Getting to the root of nodule formation.

Some plants, primarily legumes, make farmers' lives a little easier. They grow nodules on their roots that enable them to fix their own nitrogen, which farmers would otherwise have to add as fertilizer. Scientists have tried for many years--with no real success--to figure out how to get nonlegumes to form these nitrogen-fixing nodules.

Researchers know, however, that this special ability of legumes depends on rhizobium bacteria, which set up shop in the plants and secrete lipo-chitooligosaccharides (LCOs), carbohydrate-like molecules that trigger the formation of nodules.

Now, a study suggests that LCOs may be able to initiate nodule growth in nonlegumes as well. What's more, LCOs may serve as general-purpose plant growth regulators, report Horst Rohrig and his colleagues at the Max Planck Institute for Plant Breeding in Cologne, Germany, in the Aug. 11 Science.

The researchers developed a fairly simple technique for making LCOs in large quantities, they report. They then applied a very small amount of synthetic LCO to the cells of a tobacco plant, which does not grow nodules. The LCOs spurred the tobacco cells to grow and divide, an activity that usually requires the hormones auxin and cytokinin.

"We think we may be mimicking the early stages of cell division that are involved in making nodules," says coauthor Richard Walden.

LCOs appear to act like auxin, which helps control many aspects of plant growth besides nodule formation, says Walden. LCOs and auxin seem to trigger the same or similar genes.

The LCO-inspired growth in nonlegumes suggests that they and legumes share a similar growth mechanism that LCOs help turn on, the authors note.

The researchers needed only about 10 molecules of LCO per cell to initiate cell division, they report. Seeing activity with such low amounts "is powerful evidence" that LCOs are plant growth regulators, says Tom LaRue of the Boyce Thompson Institute for Plant Research in Ithaca, N.Y.

Rohrig and his colleagues don't believe that their work will necessarily lead to nitrogen-fixing tobacco plants. However, synthetic LCOs may someday increase the efficiency of nodule formation in legumes, Walden speculates.

The researchers are now testing different forms of LCOs to see what key part of the molecule triggers cell division. However, "the really hot thing is finding out what is the receptor in legumes and nonlegumes for these LCOs," says Walden.