Exclusion of inefficient Bradyrhizobium japonicum serogroups by soybean genotypes

Abstract

Soybean [Glycine max (L.) Merr.] forms a symbiosis with serogroups of Bradyrhizobium japonicum that differ in their dinitrogen fixing abilities. The objectives of this study were to identify soybean genotypes that would restrict nodulation by relatively inefficient serogroups indigenous to a large portion of the southeastern USA, and then characterize the nodulation responses of selected genotypes with specific bradyrhizobial strains under controlled conditions. From field screening trials followed by controlled single and competitive inoculations of serogroups USDA 31, 76 and 110, twelve soybean genotypes out of 382 tested were identified with varying levels of exclusion abilities. Soybean nodule occupancies and nodulation characteristics were influenced by plant genotype, environment (i.e. field or greenhouse), bradyrhizobial serogroup, and location of nodules (i.e. tap or lateral root). The cultivar Centennial sustains high seed yields even though it nodulates to a high degree with the inefficient serogroup USDA 31. In contrast, data from the released cultivars Braxton, Centennial and Coker 368 indicate that they may have been selected to exclude the inefficient serogroup USDA 76 from their tap root nodules, possibly contributing to high seed yield.

Plant Fitness Variation Caused by Different Mutualist Genotypes

“…For example, studies of domesticated legumes reveal that plants can alter the abundance of different bacteria (Nutman 1963, Chanway et al 1991. Also, certain host genotypes have been found to nodulate preferentially with particular bacteria in a heterogeneous soil population (e.g., Weiser et al 1990).…”

“…However, the extent to which any legume genotype can adaptively control the local composition of bacterial mutualists remains very much open to question. Domesticated legumes are commonly found to be extensively nodulated by bacteria that provide inferior symbiotic benefits (e.g., Bergerson 1970, Weiser et al 1990). This implies either that plants have little ability to ensure that they will encounter a superior symbiont, or that plant mechanisms to discriminate between good and bad symbionts are imperfect.…”

“…The use of a single isolate per plot forced plants to interact with a smaller range of symbiont genotypes than they are likely to encounter in nature (e.g., Weiser et al 1990). Nevertheless, this design is the simplest and most direct way to test for bacterial genotype effects on host fitness, because if plants were inoculated with mixtures of bacteria, each isolate's impact on host performance would be confounded by variation in bacterial nodule occupancy frequency.…”

Differences in numbers of nodules and lateral roots between soybean (Glycine maxL. Merr.) cultivars, Kitamusume and Toyosuzu

“…Number and average size of the nodules vary among soybean cultivars (Kanamori et al 1987;Weiser et al 1990;Sinclair et al 1991;Suganuma and Takai 1993;Ikeda 1997). Few studies have been carried out on the variation in the number and size of the nodules among cultivars, while nodulation.…”

Rj (rj) genes involved in nitrogen-fixing root nodule formation in soybean

“…It has been well documented that some soybean genotypes differentially restrict nodulation with specific strains (or serogroups) of Bradyrhizobium or Ensifer / Sinorhizobium (Cregan and Keyser 1986, Cregan et al 1989a, 1989b, Devine and Kuykendall 1996, Ferrey et al 1994, Keyser and Cregan 1987, Weiser et al 1990). This phenomenon has been attracting interest from the view of agricultural practice.…”