Abstract
The association of endophytic bacteria with their plant hosts has a beneficial effect for many different plant species. Our goal is to identify endophytic bacteria that improve the biomass production and the carbon sequestration potential of poplar trees (
Populus
spp.) when grown in marginal soil and to gain an insight in the mechanisms underlying plant growth promotion. Members of the
Gammaproteobacteria
dominated a collection of 78 bacterial endophytes isolated from poplar and willow trees. As representatives for the dominant genera of endophytic gammaproteobacteria, we selected
Enterobacter
sp. strain 638,
Stenotrophomonas maltophilia
R551-3,
Pseudomonas putida
W619, and
Serratia proteamaculans
568 for genome sequencing and analysis of their plant growth-promoting effects, including root development. Derivatives of these endophytes, labeled with
gfp
, were also used to study the colonization of their poplar hosts. In greenhouse studies, poplar cuttings (
Populus deltoides
×
Populus nigra
DN-34) inoculated with
Enterobacter
sp. strain 638 repeatedly showed the highest increase in biomass production compared to cuttings of noninoculated control plants. Sequence data combined with the analysis of their metabolic properties resulted in the identification of many putative mechanisms, including carbon source utilization, that help these endophytes to thrive within a plant environment and to potentially affect the growth and development of their plant hosts. Understanding the interactions between endophytic bacteria and their host plants should ultimately result in the design of strategies for improved poplar biomass production on marginal soils as a feedstock for biofuels.