Auxin's Action Explained

PLANT BIOCHEMISTRY

Some 70 years after the plant hormone auxin was discovered, scientists have finally identified one of the proteins targeted by this small molecule (Nature 2005, 435, 441 and 446). The long-awaited discovery of auxin's target will help plant biologists determine how plant cells detect and respond to this hormone.

Auxin (indole-3-acetic acid) plays a critical role in plant development, controlling everything from cell growth to cell division to cell specialization. Auxin causes plants to grow their roots down into the soil and their shoots up toward the sun. Strawberry plants depend on the auxin produced by their seeds to grow juicy red fruit.

"To date, no plant unable to synthesize auxin has been found," notes plant biologist Judy Callis of the University of California, Davis, in a commentary accompanying the Nature papers. Yet no one has been able to figure out exactly how cells sense the presence of auxin, because despite many years of effort, no one had found an auxin receptor capable of mediating auxin-induced changes in transcription, Callis tells C&EN.

Two independent teams of plant biologists have now discovered such an auxin receptor. Stefan Kepinski and Ottoline Leyser of the University of York, in England, and Nihal Dharmasiri, Sunethra Dharmasiri, and Mark Estelle of Indiana University each report that auxin binds to the protein TIR1, part of the multiprotein complex that tags proteins for destruction in plants. The binding of auxin to TIR1 attracts a family of proteins (the Aux/ IAAs) that otherwise prevent the activation of certain genes required for plant development. Upon binding TIR1, however, these proteins are tagged for degradation. Their destruction gives the green light to plant development.

It's likely that more auxin receptors await discovery, notes Alan M. Jones, a cell biologist at the University of North Carolina, Chapel Hill. He suggests that plant cells may rely on the previously identified auxin-binding protein ABP1 to drive auxin-regulated changes to the plasma membrane.

The discovery of a target for auxin may also affect the hunt for better herbicides, Estelle notes. Synthetic auxinlike molecules--particularly 2,4-dichlorophenoxyacetic acid (commonly known as 2,4-D)--find widespread use as herbicides. These herbicides bind TIR1, too, Estelle notes. "Now that we know what the receptor is, we can work on how auxin is interacting with the receptor. It might be possible to design new auxinlike herbicides that are more effective or specialized for particular weeds."

The impact of the discovery of this novel receptor also may be felt beyond the plant world, Estelle suggests. "This is the first instance where a small molecule directly regulates the protein degradation machinery," he notes. "It's possible that there are other instances where degradation of other proteins--potentially those that have an important role in cancer or other aspects of animal development--are regulated by small molecules."