Making plants more nutritious

Understanding why some plants like spinach store much of their calcium in a crystalline form could help scientists develop more nutritious varieties for vegetable-munching humans.

Spinach stores its calcium in calcium oxalate crystals that humans can't digest.

"Spinach has plenty of calcium, but it is tied up in calcium oxalate crystals that humans can't digest," said Dr. Paul Nakata, a USDA plant biologist who investigates calcium bioavailability at the CNRC.

According to Nakata, one cup of cooked spinach contains 244 milligrams of calcium. But because most of the calcium is in calcium-oxalate crystals, humans absorb a mere 12 milligrams (five percent).

 

When cooked, a cup of crystal-free turnip greens provides as much calcium as a cup of milk.

"On the other hand, turnip greens are nearly crystal-free and an excellent source of calcium," Nakata said. One cup of cooked turnip greens provide about the same amount of calcium as a cup of cow's milk.

To unlock the mystery of calcium oxalate crystal ormation and function, Nakata is studying a small, fast-growing plant called Medicago truncatula. His lab has inspected thousands of genetic variations of this simple plant, which like spinach, normally stores much of its calcium in crystals.

"These genetic variations look nearly identical to the naked eye," Nakata said. "But when inspected under a microscope, there are strikingly differences. Some have leaf cells packed with calcium oxalate crystals, while others are nearly crystal-free. And, in some plants the crystals appear nearly square, while in others, they are rectangular or diamond shaped."

By studying plants with different crystal structures and content, Nakata hopes to determine whether calcium oxalate crystals play an important role in helping plants adapt to stressful growing conditions or fend off attacks by pathogens and insects. He is also conducting studies to identify genes that control crystal formation.

"Since the plants that don't make crystals appear to thrive and be as healthy as those that do, we should be able to breed out or remove this characteristic from Medicago truncatula," Nakata said. "If successful, we could then turn our attention toward making calcium oxalate-rich vegetables like spinach a better source of calcium for humans."

Source: Nakata and Michele M. McConn (2000) Isolation of Medicago truncatula mutants defective in calcium oxalate crystal formation. Plant Physiology 124:1097-1104.

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