Biotechnological approaches to improve phytoremediation efficiency for environment contaminants

Abstract

The realization, that plants serve the mankind by cleanup of the toxic contaminants, is quite old, but the problems of the contaminated land sites, water bodies and ground water and spoiled air worldwide have increased many folds due to anthropogenic activities during second half of the 20th century and hence deserve special attention. The environmental concerns of government and nongovernment agencies and the people at large have increased enormously, which have paved the way for the establishment of a large number of research institutes and commercial groups to develop new techniques and technologies for rapid cleanup of the contaminants from the sites identified for alarming contaminations. Phytoremediation, as a sustainable, cost effective and potential cleanup technology over the conventional methods, has emerged very fast as an alternative technology in the last decade (see Cunningham et al. 1995; Cunningham and Ow 1996; Salt et al. 1998; Saxena et al. 1999; Macek et al. 2000; Baker et al. 2000; Morikawa and Takahashi 2000; Singh et al. 2001; Morikawa et al. 2002; Kassal et al. 2002; Dhankhar et al. 2002; Maiti et al. 2004; Prasad 2004; Datta and Sarkar 2004; Schwitzguébel 2004; Pan et al. 2005). Phytoremediation technology can be implemented in situ or ex-situ to cleanup a variety of the organic contaminants e.g.. petroleum hydrocarbons, gas condensates, crude oil, chlorinated compounds, pesticides, herbicides, explosive compounds as well as typical inorganic toxicants, such as heavy metals, metalloids, radionuclides, etc. (Morikawa and Takahashi 2000). Air pollutants like nitrogen and sulfur oxides, ozone and suspended particulate matters (SPMs) can also be ameliorated by growing efficient naturally occurring plants as well as more efficient genenetically modified plants (see Wellburn 1990; Morikawa and Takahashi 2000; Takahashi et al. 2001; Schwitzguébel 2004; Morikawa et al. 2005). Phytoremediation is considered as an aesthetically pleasing and solar energy driven cleanup technology, which causes minimal environmental disruption and in situ treatment preserves the topsoil (Morikawa and Takahashi 2000). It is inexpensive (60-80% or even less costly than conventional physiochemical methods) and useful for treating a broad range of the environmental contaminants, especially at sites with shallow or low levels of contaminants. Possibly due to their static (non-mobile) nature, plants had to evolve their survival modes even in odd environments including sites contaminated with the xenobiotic substances, which are non-essential or even harmful for them. The natural adaptations and genetic mutations have evolved a wide range of preferential or general tolerance to the toxic substances in plants. Naturally occurring tolerance to plants is based on the mechanisms like phytostabilization, rhizodegradation, phytoaccumulation, phytodegradation, phytovolatization and evapotranspiration etc. which facilitate plants various means to avoid, escape, partition or remove the toxic contaminants as an adaptation measure. Such naturally evolved potential of plants, on the other hand, can be used for cleanup purposes. Bioprospecting of the suitable plant species and genotypes having higher tolerance, agroclimatic fitness, higher biomass and faster growth cycle is needed for various kinds of the contaminants. In addition, to commercially exploite those naturally occurring plants selected for the remediation of the pollutants, some biotechnological approaches such as rhizosphere manipulations to increase bioavailability or biodegradation of the contaminants for higher uptake and rapid removal by the phytoremediator (Vassil et al. 1998; Chaudhary et al. 1998; de Souza et al. 1999; Singh et al. 2003; Saxena et al. 1999; Morikawa and Takashashi 2000; Geebelen et al. 2002; Piechalak et al. 2003; Thangavel and Subburaam 2004) and genetic engineering of plants to increase uptake, transport, partitioning, tolerance, in situ degradation, volatization or evaporation etc (Rugh et al. 1998; Zhu et al. 1999,a,b; Pilon Smits et al. 1999; Gleba et al. 1999; Zaal et al. 1999; Saxena et al. 1999; Morikawa and Takahashi 2000; Hirschi et al. 2000; Bizily et al. 2000; Hannink et al. 2001; Singh et al. 2001; Takahashi et al. 2001; Dhanker et al. 2002; Lee et al. 2003a,b; Pilon et al. 2003; Singh and Jaiwal 2003; Maiti et al. 2004; Datta and Sarkar 2004; Marikawa et al. 2002 2005; Pan et al. 2005) have been persued to increase the phtoremediation efficiency. Such biotechnological efforts are also made to resolve the specific problems for the improvement of a phytoremediator to suit to the specific contaminant(s) and site(s) to make it commercially successful. This review is an attempt to analyse such approaches and efforts in the light of the present challenges towards the alarming contaminations of toxic heavy metals, major gaseous pollutants like nitrogen oxides, sulfur oxides and organic pollutants of agrochemicals and industrial origin (Fig. 1). We have confined our discussions largely on the higher plants and focused on the need to understand the key regulatory steps and mechanisms to produce superhyperaccumualtors of commercial grade by gene technologies. We have also discussed the needs of rhizosphere manipulations of plants for their better performance. © 2007 Springer-Verlag Berlin Heidelberg.