Legumes and Rhizobium
The availability of nitrogen in the soil is a major limiting factor in agriculture, despite the fact that the atmosphere comprises 80% nitrogen. This apparent paradox is due to the fact that atmospheric nitrogen (N2) is very stable chemically and only organisms belonging to the group of prokaryotes (and no eukaryotic organisms) are capable of reducing nitrogen to a combined form which can be assimilated biologically. The most efficient nitrogen-fixing systems are those which couple the energy intensive chemical reduction of molecular nitrogen to photosynthesis. [13] |
Rhizobia are soil bacteria capable of eliciting on the roots of leguminous plants specialised root organs, known as nodules, in which they reduce dinitrogen. In this unique association between eukaryote and prokaryote the plant provides a source of energy and an ecological niche for the bacterium, which in return synthesises ammonia for the host plant. At the global scale, the Rhizobium-legume symbiosis provides a quantity of fixed nitrogen equivalent to that produced by the entire chemical fertiliser industry, and thus plays a major ecological and economic role. [14] |
The formation of nitrogen fixing nodules on legumes requires co-ordinated expression of several bacterial and plant genes. Initial stages of nodule formation require expression of specific nodulation (nod) genes by rhizobia. The nodABCFELMN gene products are involved in the synthesis of a group of signal molecules (Nod factors) that induce nodule morphogenesis. [15] |