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What makes mustard so tasty?

by Coffee Table Science
What makes mustard so tasty?
Want to take a bite?
Image credit: mutterkrause.com 

Remember our last post about how plants can actually hear caterpillars munching on their leaves? Well, if you have not read it, you can quickly get the  gist by watching this video below. Unless you know the history, you will not enjoy the post. So, all new readers, do watch the video below and others can simply skip it and head straight to know what makes mustard is so tasty. 

Before we get to the story, let me me ask you a question. What do you do when an acquaintance clings on to you everywhere you go. Might be the office, the grocery store, probably even your coffee shop and starts eating into your space. You may be patient for a while but soon you decide to do something about it. While talking is always an option, it is not necessarily the first thing that we do. Rather, our first and probably sub conscious response is to do something or say something, that might irritate this person and push him away. Don’t you agree? Apparently, our friends, the plants,  are not very different.  Their response, too, is the same. 

When a caterpillar troubles a plant such as mustard or horseradish, the plant reacts by producing irritating substances called glucosinolates that are disliked by the caterpillar and eventually, the pest stays away from the plant because the food available does not agree with its taste buds or its gut. Interestingly, these glucosinolates are something that our (human) taste buds appreciate and that is why mustard, horseradish and wasabi are something that you commonly find in salads and sandwiches. Insects like caterpillars and butterflies are a real headache for plants because of their sheer numbers and great appetite. Therefore, these plants even go ahead a step and create multiple copies of these genes that help them make these bitter fighting tools that will keep the pests away. A common roadside weed such as thale cress which is abundantly found around the world has at least 52 genes that work only for production of these glucosinolates. 
Thale cress plant has 52 genes for producing glucosinolates
Thale cress (Arabidopsis thaliana), a weed,  carries 52 genes
that can produce bitter tasting flavours so that it can keep pests away.
Image credit: Wikipedia. 
Not only this, others members such as mustard, horse radish of the Brassicaceae family, to which Thale cress, also belongs, are now known to have evolved in a way where they can produce different variants of the glucosinolates from different starting materials. (This means we have different salad dressings waiting to be explored.) So, if a particular substrate such as phenylalanine is not available, some plants can even use tryptophan or even methionine to produce a variant glucosinolate that will help it counter the pest attack. Needless to say, these plants have ensured that these genes are passed on to the next generations and the genes keep evolving to make more diverse glucosinolates as years go by.

But that is not the end of the story. It is just one side of the story. The truth is that the slow, sloppy caterpillars were not ready to give up easily. While plants were busy duplicating and distributing genes that make the bitter flavourants, caterpillars were actually developing genes that could help them digest these flavours. Through years of trial and error, these caterpillars finally managed to digest the bitterness and continue chewing on leaves. The ones who succeeded simply took to these plants and colonized them. This was the actual reason why plants had to develop alternate methods of developing glucosinolates so that they could get rid of colonizing caterpillars. 

This cycle of combating and evolving has been on for around 90 million years and is helping both these species get tougher each day. Termed Co-evolution by Ehrlich and Raven, this phenomenon was hypothesized half a century ago in the year 1964. It is safe to assume that such interactions take place all the time on the planet and are the basis for the biological diversity we can see on out planet today. Modern scientific methods are making it easier for us to track these changes even at a genetic level, something that Chris Pires and his colleagues did at the University of Missouri and published in PNAS recently. Now, you know that years of co-evolution is why mustard is so tasty. 

This also means that the taste of mustard is also likely to change over the years. But you wouldn’t know because it would be at least a few million years before one can notice it and neither you nor me will be around to taste. 

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Edger PP, Heidel-Fischer HM, Bekaert M, Rota J, Glöckner G, Platts AE, Heckel DG, Der JP, Wafula EK, Tang M, Hofberger JA, Smithson A, Hall JC, Blanchette M, Bureau TE, Wright SI, dePamphilis CW, Schranz ME, Barker MS, Conant GC, Wahlberg N, Vogel H, Pires JC, & Wheat CW (2015). The butterfly plant arms-race escalated by gene and genome duplications. Proceedings of the National Academy of Sciences of the United States of America PMID: 26100883

Ehrlich, P., & Raven, P. (1964). Butterflies and Plants: A Study in Coevolution Evolution, 18 (4) DOI: 10.2307/2406212

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