Effect of rhizosphere bacterial communities on the growth and competition of Lactuca species

Background/Question/Methods

Plants generate soil communities that feedback on plant growth and competition. These feedbacks have been implicated in plant community composition and dispersion.  We used Lactuca sativa (iceberg lettuce) and its wild progenitor Lactuca serriola to test the hypotheses that separate plant species generate unique soil communities, and that these soil communities differentially influence host, and neighboring, plant growth and competition.  

In this study we used terminal restriction length polymorphisms (TRFLPs) to differentiate the rhizosphere bacterial communities associated with the Lactuca species.  Following this, we examined the growth of each Lactuca species in sterile, non-sterile, and preconditioned soil. We preconditioned the soil with Lactuca microbial communities by growing Lactuca plants in non-sterile soil for 5 weeks before the start of the experiment. Finally we grew each Lactuca species in competition with the other, in both sterile and non-sterile soils. To do this we grew focal Lactuca plants surrounded by 4 background  Lactuca plants. Background plants generated the competition and predominant soil community faced by focal plants.  The growth and competition experiments were harvested after 5 weeks of growth and the dry weights of the focal plants were recorded.

Results/Conclusions

Our TRFLP analysis showed Lactuca sativa to have a significantly more variable soil community relative to Lactuca serriola. The preconditioning experiment showed a significant effect of plant species, soil treatment, and the interaction of the two, on focal plant mass.  Preconditioning soil caused reduced growth in both Lactuca species. However, only Lactuca serriola showed significantly increased growth in sterile soils. Similarly sterilization only increased the competitive effect of Lactuca serriola background competitors. 

These results show that Lactuca serriola’s rhizosphere communities generate a stronger negative feedback for plant growth than do the communities associated with Lactuca sativa. Furthermore Lactuca serriola has a less variable community.  In conjunction with previous work that shows most feedbacks are negative, our study suggests that selection for plants that are able to grow in dense monoculture may have released Lactuca from species-specific negative soil feedbacks. This has important implications for both agriculture and the evolution of invasive plant species.