

9.2 COMMERCIALIZATION AND ADOPTION OF GRCS

9.3 GLYPHOSATE USE

9.4 IMPACT OF GRCS ON WEEDS

9.5 MANAGEMENT OF WEED SPECIES SHIFTS

9.6 CONCLUSIONS

10 GLYPHOSATE-RESISTANT HORSEWEED IN THE UNITED STATES

10.1 BACKGROUND ON ECOLOGY OF CONYZA CANADENSIS

10.2 PREVIOUS HERBICIDE RESISTANCE IN HORSEWEED

10.3  HISTORICAL REVIEW OF GLYPHOSATE RESISTANCE

10.4 GLYPHOSATE RESISTANCE IN HORSEWEED

10.5 PHYSIOLOGICAL BASIS FOR GLYPHOSATE RESISTANCE IN HORSEWEED

10.6 GR HORSEWEED DISSEMINATION

10.7 GR HORSEWEED MANAGEMENT STRATEGIES

11 GLYPHOSATE-RESISTANT PALMER AMARANTH IN THE UNITED STATES

11.1 INTRODUCTION

11.2 PALMER AMARANTH ORIGIN, IDENTIFICATION, AND BIOLOGY

11.3 COMPETITIVE ABILITIES OF PALMER AMARANTH AND CROP YIELD LOSS

11.4 DEVELOPMENT AND SPREAD OF GLYPHOSATE RESISTANCE IN PALMER AMARANTH

11.5 GR PALMER AMARANTH IMPACTS GEORGIA COTTON

11.6 MANAGEMENT OF GR PALMER AMARANTH

12 MANAGING GLYPHOSATE-RESISTANT WEEDS AND POPULATION SHIFTS IN MIDWESTERN U.S. CROPPING SYSTEMS

12.1 INTRODUCTION

12.2 BACKGROUND ON GR CROPPING SYSTEMS

12.3 AGRONOMIC PRACTICE INFLUENCES ON WEED POPULATIONS/SHIFTS

12.4 PRESENCE OF RESISTANT WEEDS

12.5 GROWER OPINIONS

12.6 MANAGEMENT APPROACHES

12.7 ACTION PLANS FOR WEED MANAGEMENT IN GR CROPS IN THE U.S. MIDWEST CORN BELT

12.8 FINAL THOUGHTS

13 GLYPHOSATE-RESISTANT RIGID RYEGRASS IN AUSTRALIA

13.1 INTRODUCTION

13.2 EVOLUTION OF GLYPHOSATE RESISTANCE IN RIGID RYEGRASS

13.3 RESISTANCE MECHANISMS IN RIGID RYEGRASS IN AUSTRALIA

13.4 INHERITANCE OF GLYPHOSATE RESISTANCE IN RIGID RYEGRASS

13.5 FITNESS OF GLYPHOSATE-RESISTANT RIGID RYEGRASS POPULATIONS IN AUSTRALIA

13.6 MANAGEMENT OF GLYPHOSATE-RESISTANT RIGID RYEGRASS IN AUSTRALIA

13.7 CONCLUSIONS AND FUTURE PROSPECTS

14 GLYPHOSATE RESISTANCE IN LATIN AMERICA

14.1 INTRODUCTION

14.2 BROAD-LEAVED WEED SPECIES RESISTANT TO GLYPHOSATE

14.3 GRASS WEED SPECIES RESISTANT TO GLYPHOSATE

14.4 OTHER GR GRASS WEEDS

14.5 PERSPECTIVES

15 STRATEGIES FOR MANAGING GLYPHOSATE RESISTANCE— AN EXTENSION PERSPECTIVE

15.1 INTRODUCTION

15.2 EXTENSION EDUCATOR ROLL

15.3 STRATEGY

15.4 IDENTIFYING THE PROBLEM

15.5 ANSWERING FARMER QUESTIONS

15.6 CREATING AWARENESS

15.7 ENGAGING MAJOR STAKEHOLDERS

15.8 DEVELOPING BEST MANAGEMENT PRACTICES—RECOMMENDATIONS

15.9 WHERE TO FROM HERE?

16 ECONOMIC IMPACT OF GLYPHOSATE-RESISTANT WEEDS

16.1 INTRODUCTION

16.2 CHANGES IN MANAGEMENT PRACTICES SINCE THE INTRODUCTION OF GR CROPS

16.3 COSTS OF ADDRESSING GR WEEDS IN WEED MANAGEMENT PROGRAMS

16.4 ECONOMICS OF PROACTIVE VERSUS REACTIVE RESISTANCE MANAGEMENT

16.5 LOOKING AHEAD TO NEW TECHNOLOGIES

Index

GLYPHOSATE RESISTANCE IN CROPS AND WEEDS

Copyright © 2010 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

Glyphosate resistance in crops and weeds: history, development, and management / edited by Vijay K. Nandula.

p. cm.

 Includes index.

 ISBN 978-0-470-41031-8(cloth); 978-1-118-04354-7(ebk);

 1. Glyphosate. 2. Herbicide resistance. 3. Herbicide-resistant crops. 4. Plants–Effect of herbicides on. I. Nandula, Vijay K.

 SB952.G58G59 2010

 632′.954–dc22

2009054245

Dr. Stephen O. Duke for his inspiration and mentoring

and

my wife Aparna, daughter Indu, and son Ajay for their love, support, and understanding.

PREFACE

Since the discovery of its herbicidal properties in 1970 and commercialization in 1974, glyphosate has been used extensively in both croplands and non­croplands. Because of its lack of selectivity, glyphosate use was initially limited to preplant, postdirected, and postharvest applications for weed control. With the introduction of glyphosate-resistant (GR) crops in the mid-1990s, glyphosate is now widely used for weed control in GR crops without concern for crop injury. GR crops are currently grown in several countries, with particularly strong adoption in the United States, Canada, Argentina, and Brazil. The widespread adoption of GR crops has not only caused weed species shifts in these crops, but it has also resulted in evolution of GR weed biotypes. GR weed populations threaten the sustainability of glyphosate and GR crop technology, thereby jeopardizing derived benefits such as reduced fuel costs and improved soil conservation. To date, 18 weed species have evolved resistance to glyphosate worldwide. This number will most likely increase rapidly in the next few years due to increased selection pressure from glyphosate, better monitoring and detection methods, and better awareness of the problem of glyphosate resistance.

Exciting new technologies such as new generation of GR crops and multiple herbicide-resistant (HR) (including glyphosate resistance) crops are in development or approaching commercialization in the next few years, which will help manage GR weeds and reduce their spread. Modern research techniques such as weed genomics are being employed to study GR weed resistance mechanisms, fitness issues, biology, and ecology. Additional avenues of research being pursued are gene flow, population genetics, multiple resistance, modeling, GR weeds as alternative hosts for other pests, and effects on human and animal health, as well as the impact on conservation tillage. GR crop technology has revolutionized crop production in the developed world, and the benefits are gradually spilling over to the developing world.

The vast body of complex information being generated on glyphosate resistance, one of the pressing issues faced by growers and land managers, makes it hard to keep current with the topic. To sustain an effective, environmentally safe herbicide such as glyphosate and the GR crop technology well into the future, it is imperative that the issue of GR weeds is comprehensively understood. To this end, an up-to-date source of information on glyphosate resistance is essential for researchers, extension workers, land managers, government personnel, and other decision makers, so the bottom line of growers, and conservation and diversity programs is increased. I earnestly hope that this book will fill this niche.

The book is divided into 16 chapters. Chapter 1 provides an overview of more recent research on the use of the herbicide glyphosate and its environmental, toxicological, and physical aspects. Herbicide resistance is defined in Chapter 2 and several aspects related to it are introduced. Chapter 3 reviews the processes involved in the commercialization of currently grown GR crops as well as the next generation of GR and HR crops, including multiple HR traits.

Chapter 4 is a comprehensive review of GR crop development events as well as multiple HR crops that are currently approaching commercialization. Chapter 5 provides an overview of the biochemical, biological, molecular, and physiological procedures used in laboratory, greenhouse, and field research with glyphosate resistance in plants. Chapter 6 summarizes the current knowledge of biochemical mechanisms of evolved glyphosate resistance in weeds and the molecular basis behind it.

Chapter 7 examines the genetics and inheritance of the mechanisms of glyphosate resistance. A genomic approach is taken in Chapter 8, in order to gain a better understanding of the mechanisms and evolution of glyphosate resistance in weeds using GR horseweed, the first broad-leaved weed that has evolved to be resistant to glyphosate, as a model.

Chapter 9 summarizes the effect of GR corn, cotton, and soybean cropping systems on weed species shifts as well as late-season weed problems in the United States. Chapter 10 describes the history of herbicide resistance, evolution of glyphosate resistance, biology and ecology, and glyphosate resistance management in horseweed. Chapter 11 describes the unprecedented nature and magnitude of difficulty in managing GR Palmer amaranth populations. In Chapter 12, the current situation regarding GR cropping systems and weed management issues in midwestern United States is discussed.

Chapter 13 examines the development and management of GR rigid ryegrass from Australia, the first weed to evolve resistance to glyphosate. Latin America is covered in Chapter 14, which comprehensively reviews the history and current status of glyphosate resistance in weed populations there.

Chapter 15 provides insights from an extension perspective on the management of glyphosate weeds, and Chapter 16 presents an analysis of the effects of GR weeds on management costs. There is some overlap in the content presented among chapters, given the nature of the subject matter.

This book is expected to be useful to students, researchers, regulators, industry, and anyone interested in learning about glyphosate resistance around the world.

VIJAY K. NANDULA

Mississippi State University

Stoneville, MS

ACKNOWLEDGMENTS

I wish to thank Mr. Jonathan Rose, editor, John Wiley & Sons, Inc., for recognizing the need for this project and providing constant support and encouragement. I am deeply indebted to all contributing authors who have come together with the common goal of sharing historic and current information on this important subject of glyphosate resistance. I sincerely express my gratitude to all reviewers who have agreed to review and provide their input toward improving the content of the book in a very timely and efficient manner.

CONTRIBUTORS

Laura G. Abercrombie, Department of Plant Sciences, University of Tennessee, Knoxville, TN

Marion Bleeke, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO

Claire A. CaJacob, Monsanto Company, 700 Chesterfield Village Pkwy W., Chesterfield, MO

Janet E. Carpenter, Consultant; Email: [email protected]

Linda A. Castle, Pioneer Hi-Bred International, Inc., Verdia Research Campus, 700A Bay Road, Redwood City, CA

R. Eric Cerny, Monsanto Company, 700 Chesterfield Village Pkwy W., Chesterfield, MO

Michael J. Christoffers, Department of Plant Sciences, North Dakota State University, Fargo, ND; Email: [email protected]

A. Stanley Culpepper, Department of Crop and Soil Sciences, University of Georgia, Tifton, GA; Email: [email protected]

Gerald M. Dill, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO; Email: [email protected]

Greg A. Elmore, Monsanto Company, 700 Chesterfield Village Pkwy W., Chesterfield, MO

Donna Farmer, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO

Paul C. C. Feng, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO; Email: [email protected]

Leonard P. Gianessi, Crop Protection Research Institute, CropLife Founda­tion, 1156 15th Street NW, Suite 400, Washington, DC

Jerry M. Green, Pioneer Hi-Bred International, Inc., Stine-Haskell Research Center Bldg. 210, 1090 Elkton Road, Newark, DE; Email: [email protected]

Matthew D. Halfhill, Department of Plant Sciences, University of Tennessee, Knoxville, TN; and Department of Biology, Saint Ambrose University, Davenport, IA

Eric A. Haupfear, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO

Gregory R. Heck, Monsanto Company, 700 Chesterfield Village Pkwy W., Chesterfield, MO

Joy L. Honegger, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO

Jun Hu, Department of Plant Sciences, University of Tennessee, Knoxville, TN; and Institute of Plant Genomics and Biotechnology and Department of Plant Pathology and Microbiology, Texas A&M University, College Station, TX

Jintai Huang, Monsanto Company, 700 Chesterfield Village Pkwy W., Chesterfield, MO

William G. Johnson, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN

Frank Kohn, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO

Keith Kretzmer, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO

Warren M. Kruger, Monsanto Company, 700 Chesterfield Village Pkwy W., Chesterfield, MO

Christopher L. Main, West Tennessee Research and Education Center, University of Tennessee, Jackson, TN

Carol Mallory-Smith, Department of Crop and Soil Science, Oregon State University, Corvallis, OR

Marianne Malven, Monsanto Company, 700 Chesterfield Village Pkwy W., Chesterfield, MO

Susan J. Martino-Catt, Monsanto Company, 700 Chesterfield Village Pkwy W., Chesterfield, MO

Akbar Mehrsheikh, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO

John A. Miklos, Monsanto Company, 700 Chesterfield Village Pkwy W., Chesterfield, MO

Thomas C. Mueller, Department of Plant Sciences, University of Tennessee, Knoxville, TN

Vijay K. Nandula, Delta Research and Extension Center, Mississippi State University, Stoneville, MS; Email: [email protected]

Jason K. Norsworthy, Department of Crop, Soil, and Environmental Science, University of Arkansas, 1366 West Altheimer Drive, Fayetteville, AR

Micheal D. K. Owen, Department of Agronomy, Iowa State University, Ames, IA

Stephen R. Padgette, Monsanto Company, 700 Chesterfield Village Pkwy W., Chesterfield, MO

Yanhui Peng, Department of Plant Sciences, University of Tennessee, Knoxville, TN

Alejandro Perez-Jones, Monsanto Company, St. Louis, MO

Christopher Preston, School of Agriculture, Food & Wine, University of Adelaide, PMB 1, Glen Osmond SA, Australia; Email: [email protected]

Priya Ranjan, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN

Murali R. Rao, Department of Plant Sciences, University of Tennessee, Knoxville, TN

Krishna N. Reddy, USDA-ARS, Southern Weed Science Research Unit, Stoneville, MS; Email: [email protected]

R. Douglas Sammons, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis, MO

Dale L. Shaner, USDA-ARS, Fort Collins, CO; Email: [email protected]

Ken Smith, Southeast Research and Extension Center, Division of Agri­culture, University of Arkansas, Monticello, AR; Email: [email protected]

Lynn M. Sosnoskie, Department of Crop and Soil Sciences, University of Georgia, Tifton, GA

Lawrence E. Steckel, West Tennessee Research and Education Center, University of Tennessee, Jackson, TN; Email: [email protected]

C. Neal Stewart, Jr., Department of Plant Sciences, University of Tennessee, Knoxville, TN; Email: [email protected]

Patrick J. Tranel, Department of Crop Sciences, University of Illinois, Urbana-Champaign, IL

Bernal E. Valverde, Investigación y Desarrollo en Agricultura Tropical (IDEA Tropical), Alajuela, Costa Rica; and University of Copenhagen, Hojebakkegaard Allé 13, Taastrup 2630, Denmark; Email: [email protected]/[email protected]

Aruna V. Varanasi, Department of Plant Sciences, North Dakota State University, Fargo, ND

Theodore M. Webster, Crop Protection and Management Research Unit, USDA-Agricultural Research Service, Tifton, GA

Stephen C. Weller, Department of Horticulture and Landscape Architecture, Purdue University, West Lafayette, IN; Email: [email protected]

D. Wright, Monsanto Company, 800 N. Lindbergh Blvd., St. Louis

Alan C. York, Department of Crop Science, North Carolina State University, Raleigh, NC

Joshua S. Yuan, Institute of Plant Genomics and Biotechnology and Department of Plant Pathology and Microbiology, Texas A&M University, College Station, TX

1

GLYPHOSATE: DISCOVERY, DEVELOPMENT, APPLICATIONS, AND PROPERTIES

Gerald M. Dill, R. Douglas Sammons, Paul C. C. Feng, Frank Kohn, Keith Kretzmer, Akbar Mehrsheikh, Marion Bleeke, Joy L. Honegger, Donna Farmer, Dan Wright, and Eric A. Haupfear

1.1 HISTORICAL PERSPECTIVE AND MODE OF ACTION

N-(phosphonomethyl)glycine (glyphosate) is a phosphonomethyl derivative of the amino acid glycine. Glyphosate is a white and odorless crystalline solid comprised of one basic amino function and three ionizable acidic sites (Fig. 1.1). Glyphosate was actually invented in 1950 by a Swiss chemist, Dr. Henri Martin, who worked for the small pharmaceutical company, Cilag (Franz et al. 1997). The product had no pharmaceutical application and was never reported in literature. In 1959, Cilag was acquired by Johnson and Johnson, which sold its research samples, including glyphosate, to Aldrich Chemical. Aldrich sold small amounts of the compound to several companies in the 1960s for undisclosed purposes, but no claims of biological activity were ever reported. In its Inorganic Division, Monsanto was developing compounds as potential water-softening agents and over 100 related aminomethylphosphonic acid (AMPA) analogs were synthesized. When these compounds were tested as herbicides by Dr. Phil Hamm, two showed some herbicidal activity on perennial weeds. However, the unit activity was too low to be a commercial herbicide.

Figure 1.1. The structure of glyphosate.

Dr. Hamm enlisted the efforts of Monsanto chemist Dr. John Franz. He repeatedly told Dr. Franz that “he just wanted something five times as strong … that’s all.” “He convinced me to take a shot at making analogs and derivatives,” recalled Dr. Franz. “That didn’t yield anything, and I was ready to drop the project. But then I began trying to figure out the peculiarities of those two compounds, and I wondered if they might metabolize differently in the plants than the others … I began to write out metabolites … you could write a list of about seven or eight … it involved completely new chemistry. Glyphosate was the third one I made” (Halter 2007). Glyphosate was first synthesized by Monsanto in May 1970 and was tested in the greenhouse in July of that year. The molecule advanced through the greenhouse screens and field testing system rapidly and was first introduced as Roundup® herbicide by Monsanto Company (St. Louis, MO) (Baird et al. 1971).

Glyphosate inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) (Amrhein et al. 1980), which is present in plants, fungi, and bacteria, but not in animals (Kishore and Shah 1988). The enzyme catalyzes the transfer of the enolpyruvyl moiety of phosphoenolpyruvate (PEP) to shikimate-3-phosphate (S3P). This is a key step in the synthesis of aromatic amino acids, and ultimately, hormones and other critical plant metabolites. The active site of the EPSPS enzyme in higher plants is very highly conserved (CaJacob et al. 2003). The mechanism of inhibition is also unique in that the binding site for glyphosate is reported to closely overlap with the binding site of PEP (Franz et al. 1997). A diagram of the shikimate pathway and glyphosate’s inhibition site is shown in Figure 1.2. No other mode of action for glyphosate has been observed even when very high doses are applied to glyphosate-resistant (GR) soybean and canola (Nandula et al. 2007).

Figure 1.2. The site of inhibition of glyphosate from Dill (2005).