A common genetic mechanism discovered in nitrogen-fix...(ms remain little studied generally colon...)

Date:3/10/2008

ms remain little studied, generally colonize disturbed environments, such as volcanic soils or mining-affected ground, and nitrogen-poor terrains such as moraines or sandy soils. About 260 species of actinorhizal plants exist, spread among 24 genera and classified into eight families of angiosperms, flowering plants. An IRD team, jointly with a laboratory of the University of Munich, turned particular attention to the tropical tree Casuarina, or Australian pine. The first step employed molecular methods to find the sequence coding for the SymRK gene in the Casuarina genome. Once isolated, the question was whether or not Casuarina needed this gene to establish its symbiosis with the bacterium Frankia. The team therefore developed transgenic plants in which SymRK gene expression was strongly reduced. Subsequent comparison of these plants ability to form symbiotic root nodules with that of control plants showed that the plants with lowered SymRK gene expression produced only half as many root nodules as the controls. The same modified individuals also showed strongly reduced mycorrhization compared with the unaltered Australian pine. The results therefore demonstrated that the weakened SymRK gene expression produced a considerable loss of Casuarinas nitrogen-fixing ability and also a reduction in its aptitude to form mycorrhiza. More generally, these conclusions bring out the fact that, in nitrogen fixing plants, a common genetic factor seems essential for setting-up the three types of symbiotic association involving bacteria (Rhizobium or Frankia) or a mycorrhizal fungus.

Improved understanding of these genetic mechanisms could in the coming years contribute to the development of procedures for performing the transfer of the genetic material necessary for atmospheric nitrogen fixation to plants like cereals, which do not possess this faculty. Although rice, for example, establishes a symbiotic relation with a mycorrhizal fungus, it is incapable of developing nitr
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Contact: Gregory Flechet fichesactu@paris.ird.fr 33-014-803-7607 Institut de Recherche Pour le Dveloppement Source:Eurekalert

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