Phosphorus partitioning of soybean lines containing different mutant alleles of two soybean seed-specific adenosine triphosphate-binding cassette phytic acid transporter paralogs

Authors: Gillman, Jason; Baxter, Ivan; Bilyeu, Kristin

Submitted to: The Plant Genome
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2012
Publication Date: 2/6/2013
Citation: Gillman, J.D., Baxter, I.R., Bilyeu, K.D. 2013. Phosphorus partitioning of soybean lines containing different mutant alleles of two soybean seed-specific adenosine triphosphate-binding cassette phytic acid transporter paralogs. The Plant Genome. 6:1-10.

Interpretive Summary: As the human population has increased, there has been increasing attention on the effects that common agricultural and animal husbandry practices have on the environment. Soybean is used both for human and animal nutrition, and seeds are processed to simultaneously produce oil, which is used primarily by humans, and soymeal. Soymeal is a ready source of high quality protein and is frequently used as a component of animal feed mixtures, along with other grain meals. Plant seeds have the majority of their phosphorous present as phytic acid, which complexes with certain micronutrients (principally zinc, iron and calcium) as phytate. Phosphorus is a critical nutrient and non-ruminant animals are unable to efficiently digest phytic acid/phytate. As a result, plant seeds are effectively deficient in phosphorous and certain micronutrients, despite very high levels of these compounds being present. A common remedy has been supplemental additions to feed mixtures of rock phosphate, which compensates for the nutritional needs of the animal. However, this practice is not sustainable, and most of the commonly used rock phosphate is expected to be depleted within this century. Excess phosphorous in the environment is also linked to the contamination of fresh water sources, algal blooms and fish kills. A sustainable method to correct the problems associated with phytic acid has been the development of lines with genetic reductions of phytic acid (from mutant sources). Here we report on the development of a novel line, which has a remarkable 16.5-fold reduction in phytic acid content, as compared to typical soybean seeds. This line has only a trace amount of indigestible phytic acid (~2.7% of the total seed phosphorous), which is ~4x less than one of the best current low phytic acid lines. We also detail effective and efficient molecular tools to accelerate and streamline the breeding process of developing high yielding cultivars bearing this beneficial change in seed phosphorous. This new line will allow the efficient production and molecular selection of soybeans germplasm with even greater reductions of phytate, and elevation of inorganic phosphate, for improved quality soybean meal. This will provide US farmers with soybean cultivars that offer a significantly improved commercial advantage.

Technical Abstract: Seed phytate is a repository of phosphorus and minerals in mature soybean seeds (and seed meal) which presents a barrier to phosphorus and mineral bioavailability due to a lack of phytase enzyme activity in the digestive tracts of monogastric animals (e.g. humans, swine and poultry). One strategy to ameliorate these issues is the deployment of soybean varieties with reduced seed phytate. We previously identified two different recessive mutations affecting two homeologs of ATP-binding cassette phytic acid transporter (one a nonsense mutation and the other a missense mutation) as the molecular genetic cause in the EMS-induced mutant line M153. A sister low phytate EMS-induced mutant line, M766, contained a single SNP within the first intron in one transporter homeolog and a nonsense mutation in the second transporter gene. The objectives of this research were to clarify the genetics underlying the low phytate phenotype in line M766, and to determine the phenotypic effects on phosphorus partitioning in new genetic combinations of mutant alleles from M766 and M153. The results revealed that mutant alleles of both transporter genes from M766 are required for the low phytate phenotype. The inheritance of nonsense alleles affecting both transporter genes (one from M153, and one from M766) led to the production of viable seeds that contained transgressive reductions in total seed phytate and significantly higher levels of inorganic phosphate than has been reported for non-transgenic soybean material. This new information will allow the efficient production and molecular selection of soybeans with even greater reductions of phytate, and elevation of inorganic phosphate, for improved quality soybean meal.