Poplar roots host a cortege of symbiotic endo- and ectomycorrhizal fungi. In addition to these well-known root-inhabiting microbes, the sequencing of the poplar genome has revealed the occurrence of hundreds of bacterial and fungal endophytes in both roots and leaves. Some of these invasive bacteria actually help plants thrive. For example, the plant growth promoting endophytic bacterium Enterobacter sp. 638 can improve the growth of poplar on marginal soils by as much as 40%. The plant growth promoting gamma-proteobacterium was isolated from the stem of poplar (Populus trichocarpa x deltoides). Scientists at Brookhaven National Laboratory published the genome of Enterobacter on May 13, 2010 in PLoS Genetics. Their work identified a wide range of genes that help explain the symbiotic relationship between endophytic bacteria and plants. The genome sequence comprises a 4,5 Mbp chromosome and a 157 kbp plasmid. Analysis of the genome sequence, combined with metabolite analysis and quantitative PCR allowed the identification of an extended set of genes specific to the plant niche adaptation of Enterobacter. This includes genes that encode for proteins involved in survival in the rhizosphere (to cope with oxidative stress or uptake of nutrients released by plant roots), root adhesion, colonization/establishment inside the plant, plant protection against fungal and bacterial infections (siderophore production and synthesis of the antimicrobial compounds 4-hydroxybenzoate and 2-phenylethanol), and improved poplar growth and development through the production of the phytohormones indole acetic acid, acetoin, and 2,3-butanediol.
The molecular cross-talk between the partners requires a finely-tuned metabolic coordination and transcriptional regulation. The endophyte is responsible for the synthesis of a phytohormone, and a precursor for another, which poplar host is not able to produce, and the synthesis of the plant-growth promoting compounds requires in return the fate of plant-synthesized compounds, such as sucrose.
Together with inoculation of growth-promoting ectomycorrhizal fungi, such as Laccaria bicolor, Enterobacter and other endophytic bacteria can likely be exploited to improve establishment and sustainable production of poplar on marginal, non-agricultural soils.
Taghavi S, van der Lelie D, Hoffman A, Zhang Y-B, Walla MD, et al. (2010) Genome Sequence of the Plant Growth Promoting Endophytic Bacterium Enterobacter sp. 638. PLoS Genet 6(5): e1000943. doi:10.1371/journal.pgen.1000943.
Image Credit: Safiyh Taghavi and Dmytro Nykypanchuk (Brookhaven National Laboratory; image taken at the Center for Functional Nanomaterials)