Soybean-Nodulating Strains With Low Intrinsic Competitiveness for Nodulation, Good Symbiotic Performance, and Stress-Tolerance Isolated From Soybean-Cropped Soils in Argentina

Abstract

Soybean is the most important oilseed in the world, cropped in 120–130 million hectares each year. The three most important soybean producers are Argentina, Brazil, and United States, where soybean crops are routinely inoculated with symbiotic N 2 -fixing Bradyrhizobium spp. This extended inoculation gave rise to soybean-nodulating allochthonous populations (SNAPs) that compete against new inoculant for nodulation, thus impairing yield responses. Competitiveness depends on intrinsic factors contributed by genotype, extrinsic ones determined by growth and environmental conditions, and strain persistence in the soil. To assess these factors in Argentinean SNAPs, we studied 58 isolates from five sites of the main soybean cropping area. BOX-A1R DNA fingerprint distributed these isolates in 10 clades that paralleled the pHs of their original soils. By contrast, reference Bradyrhizobium spp. strains, including those used as soybean-inoculants, were confined to a single clade. More detailed characterization of a subset of 11 SNAP-isolates revealed that five were Bradyrhizobium japonicum , two Bradyrhizobium elkanii , two Rhizobium radiobacter (formerly Agrobacterium tumefaciens ), one Bradyrhizobium diazoefficiens , and one Paenibacillus glycanilyticus -which did not nodulate when inoculated alone, and therefore was excluded from further characterization. The remaining subset of 10 SNAP-isolates was used for deeper characterization. All SNAP-isolates were aluminum- and heat-tolerant, and most of them were glyphosate-tolerant. Meanwhile, inoculant strains tested were sensitive to aluminum and glyphosate. In addition, all SNAP-isolates were motile to different degrees. Only three SNAP-isolates were deficient for N 2 -fixation, and none was intrinsically more competitive than the inoculant strain. These results are in contrast to the general belief that rhizobia from soil populations evolved as intrinsically more competitive for nodulation and less N 2 -fixing effective than inoculants strains. Shoot:root ratios, both as dry biomass and as total N, were highly correlated with leaf ureide contents, and therefore may be easy indicators of N 2 -fixing performance, suggesting that highly effective N 2 -fixing and well-adapted strains may be readily selected from SNAPs. In addition, intrinsic competitiveness of the inoculants strains seems already optimized against SNAP strains, and therefore our efforts to improve nodules occupation by inoculated strains should focus on the optimization of extrinsic competitiveness factors, such as inoculant formulation and inoculation technology.

Characterization of FliL Proteins in Bradyrhizobium diazoefficiens: Lateral FliL Supports Swimming Motility, and Subpolar FliL Modulates the Lateral Flagellar System

“…B. elkanii is another important soybean symbiont that possesses only one flagellar system, similar to the B. diazoefficiens subpolar system. Furthermore, the type strain, B. elkanii USDA 76, moved equally well as B. diazoefficiens USDA 110 in semisolid medium without the addition of a viscous agent (2), and B. elkanii soil isolates were even more motile than B. diazoefficiens or Bradyrhizobium japonicum under these conditions (39), indicating that the single flagellar system of B. elkanii was enough for efficient swimming in such a tortuous environment. However, B. elkanii was unable to swim in viscous medium (2).…”

Soybean Nodulation Response to Cropping Interval and Inoculation in European Cropping Systems

“…Research in the North and South America showed that the response of inoculation with commercial Bradyrhizobia inoculants differed in soils when soybean was included in previous rotations. This introduced SNB in the soil can either enhance, stay neutral or compete with the elite strain in the commercial inoculants (Thies et al, 1991;Piccinetti et al, 2013;Iturralde et al, 2019).…”

Reaction of soybean [Glycine max (L.) Merr.] to seed inoculation with Bradyrhizobium japonicum bacteria

“…HiStick® Soy significantly increased the number and dry weight of nodules on the roots compared the location of the plantation and bacterial strain. Iturralde et al (2019) also obtained a varied number and dry weight of nodules on soybean roots depending on the strain of symbiotic bacteria. This provided the basis for selecting the best one for the production of commercial preparations.…”

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In soybean cultivation, inoculation of sowing material with bacteria fixing free nitrogen from the air is an especially important procedure (Iturralde et al 2019). This treatment is particularly recommended when native strains of symbiotic bacteria are not present in the soil, or there are too few of them (Zimmer et al 2016).

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