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Natural History, April, 2003 by Stephan Reebs
If you're lucky enough to have a backyard vegetable garden this summer, pull up a bean plant (or any other legume) and take a look at its roots. The little swellings you'll see are nodules formed by the plant and inhabited by bacteria. But don't be alarmed: there's no disease. The plant needs nitrogen to make proteins, but the nitrogen in the plant's environment takes a "raw" form the plant cannot use. That's where the bacteria come in: they "fix" the nitrogen in a molecular form that makes it available to the plant. In exchange, the plant supplies the bacteria with sugars and other compounds.
The nitrogen-fixing partnership has long been the subject of intense scientific study--if only because so many of the world's protein-rich crops are legumes: alfalfa, soybeans, and peas, to name a few. Now Rieko Nishimura, a molecular biologist at the University of California, Berkeley, and several of her colleagues at Japanese universities have provided investigators with a new tool: a mutant form of the legume Lotus japonicus, a "model" organism well known to plant geneticists around the world. The mutant form is called astray because it grows long horizontal roots that, with respect to gravity, have "gone astray."
More to the point, the astray form generates many more nodules than the plant's nonmutant form, and it makes them early in its life. Astray has aboveground abnormalities as well. For example, its stem is elongated and its color a washed-out green--typical features of plants that lack access to light. Thus the astray gene is involved both in the plant's responses to light, an attribute of the aboveground world, and in the formation of roots and nodules underground. And the gene's multiple talents offer a window into the evolution of nodulation in legumes: certain proteins that were operating in the light of day were co-opted for work within the darkness of the soil. ("A Lotus basic leucine zipper protein with a RING-finger motif negatively regulates the developmental program of nodulation," Proceedings of the National Academy of Sciences 99:15206-10, November 12, 2002)
COPYRIGHT 2003 Natural History Magazine, Inc.
COPYRIGHT 2008 Gale, Cengage Learning
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