GM could hold back the tears

Enzyme discovery may lead to tasty tear-free onions.

Onions' eye-watering powers are not connected to their flavour. Credit: © GettyImages

A new finding could lead to genetically modified onions that don't make us cry as we chop them. Researchers in Japan have identified the enzyme that releases a tear-duct-tickling chemical when an onion is cut.

Onion's tear-jerker is a compound called propanthial S-oxide. It is made by an enzyme known as lachrymatory-factor synthase, Shinsuke Imai of the House Foods Corporation in Chiba Japan, and colleagues have discovered¹. Their investigations involved a mix of genetic sleuthing and old-fashioned biochemistry.

Previous studies had suggested that onions' flavour compounds were behind the effect. Chopping was thought to make them react with a common onion enzyme, allinase, producing propanthial S-oxide.

This suggested that in order to breed or genetically engineer a non-irritant onion, scientists would have to tamper with its flavour. Indeed, onions bred to be tear-free, like the Veri Sweet vegetable recently marketed in Washington State, have a characteristically different taste.

The latest study puts paid to this idea. "The chemistry is not new," says Richard Dixon, a plant geneticist at the Noble Foundation in Ardmore, Oklahoma. "But the way it occurs in the plant is not as was first thought."

Now that a single, flavour-independent enzyme has been identified, it would be simple to create an onion in which the enzyme was absent, or suppressed, Imai's team suggest. "Anyone skilled in the art" should be able to produce such a modified onion, says Dixon.

It's not exactly what the world has been crying out for, but Dixon argues that it could be one of the first GM organisms acceptable to consumers. It won't require the addition of a foreign gene, simply the silencing of an existing one. Onion's irritant is thought to have evolved to protect the nutritious bulbs from being eaten.

For cooks the world over, this onion would have direct benefits that most existing genetically modified crops lack. "A non-lachrymatory onion would perhaps be one of the first examples where the consumer was the main beneficiary," Dixon says.