Strain selection for improvement ofBradyrhizobium japonicumcompetitiveness for nodulation of soybean

Abstract

A Bradyrhizobium japonicum USDA 110-derived strain able to produce wider halos in soft-agar medium than its parental strain was obtained by recurrent selection. It was more chemotactic than the wild type towards mannitol and three amino acids. When cultured in minimal medium with mannitol as a single carbon-source, it had one thick subpolar flagellum as the wild type, plus several other flagella that were thinner and sinusoidal. Root adsorption and infectivity in liquid media were 50-100% higher for the selected strain, but root colonization in water-unsaturated vermiculite was similar to the wild type. A field experiment was then carried out in a soil with a naturalized population of 1.8 x 10(5) soybean-nodulating rhizobia g of soil(-1). Bradyrhizobium japonicum strains were inoculated either on the soybean seeds or in the sowing furrows. Nodule occupation was doubled when the strains were inoculated in the sowing furrows with respect to seed inoculation (significant with P<0.05). On comparing strains, nodule occupation with seed inoculation was 6% or 10% for the wild type or selected strains, respectively, without a statistically significant difference, while when inoculated in the sowing furrows, nodule occupation increased to 12% and 22%, respectively (differences significant with P<0.05).

Transcriptional Control of the Lateral-Flagellar Genes of Bradyrhizobium diazoefficiens

“…The expression of each flagellar system is also different. The subpolar system seems constitutive within a range of conditions; by contrast, the expression of the lateral system requires arabinose as a carbon source or viscosity in the culture medium or the presence of obstacles in the swimming path (20)(21)(22). Hence, growth in liquid medium with mannitol as the carbon source does not permit the expression of the lateral flagella.…”

“…Although neither of these flagellar systems is required for the nodulation of soybean plants (30), bacterial motility might be essential for nodule occupation in competition against populations of compatible rhizobia in the soil (31). In earlier work, we obtained a derivative of B. diazoefficiens USDA 110 with higher motility by in vitro selection (21). The inoculation of this derivative on experimental soybean crops planted in soils with dense soybeannodulating competitor populations resulted in an enhanced nodule occupation by the derivative and promoted a higher soybean grain yield (21,31).…”

“…In earlier work, we obtained a derivative of B. diazoefficiens USDA 110 with higher motility by in vitro selection (21). The inoculation of this derivative on experimental soybean crops planted in soils with dense soybeannodulating competitor populations resulted in an enhanced nodule occupation by the derivative and promoted a higher soybean grain yield (21,31). Further studies indicated that the lateral-flagellar system of this derivative became derepressed upon culture in liquid medium with mannitol as the carbon and energy source (21,22).…”

“…The inoculation of this derivative on experimental soybean crops planted in soils with dense soybeannodulating competitor populations resulted in an enhanced nodule occupation by the derivative and promoted a higher soybean grain yield (21,31). Further studies indicated that the lateral-flagellar system of this derivative became derepressed upon culture in liquid medium with mannitol as the carbon and energy source (21,22). Therefore, the control of lateral-flagellar synthesis in this species should take into account the cell's needs on the basis of the environmental conditions in order to coordinate the activity of both flagellar systems, which would appear to be essential for the symbiotic interaction with soybean plants.…”

Sinorhizobium meliloti Chemotaxis to Multiple Amino Acids Is Mediated by the Chemoreceptor McpU

“…Bacterial chemotaxis precedes symbiosis and enables rhizobia to actively swim toward and accumulate in the host spermosphere and rhizosphere by responding to hostreleased compounds (Ames and Bergman 1981;Bergman et al 1988;Caetano-Anollés et al 1988;Dharmatilake and Bauer 1992;Malek 1989;Scharf et al 2016;Soby and Bergman 1983;Uren 2000). In particular, rhizobial motility and chemotaxis have been proven to enhance successful symbiotic interactions of Bradyhizobium japonicum, Rhizobium leguminosarum, Rhizobium trifolii, and Sinorhizobium meliloti, with soybean, pea, clover, and alfalfa, respectively (Althabegoiti et al 2008;Ames and Bergman 1981;Caetano-Anollés et al 1988;Mellor et al 1987;Miller et al 2007).…”