Perhaps the most crucial decision a plant must make is when to germinate. For once it leaves the protection of the seed, the tender shoot is at the mercy of the world. Now scientists have deciphered one of the signals involved in sprouting, and they argue that it's part of a developmental checkpoint that allows plants to put the brakes on germination for a while, if need be. Although not everyone agrees that such a checkpoint exists, the finding could one day give farmers better control of their crops.
In 2001, a team led by Nam-Hai Chua of the Rockefeller University in New York City claimed they'd found the signal that stops germination in its tracks--a transcription factor called ABI5. When the researchers gave mustard plant (Arabidopsis) seedlings abscisic acid--a stress signal--after a few days of sprouting, ABI5 kicked in and arrested growth for up to a month.
To figure out how Arabidopsis gets going again, Chua and his colleagues Luis Lopez-Molina and Sebastien Mongrand at the Centre National de la Recherche Scientifique in Bordeaux, France, looked for proteins that bind ABI5. One of these, dubbed AFP, seemed to be intimately involved: Mutants that overexpressed AFP weren't bothered by abscisic acid stress. "They just greened and grew," Lopez-Molina says. In contrast, mutant seedlings that lacked AFP took much longer to start growing again. AFP stimulates growth by escorting the growth-inhibiting ABI5 to sites of protein destruction within the cell, they speculate in the 1 February issue of Genes and Development.
The work may have practical applications. Lopez-Molina notes that transcription factors similar to ABI5 are found in wheat, rice, maize, and other crops, so it might be possible, for example, to keep plants from trying to grow if a dry spell hits by spraying them with a chemical that induces the expression of ABI5, or to spur growth using AFP if the forecast looks good.
"I like the idea that there is a checkpoint that plants use to guard or determine their developmental fate while monitoring the growth environment," says plant geneticist Jen Sheen of Harvard Medical School in Boston. Others, such as Nina Federoff of Pennsylvania State University, University Park, wonder whether the apparent checkpoint might be just a prolonging of seed development caused by the experimental conditions. For biotech, that question may not matter, notes Peter McCourt of the University of Toronto, St. George: "Sure it may be artificial, but who cares if it protects the seed?"