c Plant-associated bacteria are of great interest because of their potential use in phytoremediation. However, their ability to survive and promote plant growth in metal-polluted soils remains unclear. In this study, a soilborne Cd-resistant bacterium was isolated and identified as Enterobacter sp. strain EG16. It tolerates high external Cd concentrations (Cd 2؉ MIC, >250 mg liter ؊1 ) and is able to produce siderophores and the plant hormone indole-3-acetic acid (IAA), both of which contribute to plant growth promotion. Surface biosorption in this strain accounted for 31% of the total Cd accumulated. The potential presence of cadmium sulfide, shown by energy-dispersive X-ray (EDX) analysis, suggested intracellular Cd binding as a Cd response mechanism of the isolate. Cd exposure resulted in global regulation at the transcriptomic level, with the bacterium switching to an energyconserving mode by inhibiting energy-consuming processes while increasing the production of stress-related proteins. The stress response system included increased import of sulfur and iron, which become deficient under Cd stress, and the redirection of sulfur metabolism to the maintenance of intracellular glutathione levels in response to Cd toxicity. Increased production of siderophores, responding to Cd-induced Fe deficiency, not only is involved in the Cd stress response systems of EG16 but may also play an important role in promoting plant growth as well as alleviating the Cd-induced inhibition of IAA production. The newly isolated strain EG16 may be a suitable candidate for microbially assisted phytoremediation due to its high resistance to Cd and its Cd-induced siderophore production, which is likely to contribute to plant growth promotion.
Heavy metal contamination has become one of the most serious environmental problems in recent years. Metal extraction activities are major sources for heavy metals in the environment (1). Cadmium is highly toxic to plants, animals, microorganisms, and humans even at quite low concentrations (2, 3). Although Cd is non-redox active, it can cause oxidative stress by generating reactive oxygen species (ROS) (4), which can lead to DNA damage (5), inhibit the DNA mismatch repair system (6), and disrupt the synthesis of nucleic acids and proteins (7).Phytoremediation, a low-cost and eco-friendly technology for the decontamination of heavy-metal-polluted soils, uses plants to absorb, accumulate, and detoxify heavy metals in soil (3). However, the serious environmental stresses present in most cases of phytoremediation result in slow plant growth, low biomass production, and long time frames for remediation, limiting the usefulness of this technology (8). Beneficial plant-associated bacteria have been shown to protect plants from metal toxicity and promote plant growth. The plant growth-promoting (PGP) characteristics that most of these bacteria possess, including N 2 fixation, siderophore production, the production of plant growth hormones, such as indole-3-acetic acid (IAA), and the reduction of ethylen...
