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Symbiosis gets the nod
In the early days of genetic manipulation it was frequently suggested that it would be possible to transfer nitrogen-fixing capability to non-leguminous crop plants. However, doubts were soon raised about the ability of non-leguminous plants to provide the highly regulated environment of the root nodule in which N2-fixation occurs. Despite the role of the host plant, host genes involved in establishing the symbiosis have been the focus of less attention than the nod genes of the bacterial symbiont, but this gap is being filled by the type of work described by Tsyganov and associates (St Petersburg, Russia and Aachen, Germany; pp. 357-366). This group has previously reported that more than 40 pea genes (the Sym genes) are involved in establishing the symbiosis. This paper focuses on the initiation phase, using mutants that cannot develop root nodules (Nod-). The first indication of an 'infection' by Rhizobium is root hair curling in response to the bacterium. Even this apparently simple response involves several genes, some of which also participate in the next steps: root hair curling leads to formation of the infection thread (by which the bacteria enter the root) and at the same time cortical cells are induced to divide in an organized manner to form the nodule primordium. These two events are kept in step first because some genes are involved in both processes, and secondly because there is signalling between the two processes. By the time the nodule primordium has been established, at least 15 host genes have been involved. The picture is further complicated as we recall that there is also cross-talk between the products of these genes and those of the Rhizobium symbiont. So, while these results emphasize the difficulties of transferring N2-fixation to non-legumes, they do provide background information for enhancement of the process in legumes themselves.Professor J. A. Bryant
University of Exeter, UK
J.a.bryant{at}exeter.ac.uk
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