WA State Grape Society - Ian Burke WEED RESISTANCE ......• Genetic Variation • Selection by Herbicides – Residual herbicides… • Advantage for Selected Plants – Reproduction

WEED RESISTANCE IN GLYPHOSATE

Ian Burke

Overview

• General Terms• Current Status of Herbicide Resistance in the

PNW• Glyphosate Mode and Site of Action• Glyphosate Resistance Mechanisms• Detecting Herbicide Resistance• Slowing the Development of Herbicide

Resistance

Definitions• Herbicide tolerance is defined as the ability of a

species to survive and reproduce following herbicide treatment.

• Herbicide resistance is the heritable ability of a plant to survive and reproduce following herbicide treatment normally lethal to a susceptible (wildtype) plant.

• Site of action is the protein or enzyme that a herbicide binds or inhibits.

• Mode of action is the primary mechanism of plant death due to the binding or inhibition of a protein.

What is required to evolve herbicide resistance?

• Genetic Variation• Selection by Herbicides

– Residual herbicides…• Advantage for Selected Plants

– Reproduction• Sources of Genetic Variation include:

– Recombination of DNA– Mistakes during replication– Transposable elements– UV Radiation– Oxidative Stress– Viral Infections

weedscience.com

Species MOA

Common Groundsel C1/5 (Simizine), C3/6 (Bromoxynil) Prickly Lettuce B/2 (ALS), O/4 (Synthetic auxins)Kochia B/2 (ALS) , O/4 (Synthetic auxins)Russian Thistle B/2 (ALS) Mayweed Chamomile B/2 (ALS) Redroot Pigweed C1/5 (Photosystem II)Spiny Sowthistle B/2 (ALS) Smallseed Falseflax B/2 (ALS) Yellow Starthistle O/4 (Synthetic auxins)

Annual Bluegrass C1/5 (Photosystem II), C2/7 (Diuron), N/26 (Ethofumesate)

Downy Brome A/1 (ACCase), B/2 (ALS)

Italian Ryegrass A/1 (ACCase), B/2 (ALS), K3/15 (Flufenacet), G/9 (Glyphosate), H/10 (Glufosinate)

Wild Oat A/1 (ACCase), K1/3 (Pronamide), N/8, Z/26 (Difenzoquat, Triallate)

Current Status of Herbicide Resistance in the PNW

0

4

7

11

14

1970 1975 1980 1985 1990 1995 2000 2005 2010

A/1B/2C1/5C3/6G/9H/10K1/3N/8O/4

Current Status of Herbicide Resistance in the PNW

Glyphosate

• N-[phosphonomethyl]-glycine is an amino acid inhibitor• Broad spectrum, post emergence and non-selective• Rapidly broken down by soil organisms• A zwitterion – a neutral molecule with a positive and

negative charge• Sold as a salt, or rarely as an acid

– Isopropylene or potassium

Fate of Glyphosate in Soil

• Moderately persistent (3-130 d halflife)

• Strongly adsorbed

• Adsorption dependent on phosphate sites

• Shikimate Pathway- converts carbohydrate precursors from glycolysis into aromatic amino acids

• Glyphosate blocks enzyme EPSP synthase which prevents formation of Chorismate

Enzyme action

Effects of Glyphosate on Plants

• Glyphosate moves in both xylem and phloem• Results in slow death from lack of enzymes• Symptoms include: Stunting, Chlorosis,

Reddening and Discoloration of Terminal Growth

• Symptoms usually occur 10-14 days after exposure to glyphosate

Symptoms

Chlorotic leaves on Beans-glyphosate treatment 14 days earlier

Small leaves on Blueberry- 1 year after glyphosate drift

Injury to Tomatoes

Chlorosis to new growth on Strawberry

Walnut stem with effects of glyphosate treatment made several months earlier-proliferation of growth from the buds at internodes

Small strap-like leaves and short internodes give leaves a crowded appearance on Blueberries. Above: Symptoms on an older cane Left: New growth after glyphosate application the previous year

Types of Glyphosate ResistanceLeaf Uptake1. Epicuticular wax barrierMembrane Transport2. Reduced cell uptake3. Vacuole Sequestration4. Chloroplast exclusion

Target Enzyme - EPSPS5. Active site mutation6. Gene duplicationSacrificial (herbivory mimic)7. Hypersensitive (source

leaf)8. Amputation of sinksChemical Modification9. Metabolism

Leaf Uptake

• Epicuticular Wax Barrier

Ferreira et al. 2008

Membrane Transport

• Reduced cell uptake?

• Vacuole Sequestration of Glyphosate?

• Chloroplast exclusion?

Horseweed, Feng et al. 2004

Genetic Resistance

Target Enzyme – EPSPS5. Active site

mutation– EPSPS

P106X substitution

6. Gene amplification

Enzyme Overexpression or Gene Amplification

• Glyphosate Resistance in Weeds– Target site resistance in the EPSPS gene:

horseweed• 2 to 3 fold greater level

– Gene Amplification: • Palmer amaranth, kochia• Greater amounts of EPSPS due to gene

amplification, 5 to 160-fold more copies of EPSPS• 20 to 50 fold greater level

Pigweed vs. Monsanto – Pigweed Winning!

Sacrificial (herbivory mimic)7. Hypersensitive

(source leaf)8. Amputation of

sinks

Metabolism-based Resistance

Chemical Modification9. Metabolism

– Not reported in weeds!– Mechanism of resistance to glyphosate in

several crop species.

Italian ryegrass• Usually target site resistance

• Probably present in WA

Italian ryegrass resistance

Likely Resistance Cases

Species MOA

Prickly Lettuce G/9 (Glyphosate)Russian Thistle G/9 (Glyphosate)Horseweed G/9 Glyphosate)

Italian Ryegrass G/9 (Glyphosate)

Epilobiums, panicum and America G/9 (Glyphosate)

Kochia G/9 (Glyphosate)

Common lambsquarters G/9 (Glyphosate)

Factors indicating possible herbicide resistance:

• The level of weed control of other susceptible species

• The presence of alive plants adjacent to dead plants

• Past experience• Herbicide history• Occurrence of resistance in

the vicinity

Preventing Herbicide Resistance

• Any management action that reduces the selection pressure for resistance will reduce the rate of resistance evolution– Rotation– Manipulation of planting time– Crop competitiveness– Cultivation techniques– Short herbicide persistence– Herbicides with different mode of action

Risk of Resistance on a Per Species Basis

Moss 1998

Management Option: Low Moderate HighHerbicide mix or rotation in cropping system

> 2 modes of action

2 modes of action 1 modes of action

Weed control in cropping system

Cultural, mechanical,

and chemical

Cultural and chemical

Chemical alone

Use of same mode of action per season

Once More than once Many times

Cropping system Full rotation Limited rotation No rotationResistance status to mode of action

Unknown Limited Common

Weed infestation Low Moderate HighControl in last 3 years Good Declining Poor

Importance of Herbicide

EVERYTHING WE DO EVENTUALLY SELECTS FOR A WEED BIOTYPE THAT IS MORE COMPETITIVE FOR THAT PARTICULAR ENVIRONMENT.

Design control systems that are more diverse than the weed spectrum.

Information on Resistance

• Information on the distribution of herbicide resistance in this presentation from www.weedscience.com

• Other sites with information on herbicide resistance can be found at:– Herbicide Resistance Action Committee:

www.plantprotection.org/hrac.html– Weed Science Society of America:

www.wssa.net