The discovery of auxins: phototropism

Now that we have our seeds germinated, they must grow. Early on, people noticed that seedlings grow toward the light. This response is called phototropism (photo=light; trop=curving).

Early in the history of science, Darwin kept birds. To provide the birds with vitamins in those days, one needed to grow sprouts. A sprout contains more vitamins than a seed...why? A common species for sprouts was Phalaris or canary grass!

Darwin noted that the first leaf (coleoptile) of canary grass was very sensitive and responsive to light. He sprouted the seeds in flats and fed the mature seedlings to the birds. But before they made green leaves, the white first leaf (coleoptile) appeared and grew toward the light coming from the nearby window. Darwin was very curious about this and did a few experiments. Later scientists added to this queue of projects to elucidate the mechanism of phototropism.

Darwin, and others, obviously found that the coleoptile's tip was the light-sensitive part. The growth response to light, however, was produced further down the coleoptile.

This observation led scientists to think about hormones again. A chemical substance made in the tip would be transported down the coleoptile and the cells some distance away from the tip would respond by growing.

Fritz Went, a Dutch scientist, figured out that, if the growth stimulus was a chemical, he could trap it in a block of agar (an aqueous gel). Then he could put the block of agar on the tip of a decapitated coleoptile, and it should respond as if the block took the place of the tip.

This did not work out as expected...the coleoptiles grew straight up even with light coming strongly from the side!

He did find some coleoptiles curving as they grew, but they did not all curve toward the light! He noticed that those curving were doing so because the agar blocks were off-center; and these were curving by additional growth on the side underneath the block. He found that this response could be generated by off-center blocks applied to coleoptiles kept in the dark.

Apparently when high levels of the hormone were on one side of the coleoptile, that side would grow faster and the coleoptile would curve accordingly. The opposite side had less of the hormone and therefore grew less rapidly, accounting for the curvature.

Went called the hormone auxin (auxein: to grow). It took 20 years before this auxin was identified chemically as indole-3-acetic acid. Since then additional natural auxins have been identified.

These could induce various tissues, such as these coleoptile segments, to elongate (to grow)...

and the dose response follows a typical pattern:

You will notice immediately that it is possible for there to be too much of a good thing. You can kill plants with too much auxin. This launched a huge biochemistry search for other herbicidal auxins in the chemistry laboratories worldwide. Some of the resulting auxins are:

This kind of discovery led to structure-activity studies. It was found that at native pH a partial positive charge on a phenyl ring system and a partial negative charge on a carboxyl prosthetic group need to be separated by 0.5 nm for a compound to be an active auxin.

IBA and NAA are used in rooting compounds. 2,4-D is the active ingredient in "weed and feed" mixtures sold for lawn care. 2,4,5-T is a banned herbicide; it was the active defoliant in Agent Orange. Unfortunately a low-bidder company cut some corners in its synthesis for the government supply. It was contaminated with dioxin and caused cancer and death in people exposed to the accidental (?) mixture.

Auxin Synthesis

Auxin is synthesized from the amino acid tryptophan in one of several possible pathways.

The concentration of auxin is regulated

There are several ways that the concentration of auxin can be controlled in any tissue where the concentration is a critical factor.

Auxin transport is polar and basipetal

The transport of auxin has been studied since the middle of the 20th century when it could be labeled with radioisotopes. The typical experiment to do this is shown below.

These experiments showed that auxin is transported preferentially in the basipetal direction...acropetal transport is minimal. The polarity of transport was studied extensively by Mary Helen Goldsmith at Yale and her concept is shown below.

A natural outcome of these studies was to find compounds that could effectively prevent basipetal auxin transport. A few of the more-potent synthetic auxin transport inhibitors are shown here.

More recently, some naturally-occuring auxin transport inhibitors have been isolated and identified.

Auxin can be inactivated by conjugation

It was discovered by Bob Bandurski and others that free auxin is not the most-prevalent form in plants. Most of the auxin is found in "bound" forms in which the auxin is conjugated to an amino acid in protein structure, or to inositol or other sugars.

You might notice that the arrows have two auxin can be bound up into conjugates or retrieved from conjugated forms. This way the plant cell can keep a homeostatic level on its auxin pool.

Auxin can be degraded

Auxin can also be degraded by decarboxylation or oxidation. These two pathways to reduce the auxin pool are shown below.