Physiological Mechanisms of Glyphosate Resistance

Abstract

Glyphosate, a nonselective herbicide and also the world's most widely used herbicide, inhibits 5-enol-pyruvylshikimate-3-phosphate synthase (EPSPS), an enzyme in the aromatic amino acid biosynthetic pathway. Because of its broad-spectrum and potent weed control and favorable environmental characteristics, attempts to engineer glyphosate resistance have been intensive in the past few decades. The use of at least three different mechanisms has conferred glyphosate resistance in normally sensitive crop species. Early work focused on progressive adaptation of cultured plant cells to stepwise increases in glyphosate concentrations. The resulting cells were resistant to glyphosate because of EPSPS overexpression, EPSPS gene amplification, or increased enzyme stability. Further work aimed to achieve resistance by transforming plants with glyphosate metabolism genes. An enzyme from a soil microorganism, glyphosate oxidoreductase (GOX), cleaves the nitrogen– carbon bond in glyphosate yielding aminomethylphosphonic acid. Another metabolism gene, glyphosateN-acetyl transferase (gat), acetylates and deactivates glyphosate. A third mechanism, and the one found in all currently commercial glyphosate-resistant crops, is the insertion of a glyphosate-resistant form of the EPSPS enzyme. Several researchers have used site-directed mutagenesis or amino acid substitutions of EPSPS. However, the most glyphosate-resistant EPSPS enzyme to date has been isolated fromAgrobacteriumspp. strain CP4 and gives high levels of resistance in planta. Weeds resistant to glyphosate have offered further physiological mechanisms for glyphosate resistance. Resistant field bindweed had higher levels of 3-deoxy-d-arbino-heptulosonate 7-phosphate synthase, the first enzyme in the shikimate pathway, suggesting that increased carbon flow through the shikimate pathway can provide glyphosate resistance. Resistant goosegrass has reduced translocation of glyphosate out of the treated area. Although glyphosate resistance has been achieved by numerous mechanisms, currently the only independent physiological mechanism to give adequate and stable resistance to glyphosate for commercialization of glyphosate-resistant crops has been glyphosate-resistant forms of EPSPS.

Glyphosate influence on the physiological parameters of Conyza bonariensis biotypes

“…The increased activity of the 3-deoxy-D-arbino heptulosonate 7-phosphate synthase enzyme (DAHP) and the production of high concentrations of phenolic compounds and aromatic amino acids may allow the resistant biotype's metabolism to remain active for longer than susceptible biotypes (Pline-Srnic, 2006). However, this increased metabolism is reduced at 7 DAH for both biotypes; for the resistant biotype, there is a tendency of maintaining photosynthesis and transpiration until 14 DAH (Figure 1).…”

Gene amplification confers glyphosate resistance in Amaranthus palmeri

“…The most common glyphosate resistance mechanism selected in plant cell culture is increased EPSPS activity, typically due to gene amplification (12). There is evidence for enhanced EPSPS expression in glyphosate-resistant weeds, but no previous evidence for EPSPS gene amplification.…”

“…In crop species, resistance to glyphosate has been conferred by expression of bacterial genes that metabolize glyphosate (12), overexpression of sensitive EPSPS, expression of glyphosateresistant EPSPS from bacteria, and expression of glyphosateresistant plant EPSPS containing one or more target-site mutations (13). After step-wise glyphosate selection, EPSPS gene amplification has occurred in plant cell lines, resulting in glyphosate resistance in cell culture (12).…”

Multivariate approach in the selection of superior soybean progeny which carry the RR gene

“…Chemical control of weeds is an excellent strategy due to its practicality, efficiency and speed (GAZZIERO; VARGAS; ROMAN, 2004). Glyphosate, a non-selective herbicide, is widely used around the world, being highly effective in controlling weeds (PLINE-SRNIC, 2006). …”