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Further research is needed to examine which glyphosate-resistant crops and weeds may have acquired enhanced levels of gene Epsps and, if so, the extent to which overproduction of this gene affects crop yields and weed fitness
Plant-derived epsps genes are known to have the same origin and function as aroA genes in microbes. For a few years, starting in 1998, Monsanto sold a type of RoundUp Ready® corn (Zea mays) known as Event GA21 with a modified epsps gene from corn linked to a constitutive actin promoter from rice (Oryza sativa). The modified EPSPS protein of GA21 had a low affinity for glyphosate and differed from the native EPSPS protein by only two amino acids, including a mutation at amino acid 106, which has been reported in some glyphosate-resistant weeds
Crop genes that confer herbicide resistance are expected to become widespread in weedy rice populations if continued herbicide selection pressure purges nonresistant genotypes from these populations. Therefore, understanding the fitness costs or benefits of a transgene encoding gene Epsps and glyphosate resistance in weedy rice is important for assessing potential ecological and agronomic consequences, especially if the transgene also confers a fitness advantage in the absence of glyphosate.
Researchers tested for such effects using crop–weed hybrid progeny derived from a GE cultivar and four populations of weedy rice in regulated field experiments in China. These crop–weed hybrid lineages segregated for the presence or absence of the novel epsps transgene, and transgenic progeny exhibited very strong and consistent increases in plant growth and fecundity compared with their nontransgenic counterparts, without exposure to glyphosate. For brevity, below we refer to the transgene-positive plants as GE and nontransgenic controls as non-GE.
Originally, this epsps transgene was developed to confer resistance to glyphosate, but it also appears to provide profound and previously unrecognized pleiotropic benefits for plant growth and seed production. Given the consistency of our results, we assume that the overproduction of EPSPS and the downstream differences that we observed between GE plants and their non-GE counterparts were attributable to the over-expression of the modified transgene, epsps, rather than other tightly linked genes from the cultivated parent, although this possibility cannot be ruled out entirely. In the F2 and F3 generations, the only difference between GE and non-GE crop–weed progeny was the presence or absence of the inserted epsps construct, as well as the selectable marker gene and any crop-specific genes that may be linked to the transgene insertion site. The F1 generation included progeny from two very similar crop donors, one GE and the other not, and very similar benefits were associated with the novel transgene compared with the F2 and F3 generations.
Because the shikimic acid pathway affects carbon flow and biosynthesis, the over-expression of epsps may have many interrelated effects on plant growth and development. It is noteworthy that the GE plants in our study also had increased concentrations of the key amino acid Trp, which is a precursor of the growth hormone IAA and many other compounds. Other products and secondary metabolites were probably also affected, but were not measured here. Rsearchers also found that GE F3 plants had greater photosynthetic rates in the glasshouse, and increased levels of seed germination in a growth chamber experiment compared with non-GE controls.
Results from field experiments further demonstrate beneficial effects of the epsps transgene.
During two growing seasons, researchers found enhanced production of tillers, panicles, and seeds in transgenic segregants, probably as a result of the cascading effects of overproduction of Epsps. Remarkably, the epsps transgene was associated with increases of 48–57% in total seeds per plant of F2 crop–weed lineages when grown in pure cultivation; even greater benefits of the transgene were seen in the mixed cultivation plots.
In the near future, researchers expect that transgenic crop species that over-express the epsps gene will be developed commercially to obtain glyphosate resistance and/or improved agronomic performance.
Although pollen- and seed-mediated gene flow can have rapid and direct evolutionary effects on weedy relatives of crop plants, a more widespread agronomic problem is the fact that at least 24 weed species have spontaneously evolved resistance to glyphosate in recent decades. In Amaranthus palmeri, for example, resistance is attributable to increased Epsps expression resulting from multiple copies of the epsps gene, known as gene amplification; extra copies of the epsps gene were found on every chromosome. A similar gene amplification mechanism for overproduction of Epsps was reported in Lolium perenne ssp. Multiflorum. Increased Epsps expression in resistant biotypes could have a fitness penalty, but it seems possible that some resistant biotypes could have enhanced fitness, similar to the results reported here. Clearly, both scenarios deserve further investigation.
In conclusion, the results of this study suggest that the overproduction of Epsps can significantly increase plant growth and seed production in crop–weed hybrids of rice. To our knowledge, this is one of the first demonstrations of a strong fitness benefit associated with resistance to a herbicide The findings have broad implications for plant biology, crop breeding, weed management, biotech risk assessment, and the ongoing evolution of herbicide-resistant weeds. Further research is needed to examine which glyphosate-resistant crops and weeds may have acquired enhanced levels of Epspsand, if so, the extent to which overproduction of Epsps affects crop yields and weed fitness.
by R. T.
02 september 2013, Technical Area > Science News