The Brookhaven team, led by Daniel van der Lelie, has experience with plants growing on extremely marginal soil contaminated with heavy metals and other industrial chemicals. In prior research, the group had incorporated the molecular machinery used by bacteria that degrade such contaminants into microbes that normally colonize poplar trees, and used the resulting trees to clean up the soil. An added benefit, the scientists observed, was that the microbe-supplemented trees grew faster, even when no contaminants were present.

In the current study, the scientists isolated endophytic bacteria normally resident in poplar and willow roots, and tested selected strains’ abilities to increase poplar growth in a controlled greenhouse environment. They also sequenced the genes from four selected bacterial species and screened them for the production of plant-growth promoting enzymes, hormones, and other metabolic factors that might help explain how the bacteria improve plant growth.

The plants were first washed and surface-sterilized to eliminate the presence of soil bacteria so the scientists could study only the bacteria that lived within the plant tissues—true endophytes. The plant material was then ground up so the bacterial species could be isolated.

Individual strains were then supplemented with a gene for a fluorescent protein, and were inoculated into the roots of fresh poplar cuttings that had been developing new roots in water. The presence of the endophytic bacteria was confirmed by searching for the glowing protein. Some bacterial species were also tested for their ability to increase the production of roots in the poplar cuttings by being introduced during the rooting process rather than afterward.

The scientists identified 78 bacterial endophytes from poplar and willow. Some species had beneficial effects on plant growth, others had no effect, and some resulted in decreased growth. In particular, poplar cuttings inoculated with Enterobacter sp. 638 and Burkholderia cepacia BU72 repeatedly showed the highest increase in biomass production—up to 50%—as compared with non-inoculated control plants. Though no other endophyte species showed such dramatic effects, some were effective in promoting growth in particular cultivars of poplar.

In the studies specifically looking at root formation, non-inoculated plants formed roots very slowly. In contrast, plant cuttings that were allowed to root in the presence of selected endophytes grew roots and shoots more quickly.

The analysis of genes and metabolically important gene products from endophytes resulted in the identification of many possible mechanisms that could help these microbes thrive within a plant environment, and potentially affect the growth and development of their plant host. These include the production of plant-growth-promoting hormones by the endophytic bacteria that stimulate the growth of poplar on marginal soils.

The scientists plan to conduct additional studies to further elucidate these mechanisms.

These mechanisms are of prime importance for the use of plants as feedstocks for biofuels and for carbon sequestration through biomass production.

—Daniel van der Lelie

The study was funded by the Office of Biological and Environmental Research within DOE’s Office of Science, by Brookhaven’s Laboratory Directed Research and Development Fund, and by the Flanders Science Foundation and the Institute for the Promotion of Innovation by Science and Technology in Flanders, both in Belgium.

Resources

• Safiyh Taghavi, Craig Garafola, Sébastien Monchy, Lee Newman, Adam Hoffman, Nele Weyens, Tanja Barac, Jaco Vangronsveld, and Daniel van der Lelie (2009) Genome Survey and Characterization of Endophytic Bacteria Exhibiting a Beneficial Effect on Growth and Development of Poplar Trees. Applied and Environmental Microbiology, Vol. 75, No. 3, p. 748-757 doi: 10.1128/AEM.02239-08