Stenotrophomonas maltophilia R551-3
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Image by Barbara Panessa-Warren. The bar is 100nm.

The association of endophytic bacteria with their plant hosts has been shown to have a growth-promoting effect for many plant species. Endophytic bacteria have several mechanisms by which they can promote plant growth and health on marginal, polluted soils. These include the production of phytohormones or enzymes involved in growth regulator metabolism such as ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, auxins, indoleacetic acid (IAA) or cytokinins. In addition, endophytic bacteria can help their host plants to overcome the phytotoxic effects caused by environmental contamination. When dealing with organic contaminants, the benefical effects of endophytes can be enhanced by equipping them with the appropriate degradation pathways that complement the metabolic properties of their host plant. The idea behind this concept was that it is much easier to engineer the metabolic properties of a microorganism than those of a plant. We demonstrated that endophytic bacteria equipped with the tom toluene degradation pathway can significantly improve the in planta degradation of BTEX and TCE in poplar, resulting in reduced phytotoxicity and release. Recent research by our group illustrates the potential of endophytic bacteria to increase the biomass production of poplar. Our goal is to characterize endophytic bacteria that improve the growth and phytoremediation potential of poplar on marginal, contaminated soils.

Stenotrophomonas maltophilia R551-3 was isolated from Populus trichocarpa x deltoides cv. "Hoogvorst" and represented the second most commonly found endophytic species in poplar. Identical bacteria were isolated from the poplar rhizosphere, root and stem. Based on its 16S rDNA sequence strain R551-3 is closely related to S. maltophilia K279a, which is being sequenced by the Sanger Institute. A comparison between the two genome sequences should allow identifying gene functions that are a prerequisite for the endophytic behavior of S. maltophilia R551-3 and its successful colonization of poplar. S. maltophilia represents a bacterial species of potential agronomic importance. Traits of S. maltophilia associated with biocontrol mechanisms include antibiotic production, extracellular enzyme activities such as protease and chitinase, and rhizosphere colonization potential.

Dr. Daniel van der Lelie, M.B.A.
Brookhaven National Laboratory
Biology Department, Upton, NY