Introduction heavy metal pollution

Recent Advances in Heavy Metal Bioremediation
The process of phytoremediation has gained much attention in last few years to explore molecular and biochemical pathways involve in heavy metal uptake, transport and storage in plants (Clemens et al. 2002; Pilon-Smits and Pilon, 2002; Pollard et al. 2002; Eapen & D’Souza, 2005). However, the process of phytoremediation is rather slow; an improved technique via biotechnological approach can overcome the problem. Genetic modifications in plants to enhance the efficiency of remediation technique require a deep insight into the complete mechanism of heavy metal extraction by plant.

Development of transgenic plants with increased metal selective organic acid, ligands and phytochilatins could have promising applications in heavy metal decontamination. It is well known that organic acids and peptide ligands form complexes with metals. For example, free histidine is found as metal chelator in xylem exudates of Ni hyperaccumulators, therefore, by modifying histidine concentration in xylem exudates Ni accumulating capacity of plants can be improve. Cellular targeting manipulation specifically in metal transporters and vacuoles is important since the compartmentalization of heavy metals is safe mechanism adopted by most plants without disturbing the cellular functions. Great successes have been achieved in the development of transgenic plants with enhanced heavy metal accumulating capacity but majority of genes have been transferred from other plants or organisms (Eapen & D’Souza, 2005).

To develop plant species better suited for phytoremediation of metal contaminated sites Thlaspi caerulescens has been used as source of genes by various workers (Brewer et al. 1999; Gleba et al. 1999; Lombi et al. 2001). Brewer et al. (1999), developed somatic hybrids between T. caerulescens and Brassica napus. The selected high biomass hybrids for Zn tolerance were found to capable of accumulating Zn level that would otherwise toxic to B. napus. In other study somatic hybrids from T. caerulescens and B. juncea were also able to remove significant amounts of Pb (Gleba et al. 1999). Transgenic B. juncea showed efficient affinity for Se uptake with enhanced Se tolerance than the wild species (Pilon-Smits et al. 1999, Huysen et al. 2004). Transgenic B. juncea with Se tolerance was developed by transferring the selenocysteine methyltransferase (SMT) from the A. bisulcatus (Se hyperaccumulator). SMT transgenic plants of B. juncea accumulate 60% more Se than the wild-type when grown in a contaminated soil (Zhao & McGrath, 2009; Rascio & Navari-Izz, 2011). Transgenic plants have proved to be a promising biotechnological approach, but only few field studies have been performed till now (Zhao & McGrath, 2009; Rascio & Navari-Izzo, 2011).

Application of mixed microorganisms with plant species can provide effective future measures for heavy metal decontamination. However, several obstacles need to overcome for commercial application of such treatment system (Hrynkiewicz & Baum, 2012) such as,

1. 
   Commercially cost-effective mass-production and formulation of microbial inoculums.
   
2. 
   Microbial inoculum should be relatively universal for various plants and soils and its effectiveness should be relatively easy to evaluate.
   
3. 
   Effectiveness of microbial consortium to function in natural conditions.
   
4. 
   Knowledge of possible interactions between plants and associated soil microorganisms in natural environment.
   

However, additional research is expected to overcome these problems (Rajkumar et al. 2012), for example

1. 
   Complete physiological and molecular characterization of several environmentally relevant microorganisms.
   
2. 
   Exploration of mechanism followed by microbial chelators-metal complex uptake in plants.
   
3. 
   Effects of factors influencing the solubility and plant availability of nutrients/heavy metals.
   
4. 
   Identification of signaling processes that occur between plant roots and microbes.
   
5. 
   Effect of manipulation in rhizosphere zone processes such as coinoculating ecologically diverse microorganisms on phytoremediation process.
   

Such knowledge may enable us to exploring the mechanism of metal-microbes-plant interactions and to improve the performance and use of beneficial microbes as inoculants for microbial assisted phytoremediation (Rajkumar et al. 2012).