Symbiotic Effectiveness of Rhizobial Mutualists Varies in Interactions with Native Australian Legume Genera

Abstract

Background and ObjectivesInteractions between plants and beneficial soil organisms (e.g. rhizobial bacteria, mycorrhizal fungi) are models for investigating the ecological impacts of such associations in plant communities, and the evolution and maintenance of variation in mutualisms (e.g. host specificity and the level of benefits provided). With relatively few exceptions, variation in symbiotic effectiveness across wild host species is largely unexplored.MethodsWe evaluated these associations using representatives of several legume genera which commonly co-occur in natural ecosystems in south-eastern Australia and an extensive set of rhizobial strains isolated from these hosts. These strains had been previously assigned to specific phylotypes on the basis of molecular analyses. In the first of two inoculation experiments, the growth responses of each host species was evaluated with rhizobial strains isolated from that species. The second experiment assessed performance across genera and the extent of host specificity using a subset of these strains.ResultsWhile host growth responses to their own (sympatric) isolates varied considerably, rhizobial phylotype was a significant predictor of symbiotic performance, indicating that bacterial species designations on the basis of molecular markers have ecological importance. Hosts responded in qualitatively different ways to sympatric and allopatric strains of rhizobia, ranging from species with a clear preference for their own strains, to those that were broad generalists, through to species that grew significantly better with allopatric strains.ConclusionTheory has focused on trade-offs between the provision of benefits and symbiont competitive ability that might explain the persistence of less beneficial strains. However, differences in performance among co-occurring host species could also drive such patterns. Our results thus highlight the likely importance of plant community structure in maintaining variation in symbiotic effectiveness.

Potential of indigenous bradyrhizobia versus commercial inoculants to improve cowpea (Vigna unguiculataL. walp.) and green gram (Vigna radiataL. wilczek.) yields in Kenya

“…Ndfa values of 83-98% for cowpea are in line with findings of Naab on symbiotic effectiveness, nodulation, N 2 fixation and biomass yield has been reported in other studies (Hafeez et al 2000;Thrall et al 2011). The variability observed in nodulation, shoot dry weight and BNF for both legume species in the 10 different soils can be attributed to the differences in competitive ability and effectiveness of the indigenous rhizobial populations in these soils.…”

Strategy Diversity Stabilizes Mutualism through Investment Cycles, Phase Polymorphism, and Spatial Bubbles

“…Many studies suggest that microbial populations and communities are often structurally and genetically more diverse [67], [69], considering both type or strain richness and/or genetic diversity [68], than what can be explained by local host diversity [70]. Also the effectiveness of rhizobia, such as their ability to form nodules and their capacity to fix nitrogen, varies greatly within species, and naturally, between species [27], [58], [71], [72]; similar conclusions hold for the performance of mycorrhizal interactions [26], [65], [73]. This diversity amounts to a high variety of investment strategies; in other words, less mutualistic types coexist with more beneficial mutualists in natural communities [6], [19], [21], [22], [27], [34], [36], [71].…”

The Impact of Beneficial Plant-Associated Microbes on Plant Phenotypic Plasticity

“…While the symbiosis generally is thought to be mutualistic, it can be viewed as a form of parasitism (Djordjevic et al 1987) because the symbiont requires large amounts of carbon from the host, and varying degrees of parasitism can be observed (e.g., Barrett et al 2012; Thrall et al 2011). Nevertheless, in most cases, the symbiosis increases plant fitness under N limiting conditions.…”