Science - January 4, 2011

Plants have a single reception desk for symbiosis

The way in which peas and beans fix nitrogen from the air using bacteria is similar to the symbiosis between other plants and soil fungi, according to a report in Science by molecular biologists from Wageningen. Their findings are thanks to a forgotten tree from New Guinea.

Cooperation between the Parasponia tree and the bacteria that fix nitrogen from the air
Professor Ton Bisseling's group has spent years researching papilionaceous plants such as beans and peas. These plants have developed a symbiotic relationship with rhizobium bacteria, which fix nitrogen from the air and supply it to the plant in return for nutrients. Other plant species do not have this capability. The group is searching for the unique genes that the papilionaceous plants have that make this trick possible, but so far without success.

Hope is now pinned on a discovery dating from the 1970s, when the late Anton Akkermans, a Wageningen researcher, found a tree of the genus Parasponia in New Guinea that also cooperated with rhizobium bacteria. 'Studies were carried out then but that was before the availability of genomics techniques', says researcher René Geurts. 'Later the tree was forgotten.'

Signal compounds
Geurts obtained twenty-year-old Parasponia seeds, which he studied with his colleagues to see whether the signal compounds that rhizobium bacteria use to initiate the process of nitrogen fixation in papilionaceous plants are also necessary in Parasponia. The answer was yes. He also determined the protein receptor in Parasponia that recognizes these signals. That too turned out to be the same receptor protein as in the papilionaceous plants.

Another important discovery was that this receptor protein in the tree is also responsible for communication with the mycorrhizal fungi in the soil. 'Our conclusion is that these fungi must also be producing signal compounds for the symbiosis', says Geurts. The mycorrhizal fungi improve the plant's uptake of phosphate and other nutrients from the soil. Unlike the rhizobium bacteria, these fungi cooperate with a wide range of plant species.

Find the differences
That suggests that the mechanism for nitrogen fixation is widely available. Parasponia's reception desk for the useful bacteria and fungi is provided by plant genes that also occur in non-papilionaceous plants. Even so, close relatives of Parasponia such as hemp and hops do not have this capability. That is why the molecular biologists are now going to make a genetic comparison of Parasponia and a relative without nitrogen fixation (such as the hop plant). 'The task will be to find the genetic differences. There can't be all that many.'

Only 10 million
Previous attempts to find the unique genes for nitrogen fixation in papilionaceous plants have failed, but Geurts thinks he stands a better chance. The ability of papilionaceous plants to fix nitrogen using bacteria probably developed about 60 million years ago, but Parasponia developed the trick less than 10 million years ago, according to Geurts. As a result, there will be far fewer genetic differences between two relatives. Geurts will be looking for a needle in a hay bale rather than the proverbial haystack. The molecular biologists hope to have an answer in two years' time.