Feb 15, 2008

Tearless Onions Have Their Debut, Hurdles Remain Ahead

It used to be thought that teary eyes were the price you paid to enjoy an onion.

But about five years ago, a team of Japanese researchers found that might not be true. They found a gene.

And now, researchers in New Zealand — colleagues of the Japanese — have taken it one step further. They have applied another technology, developed by an Australian, to “silence” the gene that produces the enzyme that makes the chemical that turns into sulfuric acid when it hits your eyeballs.

There still may be some flavor issues to resolve, but current estimates are that, in 10 years or so, you will be able to peel an onion and not feel a thing. And the onion will still do what onions do – make food taste really good while contributing healthful compounds to the human diet.

The scientists credited with the initial gene discovery are part of a team led by Shinsuke Imai at House Foods Corporation in Japan. The development work in New Zealand is being led by Colin Eady, senior scientist at the New Zealand Institute for Crop and Food Research in Christchurch.

There is one “glitch.” The system used for silencing the gene is considered genetic engineering, so the scientists are working within limitations that keep the onions isolated. They can’t be sold. They are not even supposed to be taste-tested by the researchers.

Whether the public will accept GMO tearless onions – whether the onions will even make it through the clearance processes – are unknowns. The record is mixed. The public accepts GMO corn, soybeans and canola, but has rejected GMO tomatoes and potatoes – or, at least, processors have rejected them in anticipation of a negative public response. A GMO plum was developed by USDA researchers and released last year, so governments themselves exhibits unresolved mixtures of acceptance and rejection.

“The advantages of tearless onions are so enormous that, once they are available in the marketplace, they will become a mainstay in household kitchens,” said Michael Havey, a USDA research geneticist and professor of horticulture at the University of Wisconsin in Madison. “I expect to see this onion dominate the market around the world. This is a real credit to Colin Eady and his research team.”

In 2002, Imai and his Japanese co-workers found the gene now called lfs, short for lachrymatory factor synthase. This gene codes for an enzyme, normally stored in vacuoles in the onion cytoplasm, that is released when the onion is cut or bruised. The enzyme reacts swiftly with sulfur-containing compounds in the onion. Once synthesized, the lachrymatory factor is very volatile.

When it reaches our eyes, it forms sulfuric acid, reason enough for a flood of tears.

Tearless onions were the highlight of the Fifth International Symposium on Edible Alliaceae last fall in the Netherlands, where the New Zealand researchers announced they had found a way to silence the gene.

Called RNAi, for RNA interference, it regulates the expression of other genes. RNAi was discovered in 1998. RNAi can silence many kinds of genes, and the New Zealanders’ contribution is in applying it to onions and also to garlic, where they are developing disease resistance and other quality attributes.

“We have been using a gene-silencing technology, called RNAi, developed by Peter Waterhouse in Australia, that allows us to retarget the plant’s own natural regulation system without expressing foreign proteins in the plant,” Eady said.

An important objection to many genetically modified plants is that they contain proteins that are not normally found in the host plant. The Bt gene, for example, when added to corn introduces a foreign substance that creates a toxin in the corn plant.

“Through RNAi, genes can be specifically shut down or turned off. By shutting down the lachrymatory factor synthase gene, we have stopped valuable sulfur compounds from being converted to the tearing agent and instead made them available for redirection into compounds, some of which are known for their flavor and health properties,” Eady said.

The research team has been unable to induce tearing by crushing their model tearless onions, Eady said. He demonstrated by holding a cut onion to his cheek.

“What we have now is a truly unique germplasm with a unique trait. We can home in and study what the consequences of this one effect are. We can detect differences in sulfur compounds known to be involved in flavor and health and actually measure them and assign to them a role.”

USDA’s Havey in enthusiastic about not only the applications to onions and garlic but to many other foods – or plants that could become foods if genes producing poisonous substances could be silenced. Hot peppers could be toned down. Poisonous alkaloid compounds in some potato cultivars could be turned off.

In many countries onions contribute a significant proportion of the daily fiber requirement, and have for some 5,000 years. Worldwide, about 44 million tons are produced each year.

“They are such a versatile and nutritious vegetable that if we can manage to get more people cooking and eating fresh onions, then that has got to be a positive outcome,” Eady said.

Onions are one of world’s the most valuable vegetable crops, Eady said. The ancient Egyptians, and many civilizations since, valued onions not only as a food but also as a medicine. Modern medical science has confirmed ancient beliefs showing that the fructan, flavonoid and sulfur compounds combine to make onion a truly functional (healthy) food. Fructans, for instance, enhance the value of the onion by preventing the growth of bacteria and lowering blood lipid and insulin levels when eaten.

The onion has an extremely large genome size, 36 times the size of the rice genome, which complicates application of key molecular technologies when studying this vegetable, Eady said.

How long before tearless onions come to market?

“It will take at least 10 years to introduce the trait into current commercial cultivars,” he said.

While genetic engineering techniques are involved, they are not being used to introduce genes into existing varieties. Thus, conventional breeding will be required to get RNAi into commercial varieties, and plant breeders will need to build upon the work that has been done.