Recent Advances Towards Improved Phytoremediation of Heavy Metal Pollution

This chapter first gave a broad overview of the application of phytoremediation technologies for the management of metal-contaminated sites. Then the interactions between plants and the microorganisms in the rhizosphere are reviewed as these could influence the potential accomplishment of these phytotechnologies. Plant-microbe interactions can be enhanced or modulated by modifying microbial population (rhizoengineering) for the remediation of pollutants present in the soils. Rhizoengineering is an innovative approach towards phytoremediation.

Success of phytoremediation greatly depends on the plant species. In addition, bioavailability of Pb in soils is regarded as the key factor limiting the efficiency of phytoextraction. Different biological, physical and chemical methods including addition of chelating agents such as EDTA and other bio-degradable chelators have been investigated. In the absence of added chelating agents in soil, only few species are able to achieve the status of a Pb hyperaccumulator. In contrast, some studies showed that plants exposed to Pb with supplement of EDTA were able to uptake from dozen to hundred times more Pb in the shoots than those treated with Pb alone. Addition of chelating agents combined with application of electric current or plant growth regulators, might greatly increase bioavailability of Pb in soils and / or plant biomass, thus ultimately the efficiency of phytoextraction. In plants treated with Pb plus EDTA, Pb deposits at the ultrastructural level were found mainly in cell walls, vacuoles and along plasma membranes in various patterns (acicular, granular and fine precipitates) in root cells of Lespedeza chinensis and L. davidii. These might be related to the transport and detoxificaiton of Pb chelates in the plants.

Nitric oxide (NO) has been shown to be an important signaling molecule in mammalian and plant physiology. The notion that exogenous application of NO in the form of a solution-based NO donor, for example, sodium nitroprusside (SNP), can counteract the toxicity of heavy metals in plants has been supported experimentally in many studies in the past decade. However, some recent studies also appeared to have casted doubts about this. Moreover, there does not appear to have been any assessment of the practical or agricultural significance of applying NO exogenously for ameliorating heavy metal toxicity in plants, particularly during postgerminative seedling growth. The main features of the relevant studies were examined critically. The issues discussed in relation to the studies of applying NO and heavy metal treatment of seedlings during postgerminative growth might also be relevant to studies at other plant growth and developmental stages. It is concluded that the agricultural significance of exogenous application of NO to alleviate heavy metal toxicity in plants remains to be established.

Environmental exposure to heavy metals such as arsenic and chromium has become a serious problem worldwide. The recent advances in the knowledge of metal hyperaccumulation by some plant species have recognized phytoextraction as a prospective proposition using hyperaccumulator plants. This paper enumerates the progress in phytoextraction of metals using fern species with emphasis on the arsenic hyperaccumulator Pteris vittata. The scopes of R&D for value addition in ferns for efficient phytoremediation application have also been discussed.

In order to be able to use phytoremediation practices successfully it is necessary to gain knowledge on the behaviour and fate of the metals in soil-plant systems. Metal hyperaccumulator plants such as Thlaspi caerulescens of the Brassicaceae family represent an excellent model to study physiological and molecular mechanisms of metal uptake, transport, accumulation and tolerance due to their physiological, morphological and genetic characteristics, and their close relationship to Arabidopsis thaliana, the general plant reference species. In this chapter, the progress that has been made in elucidating molecular and physiological mechanisms of metal hyperaccumulation in Thlaspi caerulescens (a model Zn, Cd and Ni hyperaccumulator) and its relative T. praecox (a Cd and Zn hyperaccumulator) will be reviewed briefly. A special emphasis will be placed on hyperaccumulation of cadmium and interactions of the Thlaspi spp. with symbiotic arbuscular mycorrhizal fungi as these topics still need to be more intensively explored.

Soils contaminated with toxic levels of heavy metals present serious public health hazards. A potentially green, environmentally friendly and sustainable technology is phytoextraction of soil heavy metals. Naturally occurring metal hyperaccumulating plants have been found in specific metal enriched habitats but they are not suitable for practical phytoextraction purposes. Genetic engineering is a powerful technology to improve the phytoextraction potential of non-hyperaccumulating plants that have many other desirable attributes over the naturally occurring metal hyperaccumulating plants. In this chapter, the utility of Arabidopsis thaliana, a non-hyperaccumulator of heavy metals, to gain novel insights into how the different metal resistance-related genes might operate when plants are exposed to excess soil toxic metals was discussed. It is concluded that use of A. thaliana in this way is very useful to genetic engineering for enhanced phytoextraction of soil heavy metals by non-hyperaccumulating plants.

Poplars and willows find widespread use for the phytomanagement of contaminated sites. We review the processes involved in poplar and willow phytomanagement with the aim of elucidating knowledge gaps and fertile areas for future research. Poplars and willows are fast-growing tree species reduce contaminant mobility by their high water use and stabilisation of the contaminated substrate. Root exudates and microorganisms associated with the rhizosphere of these trees promote the degradation of some organic contaminants. Some clones accumulate of toxic trace elements, such as Cd, thereby facilitating their entry into the food chain. Phytomanagement using poplars and willows can result in biomass products, such as timber, bioenergy, essential oils, organic mulches, and stock fodder. These offset the cost of the operation. There is great potential to improve the performance of phytomanagement systems using these species by exploiting their natural genetic variation and microbiological symbionts. A key unknown is the response of deep-rooted poplars to heterogeneities in soil, which may promote or reduce contaminant mobility.