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Monday, December 16, 2013


Recently, I was asked what my favorite vegetable was. With only a brief pause, I said, an onion. It's easy to overlook the humble onion as a vegetable. After all, it's so ubiquitous, but it seldom takes the starring role in a dish. And while we only think of the bulb in general when discussing onions, the whole plant is edible, from roots to flowers. When it comes to genetic modification, however, the onion presents a notable challenge. Its genome is enormous, at an estimated 15 gigabases. To put it in comparison, the human genome is a mere 3 gigabases - that is, the onion potentially contains five times the amount of genetic information that a human has.  Simplistically speaking. A little respect the next time you prepare your mirepoix then.

But the question that has hounded us for decades (this is from 1976) is set to song by Rowlf the Dog:

Why do onions make you cry?

Well, onions and other members of the Allium family are masters of sulfur metabolism. When an onion is cut, an enzyme allinase generates a set of sulfur containing compounds that produce the range of flavors in onions and garlic. That's why how an allium is cut prior to cooking affects its flavor in the end. Unfortunately, among these compounds is propanethial S-oxide, aka lachrymatory factor (I don't think either name is helpful for the lay person). Basically, it's tear gas. Some farmers restrict the uptake of sulfur to produce onions that make less lachrymatory factor, but this also reduces the flavor of the onion, too. Clever marketing will call these "sweeter" onions - I think that's up for debate.

The mystery though, is why don't other alliums, such as garlic or shallots, seem to make people cry even though they also use allinase. In 2002, scientists discovered that lachrymatory factor isn't some random by product, but is regulated by a specific enzyme (lachrymatory factor synthase or LFS), made by a specific gene. Which means that onions can be genetically modified to lose the LFS gene, and they will retain the full flavor profile, but will lack the tear gas producing capability. And in 2008, that's exactly what happened. Using a technique called RNA interference, the LFS gene in onion plants were "silenced" -voila! "tearless" onions. No doubt the New Zealand Institute for Crop and Food Research have been trying to get this to market, but hurdling the regulations for this genetically modified crop has been onerous (specially since this brings in a new technology, RNAi, into a field still debating transgenics, cisgenics, and targeted knockouts). It's been almost 6 years since the first publication - here's hoping we'll see tearless silenced onions soon.

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