Colonization on Root Surface by a Phenanthrene-Degrading Endophytic Bacterium and Its Application for Reducing Plant Phenanthrene Contamination

A phenanthrene-degrading endophytic bacterium, Pn2, was isolated from Alopecurus aequalis Sobol grown in soils contaminated with polycyclic aromatic hydrocarbons (PAHs). Based on morphology, physiological characteristics and the 16S rRNA gene sequence, it was identified as Massilia sp. Strain Pn2 could degrade more than 95 % of the phenanthrene (150 mg?L21) in a minimal salts medium (MSM) within 48 hours at an initial pH of 7.0 and a temperature of 30uC. Pn2 could grow well on the MSM plates with a series of other PAHs, including naphthalene, acenaphthene, anthracene and pyrene, and degrade them to different degrees. Pn2 could also colonize the root surface of ryegrass (Lolium multiflorum Lam), invade its internal root tissues and translocate into the plant shoot. When treated with the endophyte Pn2 under hydroponic growth conditions with 2 mg?L21 of phenanthrene in the Hoagland solution, the phenanthrene concentrations in ryegrass roots and shoots were reduced by 54 % and 57%, respectively, compared with the endophyte-free treatment. Strain Pn2 could be a novel and useful bacterial resource for eliminating plant PAH contamination in polluted environments by degrading the PAHs inside plants. Furthermore, we provide new perspectives on the control of the plant uptake of PAHs via endophytic bacteria

The degradation dynamics of phenanthrene and growth curve of strain Pn2 with phenanthrene as its sole carbon source.

<p>Error bars are ± standard deviation (n = 3).</p

Root and shoot biomass of ryegrass after inoculation for 12 days.

<p>UR (planted ryegrass in uncontaminated solution), URB (planted ryegrass and inoculated with strain Pn2 in uncontaminated solution), CR (planted ryegrass in contaminated solution), CRB (planted ryegrass and inoculated with strain Pn2 in contaminated solution). Error bars are ± standard deviation (n = 3). * indicates a significant difference between endophyte-inoculated and endophyte-free treatments (P<0.05).</p><p>Root and shoot biomass of ryegrass after inoculation for 12 days.</p

Transmission electron micrograph of strain Pn2 (×6.0K Zoom-1 HC-1 80 kV).

<p>Transmission electron micrograph of strain Pn2 (×6.0K Zoom-1 HC-1 80 kV).</p

Plate counts of strain Pn2 colonizing in plant tissues and in Hoagland solution after inoculation for 6 days and 12 days.

<p>URB (planted ryegrass and inoculated with strain Pn2 in uncontaminated solution), CRB (planted ryegrass and inoculated with strain Pn2 in contaminated solution). Error bars are ± standard deviation (n = 3). * indicates a significant difference between contaminated and uncontaminated solutions (P<0.05).</p><p>Plate counts of strain Pn2 colonizing in plant tissues and in Hoagland solution after inoculation for 6 days and 12 days.</p

The concentration and plant concentration factors of phenanthrene in Hoagland solution and in ryegrass added with 2 mg·L⁻¹ of phenanthrene in the solution.

<p>CK (contaminated solution), CR (planted ryegrass in contaminated solution), CRB (planted ryegrass and inoculated with strain Pn2 in contaminated solution). PCF (Plant concentration factor, which is calculated as pollutant content in the plant roots or shoots (C_p, mg·kg⁻¹)/the concentration of pollutant in solution (C_s, mg·l⁻¹), PCF = C_p/C_s), TF (Plant tanslocation factor, that is shoot concentration factor (SCF)/root concentration factor (RCF), TF =  SCF/RCF). Error bars are ± standard deviation (n = 3). ND means not detected. * indicates a significant difference between endophyte-inoculated and endophyte-free treatments in contaminated solutions (P<0.05).</p><p>The concentration and plant concentration factors of phenanthrene in Hoagland solution and in ryegrass added with 2 mg·L⁻¹ of phenanthrene in the solution.</p

The degradation ability of strain Pn2 for each PAH type.

<p>ND means not detected; Error bars are ± standard deviation (n = 3).</p><p>The degradation ability of strain Pn2 for each PAH type.</p