Rhizobial Position as a Main Determinant in the Problem of Competition for Nodulation in Soybean

SILVINA L. LÓPEZ- GARCÍA; TIRSO E. E. VÁZQUEZ; GABRIEL FAVELUKES; ANÍBAL R. LODEIRO

ENVIRONMENTAL MICROBIOLOGY

Blackwell Synergy

Año: 2002 vol. 4 p. 216 - 224

1462-2912

SummarySelected Bradyrhizobium japonicum strains inoculatedon soybean seeds often fail to occupy a significantproportion of nodules when a competitorrhizobial population is established in the soil. Thiscompetition problem could result from a genetic/physiological advantage of the adapted soil populationover the introduced inoculant or from a positionaladvantage, as the soil population already occupiesthe soil profile where the roots will penetrate,whereas the inoculant remains concentrated aroundthe seeds. Here, we have assessed the contributionof these factors with a laboratory model in which arhizobial population is established in sterile vermiculite.We observed that the wild-type strain B. japonicumLP 3004 was able to grow in pots with N-freeplant nutrient solution-watered vermiculite for six orseven generations with a duplication rate of at least0.7 day-1. In addition, the rhizobial population persistedfor 3 months with 106–107 colony-forming unitsml-1 of the vermiculite-retained solution. N-starved,young rhizobial cultures are more efficient in performingseveral steps along their early associationwith soybean roots. However, N starvation duringgrowth of rhizobia used for seed inoculation didnot enhance their competitiveness against a 1 monthvermiculite-established rhizobial population, whichoccupied more than 72% of the nodules. When a similarlyestablished rhizobial population was recoveredfrom the vermiculite and homogeneously suspendedin plant nutrient solution, these cells were significantlyless competitive (29% of nodules occupied)than rhizobia obtained from a fresh, logarithmicculture in a N-poor minimal medium, thus indicatingthat cell position rather than intrinsic competitivenesswas the determinant for nodule occupation.