(ArsTechnica by JOHN TIMMER).... It turns out that much of the phosphorus in plant seeds is utterly useless to us. It's stored in a compact form, the chemical phytate, which is a six-carbon ring with phosphates hanging off each of the carbons. And, despite having been exposed to phytate for countless years, most mammals have never evolved a means of digesting it. Phytate passes through our digestive systems as if it weren't there. So most of it ends up in sewage or, if the crops are fed to livestock, agricultural waste. Both of which contribute to environmental upsets in our waterways.
So the phosphate in rice is largely wasted from a nutritional standpoint. Understanding why this is a problem requires understanding how the phosphate gets there. Which, for most crops, is a chemical fertilizer. A lot of this gets wasted before reaching the plant, as the phosphate we apply tends to react with elements like iron and aluminum in the soil. Most of the phosphate that does reach the plant ends up in the seeds, which we harvest.
This means that we constantly apply more phosphate to our cropland, and most of the fertilizer we apply ends up in our rivers, lakes, and oceans, contributing to environmental problems.
Making matters worse, we're running out of our primary source of phosphorus. A review of the status of our primary source suggests it will be depleted within 50 to 100 years. We've already been forced to tap lower-quality sources that are more expensive to process. In a world where many of the poorest already struggle with food security issues, phosphate depletion is a worrying trend.
Cutting rice's ability to pack phosphorus into its seeds wouldn't hurt the crop's nutritional value, but it could cut our need for fertilizer, along with its economic and environmental consequences. So a team of Japanese researchers decided to find out how phosphorus ends up in rice grains in the first place.
After a search for genes active in the right plant tissues at the right time, the researchers came up with a relative of the gene for a protein that transports sulfate, which is chemically similar to phosphate. A purified version of the protein encoded by this gene was able to shift radioactive phosphorus across artificial membranes, demonstrating that it was a phosphate transporter.
The researchers then generated mutations in this gene and showed that the mutations make rice sensitive to phosphate starvation. When the plants were allowed to produce rice grains, the grains contained about 20 percent less phosphorous than normal rice. The amount of phytate (the phosphorus storage molecule) dropped even more, with levels down by 25 to 30 percent.
Not surprisingly, the missing phosphate was left in the plant body. Since these are left in the field after harvesting, this phosphorus is more likely to be available for the next year's planting.
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