The impact of intra-specific diversity in the rhizobia-legume symbiosis

Abstract

Rhizobia - nitrogen-fixing, root-nodulating bacteria - play a critical role in both plant ecosystems and sustainable agriculture. Rhizobia form intracellular infections within legumes roots where they produce plant accessible nitrogen from atmospheric nitrogen and thus reduce the reliance on industrial inputs. The rhizobia-legume symbiosis is often treated as a pairwise relationship between single genotypes, both in research and in the production of rhizobial inoculants. However in nature individual plants are infected by a high diversity of rhizobia symbionts. How this diversity affects productivity within the symbiosis is unclear. Here, we use a powerful statistical approach to assess the impact of diversity within the Rhizobium leguminosarum - clover symbiosis using a biodiversity-ecosystem function framework. Statistically, we found no significant impact of rhizobium diversity. However this relationship was weakly positive - rather than negative - indicating that there is no significant cost to increasing inoculant diversity. Productivity was influenced by the identity of the strains within an inoculant; strains with the highest individual performance showed a significant positive contribution within mixed inoculants. Overall, inoculant effectiveness was best predicted by the individual performance of the best inoculant member, and only weakly predicted by the worst performing member. Collectively, our data suggest that the Rhizobium leguminosarum - clover symbiosis displays a weak diversity-function relationship, but that inoculant performance can be improved through the inclusion of high performing strains. Given the wide environmental dependence of rhizobial inoculant quality, multi-strain inoculants could be highly successful as they increase the likelihood of including a strain well adapted to local conditions across different environments.

Rhizobium nodule diversity and composition are influenced by clover host selection and local growth conditions

“…In support of this hypothesis, we found that gsB strains displayed relatively highest maximum growth rates and biofilm formation in TY broth, which could have helped them to establish symbiosis more frequently compared to other genospecies strains (Figure S9). These competitive differences are in line with our previous study where we showed that R. leguminosarum genospecies A, C and E display clear differences in competitive fitness and metabolic capacities, despite not differing in their symbiotic performance on white clover (Fields, Moffat, Friman, & Harrison, 2021; Fields, Moffat, Harrison, et al, 2021). However, as gsB strains did not dominate across all varieties and crosses, it is possible that changes in genospecies frequencies were also driven by host preference for compatible symbionts.…”

Grain Legume Yield Responses to Rhizobia Inoculants and Phosphorus Supplementation Under Ghana Soils: A Meta-Synthesis

“…For soybean, all the tested rhizobia species were from the B. japonicum and B. diazoefficiens groups, irrespective of the soil and edaphic conditions and the origin of strains. The results imply that the success of inoculation is highly determined by the identity of the strain in an inoculant, rather than the origin of the strain used (Fields et al, 2021 ).…”

Engineering multifunctional rhizosphere probiotics using consortia of Bacillus amyloliquefaciens transposon insertion mutants

“…For example, if bioinoculant species produce antimicrobials, it is likely that related strains share the same antibiotic biosynthesis and resistance genes, and hence, unlikely antagonize each other ( Xia et al, 2022 ). Increasing intra-species diversity could further lead to increased bioinoculant consortia multifunctionality, which has been shown to play an important role in the functioning of microbial ecosystems ( Van Rossum et al, 2020 ; Fields et al, 2021 ; Raffard et al, 2019 ; Nicastro et al, 2020 ; Blake et al, 2021 ; Dragoš et al, 2018 ). Multifunctionality could be optimized based on ecological complementarity by assembling consortia from strains that use different niches or specialize in different functions within the same niche ( Dragoš et al, 2018 ; Martin et al, 2016 ).…”