Why Do Onions Hate You?

You know the feeling.  You’re chopping an ordinary onion when suddenly your eyes burn like the sixth and seventh circles of hell.

You want to back away from the onion, maybe rinse your eyes out with a fire extinguisher or an ice bucket challenge.  But if you acknowledge your tears, you’ll look like a wimp!

You pretend that everything is fine.  Totally fine!

giphy (8).gif
You’re not a wimp!  Nope!  No wimps here!

Why do onions hate you?

First of all, onion (Allium cepa) belongs to the genus Allium, which also includes garlic, shallots, leeks, scallions, and chives.  These plants all feature bulbs, which are giant food storage organs.

Wouldn’t giant food storage organs be easy food for predators?

giphy (3)
Birb & doggo predating on some heckin milk bones

Not if Allium plants fight back.  The sulfur compounds that make Allium veggies particularly pungent also repel predators.  Plants adjust the amount of sulfur compounds they release based on whether or not they’re being nibbled.  For example, leeks (Allium porrum) produce more sulfur volatiles after they’re attacked by leek moths.

Alliums‘ sulfur compounds repel many different insects.  In a test of insect behavior, the Asian citrus psyllid, a citrus pest insect, strongly preferred clean air to air filled with Allium sulfur compounds.  The researchers tested sulfur compounds from crushed wild onions (Allium canadense) and garlic chive (Allium tuberosum).  Garlic chive volatiles even repelled the citrus psyllid when they were mixed with tasty-smelling citrus aromas.

In fact, normal garlic (Allium sativum) is a moderately effective mosquito repellent.  But rubbing garlic juice all over your body is also an extremely effective human repellent.  (DEET™: It Beats Smelling Like a Walking Dumpster.)

giphy
The Sweet Smell of Sulfur™

 

Sulfur compounds work as a defense against predators because disulfides and trisulfides are toxic to many insects.  They also smell like decaying matter rather than food.  However, humans aren’t affected- we find sulfur compounds delicious!

But onion takes its sulfur defenses a step further.  When an onion’s bulb is cut, the individual cells break open and enzymes called alliinases spill out, causing a lot of reactions to happen all at once.  The important step is when an enzyme called lachrymatory factor synthase (LFS) converts 1-propenyl sulfenic acid into lachrymatory factor (LF).

 

Lachrymatory factor, from Eady et al, 2008, Figure 1
The chemical structure of lachrymatory factor (LF),  blatantly stolen from Figure 1 of Eady et al, 2008.

 

Lachrymatory factor (LF) is what makes you cry!  It’s a volatile compound that floats through the air.  When LF comes into contact with the surface of your eye, a very small amount of it turns into burning acid.  Your eyes freak out and start trying to dilute it.  By, well, crying.

giphy (4).gif
Walter White is not impressed by your wimpy tears.

But you don’t want to ugly-cry while you’re trying to cook a romantic dinner or take Guy Fieri to Flavortown (or both)!  Isn’t there any way to avoid looking like a wimp?

In fact, New Zealand scientists have already created an onion that doesn’t make you cry!  The genetically modified onion uses RNAi* to silence LFS.  Without LFS protein, you can’t make LF.  And no LF means no tears.  The 1-propenyl sulfenic acid that would have been made into LF is turned into other sulfur compounds instead.

So why aren’t we all eating this onion already?!?

Two reasons.

1. The taste profile of the RNAi onion is different.

C_B5r3_XgAAaqjx.jpg

LF makes you cry, but it’s also the source of the desirable heat and pungency present in onions.  Taking LF away eliminates those flavors.  Because of the additional sulfur compounds that are produced in place of LF, No-LF onions also have a sweet sulfur flavor with notes of sautéed onions, even when raw.

How do we know this?  Did the researchers actually taste the No-LF onions?

Actually, the researchers used chemical analysis and their impressions of the smell of the onion.  They weren’t legally allowed to take a bite, because tasting transgenic plants requires regulatory approval.

Which leads to the second point that…

2. It’s a GMO.

Some consumers wouldn’t feel comfortable eating a No-LF RNAi GMO onion.  Even though it would be perfectly safe, there might be an “ick” factor to an onion that doesn’t make you cry.  So it might not be a great investment for seed companies.

So how can you look less wimpy?

You can buy conventionally bred mild onion cultivars that make lower levels of LF.  You can also use a sharp knife to reduce the number of onion cells that are broken open, and consequently the amount of LF produced.  Your best bet would be to cut the onion under a vent that whisks away LF.  But you’re not guaranteed to be LF-free except with GMO onions.

So it looks like you’ll have to keep suffering.  Welcome to the inferno…

 

wimpy onion.png
Story of your life

Footnote: 

*What’s RNAi, you ask?

RNA interference, or RNAi, originally evolved as a defense mechanism.  Viruses are a problem for plants, which don’t have an immune system in the same way that humans do.  Many plant viruses contain double-stranded RNA, but plants do not.  So if a plant detects double-stranded RNA, it uses an enzyme called Dicer to chop up that RNA into tiny bits.

Dicer.gif
Dicer, aka “Edward Dicer-hands”, is the yellow guy

The tiny bits of foreign RNA then get loaded into an “RNA-Induced Silencing Complex” (RISC) that finds other RNA sequences that pair with the RNA bits and cuts them.  The cut RNA gets degraded, rather than translated into proteins.

RISC.gif
The RISC, aka “I’m Totally Not Phallic Looking, Guys”, is the new yellow blob

Through the RNAi process, the plant effectively neutralizes the genetic material of the invading virus.

Scientists exploit RNAi to specifically target sequences that they want to silence.  They give plants a carefully designed DNA sequence that makes double-stranded RNA that will be chopped up by Dicer and put into RISC.  The sequence they use corresponds to part of the RNA produced by a certain existing plant gene, so the existing plant gene’s RNA will also be targeted by RISC.  Ultimately, the effect is that the plant can’t produce the protein associated with the gene.

Plant scientists also have other methods that they use to silence genes or do other genome editing.  CRISPR and TALENs are now the most widely used.  I might mention them in other posts.

Yeah, science is complicated.  That’s why scientists get the big bucks**.

**Scientists usually do not get the big bucks.

 

Featured Image Source:

Santoso, Alex.  “Onion Reveals Its True Evil Face.”  Neatorama.com, 2014.

Sources & Further Reading: 

Burnham, Paul S.  “Propanethial s-oxide: The lachrymatory factor in onions.” Molecule of the Month, 2007(Molecule of the Month is one of the oldest chemistry pages on the internet; the homepage is still bright yellow and features WordArt) <http://www.chm.bris.ac.uk/motm/pso/psov.htm&gt;

Dugravot, S., Mondy, N., Mandon, N., Thibout, E.  2005.  Increased sulphur precursors and volatiles production by the leek Allium porrum in response to specialist insect attack.  Journal of Chemical Ecology 31.6: 1299-1314.  <http://www.chm.bris.ac.uk/motm/pso/psov.htm&gt;

Eady, C.C., Kamoi, T., Kato, M., Porter, N.G., Davis, S., Shaw, M., Kamoi, A., Imai, S.  2008.  Silencing Onion Lachrymatory Factor Synthase Causes a Significant Change in the Sulfur Secondary Metabolite Profile.  Plant Physiology 147: 2096-2106.

Mann, R.S., Rouseff, R.L., Smoot, J.M., Castle, W.S., Stelinski, L.L.  2011.  Sulfur volatiles from Allium spp. affect Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), response to citrus volatiles.  Bulletin of Entomological Research 101: 89-97.

“RNA Interference (RNAi)”.  Nature Video, 2011.   Edited by Mary Muers, Simon Fenwick and Louisa Flintoft; advised by Craig Mello. <https://www.youtube.com/watch?v=cK-OGB1_ELE&gt; (RNAi GIFs source)

Silvaroli, J.A., Pleshinger, M.J., Banerjee, S., Kiser, P.D., Golczak, M.  2017.  Enzyme That Makes You Cry–Crystal Structure of Lachrymatory Factor Synthase from Allium cepa.  ACS Chem. Biol. (The structure of LFS was recently “solved” and published this month.  Chemists now know how it turns a stable compound into very reactive LF.)

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s