Pervasive interactions between soil resources and tree hosts structure bacterial endophyte assemblages for tree seedlings in a mature tropical forest

Background/Question/Methods

The plant microbiome is a key determinant of plant health and productivity and has received considerable attention in recent years.  Leaves in particular comprise perhaps the world’s largest microbial habitat; moreover, bacteria are the most abundant colonizers of leaves.  Recent studies have suggested that bacterial community composition is an entirely independent leaf functional trait, though no study has determined the degree to which resource availability structures bacterial community assemblages among plant hosts.  We used high-throughput sequencing to quantify community composition and structure of bacterial endophytes (those within interior portions of leaves) among tree seedlings in a tropical forest.  We tested the following hypotheses: 1) The Host Tree Hypothesis: bacterial endophyte composition and structure contrasts among host tree species.  2) The Limiting Nutrient Hypothesis: pervasive interactions between limiting soil nutrients and host species structure endophyte assemblages.  To test these hypotheses, we used five tree species (Alseis blackiana, Desmopsis panamensis, Heisteria concinna, Sorocea affinis, and Tetragastris panamensis) in the shaded understory of a lowland forest in Panama.  We used a 16-year nutrient enrichment experiment on the Barro Colorado Nature Monument where nitrogen, potassium, and phosphorus were added to large plots (40 x 40 m) in a fully factorial design (n=4). 

Results/Conclusions

Overall, our results indicate that bacterial community assemblages sharply contrast among tree species.  In addition, there are significant overlaps of particular bacterial taxa among species, suggesting that a “core microbiome” exists among all species.  Moreover, soil resource treatments caused significant changes in endophytic community compositions.  Finally, interactions between nutrients and tree species structure bacterial endophyte assemblages.  To our knowledge, this is one of the first empirical studies in situ to demonstrate that soil resources and host species strongly mediate the structure of bacterial endophyte communities.