Scientists have known since 1885 that the plant growth hormone auxin exists. They've known of its dramatic effects on plant growth and development since the 1930s. But only now do scientists know how it works.
In this week's Nature, Indiana University Bloomington biologists Mark Estelle, Nihal Dharmasiri and Sunethra Dharmasiri identify TIR1 as the protein that, with auxin, influences how and when plant cells grow and divide. In the same issue, scientists in the United Kingdom report a virtually identical result.
"How auxin works has been a holy grail in plant science," said Estelle, who led the research. "This was something even Charles Darwin considered, if only in spirit. That we've all been trying to figure it out for so long makes this latest discovery very satisfying."
Estelle said the finding is important for basic plant science, but may also lead to new insight into how related proteins function in animals, including humans.
In a 2001 report (also in Nature), Estelle and colleagues showed that the protein TIR1 acts to increase the expression of certain growth-related genes.
The scientists had not known at the time that TIR1 interacts directly with auxin (also known as indoleacetic acid, or IAA). With that finding, the scientists can now envision a complete life history for the growth hormone. Auxin is produced in the tips of plant shoots and branches. The hormone travels downward toward the roots. It migrates into plant cell nuclei, where it is picked up by the TIR1 portion of a four-protein complex called SCF(TIR1). SCF(TIR1) and auxin binds to yet another protein that represses the expression of a particular set of genes. And it's this complex of auxin, SCF(TIR1) and the repressor that signals enzymes to destroy the repressor, thereby turning on the repressed gene.
"The key interaction is between TIR1 and the repressor," Estelle said. "Nihal was