Bacterial surface polysaccharides and their role in the rhizobia-legume association

The establishment of a nitrogen-fixing symbiosis is an economically important plant phenomenon. Biological reduction of dinitrogen to ammonia is among the most effective fixation systems facilitating the plant growth in nitrogen starved soils without the requirement of massive inputs of fertilizers. This process occurs in legumes roots in structures called nodules. Nodule development is induced when legumes enters into association with gram-negative soil bacteria that belong to the Rhizobiaceae family, such as Bradhyrhizobium, Mesorhizobium, Sinorhizobium, Rhizobium and Azorhizobium, all denoted with the general term, rhizobia. Rhizobia invade the nodule and differentiate into a state called bacteroid able to fix the atmospheric nitrogen under the appropriate conditions generated inside the nodule. The nodulation process is the result of a selective interaction between rhizobia and the legume family (Leguminosae or Fabaceae), which means that each legume is nodulated only by specific bacterial species. For example, Medicago sativa by S. meliloti, Glycine max by B. japonicum and Lotus spp. by M. loti. Some bacteria display broad host range such as the Rhizobium NGR234 that can nodulate more than 112 legume genera. On the other hand M. loti has a reduced host range allowing nodulation of different Lotus species (L. corniculatus, L. japonicus, L. glaber and L. uliginosus) and Leucaena leucocephala. Nodules are classified as determinate or indeterminate, depending on the legume that is nodulated. Indeterminate nodules are elongated and present a persistent meristem at their tip, which is infected by rhizobia residing in the nodule. This characteristic results in a gradient of development stages from the older tissue near the root to the growing meristem at the nodule tip. Determinate nodules lack a persistent meristem, are usually round, and don’t show a gradient of development stages as is the case of indeterminate nodules. Examples of indeterminate nodule morphology are the ones formed on roots of Medicago sativa (alfalfa), Medicago truncatula, Pisum sativum (pea), Trifolium species (clovers) and L. leucocephala, meanwhile those formed on roots of Lotus species, Glycine max (soybean) and Vicia faba (bean) are determined.