The limited stores of phosphorous (P) here on earth have been a concern for the agricultural industry for many years. Consequently, a great deal of research is being done to find ways to use P more efficiently. One proposed system for managing P use in agricultural crops is to genetically engineer them to use a form of P that weeds and common bacteria cannot consume, or even find poisonous (López-Arredondo and Herrera-Estrella 2012). Not only would such a mechanism ensure that almost all P applied to agricultural crops is used by the desired crop, but also noxious weeds would be stunted and potentially killed by the same chemical. Numerous benefits may be reaped if these modified crops can be implemented, including reduced fertilizer and pesticide use, lower food prices, and a smaller chemical load contaminating aquatic ecosystems.—Chad Redman
López-Arredondo, D. L., Herrera-Estrella, L., 2012. Engineering Phosphorus Metabolism in Plants to Produce a Dual Fertilization and Weed Control System. Nature Biotechnology 30, 889–895.
López-Arredondo and Herrera-Estrella set out to genetically modify plants, giving them the capability of using a form of P that most plants are incapable to metabolizing. For now, both crop plants and weeds use a form of P called orthophosphate (PO4–3) as an essential nutrient. However, some bacteria digest phosphite (PO3–3) and produce orthophosphate. Previous studies have identified a gene called ptxD as the reason they can utilize phosphite, and López-Arredondo and Herrera-Estrella proceeded to implant this gene in Arabidopsis plants, a model organism commonly known as mouse-ear cress. After producing these transgenic plants (plants with the ptxD gene engineered into them), the researchers attempted to grow them, along with control wild-type plants, in a medium completely lacking orthophosphate but supplemented with phosphite. They were interested in the height each type of seedling achieved over a few days, and how the root systems developed over a longer time window.
Next, López-Arredondo and Herrera-Estrella tested the growth ability of these transgenic plants against wild-type plants in greenhouse conditions. That is, in a greenhouse, both transgenic and wild-type plants were grown in sandy soils containing either orthophosphate or phosphite as their sole source of P. After a period of time, the biomass and root length of each plant in each condition were measured.
Of course, López-Arredondo and Herrera-Estrella were interested in the effectiveness of the ptxD gene in more than just mouse-ear cress. As a further step, they produced transgenic tobacco plants and grew them alongside wild-type plants in unfertilized, orthophosphate fertilized, and phosphite fertilized conditions. Importantly, all of these different soil types were sterilized so as to control for the effects of bacteria. Furthermore, the researchers measured the rate of photosynthesis of wild-type and transgenic tobacco plants under similar fertilization conditions. Photosynthesis rate is relevant because plants use P in their respiration processes.
As any genetically modified food crop inevitably raises health questions, López-Arredondo and Herrera-Estrella preformed measurements to detect the presence of phosphite in the leaves, flowers and fruits of transgenic plants. This was merely a precaution, as the US Food and Drug Administration has declared phosphite safe for animal consumption.
The next step in this procedure was to test the ptxDtransgenic plants in more realistic soil taken from active farm ground. This soil varied most from previous experiments because it contained all the microorganisms normally found in soil. Both alkali and acidic soils were used, taken from farms in Mexico, and transgenic and wild-type mouse-ear cress was raised using varied concentrations of both orthophosphate and phosphite fertilizer.
Finally, López-Arredondo and Herrera-Estrella investigated the viability of phosphite as a weed control mechanism. They tested two common weeds, false brome grass and tall morning-glory, to determine if these plants were capable of utilizing phosphite as a source of P. Upon discovering that they were not, the researchers used these plants in a greenhouse competition test against transgenic plants. The plants were grown in soil that was directly extracted from an active farm, and again various fertilizer applications were tried. Bear in mind that both the weeds and the crop were raised in the same planter in this trial. Fertilizer conditions were unfertilized, orthophosphate fertilized, and phosphite fertilized. The same procedure was conducted with transgenic mouse-ear cress and transgenic tobacco.
The many results of this study were all in line with the notion that ptxD transgenic crops may be highly beneficial if used on a large scale. The initial test to determine if transgenic plants would grow normally in the absence of orthophosphate proved successful, with the wild-type plants completely failing in the same medium. This result set up the rest of the López-Arredondo and Herrera-Estrella experiments, showing that their transgenic plants had acquired the ability to metabolize phosphite. Ensuing greenhouse tests had similar results, with transgenic and wild-type plants responding similarly to orthophosphate fertilization and no fertilization, but with the transgenic plants developing normally with phosphite as the sole source of P and wild-type plants completely dying out. Identical results were recorded for tobacco plants as well. These data are more strong evidence that genetically engineered crops with the ptxD gene are well suited to replace traditional crops using either orthophosphate or phosphite as a source of P. Moreover, there was no detectable amount of phosphite on the leaves, fruit, or flowers of the tobacco transgenic plants.
Testing of transgenic plants is non-sterile soils produced results that were in line with previous greenhouse testing. Transgenic plants matched the growth of wild-type plants over many different concentrations of orthophosphate fertilizer while demonstrating this same phenotype growing in phosphite fertilizer. Impressively, equal biomass and root length was achieved with a substantially lower concentration of phosphite fertilizer as compared to orthophosphate fertilizer.
Lastly, the greenhouse competition tests between crops and weeds ended with crops being outstripped when the planter was fertilized with orthophosphate, but transgenic crops completely smothered out all tested weed forms when treated with phosphite. These results suggest that phosphite could not only conserve the finite supply of P, but also reduce the necessity of additional herbicide chemicals given its disruptive effects on most wild-type plants.