An Explanation for Auxin, the Plant-Growth Hormone

New research appears to answer an old question about how the remarkable hormone Auxin works. Auxin is a sort of super-hormone that is the key to making plants grow.

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Two scientific teams--one in England and one in America--have cracked a puzzle that has stumped plant biologists for more than a century. They figured out a key step in how a crucial hormone causes plants to grow. Naturally, this is big news for scientists who study plants. But as NPR's Joe Palca reports, it could have implications for the animal kingdom, as well.

JOE PALCA reporting:

In springtime, a plant biologist's thoughts turn to a hormone called Auxin. And why not? Mark Estelle of Indiana University in Bloomington says without Auxin, there would be no spring.

Mr. MARK ESTELLE (Indiana University): It's hard to identify an aspect of plant growth and development which is not influenced by the hormones. So it has a really central role in growth.

PALCA: Plant biologist Ottoline Leyser takes the importance of Auxin a step further.

Ms. OTTOLINE LEYSER (University of York): I like to claim, rather rashly, that Auxin is the brain of the plant.

PALCA: Leyser says like a brain, Auxin allows a plant to respond to its environment. Auxin helps plants bend toward the light, for example. The way Auxin does that is by switching on and off genes inside plant cells.

Ms. LEYSER: What the cells are doing at any one time depends heavily on which genes it has that are active. And Auxin can make quite radical changes to which genes are active and which genes are not active in cells. And we've known that for quite a while, but we haven't understood how that happens, how a cell knows how much Auxin there is there, and that information is then translated into turning genes off and on.

PALCA: In the early 1990s, Leyser and Mark Estelle worked together in Indiana. Now Leyser is at the University of York in England. They both decided to try to find something called the Auxin receptor; that's the protein Auxin attaches to in cells that signals the cell which genes to turn on. As they report in tomorrow's edition of the journal Nature, the Auxin receptor has now been found.

Ms. LEYSER: And it turns out, much to our amazement, to be a protein that we actually knew about already, because we'd identified it as an important part of the signaling process before.

PALCA: Leyser says the receptor, called TIR1, is a very interesting protein. It reaches out from the cell and drags in other proteins that the cell can break down.

Ms. LEYSER: So it's kind of like--a frivolous example would be it's kind of like the tongue of a frog or something that shoots out, captures the fly and brings it into the frog's mouth. It's a protein that catches proteins and brings them into the protein-degradation machinery.

PALCA: And as it turns out, once those proteins the receptor pulls into the cell are destroyed, Auxin can work its magic, making the plant grow.

TIR1 may not be the only Auxin receptor; there may be others. But Mark Estelle says the discovery of TIR1 may give plant biologists some bragging rights.

Mr. ESTELLE: Those of us who work in plants always appreciate it when we discover something before the animal people do. And TIR1 and many of the other components of the system that we're talking about are very highly conserved. They're present in animal systems, as well.

PALCA: Estelle says it's possible that the TIR1 receptor may play a role in human growth, or might even help explain why cancers grow out of control. Joe Palca, NPR News, Washington.

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