By Susan Milius
More than 70 years after biologists identified the powerful plant hormone auxin, they have finally found how plant cells detect it.
Auxin plays a role in just about every aspect of plant growth, from roots to shoots. Gardeners use auxin-containing products to coax cuttings to take root and overdoses of auxin to kill weeds.
In the long search to understand how auxin works, biologists have had trouble figuring out step one: how a cell detects the hormone in the first place. In the May 26 Nature, two research groups independently report finding an auxin receptor in plant cells. It turns out to be a previously recognized molecule called transport inhibitor response 1 protein (TIR1).
One team, led by Mark Estelle of Indiana University in Bloomington, identified the receptor by using engineered insect cells to produce TIR1. The researchers then tested whether the protein would combine with auxin in a simplified environment devoid of hundreds of other plant chemicals. By adding specific compounds known to interact with auxin, the team confirmed that TIR1 binds directly to auxin.
Similar results with TIR1 made by frog cells came from the other team, led by Stefan Kepinski of the Umeå Plant Science Center in Sweden and Ottoline Leyser of the University of York in England.
The two studies suggest that auxin and its receptor spur growth by releasing cells’ parking brakes. When the hormone docks into a TIR1 protein, it prompts the cell to destroy certain inhibitors of cell-growth genes.
