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As of 2015, 26 plant species have been genetically modified and approved for commercial release in at least one country. The majority of these species contain genes that make them either tolerant to herbicides or resistant to insects. Other common traits include virus resistance, delayed ripening, modified flower colour or altered composition.
One major complication to the use of herbicides for weed control is the ability of plants to evolve herbicide resistance, rendering the herbicides ineffective against target plants. Out of 31 known herbicide modes of action, weeds have evolved resistance to 21. 268 plant species are known to have evolved herbicide resistance at least once. [59]
For example, the system of planting glyphosate-resistant seed and then applying glyphosate once plants emerged provided farmers with the opportunity to dramatically increase the yield from a given plot of land, since this allowed them to plant rows closer together.
Weeds resistant to multiple herbicide modes of action are also on the rise. [7] Before glyphosate, most herbicides would kill a limited number of weed species, forcing farmers to continually rotate their crops and herbicides to prevent resistance. Glyphosate disrupts the ability of most plants to construct new proteins.
A weed that develops resistance to one herbicide typically has resistance to other herbicides with the same mode of action (MoA), so herbicides with different MoAs, or different resistance groups, are needed. Preventative weed resistance management rotates herbicide types to prevent selective breeding of resistance to the same mode of action.
The majority of GM crops have been modified to be resistant to selected herbicides, usually a glyphosate or glufosinate based one. Genetically modified crops engineered to resist herbicides are now more available than conventionally bred resistant varieties; [20] in the USA 93% of soybeans and most of the GM maize grown is glyphosate tolerant. [21]
The version used in genetically modified crops was isolated from Agrobacterium strain CP4 (CP4 EPSPS) that was resistant to glyphosate. [15] [16] The CP4 EPSPS gene was cloned and inserted into soybeans. The CP4 EPSPS gene was engineered for plant expression by fusing the 5' end of the gene to a chloroplast transit peptide derived from the ...
For herbicide resistant plants the EPA co-ordinates with the USDA and FDA, but does not regulate the plant itself. Instead it regulates the herbicide and its use on the new cultivar. [14] The EPA examines the construct used to transform the plant and the biology of recipient plant. The sequence of the resulting protein must be described ...