Rhizobial position as a main determinant in the problem of competition for nodulation in soybean

Abstract

Selected Bradyrhizobium japonicum strains inoculated on soybean seeds often fail to occupy a significant proportion of nodules when a competitor rhizobial population is established in the soil. This competition problem could result from a genetic/ physiological advantage of the adapted soil population over the introduced inoculant or from a positional advantage, as the soil population already occupies the soil profile where the roots will penetrate, whereas the inoculant remains concentrated around the seeds. Here, we have assessed the contribution of these factors with a laboratory model in which a rhizobial population is established in sterile vermiculite. We observed that the wild-type strain B. japonicum LP 3004 was able to grow in pots with N-free plant nutrient solution-watered vermiculite for six or seven generations with a duplication rate of at least 0.7 day(-1). In addition, the rhizobial population persisted for 3 months with 10(6)-10(7) colony-forming units ml(-1) of the vermiculite-retained solution. N-starved, young rhizobial cultures are more efficient in performing several steps along their early association with soybean roots. However, N starvation during growth of rhizobia used for seed inoculation did not enhance their competitiveness against a 1 month vermiculite-established rhizobial population, which occupied more than 72% of the nodules. When a similarly established rhizobial population was recovered from the vermiculite and homogeneously suspended in plant nutrient solution, these cells were significantly less competitive (29% of nodules occupied) than rhizobia obtained from a fresh, logarithmic culture in a N-poor minimal medium, thus indicating that cell position rather than intrinsic competitiveness was the determinant for nodule occupation.

Analysis of the role of the two flagella of Bradyrhizobium japonicum in competition for nodulation of soybean

“…Thus, the identification of conditions that are a determinant for competitiveness of the inoculated rhizobia is an important goal. We proposed that the position of rhizobia in the soil profile in relation to the roots and the rhizobial motility in the soil might be two of these conditions (López‐García et al , 2002). Further studies by Kanbe et al (2007) and Althabegoiti et al (2008) indicated that B. japonicum possesses two different flagella.…”

The effect of biotic and physical factors on the competitive ability of Rhizobium leguminosarum

“…They include the ability of rhizobia to utilize specific carbon and energy sources [17,18], the overall metabolic potential of bacteria [19], bacteriocin production, resistance to bacteriocin [20], the susceptibility to plant molecular signals [21,22] and bacterial motility [23]. In addition, the legume hosts themselves [24][25][26] and the distribution of rhizobia in the soil [27] may also influence the outcome of competitive interactions between rhizobia. Furthermore, it is possible that compounds other than plant-derived flavonoids and rhizobial Nod factors may influence strain competitiveness for nodule occupancy in Rhizobiumlegume interactions [28,29].…”

Exploiting inter-organismal chemical communication for improved inoculants

“…Commercial inoculants for soybean have been found to contain high levels of NolA (Loh et al 2001). When B. japonicum cells are cultured in N-starved media, the production of exopolysaccharides, capsular polysaccharides and the expression of nodC:lacZ is reduced, which results in the production of reduced nodule number by older cultures, or in delayed nodule formation in younger culture (Lopez-Garcia et al 2001 greenhouse experiments demonstrated that N-starvation during culture of an effective rhizobia strain did not enhance their competitiveness against an established and less competitive strain (Lopez-Garcia et al 2002), suggesting that number and position are more important than genetic/physiological aspects of competitiveness for inoculated strains. Using nod gene expression to indicate Nod factor production, Loh and Lopez-Garcia's experiments (Loh et al 2001;Lopez-Garcia et al 2001) demonstrated that although Nod factor production by a culture increased as cell density increased, production per cell decreased.…”