Role of Two Plant Growth-Promoting Bacteria in Remediating Cadmium-Contaminated Soil Combined with Miscanthus floridulus (Lab.)

Abstract

Miscanthus spp. are energy plants and excellent candidates for phytoremediation approaches of metal(loid)s-contaminated soils, especially when combined with plant growth-promoting bacteria. Forty-one bacterial strains were isolated from the rhizosphere soils and roots tissue of five dominant plants (Artemisia argyi Levl., Gladiolus gandavensis Vaniot Houtt, Boehmeria nivea L., Veronica didyma Tenore, and Miscanthus floridulus Lab.) colonizing a cadmium (Cd)-contaminated mining area (Huayuan, Hunan, China). We subsequently tested their plant growth-promoting (PGP) traits (e.g., production of indole-3-acetic acid, siderophore, and 1-aminocyclopropane-1-carboxylate deaminase) and Cd tolerance. Among bacteria, two strains, Klebsiella michiganensis TS8 and Lelliottia jeotgali MR2, presented higher Cd tolerance and showed the best results regarding in vitro growth-promoting traits. In the subsequent pot experiments using soil spiked with 10 mg Cd·kg^-1, we investigated the effects of TS8 and MR2 strains on soil Cd phytoremediation when combined with M. floridulus (Lab.). After sixty days of planting M. floridulus (Lab.), we found that TS8 increased plant height by 39.9%, dry weight of leaves by 99.1%, and the total Cd in the rhizosphere soil was reduced by 49.2%. Although MR2 had no significant effects on the efficiency of phytoremediation, it significantly enhanced the Cd translocation from the root to the aboveground tissues (translocation factor > 1). The combination of K. michiganensis TS8 and M. floridulus (Lab.) may be an effective method to remediate Cd-contaminated soils, while the inoculation of L. jeotgali MR2 may be used to enhance the phytoextraction potential of M. floridulus.

Keywords: Miscanthus floridulus (Lab.); bioinoculation; metal(loid)s; phytoremediation.

Figure 1

PGP traits of bacterial isolates. (a) IAA production, (b) ACC deaminase activity, (c) siderophore production. The data were expressed as means ± standard deviation (n = 3). Different letters represent significant differences between groups at the 5% level (ANOVA and Tukey’s test). IAA: indole−3−acetic acid, ACC: 1−aminocyclopropane−1−carboxylic acid.

Figure 2

The variations of pH value after metabolizing glucose by screened bacterial isolates. The data were expressed as means ± standard deviation (n = 3) and the statistical analysis was calculated by ANOVA and Tukey’s test.

Figure 3

Neighbor-joining tree based on 16S rRNA gene sequences. (a) MR2 and (b) TS8.

Figure 4

Optical density of selected bacteria at 0, 0.18, 0.36, and 0.54 mM Cd concentration during different incubation times. (a) MR2 and (b) TS8. The data were expressed as means ± standard deviation (n = 3). Different letters represent significant differences of absorbance between different Cd concentrations between groups at the 5% level (ANOVA and Tukey’s test).

Figure 5

Influence of inoculation with TS8 and MR2 on M. floridulus height (a) and dry weight (b). “CK,” without PGPB inoculation; “MR2,” Lelliottia jeotgali MR2 inoculation; “TS8,” Klebsiella michiganensis TS8 inoculation. The data were expressed as means ± standard deviation (n = 3). Different letters represent significant differences between groups at the 5% level (ANOVA and Tukey’s test).

Figure 6

Cd concentration in plant tissues/soil and soil pH. (a) Effect of MR2 and TS8 inoculation on Cd uptake by M. floridulus root, stem, and leaf grown for two months in the contaminated soil. (b) Cd concentration in soil after phytoremediation. (c) pH value in soil after phytoremediation. The data were expressed as means ± standard deviation (n = 3). Different letters represent significant differences between groups at the 5% level (ANOVA and Tukey’s test).