Diversity and structure of endophytic bacterial communities in Redwood trees
Endophytes are bacteria that are found inside all tissues of all land plants examined so far. From studies of agricultural crops, we know they can promote plant growth and protect the host against abiotic and biotic stress. Yet, the ecological function of bacterial endophytes remains widely elusive in natural ecosystems, except in a few forest tree species where endophyte isolates have been shown to fix nitrogen. Long-lived trees in the genera Sequoia and Sequoiadendron are able to resist environmental stress over centuries and therefore provide an interesting system to explore the possible role of endophytic bacteria in tree acclimation to a variable climate. As a first step, we addressed the question of how diverse and specific the bacterial endophytes are to host species and to sites. We used 454 pyrosequencing of the 16s rRNA gene to explore the diversity of endophytic bacteria at different heights in the canopy of Sequoia sempervirens and Sequoiadendron giganteum at three different sites in California.
We recovered 60,000 sequences with 566 phylotypes at 97% similarity from 9 trees sampled at three heights (low, middle, and high branches). The endophytic communities from three individuals of Sequoiadendron giganteum and six individuals of Sequoia sempervirens were dominated by Proteobacteria (25-80%). Sequoia sempervirens were generally dominated by Methylobacterium sp. (2-35%) while Sequoiadendron giganteum were dominated by Bacillus sp. (7-40%). Methylobacterium have been found enhance plant growth and Bacillus have been found to exude antimicrobial compounds. However, no single bacterial species consistently dominated all samples of a tree species, or even all samples within a tree. Overall, we saw great variation within individuals and across individuals of the same location. These results are in stark contrast to our results from high elevation conifers, where the endophytic community was found to be much less diverse, and where all trees at a location were dominated by one single endophytic bacterial species that made up on average 40-60% of all sequences in each sample. Instead, a diverse and flexilble endophyte community may underlie the capacity of these giant trees to withstand centuries of past and future variability in climate.