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

By: James

James — Thu, 04 Feb 2010 16:16:19 +0000

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.

By: Ron

Ron — Thu, 04 Feb 2010 12:28:54 +0000

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.

By: James

James — Thu, 04 Feb 2010 05:56:12 +0000

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.

By: Ron

Ron — Thu, 04 Feb 2010 03:30:33 +0000

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.