How a Plant Growth Hormone Predicted by Darwin Shapes Plants

2262636867_a80f7eca8a.jpgThe Power of Movement in Plants

Most of a flowering plant’s development and physiology is regulated by plant hormones.

The first of these chemical signals to be isolated and characterized was auxin.

In the 1930’s, after the initial discovery of auxin by Frits Went in 1928, he, Kenneth Thimann, and Folke Skoog showed that, in addition to causing cell elongation, auxin had developmental effects: it enhanced formation of roots on cuttings and inhibited lateral buds.

But nearly 50 years before this, Charles Darwin, assisted by his son Francis, studying the phenomenon of phototropism in grass seedlings, hypothesized that some sort of internal signal was involved in the bending of the seedlings toward light. (see movie below)

This signal turned out to be the plant hormone auxin, as shown by Went.

The Darwins’ results were published in a book entitled The Power of Movement in Plants. (A copy can also be found online here.)69-1.jpg

How Auxin Grows Plants

Auxin’s most well-known function is to stimulate cell elongation in young dicot stems via the acid growth theory.

It’s within this context of plant growth stimulation that auxin has been shown to mediate both phototropism and gravitropism in plants.

That is, auxin acts as the signal between the perception of light or gravity by the plant and the plant’s response. This response usually is more growth on one side of the stem or root, leading to the observed curvature.

How Auxin Shapes Plants

As in the game Spore, one of the first things to do when building an organism is to decide what’s up/down (forward/back) – in other words, the polarity of the organism.

1mechanism.gifAuxin has been shown to help establish polarity in very young plants (embryos). Click on small photo left for more info.

A recent report in the journal Nature helps explain how this may work at the cellular level.

Physical stress and strain also affects the fate of plant cells and thus affect the shape/function of a plant stem or leaf.

081214191012.jpgPlant biologists have shown that this may also be involved in determining root patterns and how auxin may be involved.

Finally, we have more new news regarding auxin and root hairs.

Root hairs (see diagram right) are critical conduits for the uptake of water and minerals by plants. Auxin apparently increases the length of root hairs, thus increasing their effective surface area. This would likley have significant implications to agriculture.

A recent report shows that limiting the amount of auxin in localized areas of developing plant tissues may also affect the form of plants.

*A short movie of phototropism in corn seedlings can be viewed here.

Bottom line: As with most plant hormones, auxin plays multiple roles in regulating plant development, far beyond what the Darwins could have imagined.

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