**Gene Flow from Herbicide-Resistant Crops to Wild Relatives Relatives**

**Gene Flow from Herbicide-Resistant Crops to Wild** 

Sava Vrbničanin, Dragana Božić and

Sava Vrbničanin, Dragana Božić and Danijela Pavlović Danijela Pavlović Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67645

#### **Abstract**

Development of herbicide-resistant (HR) crops is way to overcome problems in weed control due to weed resistance to herbicides and absence of new herbicides with a new mode of action for their control. Three types of HR crops were developed: nontransgenic, transgenic, and multiple HR crops. Cultivation of HR crops is associated not only with many benefits (simplification of weed control, more effective and efficient weed control, higher yields, etc.) but also with various risks (development of HR weeds, development of HR volunteer crops, gene flow from HR crops to susceptible relatives, etc.). The greatest risk is gene flow from HR crops to related weed species, wild relatives or conventional crops of the same species. Unwanted gene flow could be prevented or reduced using different barriers such as isolation in space or time, protective vegetation barriers, male sterility, etc. Sunflower hybrids resistant to herbicides (imidazolinones and sulfonilureas) was developed by conventional breeding methods, and their introduction in Serbian fields has enabled a more efficient control of harmful weed species, but the presence of huge populations of weedy sunflower is the main concern associated with their cultivation, because numerous studies have confirmed gene flow from sunflower to its relatives.

**Keywords:** gene flow, herbicide resistant crops, wild relatives

## **1. Introduction**

The main aim of plant breeding is creating new varieties and hybrids, which would enable us to overcome different problems of contemporary agriculture and achieve high yields and productivity. Research in the fields of molecular genetics, biochemistry, and physiology is leading to development of plants with additional agronomic properties, such as herbicide

resistance, pathogen and pest resistance, salt and dryness tolerance, certain food quality parameters, etc. [1–4]. The predominant resistances used in crops are herbicide resistance, both in nontransgenic and in transgenic crops. Owing to the novel insights into the mechanisms and site of action of herbicides on a molecular level, and the development of new biotechnology methods, breeding of herbicide-resistant (HR) crops has been enabled. Thanks to that it is possible to use herbicides, which are preferable from agronomic, environmental, or genetic viewpoint. This new biotechnology gives many benefits in food production such as higher yield through high efficiency of weed control, less unit cost of food production, better quality through removal of existing volunteers of the some species, the possibility of using low-tillage systems, etc. But, this new biotechnology also has some disadvantages such as development herbicide-resistant weed species due to high selection pressure, potential for development of herbicide-resistant volunteer crops, risks of cross-pollination and gene flow from resistant to susceptible relatives, etc.

The focus of this chapter is review of risks associated with HR crops growing with special attention on gene flow from crops to their wild relatives. We first discuss development of HR crops and technologies of weed control based on resistant crops. Also, we briefly discuss gene flow from HR crops to their wild relatives and barriers, which can prevent it. Finally, we discuss transfer of genes responsible for resistance from sunflower hybrids (present resistant crop in Serbia and in Europe) to wild sunflower forms.

### **2. Herbicide-resistant crops**

Discovery of new herbicides, especially with a new mode of action is difficult and expensive. During the last few decades, no one herbicide with novel site of action was found and there are no expectations for its appearance in the near future [5, 6]. One way to overcome this problem was development HR crops, which provide expanding the utility of existing herbicides and improve weed control with them. The study on developing HR crops started soon after the discovery of first herbicide-resistant weeds [7, 8]. These type of crops are designed to tolerate specific broad-spectrum herbicides, which kill the surrounding weeds, but leave the cultivated crop intact. There were two directions in HR crops development, which resulted with two groups of crops: transgenic (genetically modified, GM) and nontransgenic HR crops. The first nontransgenic program for HR crops breeding transferred resistance to herbicide triazines from a *Brassica rapa* to canola [9]. Although several triazine-resistant canola varieties were developed, farmer interest for these varieties was poor due to pleiotropic effects of mutation responsible for resistance, which caused lower yielding and poorer seedling vigor [10]. Also, nontransgenic methods like whole-cell selection, mutagenesis, and plant selection from natural populations have been used for breeding of crops resistant to sulfonylurea, sethoxydim, and imidazolinone herbicides. At the same period (1980s), tools for producing transgenic crops were becoming available and many companies start to work on their development. Bromoxynil-resistant cotton was one of the first transgenic HR crops available to farmers in 1995 [11], followed by glyphosate-resistant maize, canola, cotton, soybean, and other crops known as "Roundup Ready" crops. After period of effective, simple, and inexpensive weed management with cultivation glyphosate-resistant crops, glyphosate-resistant weeds becoming a problem in weed control, which increase the use of crops resistant to glufosinate [12], followed by initiation of new approach in HR crops development, which was based on building of multiple resistance in crop plants.

resistance, pathogen and pest resistance, salt and dryness tolerance, certain food quality parameters, etc. [1–4]. The predominant resistances used in crops are herbicide resistance, both in nontransgenic and in transgenic crops. Owing to the novel insights into the mechanisms and site of action of herbicides on a molecular level, and the development of new biotechnology methods, breeding of herbicide-resistant (HR) crops has been enabled. Thanks to that it is possible to use herbicides, which are preferable from agronomic, environmental, or genetic viewpoint. This new biotechnology gives many benefits in food production such as higher yield through high efficiency of weed control, less unit cost of food production, better quality through removal of existing volunteers of the some species, the possibility of using low-tillage systems, etc. But, this new biotechnology also has some disadvantages such as development herbicide-resistant weed species due to high selection pressure, potential for development of herbicide-resistant volunteer crops, risks of cross-pollination and gene flow

The focus of this chapter is review of risks associated with HR crops growing with special attention on gene flow from crops to their wild relatives. We first discuss development of HR crops and technologies of weed control based on resistant crops. Also, we briefly discuss gene flow from HR crops to their wild relatives and barriers, which can prevent it. Finally, we discuss transfer of genes responsible for resistance from sunflower hybrids (present resistant

Discovery of new herbicides, especially with a new mode of action is difficult and expensive. During the last few decades, no one herbicide with novel site of action was found and there are no expectations for its appearance in the near future [5, 6]. One way to overcome this problem was development HR crops, which provide expanding the utility of existing herbicides and improve weed control with them. The study on developing HR crops started soon after the discovery of first herbicide-resistant weeds [7, 8]. These type of crops are designed to tolerate specific broad-spectrum herbicides, which kill the surrounding weeds, but leave the cultivated crop intact. There were two directions in HR crops development, which resulted with two groups of crops: transgenic (genetically modified, GM) and nontransgenic HR crops. The first nontransgenic program for HR crops breeding transferred resistance to herbicide triazines from a *Brassica rapa* to canola [9]. Although several triazine-resistant canola varieties were developed, farmer interest for these varieties was poor due to pleiotropic effects of mutation responsible for resistance, which caused lower yielding and poorer seedling vigor [10]. Also, nontransgenic methods like whole-cell selection, mutagenesis, and plant selection from natural populations have been used for breeding of crops resistant to sulfonylurea, sethoxydim, and imidazolinone herbicides. At the same period (1980s), tools for producing transgenic crops were becoming available and many companies start to work on their development. Bromoxynil-resistant cotton was one of the first transgenic HR crops available to farmers in 1995 [11], followed by glyphosate-resistant maize, canola, cotton, soybean, and other crops known as "Roundup Ready" crops. After period of effective, simple, and inexpensive weed

from resistant to susceptible relatives, etc.

38 Herbicide Resistance in Weeds and Crops

**2. Herbicide-resistant crops**

crop in Serbia and in Europe) to wild sunflower forms.

Significant number of crop plants resistant to different ALS (acetolactate synthase; also known as AHAS—acetohydroxyacid synthase) inhibiting herbicides were developed using conventional breeding methods (**Table 1**). These groups of herbicides have very good characteristics for utilization in weed control in HR crops, which include low use rates, broad spectrum weed control, low mammalian toxicity and environmental compatibility. Immediately after discovery of this group of herbicides, ALS resistant tobacco and maize lines were developed using tissue culture selection [13, 14], while ALS-resistant soybean developed using mutagenesis [15].


**Table 1.** Some commercialized HR crops, modified from reference [20].

After that, three technologies of weed control, which include crop resistance to this group of herbicides, were developed. The Clearfield® and the Clearfield Plus® system have been developed with the aim to grow crops resistant to IMI herbicides [16], while ExpresSun® system has been developed with the aim to grow sunflower hybrids resistant to tribenuron-methyl [17]. As there is no "alien" genes introduced into these crops, this group of HR crops is not considered as transgenic and has been accepted in countries where the cultivation of GM crops is prohibited [16], like many European countries, as well as in Serbia.

Transgenic (GM) crops developed based on the use of different transgenes, mainly responsible for resistance to glyphosate, which introduced into many crop species (**Table 1**). These crops became popular thanks to simplification of weed control and reduction of production costs, making the crop more profitable. Between more than a hundred GM products, which have been authorized for commercialization only 13 are crops [18]. The main GM crops are maize, soybean, cotton, and rapeseed, which grow on more than 90 million ha distributed in 14 countries in which these crops have been authorized [19]. These crops are grown in America, Australia, China, South Africa, but distribution is the highest in the USA, where it covers more than 49.8 million ha [19]. In Europe, GM crops (maize, rapeseed, endive, soybean, and flowers) adopted for the production and/or consumption only in few countries, between which Spain is major producer, growing GM maize on more than 100,000 ha [19].

New approach in development of HR crops is technology, which combines glyphosate resistance with resistance to other herbicides resulting in multiple HR crops (**Table 2**). This technology developed with the aim to overcome increasing development of multiple HR weeds and based on engineering crops that are able to express multiple HR traits and tolerate multiple herbicides. This new concept using stacked (contains more than one transgene) genes as a tool for postoccurrence and future resistance management is the equivalent to using a single herbicide in case when weed is already resistant to one member of a dual stack [21]. Appropriate transgene stacks should delay resistance longer than approach, which use each component separately and sequentially because each weed resistant to either herbicide will be killed by the other herbicide in the stack. However, that stacking multiple HR into crops may or may not delay the evolution of herbicide resistance because effectiveness of the transgene stacks depends on the management decisions


**Table 2.** Multiple HR crops under development [20].

and adoption of the accompanying stewardship programs [21]. Namely, it depends on the effectiveness of each included herbicide in control of each target weed species. Some soybean multiple resistant cultivars have recently been approved for commercial use, such as cultivars resistant to glyphosate, glufosinate, and 2,4-D, as well as resistant to glyphosate and dicamba [21]. Except that it is possible to develop stacks of transgenes for different traits. For example, maize containing transgenes for resistance to insects and to herbicides is commercialized [22].

After that, three technologies of weed control, which include crop resistance to this group of herbicides, were developed. The Clearfield® and the Clearfield Plus® system have been developed with the aim to grow crops resistant to IMI herbicides [16], while ExpresSun® system has been developed with the aim to grow sunflower hybrids resistant to tribenuron-methyl [17]. As there is no "alien" genes introduced into these crops, this group of HR crops is not considered as transgenic and has been accepted in countries where the cultivation of GM crops is

Transgenic (GM) crops developed based on the use of different transgenes, mainly responsible for resistance to glyphosate, which introduced into many crop species (**Table 1**). These crops became popular thanks to simplification of weed control and reduction of production costs, making the crop more profitable. Between more than a hundred GM products, which have been authorized for commercialization only 13 are crops [18]. The main GM crops are maize, soybean, cotton, and rapeseed, which grow on more than 90 million ha distributed in 14 countries in which these crops have been authorized [19]. These crops are grown in America, Australia, China, South Africa, but distribution is the highest in the USA, where it covers more than 49.8 million ha [19]. In Europe, GM crops (maize, rapeseed, endive, soybean, and flowers) adopted for the production and/or consumption only in few countries, between which Spain is major producer, growing GM

New approach in development of HR crops is technology, which combines glyphosate resistance with resistance to other herbicides resulting in multiple HR crops (**Table 2**). This technology developed with the aim to overcome increasing development of multiple HR weeds and based on engineering crops that are able to express multiple HR traits and tolerate multiple herbicides. This new concept using stacked (contains more than one transgene) genes as a tool for postoccurrence and future resistance management is the equivalent to using a single herbicide in case when weed is already resistant to one member of a dual stack [21]. Appropriate transgene stacks should delay resistance longer than approach, which use each component separately and sequentially because each weed resistant to either herbicide will be killed by the other herbicide in the stack. However, that stacking multiple HR into crops may or may not delay the evolution of herbicide resistance because effectiveness of the transgene stacks depends on the management decisions

prohibited [16], like many European countries, as well as in Serbia.

**Herbicide types Crops**

Glyphosate, glufosinate, 2,4-D and ACCase inhibitors Maize

**Table 2.** Multiple HR crops under development [20].

Glyphosate and glufosinate Soybean, maize, cotton

Glyphosate and ALS inhibitors Soybean, maize Glyphosate, glufosinate and 2,4-D analogs Soybean, cotton Glyphosate, glufosinate and dicamba Soybean, cotton Glyphosate, glufosinate and HPPD inhibitors Soybean, cotton

maize on more than 100,000 ha [19].

40 Herbicide Resistance in Weeds and Crops
