Serological and Ecological Characteristics of a Nodule-Dominant Serotype from an Indigenous Soil Population of Rhizobium leguminosarum bv. trifolii

Abstract

Although at least 13 antigenically distinct serotypes of Rhizobium leguminosarum bv. trifolii exist in an Abiqua silty clay loam soil, one serotype, AS6, occupies .50% of the root nodules formed on field-grown subclover and between 33 and 78% of the nodules formed on five annual clover species grown in the same soil under laboratory conditions. The dominance of subclover nodules by serotype AS6 was reproducible over a 4-year sampling period and throughout the entire 200by 100-m pasture examined. Serotype AS6 was composed of three antigenically distinct subtypes (AS6-a, AS6-b, and AS6-c). Each subtype contributed about one-third of the AS6 isolates recovered from nodules of field-grown subclover plants and maintained similar population densities in nonrhizosphere and rhizosphere soil. Rhizobia with the AS6 antigenic signature accounted for from 20 to 100% of the soil populations of R. keguminosarum in arable and pasture soils under legumes throughout the state of Oregon. Over a 12-month period, the population densities of the serotype AS6 complex and three minor nodule-occupying serotypes (AG4, AP17, and AS21) were measured in the rhizospheres of field-grown subclover and orchard grass and in nonrhizosphere Abiqua soil. Regardless of season or serotype, the orchard grass rhizosphere effect was minimal, with the ratio between rhizosphere and nonrhizosphere serotype population densities ranging between 2.5 (midsummer) and 10.5 (spring). In contrast, the magnitude of the subclover rhizosphere effect varied seasonally and among serotypes. Between October and December the ratios for all serotypes were similar (12.5 to 25.5). However, in the spring (April and May), the magnitude of the rhizosphere effect varied among the indigenous serotypes (ratios, 10.5 to 442) and for minor nodule-occupying serotypes AS21 (ratio, 442) and AP17 (ratio, 47) was as great as, or even greater than, the magnitude of the rhizosphere effect observed with the AS6 complex (ratio, 65.5).

Analysis of genetic structure in soil populations of Rhizobium leguminosarum recovered from the USA and the UK

“…Because soil bacteria are generally resilient to stress (Paul & Clark 19891, and because most of the soil volume is a nutrient poor, highly porous and discontinuous matrix, soil may favour the persistence of a large number of genotypic variants of a species and foster the develop ment of nonclonal population structure (Bottomley 1993). Alternatively, soil contains nutrient-rich microenvironments such as the root-soil interface where sufficient energy exists to allow superior genotypes to undergo clonal expansion by binary fission (Leung et al 1994a). Since enhanced microbial growth in soil microenvironments is invariably accompanied by an enhanced rate of predation by protozoans and nematodes (Christensen et nl.…”

Genetic diversity of an Italian Rhizobium meliloti population from different Medicago sativa varieties

“…In symbiotic rhizobacteria like Rhizobium species, the evolution of the population structure may be influenced by environmental conditions like biological barriers to gene exchange or geographical isolation and also by the type of soil or the genotype of the host plant (8). It is known that legumes can be preferentially nodulated by one of two rhizobial strains even if the preferred strain is outnumbered by the other strain (28,44). However, little is known about the relationships between the genetic polymorphism within a Rhizobium population and the preferences for nodulating particular genotypes of compatible host plants (28).…”

“…It is known that legumes can be preferentially nodulated by one of two rhizobial strains even if the preferred strain is outnumbered by the other strain (28,44). However, little is known about the relationships between the genetic polymorphism within a Rhizobium population and the preferences for nodulating particular genotypes of compatible host plants (28). One additional element that can also play a critical role in the evolution of Rhizobium populations is the occurrence of large plasmids that can have an evolutionary history different from the evolutionary history of the strains that they live in (39,47).…”

“…Most studies of the population genetics of Rhizobium and Bradyrhizobium species have been carried out by using serological or multilocus enzyme electrophoresis techniques (3,8,9,11,12,28,(40)(41)(42). More recently, techniques based on DNA analysis have been used with or instead of multilocus enzyme electrophoresis (4, 7, 9, 12, 24-27, 45, 47).…”

Competition for Nodulation

“…However, the rhizosphere effect, i.e. the ratio between rhizosphere and non rhizosphere population densities increased much more among the rare serotypes than in the dominant one in spring, while this effect was similar in other seasons (Leung et al, 1994b). Moreover, in a study carried out by Laguerre et al (2003) in France, a difference between fave bean and pea was observed for the relationship of dominance in soil or nodules in the R. leguminosarum bv viceae population.…”