Interactions in rhizosphere for bioremediation of heavy metals

Abstract

Contamination of soil by release of heavy metals from various industrial effluents, agrochemicals and other sources presents a major threat to the soil environment. Heavy metal compounds are difficult to degrade by biological processes and persist in the environment for a long time. The elevated concentration of heavy metals in soils adversely affects the growth of plants, microbial populations and symbiosis and, therefore, crop yields. Bioremediation techniques are often applied to clean up soil pollutants. Recent techniques used for removal of heavy metal pollution from soil include the use of plant growth-promoting rhizobacteria (PGPR) in the rhizosphere, and their metabolic activities in bioremediation. In this review, we describe how heavy metals can be cleaned from the soil environment with the help of plant-microbe interaction. The plants also play a vital role in degradation of heavy metals. Exudates from the plant root help to stimulate the bacteria for survival and help in degradation of heavy metals in the environment. The plant root also helps the bacteria to spread and penetrate into the soil layers. Therefore, the bioremediation process stimulated in the rhizosphere is also referred as rhizoremediation. PGPR in association with plants roots provide beneficial effects on plant growth and also provide nutrition through mechanisms such as N 2 fixation, phytohormone production and siderophores. The inoculation of soil with heavy metal-tolerant bacteria through the seeds of plants could be important and improve rhizoremediation techniques. The role of mycorrhiza and root exudates of host plants in the bioremediation process is also discussed.