Rhizobial mediation of Acacia adaptation to soil salinity: evidence of underlying trade-offs and tests of expected patterns


*Correspondence author. E-mail: peter.thrall@csiro.au


  • 1The ecological importance of host–soil symbiont associations for plant community structure and dynamics has been recently highlighted, particularly with regard to ecological and evolutionary responses along environmental gradients.
  • 2We used a range of native Australian shrubby legumes (Acacia spp.) and associated root-nodule forming bacteria (rhizobia) in laboratory and glasshouse studies to investigate the ecology of Acacia–rhizobial interactions with respect to soil salinity, a major environmental stressor in many parts of the world.
  • 3Analysis of laboratory growth data provided clear evidence of a trade-off in that growth rate of individual rhizobial isolates was reduced with increasing salt tolerance in culture.
  • 4A large replicated glasshouse inoculation trial using 40 strains of rhizobia and nine species of Acacia that varied in their ability to grow in saline soils found strong evidence of host specificity, but neither the average growth promotion of host plants nor the specificity of growth promotion was related to salt tolerance of the isolates.
  • 5In a second glasshouse experiment, we used a subset of salt-sensitive and salt-tolerant rhizobia and six Acacia species to evaluate performance and symbiotic effectiveness across different levels of soil salinity. Overall, we found no evidence of a relationship between rhizobial salt-tolerance (as measured in the laboratory) and impact on host growth performance, either in saline or non-saline soils, and there was no evidence that salt-tolerant rhizobia perform better in more saline environments.
  • 6Synthesis. More salt tolerant Acacia spp. were less responsive in growth to rhizobial inoculation than salt sensitive hosts, implying that evolution towards reduced dependence on the symbiosis may facilitate adaptation to salt, raising a general question regarding the extent to which shifts in dependence on microbial symbionts underlies plant adaptation to other environmental gradients.