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Gene Flow and Herbicide Resistance
Gene Flow and Herbicide Resistance AJ Jhala, University of Nebraska–Lincoln, Lincoln, NE, USA SZ Knezevic, Northeast Research and Extension Center, University of Nebraska–Lincoln, Concord, NE, USA Ó 2017 Elsevier Ltd. All rights reserved.
Glossary Biotype A group of plants within a species with biological trait(s) that are not common to the population as a whole. Cross-resistance A weed biotype resistant to more than one herbicide having the same mode of action. Gene flow The process of successful transfer of genetic information between different individuals, populations, and generations (to progeny) and across spatial dimensions. Herbicide-resistant weed Inherited ability of a weed to survive and reproduce following exposure to an application rate of herbicide normally lethal to the wild type.
Herbicide-tolerant weed Inherent ability of a weed to survive and reproduce after herbicide treatment. Multiple resistance Resistance of a weed species to more than one herbicide having different modes of action in which more than one basis for resistance is involved. Pollen-mediated gene flow The transfer of genetic information between and within plant species via pollen. Seed-mediated gene flow The transfer of genetic information/trait(s) via seed movement and/or dissemination due to wind, water, humans, birds and animals, or mechanical means (e.g., shattering). Tolerance Survival of a population of a weed species following a herbicide dosage lethal to other species.
Abbreviations PMGF Pollen-mediated gene flow SMGF Seed-mediated gene flow
ACCase Acetyl-CoA carboxylase ALS Acetolactate synthase HPPD 4-Hydroxyphenylpyruvate dioxygenase
Introduction
Herbicide-Resistant Weeds
Agriculture is essential for providing food, feed, and fiber worldwide. Crop production is a type of agriculture that includes many different practices, depending on whether industrial or subsistence farming systems are used. As of 2015, more than seven billion people in the world rely on agriculture for the vast majority of their plant-derived necessities. Over the past 50 years, management of agricultural fields has been driven by economic returns and technical advancements in crop genetics and synthetic pesticides. Improvements in crop plants through biotechnology and genetic engineering are being taking place and being carefully assessed for their fit in specific production systems. For example, the cropping systems in North and South America over the past 20 years have been dominated by the development and widespread adoption of herbicide-resistant crops, promoting the use of limited herbicides, including glyphosate and glufosinate in glyphosate- and glufosinate-resistant crops, respectively. Although this technology has provided excellent weed control over the past several years, this intensive crop production system has become primarily dependent on herbicides for weed control. Most importantly, the loss or reduction in the utility of residual herbicides or herbicides with different modes of action is a concern to many involved in various aspects of weed control.
Weed resistance to herbicide is not unique. Resistance has been documented in several other species: for example, insects resistant to insecticides and bacteria resistant to antibiotics were documented well before weeds became resistant to herbicides. Since the first report of wild carrot (Daucus carota L.) resistant to 2,4-dichlorophenoxyacetic acid in 1957, resistance to herbicides has been reported in over 400 biotypes worldwide. About 10 new herbicide-resistant weed biotypes are reported each year. Of 400 biotypes, 30% (153 biotypes) are resistant to acetolactate synthase (ALS) inhibitors, 20% (72 biotypes) are resistant to photosystem II inhibitors, and 11% (46 biotypes) are resistant to acetyl-CoA carboxylase (ACCase) inhibitors. Thus, herbicides belonging to these three modes of action account for 60% of all herbicide-resistant weed biotypes. The newest type of herbicide resistance is resistance to glyphosate, the most used herbicide worldwide. As of 2015, 32 weed species worldwide have been reported to be resistant to glyphosate, including 15 species in the United States. Registered herbicides are classified into different groups based on their sites of action. The herbicide mode of action refers to the general biological process that a herbicide uses to disrupt plant growth, produce symptoms, and kill susceptible weeds. Sometimes, the ‘site of action’ is used interchangeably with the ‘mode of action,’ but specifically speaking, site of action refers
Encyclopedia of Applied Plant Sciences, 2nd edition, Volume 3
http://dx.doi.org/10.1016/B978-0-12-394807-6.00234-3
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Weeds and Competition j Gene Flow and Herbicide Resistance
to the exact site or enzyme where a herbicide acts to disrupt the growth of plants. Knowledge of herbicide groups and modes of action is important because herbicide resistance evolves not only in response to repeated use of the same herbicide but also with the recurring use of herbicides belonging to the same mode of action. Thus, a rational and judicious use of available herbicides, along with other weed control methods, requires being aware of and selecting herbicides with different modes of action. Multiple herbicide resistance occurs when a weed species is no longer controlled by two or more herbicides with different modes of action applied at labeled rates, when previously these herbicides would have provided effective control at the same application rates. In other words, a weed species with multiple resistance has acquired resistance against herbicides belonging to different chemistries. This is due to overreliance or continuous use of two or more selected herbicides over time. Multiple resistance has been reported in several weed species. The resistance of Palmer amaranth (Amaranthus palmeri) to 4-hydroxyphenylpyruvate dioxygenase–inhibiting herbicides (mesotrione, tembotrione, and topramezone) and PS II–inhibiting herbicide (atrazine) has recently been confirmed in a continuous seed corn production field in Nebraska. Similarly, common waterhemp in Nebraska has been confirmed resistant to triazine, ALS inhibitors, and glyphosate. In fact, a population of common waterhemp in Illinois has even been confirmed resistant to four herbicides with different modes of action (atrazine, lactofen, ALS inhibitors, and glyphosate). In the future, if integrated weed management options are not adopted, expect complex weed resistance issues that will reduce herbicide options.
enhanced ability to metabolize a herbicide can potentially inactivate the herbicide before it can reach its site of action within the plant.
Mechanisms of Resistance: There are Five Mechanisms that Account for Herbicide Resistance
Gene Flow and Weed Resistance
Alterations in the Target Site of the Herbicide A herbicide has a specific site of action within the plant where it acts to disrupt a particular physiological process. If this target site is altered, then the herbicide is unable to kill the plant. Target site resistance is the result of a modification in the herbicide-binding site (usually an enzyme), which prevents a herbicide from binding effectively. If the herbicide cannot bind to the enzyme then it cannot inhibit the enzyme and the plant survives. Target site resistance is the most common resistance mechanism. Some believe in the natural selection theory, suggesting that herbicide-resistant weeds have always occurred at extremely low numbers within a particular weed species. When a herbicide effectively controls the majority of susceptible population of a species, only those plants that possess a resistance trait can survive and produce seed for future generations. This theory of resistance development has several parallels to Darwin’s theory of survival of the fittest, suggesting that plants less adapted do not survive, and only the fittest plants produce seed. The seeds from resistant biotypes ensure that the resistant trait is transferred to future generations. Thus, if the same herbicide is used repeatedly, the resistant biotypes continue to expand.
Enhanced Metabolism of the Herbicide Some plants may use their own metabolic activity to detoxify a foreign compound such as a herbicide. A weed with an
Compartmentation of the Herbicide Plants are capable of sequestering foreign compounds within their cells or tissues to prevent the compounds from causing harmful effects. When a herbicide is placed within a restricted compartment, it cannot reach its site of action and is thus unable to kill the plant. About 18 weed biotypes show resistance to the bipryridylium herbicides, paraquat and diquat. One of the proposed mechanisms for resistance is that the resistant biotypes restrict the movement of the herbicides and prevent the herbicides from reaching the target site.
Decreased Absorption and/or Translocation Decreased absorption and/or translocation can cause resistance because herbicide movement is restricted and the herbicide does not reach its target site of action in a concentration sufficient to cause death.
Gene Amplification/Overexpression Gene amplification/overexpression is the most recently described resistance mechanism, and causes resistance by increasing the production of the target enzyme, effectively diluting a herbicide in relation to the target site.
Gene flow via pollen and/or seed dispersal from resistant weeds may also provide a source of resistant genes in a susceptible population.
Pollen-Mediated Gene Flow Resistant genes may disperse naturally by pollen via wind, insects, or other pollinators. This process is known as pollen-mediated gene flow (PMGF). Several factors and their interactions influence the frequency and distance of PMGF, including reproductive biology of the weed species, pollen viability and longevity, flowering synchrony and pollen production, wind speed and direction, as well as type and presence of pollination vectors. Gene flow is a natural phenomenon not unique to weed species, but common to all flowering plants. However, after the evolution of herbicide-resistant weeds, PMGF is believed to be an important avenue for the spread of resistance within and between weed species. The dissemination of herbicideresistant genes through pollen is more common in weed species that are obligate outcrossing or dioecious (separate male and female plants), such as Palmer amaranth (A. palmeri S. Wats.) and common waterhemp (Amaranthus rudis).
Palmer Amaranth Palmer amaranth is a broadleaf, tall, upright, and dioecious summer annual weed native to northern Mexico, southern Arizona, and California. It was first identified by Sereno Watson
Weeds and Competition j Gene Flow and Herbicide Resistance in 1877 in California along the banks of the Rio Grande river. Palmer amaranth has since spread into the mid-south and southeastern United States, where it is a competitive weed in row crop production. Interference and competition from Palmer amaranth has significantly reduced growth and yield of corn, cotton, soybean, and peanut. The profitability of row crop production in the United States has greatly improved since the commercialization of glyphosate-resistant crop cultivars; however, the repeated use of glyphosate in glyphosateresistant crops has resulted in the evolution of glyphosateresistant Palmer amaranth, one of the most serious weeds in the southeastern and mid-south United States. Researchers have reported Palmer amaranth biotypes resistant to both glyphosate and ALS-inhibiting herbicides. Palmer amaranth is dioecious and pollinated by wind; therefore, glyphosate resistance can be spread by wind and the trait is highly mobile. Additionally, Palmer amaranth pollen grains are small, fairly smooth, settle slowly, and are more likely to be transported away from the paternal plant than larger or more highly ornamented pollen grains. Research has been conducted in Georgia to determine whether glyphosate-resistant trait can be transferred via PMGF from a glyphosate-resistant Palmer amaranth source to a glyphosate-susceptible sink. Glyphosateresistant Palmer amaranth plants were transplanted at the center of a 30-ha cotton field, and susceptible plants were transplanted into plots at a distance up to 300 m in each of the four cardinal and ordinal directions. At the end of the season, seeds were harvested from mature female glyphosate-susceptible plants. The seeds were grown and plants were treated with glyphosate when they were 5–7 cm tall. Results suggested that 50–60% of the offspring at 1 and 5 m distances were resistant to glyphosate, whereas 20–40% of the offspring were resistant at the furthest distances. Results from this study demonstrated that glyphosate-resistant Palmer amaranth could be dispersed up to 300 m under natural field conditions, and that the widespread occurrence of glyphosate-resistant Palmer amaranth is due in part to the movement of pollen between spatially segregated populations. A similar study is being conducted in Nebraska to determine PMGF from glyphosate-resistant common waterhemp to glyphosate-susceptible common waterhemp under field conditions, and transgene movement was detected at up to 50 m from the pollen source.
Lolium rigidum Weedy ryegrasses, Lolium rigidum Gaud. is an economically important weed that represents the most serious case of herbicide resistance in Australian cropping systems. It is an anemophilous, self-sterile, cross-pollinated species with the potential for prolific pollen production. Herbicide-resistant L. rigidum infests millions of hectares of southern Australian cropping systems. A field experiment at landscape level was conducted with herbicide-susceptible L. rigidum transplanted into a bushland environment at increasing distances from a large commercial field infested with herbicide-resistant L. rigidum. Herbicide resistance was used as a marker to quantify distance and frequency of PMGF. Results suggested that PMGF occurred at 3000 m, maximum tested distance, from the pollen source. Thus, longdistance PMGF is possible in L. rigidum and can spread herbicide-resistant traits across fields.
449
Seed-Mediated Gene Flow In addition to pollen, resistance genes may also be spread by seeds. Seed dispersal among weed populations can occur through seed shattering and natural vectors, such as wind, water, insect herbivores, or mammals, as well as by humans. Growers often neglect the cleaning of farm machinery after operation and spread herbicide-resistant weed seeds during seeding, tillage, and/or harvesting operations in different fields. Understanding the contribution of gene mutation versus gene flow by pollen and/or seed dispersal is important because it determines the success of different management practices (e.g., reducing the intensity of herbicide selection versus limiting the movement of resistant seeds) in preventing the spread of resistance to adjacent fields, farms, and across agricultural landscapes. In general, seed traits that encourage dispersal from the mother plant help seeds stay airborne or catch the fur of animals, or hard seed coat to survive the digestive tracts of animals would be more subject to spread by seed movement. Additionally, the seeds of certain weed species mimic the seeds of crops, thus aiding in their dispersal by humans. Several weed seeds also have the ability to persist in the soil by remaining dormant for several years before germinating, further increasing the chances of seed-mediated gene flow. Finally, resistant genes in highly self-pollinated weed species such as horseweed (Conyza bonariensis (L.) Cronq.) and common lambsquarters (Chenopodium album L.), are primarily spread by the movement of seeds.
See also: Plants and the Environment: Invasive Plant Species. Weeds and Competition: Integrated Weed Management; Thermal Weed Control.
Further Reading Beckie, H.J., Hall, L.M., 2008. Simple to complex: modeling crop pollen-mediated gene flow. Plant Sci. 175, 615–628. Busi, R., Yu, Q., Barrett-Lennard, R., Powles, S., 2008. Long distance pollen-mediated flow of herbicide-resistance genes in Lolium rigidum. Theor. Appl. Genet. 117, 1281–1290. Jhala, A.J., Bhatt, H., Topinka, K., Hall, L.M., 2011. Pollen-mediated gene flow in flax (Linum usitatissimum L.): can genetically engineered and organic flax coexist? Heredity 106, 557–566. Knezevic, S., 2007. Herbicide tolerant crops: 10 years later. Maydica 52, 245–250. Mallory-Smith, C., Zapiola, M., 2008. Gene flow from glyphosate-resistant crops. Pest Manage. Sci. 64, 428–440. Poppy, G.M., Wilkinson, M.J., 2005. Gene Flow from GM Plants. Blackwell Publishing Ltd, Oxford, UK. Sosnoskie, L.M., Webster, T.M., Kichler, J.M., et al., 2012. Pollen-mediated dispersal of glyphosate-resistance in Palmer amaranth under field conditions. Weed Sci. 60, 366–373.
Relevant Websites http://www.gmo-compass.org/eng/home/ – GMO Compass: A website supported by the European Union within the European Commission’s 6th Framework Program. http://www.hracglobal.com/ – Herbicide Resistance Action Committee. http://www.isaaa.org/ – International Service for the Acquisition of Agri-biotech Applications (ISAAA). http://weedscience.org/ – International Survey of Herbicide Resistant Weeds. http://takeactiononweeds.com/ – Take Action on Weeds. http://wssa.net/ – Weed Science Society of America.
Glossary Biotype A group of plants within a species with biological trait(s) that are not common to the population as a whole. Cross-resistance A weed biotype resistant to more than one herbicide having the same mode of action. Gene flow The process of successful transfer of genetic information between different individuals, populations, and generations (to progeny) and across spatial dimensions. Herbicide-resistant weed Inherited ability of a weed to survive and reproduce following exposure to an application rate of herbicide normally lethal to the wild type.
Herbicide-tolerant weed Inherent ability of a weed to survive and reproduce after herbicide treatment. Multiple resistance Resistance of a weed species to more than one herbicide having different modes of action in which more than one basis for resistance is involved. Pollen-mediated gene flow The transfer of genetic information between and within plant species via pollen. Seed-mediated gene flow The transfer of genetic information/trait(s) via seed movement and/or dissemination due to wind, water, humans, birds and animals, or mechanical means (e.g., shattering). Tolerance Survival of a population of a weed species following a herbicide dosage lethal to other species.
Abbreviations PMGF Pollen-mediated gene flow SMGF Seed-mediated gene flow
ACCase Acetyl-CoA carboxylase ALS Acetolactate synthase HPPD 4-Hydroxyphenylpyruvate dioxygenase
Introduction
Herbicide-Resistant Weeds
Agriculture is essential for providing food, feed, and fiber worldwide. Crop production is a type of agriculture that includes many different practices, depending on whether industrial or subsistence farming systems are used. As of 2015, more than seven billion people in the world rely on agriculture for the vast majority of their plant-derived necessities. Over the past 50 years, management of agricultural fields has been driven by economic returns and technical advancements in crop genetics and synthetic pesticides. Improvements in crop plants through biotechnology and genetic engineering are being taking place and being carefully assessed for their fit in specific production systems. For example, the cropping systems in North and South America over the past 20 years have been dominated by the development and widespread adoption of herbicide-resistant crops, promoting the use of limited herbicides, including glyphosate and glufosinate in glyphosate- and glufosinate-resistant crops, respectively. Although this technology has provided excellent weed control over the past several years, this intensive crop production system has become primarily dependent on herbicides for weed control. Most importantly, the loss or reduction in the utility of residual herbicides or herbicides with different modes of action is a concern to many involved in various aspects of weed control.
Weed resistance to herbicide is not unique. Resistance has been documented in several other species: for example, insects resistant to insecticides and bacteria resistant to antibiotics were documented well before weeds became resistant to herbicides. Since the first report of wild carrot (Daucus carota L.) resistant to 2,4-dichlorophenoxyacetic acid in 1957, resistance to herbicides has been reported in over 400 biotypes worldwide. About 10 new herbicide-resistant weed biotypes are reported each year. Of 400 biotypes, 30% (153 biotypes) are resistant to acetolactate synthase (ALS) inhibitors, 20% (72 biotypes) are resistant to photosystem II inhibitors, and 11% (46 biotypes) are resistant to acetyl-CoA carboxylase (ACCase) inhibitors. Thus, herbicides belonging to these three modes of action account for 60% of all herbicide-resistant weed biotypes. The newest type of herbicide resistance is resistance to glyphosate, the most used herbicide worldwide. As of 2015, 32 weed species worldwide have been reported to be resistant to glyphosate, including 15 species in the United States. Registered herbicides are classified into different groups based on their sites of action. The herbicide mode of action refers to the general biological process that a herbicide uses to disrupt plant growth, produce symptoms, and kill susceptible weeds. Sometimes, the ‘site of action’ is used interchangeably with the ‘mode of action,’ but specifically speaking, site of action refers
Encyclopedia of Applied Plant Sciences, 2nd edition, Volume 3
http://dx.doi.org/10.1016/B978-0-12-394807-6.00234-3
447
448
Weeds and Competition j Gene Flow and Herbicide Resistance
to the exact site or enzyme where a herbicide acts to disrupt the growth of plants. Knowledge of herbicide groups and modes of action is important because herbicide resistance evolves not only in response to repeated use of the same herbicide but also with the recurring use of herbicides belonging to the same mode of action. Thus, a rational and judicious use of available herbicides, along with other weed control methods, requires being aware of and selecting herbicides with different modes of action. Multiple herbicide resistance occurs when a weed species is no longer controlled by two or more herbicides with different modes of action applied at labeled rates, when previously these herbicides would have provided effective control at the same application rates. In other words, a weed species with multiple resistance has acquired resistance against herbicides belonging to different chemistries. This is due to overreliance or continuous use of two or more selected herbicides over time. Multiple resistance has been reported in several weed species. The resistance of Palmer amaranth (Amaranthus palmeri) to 4-hydroxyphenylpyruvate dioxygenase–inhibiting herbicides (mesotrione, tembotrione, and topramezone) and PS II–inhibiting herbicide (atrazine) has recently been confirmed in a continuous seed corn production field in Nebraska. Similarly, common waterhemp in Nebraska has been confirmed resistant to triazine, ALS inhibitors, and glyphosate. In fact, a population of common waterhemp in Illinois has even been confirmed resistant to four herbicides with different modes of action (atrazine, lactofen, ALS inhibitors, and glyphosate). In the future, if integrated weed management options are not adopted, expect complex weed resistance issues that will reduce herbicide options.
enhanced ability to metabolize a herbicide can potentially inactivate the herbicide before it can reach its site of action within the plant.
Mechanisms of Resistance: There are Five Mechanisms that Account for Herbicide Resistance
Gene Flow and Weed Resistance
Alterations in the Target Site of the Herbicide A herbicide has a specific site of action within the plant where it acts to disrupt a particular physiological process. If this target site is altered, then the herbicide is unable to kill the plant. Target site resistance is the result of a modification in the herbicide-binding site (usually an enzyme), which prevents a herbicide from binding effectively. If the herbicide cannot bind to the enzyme then it cannot inhibit the enzyme and the plant survives. Target site resistance is the most common resistance mechanism. Some believe in the natural selection theory, suggesting that herbicide-resistant weeds have always occurred at extremely low numbers within a particular weed species. When a herbicide effectively controls the majority of susceptible population of a species, only those plants that possess a resistance trait can survive and produce seed for future generations. This theory of resistance development has several parallels to Darwin’s theory of survival of the fittest, suggesting that plants less adapted do not survive, and only the fittest plants produce seed. The seeds from resistant biotypes ensure that the resistant trait is transferred to future generations. Thus, if the same herbicide is used repeatedly, the resistant biotypes continue to expand.
Enhanced Metabolism of the Herbicide Some plants may use their own metabolic activity to detoxify a foreign compound such as a herbicide. A weed with an
Compartmentation of the Herbicide Plants are capable of sequestering foreign compounds within their cells or tissues to prevent the compounds from causing harmful effects. When a herbicide is placed within a restricted compartment, it cannot reach its site of action and is thus unable to kill the plant. About 18 weed biotypes show resistance to the bipryridylium herbicides, paraquat and diquat. One of the proposed mechanisms for resistance is that the resistant biotypes restrict the movement of the herbicides and prevent the herbicides from reaching the target site.
Decreased Absorption and/or Translocation Decreased absorption and/or translocation can cause resistance because herbicide movement is restricted and the herbicide does not reach its target site of action in a concentration sufficient to cause death.
Gene Amplification/Overexpression Gene amplification/overexpression is the most recently described resistance mechanism, and causes resistance by increasing the production of the target enzyme, effectively diluting a herbicide in relation to the target site.
Gene flow via pollen and/or seed dispersal from resistant weeds may also provide a source of resistant genes in a susceptible population.
Pollen-Mediated Gene Flow Resistant genes may disperse naturally by pollen via wind, insects, or other pollinators. This process is known as pollen-mediated gene flow (PMGF). Several factors and their interactions influence the frequency and distance of PMGF, including reproductive biology of the weed species, pollen viability and longevity, flowering synchrony and pollen production, wind speed and direction, as well as type and presence of pollination vectors. Gene flow is a natural phenomenon not unique to weed species, but common to all flowering plants. However, after the evolution of herbicide-resistant weeds, PMGF is believed to be an important avenue for the spread of resistance within and between weed species. The dissemination of herbicideresistant genes through pollen is more common in weed species that are obligate outcrossing or dioecious (separate male and female plants), such as Palmer amaranth (A. palmeri S. Wats.) and common waterhemp (Amaranthus rudis).
Palmer Amaranth Palmer amaranth is a broadleaf, tall, upright, and dioecious summer annual weed native to northern Mexico, southern Arizona, and California. It was first identified by Sereno Watson
Weeds and Competition j Gene Flow and Herbicide Resistance in 1877 in California along the banks of the Rio Grande river. Palmer amaranth has since spread into the mid-south and southeastern United States, where it is a competitive weed in row crop production. Interference and competition from Palmer amaranth has significantly reduced growth and yield of corn, cotton, soybean, and peanut. The profitability of row crop production in the United States has greatly improved since the commercialization of glyphosate-resistant crop cultivars; however, the repeated use of glyphosate in glyphosateresistant crops has resulted in the evolution of glyphosateresistant Palmer amaranth, one of the most serious weeds in the southeastern and mid-south United States. Researchers have reported Palmer amaranth biotypes resistant to both glyphosate and ALS-inhibiting herbicides. Palmer amaranth is dioecious and pollinated by wind; therefore, glyphosate resistance can be spread by wind and the trait is highly mobile. Additionally, Palmer amaranth pollen grains are small, fairly smooth, settle slowly, and are more likely to be transported away from the paternal plant than larger or more highly ornamented pollen grains. Research has been conducted in Georgia to determine whether glyphosate-resistant trait can be transferred via PMGF from a glyphosate-resistant Palmer amaranth source to a glyphosate-susceptible sink. Glyphosateresistant Palmer amaranth plants were transplanted at the center of a 30-ha cotton field, and susceptible plants were transplanted into plots at a distance up to 300 m in each of the four cardinal and ordinal directions. At the end of the season, seeds were harvested from mature female glyphosate-susceptible plants. The seeds were grown and plants were treated with glyphosate when they were 5–7 cm tall. Results suggested that 50–60% of the offspring at 1 and 5 m distances were resistant to glyphosate, whereas 20–40% of the offspring were resistant at the furthest distances. Results from this study demonstrated that glyphosate-resistant Palmer amaranth could be dispersed up to 300 m under natural field conditions, and that the widespread occurrence of glyphosate-resistant Palmer amaranth is due in part to the movement of pollen between spatially segregated populations. A similar study is being conducted in Nebraska to determine PMGF from glyphosate-resistant common waterhemp to glyphosate-susceptible common waterhemp under field conditions, and transgene movement was detected at up to 50 m from the pollen source.
Lolium rigidum Weedy ryegrasses, Lolium rigidum Gaud. is an economically important weed that represents the most serious case of herbicide resistance in Australian cropping systems. It is an anemophilous, self-sterile, cross-pollinated species with the potential for prolific pollen production. Herbicide-resistant L. rigidum infests millions of hectares of southern Australian cropping systems. A field experiment at landscape level was conducted with herbicide-susceptible L. rigidum transplanted into a bushland environment at increasing distances from a large commercial field infested with herbicide-resistant L. rigidum. Herbicide resistance was used as a marker to quantify distance and frequency of PMGF. Results suggested that PMGF occurred at 3000 m, maximum tested distance, from the pollen source. Thus, longdistance PMGF is possible in L. rigidum and can spread herbicide-resistant traits across fields.
449
Seed-Mediated Gene Flow In addition to pollen, resistance genes may also be spread by seeds. Seed dispersal among weed populations can occur through seed shattering and natural vectors, such as wind, water, insect herbivores, or mammals, as well as by humans. Growers often neglect the cleaning of farm machinery after operation and spread herbicide-resistant weed seeds during seeding, tillage, and/or harvesting operations in different fields. Understanding the contribution of gene mutation versus gene flow by pollen and/or seed dispersal is important because it determines the success of different management practices (e.g., reducing the intensity of herbicide selection versus limiting the movement of resistant seeds) in preventing the spread of resistance to adjacent fields, farms, and across agricultural landscapes. In general, seed traits that encourage dispersal from the mother plant help seeds stay airborne or catch the fur of animals, or hard seed coat to survive the digestive tracts of animals would be more subject to spread by seed movement. Additionally, the seeds of certain weed species mimic the seeds of crops, thus aiding in their dispersal by humans. Several weed seeds also have the ability to persist in the soil by remaining dormant for several years before germinating, further increasing the chances of seed-mediated gene flow. Finally, resistant genes in highly self-pollinated weed species such as horseweed (Conyza bonariensis (L.) Cronq.) and common lambsquarters (Chenopodium album L.), are primarily spread by the movement of seeds.
See also: Plants and the Environment: Invasive Plant Species. Weeds and Competition: Integrated Weed Management; Thermal Weed Control.
Further Reading Beckie, H.J., Hall, L.M., 2008. Simple to complex: modeling crop pollen-mediated gene flow. Plant Sci. 175, 615–628. Busi, R., Yu, Q., Barrett-Lennard, R., Powles, S., 2008. Long distance pollen-mediated flow of herbicide-resistance genes in Lolium rigidum. Theor. Appl. Genet. 117, 1281–1290. Jhala, A.J., Bhatt, H., Topinka, K., Hall, L.M., 2011. Pollen-mediated gene flow in flax (Linum usitatissimum L.): can genetically engineered and organic flax coexist? Heredity 106, 557–566. Knezevic, S., 2007. Herbicide tolerant crops: 10 years later. Maydica 52, 245–250. Mallory-Smith, C., Zapiola, M., 2008. Gene flow from glyphosate-resistant crops. Pest Manage. Sci. 64, 428–440. Poppy, G.M., Wilkinson, M.J., 2005. Gene Flow from GM Plants. Blackwell Publishing Ltd, Oxford, UK. Sosnoskie, L.M., Webster, T.M., Kichler, J.M., et al., 2012. Pollen-mediated dispersal of glyphosate-resistance in Palmer amaranth under field conditions. Weed Sci. 60, 366–373.
Relevant Websites http://www.gmo-compass.org/eng/home/ – GMO Compass: A website supported by the European Union within the European Commission’s 6th Framework Program. http://www.hracglobal.com/ – Herbicide Resistance Action Committee. http://www.isaaa.org/ – International Service for the Acquisition of Agri-biotech Applications (ISAAA). http://weedscience.org/ – International Survey of Herbicide Resistant Weeds. http://takeactiononweeds.com/ – Take Action on Weeds. http://wssa.net/ – Weed Science Society of America.