Nitrogen is one of the most important plant nutrients. However, some crops require little or no added nitrate, because their roots are infected by soil bacteria that are able to take in atmospheric nitrogen.
Both parties in the symbiotic relationship benefit; the plant gets nitrogen from the bacteria, and the bacteria get other nutrients from the plant.
Researchers at Glasgow University are interested in altering nitrogen-fixing bacteria to extend the range of plants that can be infected. One of the long-term objectives is to infect crop plants such as wheat and barley, which normally require the addition of nitrate fertilisers for healthy growth.
The cost savings could be enormous. Each of the 1.9 million hectares of wheat grown in the UK each year is dressed with 180kg, or pounds 55 of nitrate fertiliser - a cost of more than pounds 100m for fertilising wheat alone.
The most widely studied nitrogen-fixing association is that between leguminous plants such as peas and beans and the bacterium Rhizobium.
But it is the less well-known bacterium Frankia that is arousing most interest as a potential partner for plants that cannot fix nitrogen. This is because it can interact with a much wider range of host plants than Rhizobium, covering species as diverse as alder trees and the garden plant Ceanothus. These have little in common apart from their ability to form a symbiotic relationship with Frankia.
Scientists think this degree of so-called host range promiscuity implies it may be relatively easy to engineer Frankia to enable it to infect plants outside its usual host range.
The first step is to try to pinpoint the gene, or genes, controlling Frankia's ability to pick out and infect the roots of a host plant. The researchers are studying various strains of Frankia to find those that are best at infecting existing host plants. Relying on trial and error, they will then insert genes from this strain into a less infectious Frankia strain, in an attempt to boost its infectivity, and so identify the genes that control infection.
Plants infected with Frankia are already used to improve the condition of poor-quality soils. Alder trees, for example, are grown in conifer plantations specifically for their nitrogen- fixing abilities. Altering Frankia so that it could infect conifers would allow planting density to be increased.
One snag: providing crops with nitrogen by infecting them with Frankia could increase nitrate pollution if it turned out that the nitrogen ended up in the root system rather than in the harvested parts of the plant.
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