How Do Our Gardens Grow? Researchers Find a Clue
CHICAGO How do flowers know to bend toward the sun, plant shoots to grow up, roots to grow down and strawberry seeds to grow red, luscious fruit?
Figuring out the exact chemical and genetic steps that command plant cells to grow or divide in response to light or gravity has been one of the longest-standing puzzles of biology.
But now plant scientists are abuzz over a solution reported in Wednesday's issue of the British journal Nature by research teams from Indiana University led by Mark Estelle and a group headed by Ottoline Leyser at the University of York, England, working independently.
They found that the hormone auxin, which is released with exposure to the sun or the pull of the Earth's gravity, combines with a protein called TIR1 to activate a plant's growth genes.
As the discovery opens a new understanding of how plants grow, it could lead to improved crop yields and, perhaps, strawberries as big as softballs. It is also likely to heighten the debate over genetically modified foods.
"This is indeed one of the most important findings in plant biology for many years," said University of Chicago plant biologist Jocelyn Malamy. "These two papers represent a huge step forward--one that was a surprise to many plant biologists--and will have a tremendous impact on plant science."
Charles Darwin had speculated about the existence of a plant growth hormone in his 1880 book "The Power of Movement in Plants." But even after scientists in the Netherlands discovered auxin (derived from the Greek word auxein, to grow) in the mid-1920s, no one could figure out how exactly it triggered plant growth.
Estelle has been trying to solve that puzzle for 20 years. He finally succeeded by studying gene mutations in a plant called Arabidopsis thaliana. Scientists have identified all the genes in Arabidopsis, which is related to broccoli and cauliflower.
Four years ago, Estelle and his team identified the TIR1 protein. Now they are reporting the critical discovery that auxin combines with TIR1 to release the chemical brakes holding back genes that regulate cell growth.
The researchers have mapped auxin's life cycle. The growth hormone is produced in the tips of shoots and branches and in young leaves, where growth is the fastest, though it also travels to other parts of the plant. Roots get a double supply because root tips also make their own auxin.
Auxin enters plant cells to combine with TIR1, and together they remove the proteins locking up the cells' growth genes. After auxin has helped spur growth, it detaches from the genes and is chemically broken down, allowing new chemical brakes to take effect and stop growth.
Despite the new understanding of the machinery of plant growth, auxin remains a big mystery because it does so many other things that have yet to be explained.
"What they found is a central mechanism that eventually is going to be important in understanding how plants perform," said Hans Kende, a plant hormone expert at Michigan State University and a member of the National Academy of Sciences.
"It is not going to have a direct effect on biotechnology in terms of now we can make plants bigger or more efficient," he said. "But down the road it might."
Estelle and other scientists say the new knowledge about plant growth could make food more abundant and nutritious for people around the world.
"The more we understand about how plants grow and develop, the more opportunities there are to manipulate plant growth and development using the tools of genetic engineering," Estelle said.
That possibility is likely to increase concern among opponents of genetically modified foods, who worry that manipulating plant genes may become easier and more widespread. One of their main fears is that genes put into or modified in one plant species may accidentally contaminate other species and cause serious environmental problems.
Although biologists have long been familiar with the growth patterns of plants, the new findings explain the mechanism underlying those behaviors. Flowers, for instance, rotate to face the moving sun because auxin is turned on in some parts of the plant stem and turned off in others depending on the sun's position, Estelle said.
If the sun is shining on the left, auxin is turned on in cells on the right side of the plant, causing the cells to grow and elongate on the right, which bends the flower toward the left and the sun. As the sun moves across the sky, different cells are affected, thereby continually changing the tilt of the flower.
Some plant cells are sensitive to light and others to the pull of gravity; auxin spurs plant shoots to reach for the sun and root tips to grow downward where water and nutrients are found.
Seeds grow into fruit because they make their own auxin, which causes the tissue surrounding them to grow into the flesh of the fruit. The hormone is also responsible for timing when fruit falls from trees. As fruit ripens, the growth hormone declines in the stem securing the fruit, and eventually the stem weakens enough to break. A similar process is at work in the fall to detach leaves from trees.
Auxin is also the reason why cutting off the top of a bush makes it denser. Stopping auxin production in the shoots accelerates hormone production in lower branches.
Despite the new discoveries, auxin hangs on to other mysteries. How, for example, are large doses able to kill broad-leaf weeds, such as dandelions, but leave grass unharmed? A synthetic form of auxin, 2,4-D, became the first effective herbicide and is widely used today.
"Plant growth is a very complicated process," Estelle said. "We've made a big advance in terms of understanding how plants grow, but it's definitely not the whole story."
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Source: Knight Ridder/Tribune Business News