International Research Journal of Plant Science (ISSN: 2141-5447) Vol. 2(8) pp. 220-232, August, 2011 Available online http://www.interesjournals.org/IRJPS Copyright © 2011 International Research Journals Review Research Paper Role of Rhizobacteria in phytoremediation of heavy metals: An overview Qurban Ali*, Muhammad Ahsan 1 , Ihsan Khaliq 1 , Mehboob Elahi 2 , Shafaqat Ali 3 , Fawad Ali 1 and Muhammad Naees 3 1 Department of Plant Breeding and Genetics, University of Agriculture Faisalabad, Pakistan. 2 Pakistan Nestle Pvt. (Ltd), 3 Department of Environmental Sciences, GC University, Faisalabad, Pakistan. Accepted 28 June, 2011 Heavy metal pollution of soil is a significant environmental problem and has its negative impact on human health and agriculture. Rhizosphere, as an important interface of soil and plant, plays a significant role in phytoremediation of contaminated soil by heavy metals, in which, microbial populations are known to affect heavy metal mobility and availability to the plant through release of chelating agents, acidification, phosphate solubilization and redox changes, and therefore, have potential to enhance phytoremediation processes. Phytoremediation strategies with appropriate heavy metal-adapted rhizobacteria have received more and more attention. A heavy metal-resistant bacterial strain was isolated from heavy metal-contaminated soils and identified as Burkholderia sp. J62 based on the 16S rDNA gene sequence analysis. Strain J62 was able to colonize and develop in the rhizosphere soil of maize and tomato after root inoculation and was able to promote the growth of maize and tomato. The application of strain J62 effectively increased the bioavailability of Pb and Cd in the rhizosphere soils and promoted the growth of maize and tomato plants, consequently increasing the total Pb and Cd uptakes of the plants even under nonsterile conditions. The isolate was found to exhibit different multiple heavy metal and antibiotic resistance characteristics. Inoculation with the isolate was found to increase the biomass of maize and tomato plants. Increase in tissue Pb and Cd contents varied from 38 to 192% and from 5 to 191% in inoculated plants growing in heavy metal-contaminated soils compared to the uninoculated control, respectively. Phytoremediation could clean up the heavy metal-contaminated soil. In addition, although strain J62 significantly increased the Pb and Cd uptakes by the maize and tomato, the total Pb and Cd accumulation of the plants is low. Keywords: Rhizobacteria, burkholderia sp., phytoremediation, heavy metals, rhizosphere, solubilization. INTRODUCTION Heavy metals are conventionally defined as elements with metallic properties (ductility, conductivity, stability as cations, ligand specificity, etc.) and an atomic number >20. The most common heavy metal contaminants are Cd, Cr, Cu, Hg, Pb and Ni. Metals are natural components in soil with a number of heavy metals being required by plants as micronutrients. "Heavy metals" are chemical elements with a specific gravity that is at least 5 times the specific gravity of water. The specific gravity of water is 1 at 4°C (39°F). Simply stated, specific gravity is *Corresponding author: E-mail qurbanalisaim5@yahoo.com, saim_1692@yahoo.com +923219621929) a measure of density of a given amount of a solid substance when it is compared to an equal amount of water. Some well-known toxic metallic elements with a specific gravity that is 5 or more times that of water are arsenic, 5.7; cadmium, 8.65; iron, 7.9; lead, 11.34; and mercury, 13.546 (Lide, 1992). There are 35 metals that concern us because of occupational or residential exposure; 23 of these are the heavy elements or "heavy metals": antimony, arsenic, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, gold, iron, lead, manganese, mercury, nickel, platinum, silver, tellurium, thallium, tin, uranium, vanadium, and zinc. Interestingly, small amounts of these elements are common in our environment and diet and are actually necessary for good health, but large amounts of any of them cause acute or