CONICET | Buscador de Institutos y Recursos Humanos

Competition for nodulation Julieta Perez-Gimenez, Juan Ignacio Quelas and Anibal Lodeiro IBBM-Facultad de Ciencias Exactas, Universidad Nacional de La Plata-CONICET. Calles 47 y 115 (1900) La Plata, Argentina. N nutrition in soybean crops is dual: combined N may be obtained from the soil, or NH3 may be synthesized by reduction of atmospheric N2 in a symbiosis with different bacteria known as rhizobia, in particular, Bradyrhizobium japonicum. Since N2 fixation is more economic and ecologically sustainable than fertilization with chemical N compounds such as urea, soybean crops are routinely inoculated with selected strains of B. japonicum with high N2-fixing performance. Being a biological process, symbiotic N2 fixation is sensitive to additional limitations with respect to N fertilization. One important limitation is the competition exerted by the soil microbiota. Symbiotic N2 fixation requires the invasion of roots by the rhizobia, and the formation of specialized organs known as root nodules. Each nodule is normally invaded and occupied by a single clone of rhizobia, and at lower frequency, by two clones. The maximum number of nodules formed in a soybean root is in the order of 102, and therefore this is the range of rhizobial clones that get the opportunity to occupy these nodules. However, this same root is in contact with at least 106 rhizobia colonizing its surface and its surroundings, and therefore only 1 clone every 10,000 is successful in establishing itself in the nodules. This creates a strong competition between the inoculated strains and those rhizobia resident in the soil for the invasion of the root nodules. This problem becomes very serious when rhizobia from the soil are poor N2-fixers, a situation rather common, since soil, contrarily to humans, does not select for high N2-fixing performance. Therefore, several measures were proposed to counteract the problem of competition for nodulation. This problem is very complex, since the plant, the soil, the local rhizobial population, and the inoculant play key roles. In addition, this system is under the influence of multiple biotic and abiotic factors that exert a different effect on each part of the system. Hence, a deep knowledge of the system and the factors that affect it is required to rationally manage the problem of competition for nodulation. Several years of studies indicated that effective measures are the manipulation of compatibility between plant and bacterial genotypes, the improvement of local bacterial genotypes, the inoculant formulation, the inoculant application technology, and the bacterial motility in the soil.

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