Researchers at Duke University Medical Center have traced the biochemical pathway by which plants build a compound that compromises the quality of corn and soybeans as an animal feed. Their studies indicate that it is feasible to engineer such plants to significantly improve their quality as animal feeds -- a potentially important boon to the hog and poultry industries, said the researchers
The researchers, led by Howard Hughes Medical Institute investigator John York, published their findings the week of August 15 in the online Early Edition of the Proceedings of the National Academy of Sciences.Lead author on the paper was Jill Stevenson-Paulik in the York laboratory. Their research was supported by the National Institutes of Health.
In their studies, the researchers sought to understand the biochemical pathway that leads to the synthesis in plants of the chemical called phytate. In the plant, this molecule is a regulator of signaling in the cell; and in seeds, it acts as a phosphate storage molecule.
Phytate also acts as an "antinutrient" for animals, mainly pigs and chickens, that consume such grains, said Stevenson-Paulik. "Phytate is a very abundant compound in plant seeds that compromises the nutrition in the animals that consume it as their main food source. It binds such minerals as calcium and iron very well, and since it is not digested, animals that consume grains with phytate will lose these minerals as the phytate passes through their gut."
What's more, said the researchers, such excreted phytate contributes to environmental phosphorus pollution, because it washes into surface waters causing the abnormal growth of aquatic plant life called eutrophication.
According to Stevenson-Paulik, creating low-phytate strains of feed grains was hindered by the lack of knowledge about the later biochemical pathways by which phytate is synthesized in plants.
In their studies, Stevenson-Paulik, York and their colleagues drew on their previous studies in yeast that enabled them to understand the biochemical pathways for producing phytate. Using those insights, they searched for counterpart genes in the mustard plant Arabidopsis -- a widely used model plant in genetic studies.
Their analysis revealed that the genes for two particular enzymatic regulatory switches, called kinases, were central to the final steps of phytate synthesis. What's more, they found that genetic mutations that knocked out both these switches -- called AtlPK1 and AtlPK2β -- nearly eliminated phytate production in the resulting Arabidopsis seeds.
Said York, "Perhaps one of the most important aspects of Jill's work is the finding that it wasn't just knocking out the last step in phytate synthesis that was important. Knocking out the last two steps really reduced seed phytate. And what was very unexpected and quite significant is that just knocking out one gene resulted in a buildup of toxic precursor compounds in the seeds." Also, found the researchers, the phytate-eliminating mutations did not compromise seed yield, and also increased the phosphate levels in the seeds.
"The amount of free phosphate in the double mutant is dramatically increased over what is found in nature," said Stevenson-Paulik. "And that has a great benefit in terms of nutrition, because it provides more available phosphorous for the animals that would eat grains with such properties."
According to York, a patent on the low-phytate strains has been applied for, and discussions have been initiated with feed companies about production of such grains. "The next step is to move this process into a commercial environment so that companies can begin producing low-phytate strains in their crop line," he said.
Besides York and Stevenson-Paulik, other co-authors of the paper were Robert Bastidas, Shean-Tai Chiou, all in the Duke Department of Pharmacology and Cancer Biology, and Roy Frye of the Pittsburgh VA Medical Center.