De. Crowley et al., RHIZOSPHERE EFFECTS ON BIODEGRADATION OF 2,5-DICHLOROBENZOATE BY A BIOLUNINESCENT STRAIN OF ROOT-COLONIZING PSEUDOMONAS FLUORESCENS, FEMS microbiology, ecology, 20(2), 1996, pp. 79-89
A recombinant strain of bioluminescent Pseudomonas fluorescens 2-79 RL
D containing a catabolic pathway for degradation of 2,5-dichlorobenzoa
te (2,5-DCB) was monitored in soil microcosms to, examine the influenc
e of plants on its growth and activity in a contaminated soil. Recombi
nant P. fluorescens 2-79 RLD was generated by mating a versatile chlor
obenzoate utilizer, P. putida P111, containing plasmid pPB111, with a
bioluminescent strain of P. fluorescens that had been transformed prev
iously with a Tn7-liuxCDABE marker. Plasmid pPB111 contains genes enco
ding for a chlorobenzoate-1,2-dioxygenase that converts ortho-chlorobe
nzoates to their corresponding catechols. DNA hybridization experiment
s and cell-free extract assays with parental and recombinant P. fluore
scens 2-79 RLD suggested that the reaction product of the plasmid pPB1
11 encoded chlorobenzoate dioxygenase was degraded by an endogenous ca
techol dioxygenase in P. fluorescens 2-79. After introduction of P. fl
uorescens 2-79 RLD into soil containing 10 mg kg(-1) 2,5-DCB, normally
recalcitrant 2,5-DCB was degraded rapidly over a period of 2 to 4 day
s in soil with plants. In contrast, 2,5-DCB disappearance in nonplante
d soil was significantly slower, requiring 7 days in one experiment, a
nd more than 2 weeks in a second experiment. Population numbers of the
degrader were similar in planted and nonplanted soil for the first 7
days, but declined in nonplanted soils by day 14. Physiological status
, measured using an assay based on lag-phase duration, was similar in
planted and nonplanted soils at day 2, but rapidly declined in nonplan
ted soil by day 7. At day 14, plasmid stability in P. fluorescens 2-79
RLD was significantly greater in rhizosphere soil; only 10% of P. flu
orescens 2-79 RLD cells in rhizosphere soil had lost the ability to de
grade 2,5-DCB, versus 94% of the cells in nonplanted soil. The plasmid
also was transferred to indigenous bacteria in both planted and nonpl
anted soils, as determined by the appearance of novel degraders. The r
esults demonstrate that the presence of plants promoted rapid degradat
ion of DCB and provided a niche that enhanced maintenance of plasmid p
PB111 in the degrader bacterium.