Abstract
Metal toxification has remained one of the problems with the advent of industrial revolution. Plant based remediation are showing increasing promise for use in soils contaminated with organic and inorganic pollutants. A large number of plant families has been identified which has shown significant result in detoxification of heavy metals. Hyperaccumulator plant is capable of sequestering heavy metals in their shoot tissues. High tolerance to HM toxicity is dependent on a reduced metal uptake or increased internal sequestration, which depends on plant and environmental condition. Recent progresses on understanding cellular/molecular mechanisms of metal tolerance by plants are reviewed. This chapter aims to focus on molecular mechanism involved in heavy metal detoxification and tolerance by plants. A different method by which plant effectively converts toxic metal in less toxic compounds has been explained in this chapter. Further, mode of accumulation and sequestration of metals have been explained which are utilized by hyper accumulators.
TopIntroduction
Phytoremediation can be defined as the process of utilizing plants to absorb, accumulate, detoxify and render harmless, contaminants in the growth substrate (soil, water and air) through physical, chemical or biological processes (Cunningham & Berti, 1993). Plant based remediation are showing increasing promise for use in soils contaminated with organic and inorganic pollutants. For removal of contaminants from soil, phytoremediation involves different processes, such as enzymatic degradation, that potentially lead to contaminant detoxification (Van Aken, 2008). However, despite great promise, rather slow removal rates and potential accumulation of toxic compounds within plants might have limited the application of phytoremediation (Eapen, Singh & D’souza 2007).
Phytoremediation is proposed as a cost-effective alternative for the treatment of contaminated soils. Topsoil would be preserved and the amount of hazardous material reduced significantly (Ensley, 2000). In recent years, public concerns relating to ecological threats caused by heavy metal led to intensive research of new economical plant based remediation of contaminated soil namely chemical, physical and microbiological methods are costly to install and operate (Danh, Truong, Mammucari, Tran & Foster, 2009). The ability to hyperaccumulate metals in above ground tissue without phytotoxic effects has evolved in almost 500 plant species mainly those in the Brassicaceae family (Kramer, 2010). At least three processes make a major contribution to the ability to hyperaccumulate/hypertolerate metals.
- 1.
Enhanced root uptake and loading into xylem.
- 2.
Superior root to shoot translocation and
- 3.
Efficient detoxification via chelation and sequestration predominantly within leaf cell vacuoles (Clemens, kim, Neumann, & Schroeder, 2002).
Table 1. | Treatment | Cost ($/Ton) | Additional Factors |
| Vitrification | 75-425 | Long-term monitoring |
| Landfilling | 100-500 | Transport/excavation/monitoring |
| Chemical treatment | 100-500 | Recycling of contaminants |
| Electrokinetics | 20-200 | Monitoring |
| Soil washing | 120-200 | Transport/Monitoring |
| Low temp. thermal Desorption | 45-200 | Transport/Monitoring |
| Incineration | 200-600 | - |
| Vitrification | 700 | - |
| Pneumatic fracturing | 8-12 | - |
| Excavation/Retrieval Disposal | 270-460 | Transport/Monitoring |
| Disposal alone | 35-60 | - |
Saxena, 1999.
Key Terms in this Chapter
Phytoremediation: It is method of applying plants for in situ removal or degradation of contaminants into non-toxic form or less toxic form.
Translocation: The process by heavy metals or contaminants is Trans located to shoots after intake by roots. Translocation factor has been found prominent in estimating translocation.
Heavy-Metals: Class of toxicants that cannot be broken down to non-toxic form easily. It has high density and high atomic number. These metals keep on accumulating in system posing serious threat to environment and organisms.
Detoxification: The process by which contaminants are converted to less toxic or non-toxic form of compounds. Metallothineins and phytochelatins play a prominent role in detoxification process.
Sequestration: Heavy metal ions are disposed of from cytosol by sequestration. Vacuole plays a vital role in it. Mediators of sequestration are transporters in the respective membrane.
Transporters: Heavy metals enter plant through specific metal transporters. Proteins like ZIP14, ZNT1, IRT1, COPT1, tVramp-1/3/4 and LCT1 on the plasma membrane-cytosol interface; ZAT, ABC type, AtMRP, HMT1, CAX2 seen in vacuoles; and RAN1 play a vital in transport phenomenon.
Accumulation Mechanism: Plants adapt different mechanism for transport and accumulation of heavy metals via xylem and phloem transport. Vacuoles play a vital role in accumulation of heavy metals.
Hyperaccumulator: Plants which are capable of growing in high metal concentration and have capability to detoxify it by different mechanisms. A large number of plant families have been identified which can tolerate metal contaminants.
Cellular Mechanism: It involves all those transport ways through which xylem transport heavy metal from soil to different parts of plant including uptake, sequestration and translocation methods.