Please cite this article in press as: Y. Ma, et al., Improvement of plant growth and nickel uptake by nickel resistant-plant-growth promoting bacteria, J. Hazard. Mater. (2009), doi:10.1016/j.jhazmat.2008.12.018 ARTICLE IN PRESS G Model HAZMAT-9242; No. of Pages 8 Journal of Hazardous Materials xxx (2009) xxx–xxx Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Improvement of plant growth and nickel uptake by nickel resistant-plant-growth promoting bacteria Ying Ma ∗ , Mani Rajkumar, Helena Freitas Centre for Functional Ecology, Department of Botany, University of Coimbra, Coimbra 3000, Portugal article info Article history: Received 14 July 2008 Received in revised form 10 November 2008 Accepted 2 December 2008 Available online xxx Keywords: PGPB Nickel Siderophores IAA Phytoremediation abstract In this study, among a collection of Ni-resistant bacterial strains isolated from the rhizosphere of Alyssum serpyllifolium and Phleum phleoides grown on serpentine soil, five plant growth-promoting bacteria (PGPB) were selected based on their ability to utilize 1-aminocyclopropane-1-carboxylate (ACC) as the sole N source and promote seedling growth. All of the strains tested positive for indole-3-acetic acid (IAA) production and phosphate solubilization. In addition, four of the strains exhibited significant levels of siderophores production. Further, the efficiency of PGPB in enhancing Ni solubilization in soils was analyzed. Compared with control treatment, inoculation of PGPB strains significantly increased the con- centrations of bioavailable Ni. Furthermore, a pot experiment was conducted to elucidate the effects of inoculating Ni-resistant PGPB on the plant growth and the uptake of Ni by Brassica juncea and B. oxyrrhina in soil contaminated with 450 mg kg -1 Ni. Psychrobacter sp. SRA2 significantly increased the fresh (351%) and dry biomass (285%) of the B. juncea test plants (p < 0.05), whereas Psychrobacter sp. SRA1 and Bacillus cereus SRA10 significantly increased the accumulation of Ni in the root and shoot tissues of B. juncea compared with non-inoculated controls. This result indicates that the strains SRA1 and SRA10 facilitated the release of Ni from the non-soluble phases in the soil, thus enhancing the availability of Ni to plants. A significant increase, greater than that of the control, was also noted for growth parameters of the B. oxyrrhina test plants when the seeds were treated with strain SRA2. This effect can be attributed to the utilization of ACC, solubilization of phosphate and production of IAA. The results of the study revealed that the inoculation of Ni mobilizing strains Psychrobacter sp. SRA1 and B. cereus SRA10 increases the effi- ciency of phytoextraction directly by enhancing the metal accumulation in plant tissues and the efficient PGPB, Psychrobacter sp. SRA2 increases indirectly by promoting the growth of B. juncea and B. oxyrrhina. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Heavy metal pollution of soils is becoming one of the most severe environmental hazards and has negative impact on human health and agriculture. Elevated levels of heavy metals not only decrease soil microbial activity, soil fertility and yield losses [1], but also threaten human health through the food chain [2]. Phytoextraction is emerging as a potential cost effective solution for the remediation of heavy metal-contaminated soils in opposition to the conven- tional chemical and physical remediation technologies that are generally too costly and often harmful to soil characteristics [3,4]. Some plant species (identified as hyperaccumulators) growing in heavy metal-contaminated sites have been found with the ability to accumulate unusually high concentrations of heavy metals with- out impacting on their growth and development [5]. However, most hyperaccumulators identified so far are not suitable for field phy- ∗ Corresponding author. Tel.: +351 239 855243; fax: +351 239 855211. E-mail address: cathymaying@yahoo.com.cn (Y. Ma). toremediation applications due to their small biomass and slow growth [6]. Moreover, the metals at elevated levels are generally toxic to most plants impairing their metabolism and reducing plant growth. These properties have an adverse impact on the poten- tial for metal phytoextraction and restrict the employment of this technology. Thus, the development of phytoremediation strategies for heavy metal-contaminated soils is necessary. In this regard, interactions among metals, rhizosphere microbes and plants have attracted attention because of the biotechnological potential of microorganisms for metal removal directly from polluted soils or the possible transfer of accumulated metals to higher plants [7]. Further, the rhizosphere provides a complex and dynamic microen- vironment where microorganisms, in association with roots, form unique communities that have considerable potential for plant growth promotion [8] and detoxification of hazardous waste com- pounds [9,10]. In addition, certain metal resistant microorganisms can affect on trace metal mobility and availability to the plants through release of chelating agents, acidification, phosphate solubi- lization and redox changes [11,12]. Therefore, improvement of the interactions between plants and beneficial rhizosphere microbes 0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.12.018