Intimate chemistry of a symbiotic odd couple.

Intimate chemistry of a symbiotic odd couple

Symbiosis, like a happy marriage between seemingly mismatched lovers, succeeds through a subtle chemistry. Each partner benefits, even though they may differ so radically that one wonders how they ever communicate.

By eavesdropping on a chemical conversation between one symbiotic couple -- the soybean plant and its bacterial sidekick -- two biochemists have discovered a surprisingly intimate collaboration.

In the March 8 SCIENCE, Indu Sangwan and Mark R. O'Brian from the State University of New York at Buffalo report that they soybean and the bacteria seem to share inthe making of a vital molecule called heme. "It's the only example I know of where a higher organism supplies an intermediate product in a pathway of another organism," says biochemist Winston J. Brill, who has studied these bacteria and who now runs a scientific consulting firm in Madison, Wis.

Soybean plants, like many other legumes, thrive in poor soils because they link up with bacteria that convert air's nitrogen into a form the plant can use. In return, the plant encases the nitrogen-fixing microbes within pea-sized nodules along its roots and keeps them supplied with nutrients.

To fix nitrogen, the bacteria need lots of energy and oxygen -- and, consequently, lots of heme. They use the molecule to make energy-generating proteins called cytochromes. Nodules hold a rich supply of heme, which helps transport exygen and tints the nodule interior red.

Scientists have long wondered where all this heme originates. does it come from the plant, the bacteria, or both? The nodules seem to contain heme only when bacteria are also present, suggesting the plant relies on supplies from its microbial partners. To find out more about the bacteria's role, O'Brian and Sangwan mixed a genetically altered strain of the nitrogen-fixing Bradyrhizobium japonicum with newly germinated soybean seedlings. The mutant microbes could not make a chemical called ALA (delta-aminolevulinic acid), the initial precursor for heme.

The researchers found that the bacteria could not make heme on their own, but heme was still produced in the nodules. "What it hints at is that both the plant and the bacteria play a role in making the heme," says Brill.

"We'd love to know if this is what happens normally," adds Mary Lou Guerinot, a molecular geneticist at Dartmouth College in Hanover, N.H., who engineered the mutant strain in 1986 and proposed that soybean might "rescue" heme-deficient bacteria.

O'Brian thinks that even unaltered B. japonicum may rely on its plant partner for some ALA and may somehow prod the plant into producing more of it. For example, he and Sangwan found that the bacteria produce about the same quantity of ALA whether or not they have linked up with a plant, but the amount of heme increases when the bacteria live symbiotically. In soybeans with bacterial collaborators, "the ability to make ALA is almost 10 times as good as in a plant that has not seen any bacteria," O'Brian explains. "The bacteroid [symbiotic bacterium] seems to be telling the plant to turn up the [ALA-making] activity."