Engineering Resistance to Herbicides

Abstract

Crop resistance can be generated through a selection process or the introduction of a gene via transformation. We will refer to the latter as engineered herbicide resistance. The literature contains numerous studies on engineered plant resistance to various herbicide classes. There is a large distinction between demonstrating a resistance phenotype in plants versus developing a commercial resistant crop that requires consistent performance under varying environmental conditions. Creating an herbicide resistant crop must successfully navigate a maze of key technical issues while satisfying the practical product goals of agronomic equivalence, weed management, economic feasibility, governmental regulatory compliance and public acceptance. Most of our experience has been derived from developing crop resistance to glyphosate. In this review, we will utilise these experiences to describe the essential studies required for the development of a commercial herbicide‐resistant crop with emphasis on the development of glyphosate resistance.

Evolution of a Double Amino Acid Substitution in the 5-Enolpyruvylshikimate-3-Phosphate Synthase in Eleusine indica Conferring High-Level Glyphosate Resistance

“…We have fitness studies underway with the wild-type, P106S (rr), and TIPS (RR, Rr) EPSPS mutants. However, if this fitness cost from decreased catalytic efficiency is offset, for instance, by gene duplication of the TIPS gene as required for commercial crops (CaJacob et al, 2004), then evolution of Roundup Ready-like E. indica may be expected in nature, especially in species exhibiting EPSPS gene amplification (for review, see Sammons and Gaines, 2014), where the tandem repeat nature of the duplication (Jugulam et al, 2014) may facilitate incorporation of the necessary point mutations, and gene duplication is free of fitness cost (Vila-Aiub et al, 2014). Therefore, the evolutionary recipe to high-level glyphosate resistance in weedy plant species under glyphosate selection may have these primary components:…”

Recent Advancements in Gene Expression and Enabling Technologies in Crop Plants

“…Glyphosate is absorbed readily through plant surfaces (Kirkwood et al 2000). Once taken up by plant leaves, the unique physicochemical properties of glyphosate allow its systemic translocation via the phloem, in a manner similar to that of the photoassimilate sucrose, to rapidly growing and stronger metabolic sink tissue (CaJacob et al 2004). Some early work indicated that most crops and weed species lack endogenous mechanisms for catabolism or detoxification of glyphosate, although a number of plant and weed species, mostly legume, are known to have biochemical activities that could convert glyphosate into less toxic compounds such as aminomethylphosphonic acid (AMPA; Reddy et al 2004Reddy et al , 2008.…”

“…Two basic strategies have been used to engineer GT crops, (a) expression of a transgene encoding a glyphosate-insensitive target enzyme, and (b) expression of transgene-encoding enzymes for glyphosate deactivation or detoxification (CaJacob et al 2004;Feng et al 2010a;Pollegioni et al 2011). The discovery of the chloroplast-localized enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EP-SPS) as the sole target of glyphosate (Steinrücken and Amrhein 1980) offered an opportunity for engineering GT crops through glyphosate-insensitive EPSPSs.…”

“…CP4 (known as the cp4-epsps gene) was isolated from the waste stream of a glyphosate-manufacturing site. CP4-EPSPS is a unique class II enzyme that is highly insensitive to glyphosate, but maintains a normal affinity for PEP (CaJacob et al 2004;Barry et al 1992), thereby ensuring continued production of essential aromatic amino acids in GE crops transformed to contain CP4-EPSPS. The first-generation GT soybean, Roundup Ready® soybean (Event 40-3-2), expressed a cp4-epsps transgene, and was commercialized in 1996.…”

“…The first-generation GT soybean, Roundup Ready® soybean (Event 40-3-2), expressed a cp4-epsps transgene, and was commercialized in 1996. The cp4-epsps gene has been the most commonly used transgene in engineering GT crops by the insensitive-EPSPS approach, due to its unique properties (CaJacob et al 2004). Other first generation GT crops that were generated by plant expression of CP4-EPSPS include Roundup Ready® cotton (event MON1445), Roundup Ready® canola (event RT73), Roundup Ready® sugar beet (event H7-1), and Roundup Ready® alfalfa (events J101 and J163; CaJacob et al 2004;Feng et al 2010a;Green 2012).…”