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We got to genetics in class today and the story of the shrunken 2 gene

The origin story of shrunken2, the gene behind much of the sweet corn we eat today. Pictures of the phenotype of CAL mutants in arabidopsis (the gene I mentioned last week for its role in differentiating between broccoli and cauliflower).

Arabidopsis that carries broken copies of both the AP1 (apetala1) and CAL (cauliflower) genes. The flower bearing stems have been replaced by these cauliflower-head-like growths. Image from "Genome-Wide Analysis of Gene Expression during Early Arabidopsis Flower Development" by Frank Wellmer et al (in PLOS Genetics a creative commons licensed journal). Article here:

Just in time for me to put together my worksheet for Thursday! I’ve managed to work in the CAL gene, which I talked about last week in my discussion of Cruciferous vegetables:

Cauliflower plants (and broccoflower plants) have broken copies of the CAL gene, which (when it isn’t broken) is helps the plant decide to switch from producing stems that were bear flowers to the flowers themselves. Without a functional version of CAL, cauliflowers just keep making denser and denser stems, producing the distinctive heads of cauliflower. If you have journal access, you can read more about the CAL gene at this science paper:

I also threw in a question that uses the shrunken2 gene (one of the two most common genes that convert normal starchy corn into sweet corn). From the question in question:

Note the shriveling of the yellow kernels that carry two broken copies of the shrunken2 gene, the purple kernels carry either one broken and one working copy of shrunken2, or two working copies. The change in color is controlled by another gene nearby on the same chromosome, shrunken2 itself has no effect on the color of corn kernels. Photo credit goes to MG Neuffer and MaizeGDB.

Corn kernels without a working copy of the shrunken2 gene can’t convert very much of the sugar provided by photosynthesis in the leaves of the corn plant into starch. Instead, sugar itself accumulates in the kernel making the corn taste quite sweet.

When sugary corn kernels are dried, they shrivel up, while starchy ones remain relatively round and smooth. This has to do with the fact that sugars are water soluble while starch is not. So, as I understand it, corn kernels with more sugar are also a greater percentage water than corn kernels that are made mostly of starch.

The mutant form of shrunken2 was identified by John Laughnan, a maize geneticist at the University of Illinois Urbana-Champaign. The story of the discovery as told in Maize Genetics and Breeding in the 20th Century by Peter Peterson and Angelo Bianchi:

According to historical sources [E.H.C.] [This is Edward H. Coe], serendipity played a part in a practical discovery (1953) from which many sweet corn worshipers now benefit. Soaking and chewing upon a corn seed to aid in concentration is a pervasive but minor indiscretion in the profession, generally conducted surreptitiously and especially embraced when seeking rare mutations or recombinants. Muttering, so it is said, “that’s shrunken, too,” and “super, it’s sweet!” [John Laughnan] came upon the now popular and widely grown, high-sugar Super Sweet type. When next the reader has a table ear with butter (or better, corn oil margarine) and salt, it might be gratefully remembered that the sh2 factor is so close to A1 that it was originally attractive as a marker in intensive genetic analysis-else it might yet be only a phenotypic curiosity.

One of the things that sucks about moving into the area of comparative grass genomics is losing of the feeling of following in the footsteps of generations of maize geneticists. The maize community has, for lack of a better word, a sense of history.

4 replies on “We got to genetics in class today and the story of the shrunken 2 gene”

I think it has more to do with the amount of water associating with the starch versus sugars. Last I checked, starch was soluble in water (ever dissolved a starch packing peanut?).

The maize community has a good sense of history because it is relatively old and developed as a very cooperative community. IMHO, of course. 🙂

Hmm. Now that you bring it up I decided I needed to look into it and it turns out starch is relatively insoluble in water at low temperatures, but more soluble at higher ones. I’m reasonably certain that the starch graduals plants use as energy stores (both day to day, and in seeds and storage tissues) are a reasonably insoluble form of starch.

It is important for a plant to be able to store energy in non-soluble forms, because building up reserves of energy in forms soluble in water (like sugars) would mean dealing with big changes in solute potential and osmotic pressure. But let me know if you think I’m barking up the wrong tree with this line of reasoning.

I completely agree with your assessment of the maize community.

You are right that starch is relatively insoluble at lower temperatures. I think the driving force in the difference in water between the two types of kernels is comparing a polymer to monomers. And the point about storing reserves in a manner that sequesters them in an “inert” state is reasonable way to think about it. The plant can obviously deal with the differences between free sugar and starch pretty well when it is filling the kernels. Do both kinds of kernels load the same amount of sugar (I can’t remember)? I don’t remember noticing a big difference between shrunken and normal kernels until drying, but that may have been on oversight on my part. I do remember having to be a bit more careful trying to get them growing again.

And of course, all of this depends on the level of student you’re talking to. You need to have better answers the deeper you are in a subject.

D’oh, you’re right polymers and monomers is a big part of it. Even if it’s technically dissolved a starch granule would be effectively one molecule for the purposes of calculating solute potential, while the sugars that make up that granule would have a much bigger effect simply by being more lots of separate little molecules (monomers).

The couple of times I worked with shrunken mutants, I couldn’t tell the difference between mutant and wild type until drying the ears either. Since sugar and starch have approximately the same density converting from sugar to starch doesn’t change the density of the stored energy, so the fact that once the water is removed by drying the shrunken2 kernels are smaller makes me think shrunken2 kernels are able to load less total sugar over development than non-shrunken2 kernels.

An important part of being a sink tissue (seeds, storage roots, etc) is keeping the amount of dissolved sugar in a group of cells low (by quickly converting it into), which facilitates the flow of more sugars out of the phloem and into those cells. So if shrunken2 kernels really don’t contain as much total sugar/starch as wild-type kernels, it would make sense why they weren’t as successful at loading sugar, since the sugar would have to be loaded from a lower concentration tissue into a higher concentration one (which plants can do, but it’s the difference in effort between rolling a barrel down a hill, and up one).

Right now I’m guessing my students haven’t learned enough about plants to ask the difficult questions, but since I hope to teach more advanced classes (and I already get some pretty insightful questions in office hours) it make sense to make sure I understand the details of this pretty well, and you’re definitely helping with that, thanks Ron.

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