Texas A&M University researchers have developed cotton plants that utilize a form of phosphorus that allows them to outcompete weeds, particularly Palmer amaranth/pigweed, thus offering “a novel alternative” to herbicides that are becoming increasingly ineffective as more weed species become resistant to glyphosate and other widely-used chemistries.

Weed resistance and weed control are the No. 2 and No. 3 concerns of U.S. cotton producers after input costs.

Dr. Keerti S. Rathore, professor at the Texas A&M Institute for Plant Genomics and Biotechnology and leader of the research team, says the cotton plants are genetically engineered to express ptxD, a bacterial gene, that allows them to use phosphorus supplied in the form of phosphite (PO3).

“Normally, phosphorous is supplied to cotton plants in the form of orthophosphate (PO4, the metabolizable form),” he says. “These cotton plants are able to convert PO3 to PO4, so they grow normally. But weeds, like most other organisms, can’t use PO3 as a source of phosphorus, and are severely suppressed.”

“Our researchers have addressed an issue that costs producers billions of dollars. This is an economical, environmentally safe, and sustainable solution."—Dr. Patrick Stover, Texas A&M vice chancellor of agriculture and life sciences and AgriLife Research acting director

In a paper published in PNAS, one of the world’s most-cited and comprehensive multi-disciplinary scientific journals, the A&M research team noted that chemical weed control options “are rapidly shrinking due to the recent rise in the number of herbicide-resistant weeds in crop fields, with few alternatives on the horizon. There is an urgent need for alternative weed suppression systems, especially non-herbicidal ones, to sustain crop productivity, while reducing our dependence on herbicides and tillage.”

AN EFFECTIVE ALTERNATIVE
The new cotton plants allow for a selective fertilization system, based on phosphite as the sole source of phosphorus, while offering an effective alternative for suppressing weed growth, the paper notes.

“We believe this system is one of the most promising technologies of recent times, that can help to solve many of the biotechnological, agricultural, and environmental problems we encounter,” Rathore says.

Results of the research have shown:

• Cotton plants with the ptxD gene can utilize phosphite as a sole source of phosphorus.
• The ptxD gene/phosphite system is highly effective in controlling the growth of glyphosate-resistant Palmer amaranth/pigweed. While the cotton plants fertilized with phosphite “were significantly taller, had more leaves, and produced higher biomass,” pigweed plants fertilized with phosphite “were severely stunted and exhibited typical symptoms of phosphorus deprivation in the form of leaf bleaching.” Growth parameters of the pigweed plants were significantly reduced, including plant height, and shoot biomass was reduced fourfold.
• The ptxD gene/phosphite fertilizer system is highly effective in suppressing weeds in low phosphorus, natural field soils as tested in growth chamber and greenhouse, with cotton plants showing faster growth and greater shoot biomass. Pigweeds showed stunted growth and a 12-fold lower shoot biomass compared to those growing with metabolizable phosphorus.

INCREASINGLY CHALLENGING PROBLEM
Palmer amaranth/pigweed is “the most noxious glyphosate-resistant weed affecting cotton production throughout the southern United States,” the paper notes, “and for populations resistant to multiple herbicides, it is an increasingly important problem” not only reducing lint yield and quality, but also slowing mechanical harvesting two-fold to 3.5-fold.

The system “represents one of the most promising technologies of recent times” toward helping solve many of the agriculturaland environmental problems incurred in cotton production.

With the rapid loss of effective herbicide options, the researchers write, “growers are turning toward old, expensive practices, such as hand weeding, as well as environmentally destructive practices such as tillage cultivation … While some pigweed populations have apparently evolved different and independent mechanisms for resistance to glyphosate, it is less likely that weeds will develop an ability to utilize phosphite, because the only way they can escape the suppressing effects of phosphite in a low phosphorus environment is by gaining an ability to oxidize it into the metabolizable form of phosphorus.”

The system, the researchers say, offers a way to successfully grow transgenic cotton at the expense of glyphosate-resistant pigweed — “a powerful alternative for crop production using a non-toxic chemical compound.”

In addition to being a highly promising strategy to suppress weed growth, it has been reported that lower amounts of phosphorus fertilizer are required to achieve equivalent yields from transgenic plants capable of metabolizing phosphite when it is supplied in the fertilization regimen instead of the usual orthophosphate.

IMPORTANT ECOLOGICAL BENEFITS
The researchers note that the ptxD/phosphite system “should provide important ecological benefits” by reducing water eutrophication and hypoxia caused by the runoff of orthophosphate and nitrogen fertilizers into water bodies.

The system “represents one of the most promising technologies of recent times” toward helping solve many of the agriculturaland environmental problems incurred in cotton production, the report says.

“Our researchers have addressed an issue that costs producers billions of dollars,” said Dr. Patrick Stover, vice chancellor of agriculture and life sciences and AgriLife Research acting director, in a university news release. “This is an economical, environmentally safe, and sustainable solution."

Field trials for the system are planned, Rathore says. “Our Mexican collaborators have a patent on the basic idea of using the ptxD/phophite to control the weeds. What we have done in our latest research is to provide a proof of concept in cotton. The next steps will be to generate 50 to100 new cotton lines with the ptxD gene, and select one or two lines based on performance, transgene integration site and expression, etc. Then the selected line can be patented. It is most likely to be a joint patent.”

Team members in addition to Dr. Rathore were Dr. Devendra Pandeya, Dr. Madhusudhana Janga, Dr. Muthu Bagavathiannan, and LeAnne Campbell, all with Texas A&M AgriLife at College Station, and Dr. Damar López-Arredondo and Dr. Priscilla Estrella-Hernandez at StelaGenomics Inc., and Dr. Luis Herrera-Estrella at the Center for Research and Advanced Studies of the National Polytechnic Institute at Irapuato, Mexico.

The research was funded in part by Cotton Incorporated.