Distinctive rhizoplane bacterial communities in tropical trees

Little is known of the bacterial communities associated with tropical rainforest tree root surfaces. We analysed the rhizoplane bacterial community of shallow roots of four species of native trees in a rainforest arboretum in Malaysia, using 454-pyrosequencing of the 16S RNA gene. The bacterial community of the rhizoplane was found to be distinct from that of soil from the same depth (5-10 cm) in surrounding forest. The rhizoplane community is dominated by a similar spectrum of bacterial phyla to bulk soil, particularly acidobactera and alpha- and beta- proteobacteria. However, lower level taxonomic composition is more distinctive. The rhizoplane bacterial community for each of the four tree species sampled clustered separately from one another on an ordination, and also separately from soils of the surrounding rainforest. It is unknown whether the bacterial species involved tend to be commensal, parasitic or mutualistic, and whether they are linked to the tree roots themselves or to fungal symbionts of the trees. The bacterial genus Burkholderia is especially abundant on the rhizoplane of all tree species sampled, and forms a community distinct to each tree species. Given the known N-fixing and growth-promoting abilities of Burkolderia in crop plants, it appears possible that these are N-fixing mutualists in species-specific relationships with their host trees. We also compared the phyllosphere and rhizoplane flora of two tree species, Dyera and Gnetum. There is little evidence of similarity between the rhizoplane and phyllosphere bacterial community of each tree species, with the exception of Gnetum having an anomalously high abundance of the genus Elizabethkingii in both rhizoplane and phyllosphere. This suggests the possibility of a mutualism between this bacterium and its host plant. Such host-specific relationships between trees and bacteria might have implications for understanding recruitment and structuring of tree species diversity in tropical forests, and practical implications for the tropical forestry industry in terms of growing young trees from seedlings.