By LEE BOWMAN
Scripps Howard News Service
May 25, 2005
Using different approaches, two teams independently identified a receptor for the plant hormone auxin, a substance first reported to move within plants by Darwin himself in 1880. To date, no member of the plant family has been found that's unable to synthesize auxin, named after the Greek word for growth.
The hormones affect numerous plant processes, including cell division, elongation, the loss of leaves in autumn and the formation of buds, roots, flowers and fruit, as well as the direction and shape that plants grow - toward a light source, for instance.
Auxin also promotes fruit development. For example, auxin produced by the developing seeds of a strawberry promotes the growth of a red and juicy fruit. If the seeds are removed, the fruit is stunted.
The chemical makeup of the hormone itself was teased out in the 1930s, and horticulturalists have used it for decades both as a weed-killer - too much auxin makes plants die - and as a agent to promote growth, such as root development in plant cuttings.
But the mechanism of how the hormone did its job has been elusive.
"How auxin works has been a holy grail in plant science," said Indiana University Bloomington biologist Mark Estelle, who led the research team there. "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."
What Estelle's team and Stefan Kepinski and Ottoline Leyser of the University of York and the Umea (Sweden) Plant Center, report Thursday in the journal Nature is that auxin directly binds to a protein called TIR1 and sets in motion the destruction of a family of proteins that normally block activation of genes required for plant growth.
In an earlier Nature study, Estelle's group had showed that TIR1 acts to increase the expression of certain growth-related genes, but had not known that it interacted directly with auxin.
Estelle said the new finding completes the life history for the growth hormone and is important not only for basic plant science, but may also lead to new insights into how related growth-regulating hormones behave in animals, including humans.
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