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 able to get that to work in a test tube, but the interaction would only occur if he added auxin to the solution."
Auxin may stick around awhile, but it is eventually broken down by natural cell processes.
The scientists used the common plant model Arabidopsis thaliana (wall cress) for their experiments. They also expressed TIR1 in insect cell cultures, to make sure that the binding of TIR1 to auxin is not directly influenced by other plant cell proteins.
Auxins influence a wide variety of plant processes. For example, they are the hormones that cause roots to grow downward and flowers to track the moving sun. Auxins also stop the growth of lateral branches. Since auxins are produced in shoot tips, lopping off the tip can cause numerous side shoots to appear, giving a plant a denser, more sculpted look.
Auxin also promotes fruit development. In strawberries, for example, auxin produced by the developing seeds promotes the growth of a red and juicy fruit.
Another protein called ABP1 (Auxin Binding Protein 1) has previously been shown to bind auxin. But unlike TIR1 and auxin, it remains unclear whether the pairing of ABP1 and auxin actually stimulates plant growth.
The research reported by Estelle, Dharmasiri and Dharmasiri was funded by grants from the National Science Foundation, the National Institutes of Health and the U.S. Department of Energy.
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