The mysterious relationship between legume-type plants and the nitrogen-fixing bacteria they harbour is to be investigated in a Japanese genome-sequencing project.
The nitrogen-grabbing function of the bacteria, which act in beans and other legumes, is of particular interest to researchers because it might replace high-nitrogen fertilizers as a means of accelerating plant growth. And nitrogen pollution of land and waterways is emerging as a major problem in Europe, east Asia and other intensively farmed regions of the world.
Japan's Kazusa DNA Research Institute in Chiba, a small agricultural district outside Tokyo, has already sequenced the genome of one bacterium involved, Mesorhizobium loti. It is now taking on the task of sequencing the associated legume, Lotus japonicus.
Satoshi Tabata, who heads the project, believes that unravelling the two genomes will lead to a better understanding of nitrogen fixation — the conversion of inorganic nitrogen from soil into organic nitrogen that plants can use for growth. Researchers may eventually use the genetic information to optimize nitrogen fixation in legumes, and perhaps apply it to other plants.
Plants cannot themselves fix nitrogen, but legumes and some other plant varieties have developed a symbiotic relationship with bacteria that can absorb nitrogen from the soil. The bacteria receive shelter and nutrients from the plant in exchange for nitrogen.
Researchers believe the fixation process involves several steps. First, the plant and bacterium undergo a recognition process, in which each notes the each other's presence by emitting chemical signals. The plant reacts to the bacterial signal by developing root 'nodules'. Once the bacteria have been taken up into the specialized nodule cells, they begin nitrogen fixation.
Tabata says the project will help to identify the genes that enable the plant and bacterium to recognize each other and the plant genes that control nodule formation.
Ultimately, some of these genes could be used in unrelated plants, such as corn and wheat, spurring 'natural' nitrogen fixation and alleviating the need for the current heavy use of high-nitrogen fertilizers.
Tabata's approach to the sequencing problem is to focus on sequences in the genome that are similar to genes of known function. This will provide useful information even before the genome is complete, he says. Jens Stougaard of the Laboratory of Gene Expression at the University of Aarhus in Denmark, which studies gene function by examining mutated versions of L. japonicus and other plants, says Kazusa's work is “laying out the path for the rest of us to follow”.
Furthermore, according to Tabata, L. japonicus could serve as a model for other legumes that are agriculturally important but difficult to study. Its genome is only about 450 megabases, about two-fifths the size of its more celebrated cousin, the soy bean. And its fast maturation and small size make it a convenient experimental plant.
But at the current rate it could take 15 years to complete the sequence. The project is funded by the Chiba regional government. Tabata says he would like other groups to get involved, but finds most national governments want to focus directly on crop plants.