Rare microbes drive the expression of plant root traits

Background/Question/Methods

The plant-associated microbiome can alter plant function and traits in the same way that the gut microbiome influences human health.  For example, certain strains of bacteria living in plant roots can cause the plant to elongate its root system or to allocate more growth to shoots at the expense of roots. While several studies have explored how individual bacterial stains can alter plant trait expression, few have looked a how community composition may alter plant trait expression. Here we ask: Does the composition of the microbial community alter the expression of key plant traits?

We constructed root endophytic bacterial communities on a single clone of Populus deltoides to explore how changes in community composition alter plant morphological and physiological traits. Using four distinct strains of bacteria, whose individual presences lead to the expression of different traits, we inoculated communities in different combinations and let them grow for five weeks. At harvest, we measured microbial abundance as well as plant biomass production, the root:shoot ratio, the specific root length, and root branching.

Results/Conclusions

We found that, overwhelmingly, changes in plant traits were correlated with the presence of a single member of the community. Further, the dominant bacterial strain in the community, which contributed up to 99% of relative community abundance, appeared to have little influence over the plant traits measured. For example, specific root length decreased by 60% when our minor member Pseudomonas strain was present; whereas, when Pseudomonas was excluded, the dominant strain produced the highest measured specific root length of 1.60 cm mg^-1. Our data indicate that when plants interact with their bacterial microbiome rare species can dictate plant phenotype. Additionally, our microbial strains interact to produce plant trait values in non-additive ways. Our data, while taken from a very simple system, suggest that rare microbes may be important in preserving plant function in ecosystems undergoing change.