What Methods Are Used to Help Plants Protect Themselves Against Pests?

Pesticides, used to control plant pests, are needed because plants cannot move to avoid pests. Although useful in some cases, pesticides (i.e., herbicides, insecticides, and fungicides) are costly to the farmer and can be damaging to the environment and to humans. Herbicides must distinguish between desirable crop plants and undesirable weedy species, for example between commercial rice and red rice (both Oryza sativa); the latter is the most troublesome weedy species in many rice-growing regions of the world (1).

Crop tolerance to herbicides is achieved (a) by mutations that render a plant not susceptible to the herbicide or (b) through the introduction of transgenes. An example of the first approach was the identification of varieties that, after treatment with a chemical mutagen, were tolerant to imidazolinone herbicides; these Clearfield® varieties (2) are tolerant to herbicides, such as Pursuit® and Raptor®. In the second approach to be commercialized, GE crops, tolerant to glyphosate or Roundup®, were engineered with a bacterial gene encoding a target enzyme, 5-enol-pyruvylshikimate-3- phosphate synthase (EPSPS), which confers tolerance to the herbicide (3). More recently, other commercialized HT GE crops were created with tolerance to glufosinate or Liberty® by introducing phosphinothricin-Nacetyltransferase (pat) or bialaphos resistance (bar) genes from Streptomyces sp. that encode enzymes that detoxify the herbicide’s active ingredient (4). The leading commercialized insect-tolerant GE crops have genes from the soil bacterium Bacillus thuringiensis, which encode pesticidal Cry proteins that protect the plant against specific insect pests (see “Will the Widespread Use of Bt Crops Lead to the Development of Insect Resistance to Bt?“).  

The first GE HT crops, cotton, corn, and soybean, have been grown commercially in the United States since 1995 (5). In 1996 HT soybean comprised 7% of total U.S. soybean acreage, compared with 92% in 2008 (Table 1; 6). HT soybeans and cotton are the most widely and rapidly adopted GE crops in the United States, followed by insect-resistant Bt cotton and corn, which were also approved for commercial production in 1995. In 1996, Bt cotton was estimated to compose 15% of U.S. cotton acreage or 1.8 million acres (0.73 million hectares) (7), and Bt corn was grown on approximately 1% of the U.S. corn acreage (Table 1; 8, 9). Since 1996, both Bt corn and cotton crops have been widely adopted, and, as individual traits, represent 17% (10) and 18% (7), respectively, of cultivated U.S. acreage in 2008. However, these percentages represent varieties with individual traits and account for only part of the adoption because of stacked traits, i.e., introducing HT and Bt traits in the same plant (see “Will the Widespread Use of Bt Crops Lead to the Development of Insect Resistance to Bt? “).  In 2008 stacked varieties of corn made up 40% of acreage and stacked varieties of cotton comprised 45% of acreage; in combination with individual traits this adoption accounts for 80%of corn and 86% of cotton (Table 1). No stacked traits presently exist in commercial soybean varieties.

References:

1. Webster TM. 2000. The southern states 10 most common and troublesome weeds in rice. Proc. South Weed Sci. Soc. 53:247–74      

2. Croughan TP. 2005. Resistance to aminoacetohydroxyacid synthase-inhibiting herbicides. U.S. Patent No. 50,198,705       

3. Barry G, Kishore G, Padgette S, Taylor M, Kolacz K, et al. 1992. Inhibitors of amino acid biosynthesis: strategies for imparting glyphosate tolerance to crop plants. In Biosynthesis and Molecular Regulation of Amino Acids in Plants, ed. BK Singh, HE Flores, JC Shannon, pp. 139–45. Madison, WI: Am. Soc. Plant Physiol.     

4. Wehrmann A, Van Vliet A, Opsomer C, Botterman J, Schulz A. 1996. The similarities of bar and pat gene products make them equally applicable for plant engineers. Nat. Biotechnol. 14:1274–78      

5. Center for Environmental Risk Assessment (CERA). 2011. GM Crops Database http://www.cera-gmc.org/?action=gm_crop_database. Last accessed 2011-11-25. PDF
Database for querying safety information on genetically engineered plants and plants with novel traits produced using accelerated mutagenesis and plant breeding.

6. USDA Econ. Res. Serv. 2008. Adoption of genetically engineered crops in the U.S.: Soybean varieties. http://www.ers.usda.gov/data/biotechcrops/extentofadoptiontable3.htm. Last accessed 2011-12-9. PDF

7. USDA Econ. Res. Serv. 2008. Adoption of genetically engineered crops in the U.S.: Upland cotton varieties. http://www.ers.usda.gov/data/biotechcrops/ExtentofAdoptionTable2.htm. Last accessed 2011-12-9. PDF

8. Carpenter J, Felsot A, Goode T, Hammig M, Onstad D, Sankula S. 2002. Comparative environmental impacts of biotechnology-derived and traditional soybean, corn, and cotton crops. Counc. Agric. Sci. Technol. (CAST) June:1–189      

9. Fernandez-Cornejo J, McBride WD. 2002. Adoption of bioengineered crops. USDA Econ. Res. Serv., Agric. Econ. Rep. No. 810. http://www.ers.usda.gov/publications/aer810/. Last accessed 2011-12-9. PDF

10. USDA Econ. Res. Serv. 2008. Adoption of genetically engineered crops in the U.S.: Corn varieties. http://www.ers.usda.gov/data/biotechcrops/extentofadoptiontable1.htm. Last accessed 2011-12-9. PDF

Updated 2/16/12