Endophytic bacteria strongly reduce TCE evapotranspiration during phytoremediation in the field

Abstract

Phytoremediation of volatile organic contaminants is often unsatisfactory because plants and their rhizosphere do not completely degrade these compounds resulting in evapotranspiration through the leaves causing secondaru contamination of the ambient air and by consequence undermining the merits of phytoremediation. Under laboratory conditions, endophytic bacteria equipped with the appropriate degradation pathway can be used to improve the in planta degradation of organic contaminants resulting in a decreased phytotoxicity and a reduction of evapotranspiration. We report the first in situ inoculation of poplar trees growing on a TCE-contaminated site with the TCE degrading endophytic strain Pseudomonas putida W619. Pseudomonas putida W619 was selected as an ideal candidate for inoculation because it is a root endophyte of poplar that was equipped with the pTOM plasmide coding for TCE degradation via natural gene transfer. Furthermore, under laboratory conditions, inoculation of poplar cuttings with P. putida W619 resulted in(a) a remarkably high plant growh promotion, (b) decreased activities of glutathione reductase in the roots, and superoxide dismutase in the roots and the leaves, and (c) a storng reduction in stomatal resistance, all indicating an improved plant fitness in comparison with non-inoculated cuttings. To allow inoculation of P. putida W619 in the field, at the moment of planting, some of the poplar trees were equipped with a drainage tube in the rooting zone. Three months after inoculation, the amount of TCE evapotranspiring through the leaves was compared for non-inoculated and inoculates poplar trees. The in situ inoculation with strain P.putida W619 resulted in 90% reduction of TCE evapotranspiration under field conditions. This was achieved by succesful establishment and enrichment of P. putida W619 as a poplar root endophyte, and by further horizontal gene transfer of TCE metabolism to members of the endogenous endophytic population.