29 March 1996


PESTS and diseases of legumes could be controlled in the future by genetically engineered bugs in root nodules.

"The work is still at a very early stage, but we have succeeded in achieving some level of control of the beetle, Sitona, under laboratory conditions," says Lance Mytton, IGERs nitrogen fixation unit group leader.

This is done by introducing the Btt Toxin gene to the chromosomes of a rhizobium bacterium that is found in many soils. This has no effect until the bug forms a nodule on the root of a legume. When it does, the gene is triggered and it starts producing a toxin. The poison is not translocated through the plant, but is available to kill the larval form of the beetle, which feeds on legume roots.

Rules governing use of genetic engineering in biological control means that experimental work has to be conducted under strict containment until the procedure is shown to be completely safe. The next stage will be to see if it is possible to control clover disease pathogens in the same way.

More immediately, grass producers will soon have the opportunity to buy clover seed inoculated with more efficient nitrogen fixing rhizobia. New super rhizobium strains, bred by conventional selection techniques rather than genetic engineering, are now available for marketing to seed suppliers.

"We surveyed how much nitrogen was being fixed in different lowland swards and found it was frequently a lot less than it should have been, even though the clover looked dense. The problem is that there are big populations of native rhizobia competing with inoculated strains to form nodules on clover roots."

Aggressive native bugs, which are far less efficient fixers of atmospheric nitrogen, form 70% of the nodules. The new improved inoculant strains which should be commercially available in 1997, are so competitive that they regularly form 70% of the nodules. They also have an improved shelf life.

"There is a difference between the nitrogen fixing ability of different clovers, but legumes are only as good as the rhizobium in their root nodules."

Dr Mytton is sceptical about the idea of breeding grasses and cereals that get much of their nitrogen requirement from rhizobium nodules on their roots. He accepts that some free living bugs can be persuaded to fix nitrogen close to the roots of non-legumes, but without the mutually beneficial symbiotic relationship that exists between rhizobia and clover.

That said, new research at Aberystwyth into the importance of oxygen levels in efficient nitrogen fixation involves using the tropical legume sespania, which forms rhizobium nodules on its stems. These are green and can photosynthesise, an ability that it might be possible to exploit in agriculture, or bio-fuel production.