Reducing phytate in peas
Most phosphorus in the seeds of crops is stored in the form of phytate, a molecule not well digested by humans and many other animals. Phosphorus excretion is one of the major pollutants of surface waters in many locations around the world. Important micronutrients, such as iron and zinc, bind to phytate and are also excreted, potentially leading to micronutrient deficiencies in humans and animals.
Low-phytate mutants have been developed in several crop species, including corn, soybean, and barley, to address these problems. In these species, the pathway responsible for phytate production is blocked, and the majority of phosphorus accumulates in an inorganic, biologically available form.
Scientists from the University of Saskatchewan and USDA-ARS have researched the development of low-phytate pea, the most widely produced legume crop in Canada. Their study was published in the January-February issue of Crop Science.
The team selected the CDC Bronco field pea cultivar for development of low-phytate pea. Seeds were exposed to sodium azide before planting. In the following generation, 5000 single plants were harvested and a subsample of seeds from the plants was tested for the low-phytate trait using a colorimetric assay. This procedure involves measuring how much light a solution absorbs to determine its contents. Normal phytate barley seeds, low-phytate barley seeds, and CDC Bronco seeds were used as checks.
Twenty pea plants had a relatively high concentration of inorganic phosphorus. These were considered potential low phytate lines. Seeds from these 20 plants were evaluated further using high-performance liquid chromatography (HPLC) assay, which involves separating dissolved compounds in a solution and analyzing the results. Two lines were found that had consistent expression of the low phytate trait. Seed of these two lines was multiplied for subsequent field trials.
Field trials were conducted at three locations in both 2009 and 2010 to compare the two low-phytate pea lines to their parent line and two other common varieties. Harvested seeds of the two low-phytate lines did not differ significantly from CDC Bronco in total phosphorus concentration. However, the lines did have substantially lower phytate concentration and greater inorganic phosphorus concentration than CDC Bronco. Phytate phosphorus as a proportion of total phosphorus was 32-33% for the two low phytate lines, in comparison to 78% for CDC Bronco. The two other check cultivars, Cutlass and CDC Golden, had similar total phosphorus, phytate phosphorus, and inorganic phosphorus concentrations compared to CDC Bronco.
The two low-phytate lines were similar to CDC Bronco in terms of agronomic performance, with a few notable differences. The low-phytate lines had grain yields 8-14% lower than that of CDC Bronco on average, and their seed weight was 6% lower.
“We expect that crossing and backcrossing the low-phytate lines with improved field pea varieties, followed by recurrent selection for yield, will allow for the selection of low-phytate lines with normal yield, maturity, and seed weight,” says Tom Warkentin, University of Saskatchewan professor and lead author of this study.
This research demonstrates that developing a low-phytate pea line can significantly reduce phytate phosphorus content that is harmful to humans and many animals. However, future research is needed to develop a low-phytate pea line that maintains agronomic performance on par with standard pea cultivars.