The American beachgrass, Ammophila breviligulata, is a key colonizer of sand dunes and it is considered to be an important dune architect along barrier islands of the North Atlantic coast. Restoration of beachgrass through replanting efforts after storms are sometimes unsuccessful, and could greatly benefit from a better understanding of the biotic and abiotic factors that affect the growth of this plant. In particular, very little is known about the microbiomes associated with beachgrass, specially in terms of the plant-microbe interactions within (endosphere) and on the sand adjacent to the root (rhizosphere). In this work, we have used culture-dependent and -independent techniques to characterize the bacterial communities associated with healthy and unhealthy replanted grasses in an effort to identify health-promoting members of the community. We have used Illumina 16S rRNA gene sequencing to characterize the bacterial community composition within bulk soil, rhizosphere and endosphere samples of A. breviligulata obtained across six different New York Barrier Island beaches. In addition, we have isolated plant-growth promoting bacteria from these environments and characterized their ability to produce plant hormones as well as other molecular mechanisms of interaction with the plant.
Microbial communities across all six sites and different root microenvironments were dominated by Proteobacteria, Actinobacteria and Bacteroidetes. At the genus level, rhizosphere communities seem to be dominated by Pseudomonas, Sphingomonas, Rhodanobacter, Rhizobium and Dyella species within the Proteobacteria, and Streptomyces, Kribbella and Promicrospora species within the Actinobacteria. When comparing healthy and unhealthy replanting sites, the proportions of Actinobacteria are lower while those of Bacteroidetes increase. Microbiomes were also found to display similar community structures at the root microenvironment level across all sites, with higher proportions of Bacteroidetes in the endosphere and rhizosphere as compared to the surrounding bulk soil, while Acidobacteria and Firmicutes were found to be conversely more abundant in bulk soil. We have also obtained isolates belonging to the genera Pseudomonas, Sphingomonas, Herbaspirillium, Dyella, Pantoea and Variovorax which were selected due to their high levels of production of the plant hormone indole-3 acetic acid ranging between 100 and 200 µg/ml, as well as the ability to produce siderophores. Many of these genera have also been identified amongst the most dominant in the beachgrass rhizosphere during the early stages of dune formation, which could be indicative of their role in the promotion of root growth at this stage.