| Soil contamination caused by heavy metals has a posed serious threat to farmlands adjacent to mining areas. Microbe-assisted phytoremediation is considered as a cost-efficient, simple and environment-friendly technology applied to the bioremediation of heavy metal-contaminated soil. Therefore, it is urgent to screen the bioremedia-tion-effective microbe-plant system and is important to study the mechanism by which mi-crobe-plant system carry out bioremediation.In this study, we inoculating Rhizobium sp.W33 into rhizosphere soil, and analyzed the effect of Rhizobium sp.W33 on seven different kinds of plants, including ryegrass(Lolium perenne L.), hybrid Pennisetum(Pennisetum americanum × P. purpureum CV.23A × N51), tall fescue(Festuca elata Keng ex E. Alexeev), alfalfa (Medicogo sativa L.), Crotalaria rotundifolia(Crotalaria mucronats Desv.), oilseed rape(Brassica campestris L.), and Indian mustard(Brassica juncea), in terms of the biomass and copper uptake. Compared with the values for the control group, the dry weight of alfalfa root decreased by 1.4%, whereas the biomass of the root of the other six plants all increased, ranging from 9.3% to 71.4%. In particular, the dry weight of ryegrass and tall fescue root increased sig-nificantly (p< 0.05) by 31.3% and 27.9%, respectively. The Cu concentration mainly ac-cumulated in the root of ryegrass, Pennisetum, tall fescue and Crotalaria reached 1000 mg/kg,or even higher, an amount that is greater than that in the shoot. Compared with the values for the control group, the Cu content of rape, ryegrass and alfalfa root increased sig-nificantly (p< 0.05) by 45.6%,25.5% and 14.4%. The Cu content of ryegrass shoots in-creased significantly (p< 0.05) by 70.1%. On the basis of the above analysis, we conclud-ed that the inoculation of Rhizobium sp.W33 into rhizosphere soil of ryegrass can signifi-cantly promote the uptake and enrichment of Cu by ryegrass.The effects of Rhizobium sp.W33 on the type and content of organic acids existing in the rhizosphere soil of the seven kinds of plants differed. It revealed that malic acid is pre-sent as the one predominat organic acid, followed by oxalic acid and citric acid. By com-paring with the value of the control, it showed that the malic acid content of ryegrass, Indi-an mustard, tall fescue decreased by 42.6% to 85.6%. Meanwhile, the oxalic acid content of ryegrass, tall fescue and Indian mustard decreased by 21.4%,52.3% and 61.7%, re-spectively. Noticeably, citric acid and succinic acids accounted for a relatively small pro-portion of the total organic acids content. Further analysis revealed that a significantly neg-ative correlation exists between the Cu content of ryegrass roots and shoots as well as be-tween the content of oxalic acid and malic acid in the rhizosphere soil.The total phenolic content which increased by at least 25%, was observed in the rhizosphere soil of rape, Pennisetum, tall fescue and alfalfa. These observations are signifi-cantly different from observation of the control treatment. The soluble carbohydrate content in the rhizosphere soil of Crotalaria, ryegrass, Indian mustard and Pennisetum significantly increased (p< 0.05) by 17.7% to 43.8%. The catalase activity present in the rhizosphere soil of ryegrass, pennisetum and oilseed rape increased significantly (p< 0.05). The change in the invertase activity of the rhizosphere soil is complex and elusive. A significantly posi-tive correlation appeared between the Cu content of ryegrass roots and shoots, as well as between soluble carbohydrate content and catalase activity in the rhizosphere soil.Under the three diffenent concentrations of Cu (0 mg/L,4 mg/L and 8 mg/L, respec-tively), Rhizobium sp W33-gfp was found to colonize on the root surface of ryegrass stably. However, the population of Rhizobium sp. W33-gfp gradually decreased and then changed stably over time.Under the three diffenent concentrations of Cu (0 mg/L,4 mg/L and 8 mg/L, respec-tively), the biomass in the roots and shoots of ryegrass significantly increased (p< 0.05), with respect to control treatment. The increasing trend relative to the biomass was nega-tively influenced by the increase in Cu concentration. Under Cu concentrations of 4 and 8 mg/L, the Cu content that accumulated in the ryegrass roots inoculated with Rhizobium sp. W33-gfp significantly increased (p< 0.05), exceeding 1000 mg/kg.With the above-mentioned treatment, an unknown acid and one kind of malic acid was secreted from within the ryegrass roots. Relative to the control treatment, the content of malic acid significantly decreased (p<0.05). However, the contents of the total phenolic and water soluble sugar of ryegrass significantly increased within unit time(p<0.05). Ac-cording to the results (copper concentration of 4 mg/L), a significantly positive correlation (p<0.05) exists between the Cu content of ryegrass roots and soluble sugar content, whereas a significantly negative correlation (p<0.01) exists between Cu and malic acid contents.These data revealed the most effective and potential microbe-assisted phytoremedia-tion partnership involved in copper-contaminated soil was ryegrass associated with Rhizo-bium sp. W33. By combining laser scanning confocal microscope with green fluorescent protein technology, it was concluded that Rhizobium sp. W33 could colonize the root sur-face of ryegrass. The analysis concerning the root exudates indicated that the ability of ryegrass to absorb copper significantly correlate with the content of oxalic acid, malic acid, water-soluble carbohydrates, as well as enzymatic activity of catalase, all of which played an important role in bioremediation for copper-contaminated soil mediated by Rhizobium sp. W33-ryegrass partnership. |
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