Repression of Pseudomonas putida phenanthrene‐degrading activity by plant root extracts and exudates

Abstract

The phenanthrene-degrading activity (PDA) of Pseudomonas putida ATCC 17484 was repressed after incubation with plant root extracts of oat (Avena sativa), osage orange (Maclura pomifera), hybrid willow (Salix alba x matsudana), kou (Cordia subcordata) and milo (Thespesia populnea) and plant root exudates of oat (Avena sativa) and hybrid poplar (Populus deltoides x nigra DN34). Total organic carbon content of root extracts ranged from 103 to 395 mg l(-1). Characterization of root extracts identified acetate (not detectable to 8.0 mg l(-1)), amino acids (1.7-17.3 mg l(-1)) and glucose (1.6-14.0 mg l(-1)), indicating a complex mixture of substrates. Repression was also observed after exposure to potential root-derived substrates, including organic acids, glucose (carbohydrate) and glutamate (amino acid). Carbon source regulation (e.g. catabolite repression) was apparently responsible for the observed repression of P. putida PDA by root extracts. However, we showed that P. putida grows on root extracts and exudates as sole carbon and energy sources. Enhanced growth on root products may compensate for partial repression, because larger microbial populations are conducive to faster degradation rates. This would explain the commonly reported increase in phenanthrene removal in the rhizosphere.

Nature and role of root exudates: Efficacy in bioremediation

“…The attraction of competent bacteria to the root zone may improve bioavailability and increase PAH degradation in the rhizosphere. Subjugation of the phenanthrene degrading activity of P. putida following exposure to root extracts and exudates recommended that enzyme induction may not occur during rhizodegradation of PAHs (Rentz et al, 2004). Related studies observed repression of PAH catabolic genes on a per cell basis and demonstrated greater naphthalene degradation by cultures grown on root products compared to naphthalene, supporting the notion that prolific microbial growth provides improved degradation in the rhizosphere (Kamath et al, 2004).…”

The Target for the Pseudomonas putida Crc Global Regulator in the Benzoate Degradation Pathway Is the BenR Transcriptional Regulator

“…There is scattered evidence indicating that, when confronted by mixtures of aromatic compounds in sufficient concentrations, some soil bacteria metabolize certain aromatics preferentially over others. For example, root extracts from several plants, which contain several aromatic compounds, can inhibit the assimilation of phenanthrene by a strain of P. putida (33). Similarly, benzoate inhibits the catabolism of phenol in Ralstonia eutropha (1) and represses 4-hydroxybenzoate degradation genes in P. putida (29).…”

Uptake of selected PAHs from contaminated soils by rice seedlings (Oryza sativa) and influence of rhizosphere on PAH distribution

“…C for 48 h and preserved in a refrigerator at À20 C. The procedure used to extract PAHs from the soils was a modification of those by Kipopoulou et al (1999), Simonich and Hites (1994), and Gao et al (2005). Specifically, 4 g (dry weight) of the ground soil particles (0.2 mm aperture) were extracted twice using an ultrasonic crusher (JY92-2D, China) with 40 mL of mixed dichloromethane and acetone (5:3 v/v) after the soil samples were soaked overnight in 40 mL of the above solvent.…”

“…Due to carcinogenic, mutagenic, and teratogenic properties (Brady et al, 2003), their occurrence frequency (Jones et al, 1989;Wennrich et al, 2002;Tao et al, 2004) and their environment persistence (Fismes et al, 2002;Bi et al, 2003), PAHs are of a particular concern to human and environmental health. Conceivably, plants could be exploited to aid in the removal of PAHs and other contaminants from soils via two main mechanisms: (1) the accumulation by, and the subsequent metabolism in, plant tissues following the contaminant uptake by plant roots (Fismes et al, 2002;Wild et al, 2005) and (2) the enhanced microbial activity induced by root exudation of enzymes to transform and/or mineralize contaminants (Rentz et al, 2004;Kamath et al, 2005). Thus, the bioavailability of soil associated PAHs and plant uptake capacity would greatly influence the efficiency of PAH phytoremediation.…”