Keywords

4.1 Introduction: Background of Heavy Metal Pollution

Heavy metals being toxic and bioaccumulative in nature, are environmental pollutants with prolonged persistence in the environment, thus leading to detrimental effects on floral wealth and human health (Rzymski et al. 2014). They are metals possessing a specific density of more than 5 g cm−3 and have adverse impacts on the life and environment (Järup 2003). Some metals known as micronutrients, (copper, iron, manganese, molybdenum, nickel and zinc) play a vital role in the normal functioning of plant cells such as biosynthesis of nucleic acids, chlorophyll, carbohydrates, secondary metabolites, stress resistance and maintenance of biological membranes as well as overall growth of the plants (Rengel 2004). However, when their internal concentration transcends a certain threshold limit, they negatively influence plant growth and become toxic, forming a bell-shaped dose–response relationship (Marschner 1995). Moreover, the concentration of heavy metals is generally location-specific, subjected to the source of individual pollutants.

As per the World Health Organization (WHO), the common toxic ‘heavy metals’ of public health concern are arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), mercury (Hg), nickel (Ni), lead (Pb), selenium (Se), manganese (Mn), copper (Cu) and molybdenum (Mo). The standards for heavy metals in soil, plant and water as per Bureau of Indian standards (BIS) and WHO have been presented in Table 4.1.

Table 4.1 Normal and critical range of heavy metals in soil, plant and water
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4.1.1 World Status

Rapid industrialization and exponential increase in the human population has increased the discharge of massive loads of heavy metal pollutants in the environment (Zhang et al. 2020). Globally, around 500 M ha of our land resources are facing the problem of soil contamination ended up with higher concentrations of heavy metals compared to the regulatory levels (Liu et al. 2018). Industries and other human activities discharge approximately 2 million tons/day of sewage and effluents into the water bodies making them unfit for various agricultural and other activities. Fly ash dumping sites of coal-based thermal power stations are also a major source of heavy metal pollution around the world (Pandey and Singh 2010; Pandey 2020). In developing nations, the situation is more critical where about 90% of sewage and 70% of industrial wastes (generally untreated/partially treated) are being discharged into surface water resources (Anonymous 2010). Over the past few years, the annual global release of heavy metals has surpassed 0.2 lacs MT for Cadmium, 9.3 lacs MT for Copper, 7.83 lacs MT for lead and 1.35 lacs MT for Zinc (Thambavani and Prathipa 2012). Further, heavy metal poisoning has become a universal public health concern. Heavy metal pollution in soils also has tremendously impacted the global economy, which has annually been estimated to be beyond US$10 billion.

4.1.2 Indian Status

The data available on the nature and extent of metal pollution and its impact assessment on the plant, soil and human health is not very conclusive in India. However, as per Indian central pollution control board (CPCB 2009), approximately 38,254 megaliters per day (MLD) of sewage and 25,000 MLD of untreated industrial wastewater generated from urban areas are released into the surface water bodies, wreaking degradation of the quality of water resources. Bhardwaj (2005) has estimated that by 2050, wastewater generation in India is going to be around 1,22,000 MLD. The utilization of such wastewater loaded surface water sources for irrigation purposes in agricultural fields has magnified the heavy metals concentrations in soils of agricultural fields particularly those situated in the vicinity of urban areas (Saha and Panwar 2013). However, the heavy metal accumulation in the soils will vary depending upon the source, concentration and duration of application (Rattan 2005). Usage of sewage water as irrigation for 20 years successively may result in significant accumulation of zinc (2.1 times), copper (1.7 times), iron (1.7 times), nickel (63.1%) and lead (29%) in the soils as compared to soils irrigated with tube well water (Simmons 2006). Such unrestricted transfer of heavy metals in arable land through wastewater irrigation will trigger more metal uptake by crops and will enter the food chain (Rattan 2002).

4.2 Sources of Heavy Metal Pollution

Geogenic and anthropogenic activities are mainly responsible for heavy metal pollution in the environment. Geogenic processes such as biogenic, terrestrial, volcanic processes, erosion, leaching and meteoric are the main sources of heavy metals in the environment (Muradoglu et al. 2015). While, industrialization, urbanization and modernization of the agricultural sector are substantially contributed to the release of heavy metal pollutants into the surrounding which gets deposited on the soil through natural processes of sedimentation and precipitation. In addition, anthropogenic processes such as irrigation with sewage and industrial wastewater, mining activities, fly ash disposal, excessive application of pesticides and fertilizers, have disturbed the natural balance of geochemical cycles, which in turn has resulted in the entry of heavy metals into the soil (Zhang et al. 2011; Dixit et al. 2015). The major contributors of heavy metals in the environment are listed in Table 4.2.

Table 4.2 Major sources of heavy metals
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4.3 Plant Assisted Bioremediation: Techniques/Strategies

Plant assisted bioremediation involves the symbiotic relationship between rhizospheric microorganisms and the plant roots (Kumar et al. 2017). The symbiotic relationship intensifies bioavailability of the heavy metals and stimulates absorption capacity of the roots. Remediation of metal-polluted soil by soil microbes especially the rhizospheric population is known as rhizoremediation (Kuiper et al. 2004). Rhizoremediation involving plant growth-promoting rhizobia, mycorrhiza and other microorganisms is very efficient in promoting plant biomass and thus its efficiency to stabilize and remediate metal-polluted soil (Jing et al. 2007). Plant roots release exudates, may be enzymatic or non-enzymatic that modify the soil environment and habitat to numerous microorganisms. Rhizosphere plays a great role in the remediation of metal polluted soil. Heavy metals can only be transformed via several processes such as sorption, methylation, complexation or change in valence oxidation state, affecting their mobility and bioavailability. Microbes have an important role in the processes like carbon sequestration, plant growth, productivity and phytoremediation. Microorganisms (bacteria, fungi and microalgae) along with plants are the potential agents of bioremediation. They enhance the plant growth through different enzymatic activities, nitrogen fixation and reducing the ethylene production (Pandey and Singh 2019). Bacteria respond to the heavy metals and the molecules generated through oxidative stress in different ways. These are entrapped in the capsules, transported through heavy metals by the cell membrane, absorbed on the cell walls, precipitated or oxidized/reduced (Singh et al. 2010). The microbial response to heavy metals is important in harnessing them as potential candidates for remediation of metal polluted soils (Hemambika et al. 2011). Plant growth-promoting bacteria (PGPB) also known as growth-promoting agents are now assessed for their metal detoxifying potential in remediating metal-polluted soils (Ahemad 2014). Fungi are important as these augment the phytoremediation by changing the bioavailability of metal through different ways like modifying the pH of the soil, production of different chelators, and controlling the redox reaction etc. (Ma et al. 2011a, b). Also, a high surface-to-volume ratio make bacteria a potential biosorbing agents. While, the plants absorb these metals and translocate them to various plant tissues and organs. Plants remediate the heavy metal polluted soil by adopting different techniques/strategies such as phytoextraction, phytostabilization, phytovolatilization, phytostimulation (Pandey and Bajpai 2019; Pathak et al. 2020). These are described as:

Phytoextraction: Phytoextraction is the process of the uptaking and storing of heavy metals from the soil by the plants (McGrath 1998). There are two fundamental ways of phytoextraction:

  • Natural: The natural way of removal of heavy metals by the plants, also known as unassisted phytoremediation.

  • Assisted: Microbes, plant hormones and chelating agents assist the plant in the remediation of heavy metal polluted soils (Malik et al. 2022).

Natural phytoremediation can be accomplished by either (1) hyperaccumulator plants or (2) genetic engineering of the plant with certain characteristics of hyperaccumulators for the accomplishment of phytoextraction (Chaney et al. 2005). The hyperaccumulator plants are the plants whose tissues can contain certain heavy metals from 1000 to 10,000 mg kg−1 (Black 1995). They can collect and concentrate the heavy metals in the harvestable tissues, biomass without affecting the plant growth. The heavy metal concentration in the hyperaccumulator plants is approximately 100 times higher compared to the ordinary plants. It is approximately 1000 mg kg−1 for arsenic and nickel, 100 mg kg−1 for cadmium and 10,000 mg kg−1 for zinc and manganese. The most prominent examples of hyperaccumulator plants are Arabidopsis, Alyssum, Noccaea and the members of Brassicaceae family.

Phytostabilization: Phytostabilization involves complexation, precipitation, sorption or metal reduction (Ghosh and Singh 2005). Plants restrict the movement of the metals in the roots by the assimilation, aggregation, adsorption and precipitation. They also help to avoid movement of the metals through water, wind, drainage and dispersion of soil (USEPA 1999). The phytostabilization stabilizes the metal contaminant rather than translocating it to the edible parts, that in turn can reach human beings (Prasad and Freitas 2003). In this, there is the aggregation of metal by roots or root exudates that immobilize and lower the accessibility of the soil pollutants. Chromium and lead are toxic metals that are remediated by phytostabilization. The proficiency of the phytostabilization is increased by the addition of nutrients to soil viz. lime and phosphate. Brassica juncea has been reported to be an efficient Phytostabilizer as it accumulates chromium in the roots (Bluskov and Arocena 2005).

Phytovolatilization: The release of metal pollutants to the atmosphere by the plants in altered or unaltered form after metabolic and transpirational pull is called phytovolatilization (USEPA 1999). Selenium, arsenic and mercury are the main metal pollutants that can be remediated through phytovolatilization (Dietz and Schnoor 2001).

4.4 Significance of Plant Assisted Bioremediation of Heavy Metal in Agriculture

Agriculture is the main backbone of the Indian economy and socio-political stability. With approximately 7% of the growth, Indian economy is the 7th largest in the world. The contribution of agriculture and its allied sectors was 51.81% during 1950–51, which declined to 18.20% by 2013–2014 and now it is approximately 14.39% (2018–19). These figures are still higher than most of the countries. Soil quality is one of the main contributing factors for sustainable agriculture. Sustainability is defined as the living within the regenerative capacity of the biosphere (Wackernagel et al. 2002). Inappropriate agricultural management practices, excessive use of a large number of chemicals, insecticides, pesticides, sludge and manure are attributing declination of soil quality. Developmental activities such as industrialization, urbanization and transportation are competing with natural resources, and impacting soil quality and biodiversity (Godfray et al. 2010). Despite several adverse effects of industries on the environment, they are considered important because of the unlimited human desires. The dependency of agriculture and industries on each other and their impact on the environment and human life is depicted in Fig. 4.1. Different anthropogenic activities have become the main reason for the deterioration of natural resources (soil + water). In the long run, polluted soil and water will not be suitable to grow the food which will directly or indirectly impact the socio-economic condition of the country (Saha et al. 2017). In the agroecosystem, agriculture and industries are the main reason for soil pollution by heavy metals such as cadmium, chromium, lead, arsenic, nickel and mercury. Introduction of the above metals in the soil environment and their impact on soil is quite alarming, as:

Fig. 4.1
A model diagram indicates the agricultural and industrial practices and their effects on the natural world and on humans.

Relationship of agriculture and industrial activities affecting natural resources and humans

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  • Entry of these metals in the agroecosystem results in degradation of soil structure and affects moisture retention in the soil profile.

  • Heavy metals interact with soil components, microorganisms (nitrogen transformation, mineralization/immobilization etc.), root cells and affect the transformation of soil nutrients and their uptake.

  • Build-up of salinity problem in the polluted soil along with the effect on water and nutrient uptake.

  • Heavy metals lead to alkalinity development, result in more ammonia volatilization losses.

  • The agronomic efficiency and partial factor productivity of polluted soil are normally lower than the unpolluted soil.

  • The shelf life of the crops irrigated with industrial wastewater is lower than irrigated with fresh water.

  • Heavy metal pollution results in huge ecological disturbances

4.5 Role of Microflora and Flora in Plant Assisted Bioremediation

Plant assisted bioremediation is an eco-friendly approach, encompassing the complex phenomenon of interaction between the microbes and plant genotype with its biotic and abiotic environment. The most important components of plant assisted bioremediation consist the in situ selection of genotypes and symbiotic microorganisms in the rhizosphere (having the capability of degrading the organic contaminants completely). Further, the identification of candidate genes and alleles linked with biochemical and physiological processes also has a key role in the development of a potential plant assisted bioremediation strategy. High levels of metal extraction and translocation to shoots and organic degradation are keys to develop an efficient phytoremediation measure. A most promising approach to substitute the costly remediation technologies is the use of plants assisted by microbes to clean up heavy metal polluted soils and water (Malik et al. 2022). Therefore, the selection of appropriate microbes and plant species is a prerequisite for effective remediation of heavy metal pollution.

4.5.1 Potential Microbes Involved in Bioremediation

Microorganisms help in the uptake of heavy metals through both active (bioaccumulation) and passive (adsorption) modes. Microbes (bacteria, fungi and algae) have been utilized to remediate the contaminated sites. The high surface to volume ratio, ubiquitous nature, the capability of growing in extreme conditions and active chemisorption sites make bacteria a potential candidate for bioremediation (Srivastava et al. 2015; Mosa et al. 2016). Higher absorption, uptake and recovery capacity of fungi make them suitable biosorbents for the remediation of toxic metals (Fu et al. 2012). Also, algae compared to other biosorbents produce high biomass. The high sorption capacity of algae and the presence of various metal binding chemical groups like hydroxyl, carboxyl, phosphate and amide make them a suitable contender for remediation of heavy metals (Abbas et al. 2014). The microorganisms involved in bioremediation of heavy metals are summarized in Table 4.3.

Table 4.3 Potential microbes for remediation of heavy metal pollution
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Numerous researchers have reported bacterial accumulation and sorption along with other plant growth promoting features responsible for the enhanced plant growth in polluted soils (Ma et al. 2011a, b; Kumar et al. 2009). Higher accumulation of heavy metals in plants without having any phytotoxicity is due to decreased internal availability of metals or metalloids and higher rhizospheric plant bioavailability (Deng et al. 2013a, b; Weyens et al. 2010). Nickle uptake by Alyssum murale was significantly enhanced by Sphingomonas macrogoltabidus, Microbacterium liquefaciens and Microbacterium arabinogalactanolyticum inoculation compared to the un-inoculated control (Abou-Shanab et al. 2003). Correspondingly, the inoculation of Phaseolus vulgaris with Pseudomonas putida KNP9 protected it from metal toxicity (lead and cadmium) and improved its growth with respect to controls (Tripathi et al. 2005). Therefore, the application of metal remediating plant growth-promoting bacteria (PGPB) along with plant growth promoting activities makes the remediation process more effective and efficient (Glick 2012). The utilization of the mining sites with the higher concentration of heavy metals is a global challenge to environmental sustainability (Ahirwal and Pandey 2021). In this direction, researchers demonstrated that the Pseudomonas aeruginosa-HMR1 removes heavy metals and exhibits plant growth-promoting attributes. Thus, the P. aeruginosa-HMR1 can be used for the restoration of mining lands for forestry, ornamental plants and agricultural purposes (Bhojiya et al. 2021).

Fungi have been found to have more tolerance to metals than bacteria (Deng and Cao 2017; Deng et al. 2013a). Fungi easily reach the microsites that are not accessible to the plant roots and thus can compete with other microbes for food and metal uptake. These protect the plant roots from directly interacting with metals and increase the soil hydrophobicity, thus hindering metal transport. Moreover, the extended mycelia formation by fungi also makes them suitable for bioremediation. In metal-polluted soils, various fungi like Aspergillus, Trichoderma and the arbuscular mycorrhizae (AM) have demonstrated the capacity to improve the phytoremediation process (Deng et al. 2011, 2013a). These fungi have high capability of immobilization of toxic/heavy metals by forming either the insoluble compounds, chelation or through biosorption. The fungal species and ecotype greatly affect phytoremediation efficiency. Some examples of bioremediation of heavy metals are given in Table 4.4.

Table 4.4 Plant assisted microflora involved in bioremediation of heavy metals
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4.5.2 Potential Plants Involved in Bioremediation

The potential phytoextractive plant species has the ability to accumulate the high content of the metals into the aboveground biomass without showing any toxicity symptoms. Their potential of phytoextraction can be enhanced by the use of fast-growing hyperaccumulator tree species with extensive root systems, thus ensuring its economic and environmental feasibility. Remarkable genetic variability has been reported to exist among plants of Salicaceae species adapted to soil of varying level metal contaminants (Dickinson and Pulford 2005; Puschenreiter et al. 2010; Marmiroli et al. 2011; Yang et al. 2015). In addition to this, adoption of native fast growing tree species may provide us with a better possible solution. Some of the plants suitable for plant assisted bioremediation are given in Table 4.5.

Table 4.5 Potential plants for remediation of heavy metal pollution from soil
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4.6 Mechanism of Plant Assisted Bioremediation

The mechanism for the plant assisted bioremediation involves bioavailability, uptake, translocation, sequestration and different defence mechanism that can help to develop heavy metal stress-resistant cultivars and highly efficient plant species for phytoremediation in harmony with microflora through genetic engineering technologies.

4.6.1 Bioavailability

It is defined as a part of the total elemental concentration available to plants that determines the uptake and accumulation of heavy metal ions in plants. Heavy metals exist in soils with several degrees of fractions i.e. soil solution form, soluble metal complexes and free metal ions forms. Several factors that determine the bioavailability of heavy metal elements are environmental conditions (moisture, temperature and oxidation state), soil properties (pH and organic matter) and enhanced biological activity by microbes (Yang et al. 2012; Bravin et al. 2012). These factors regulate the release of heavy metals into the soil and influence the plant uptake from soils. Environmental factors like high temperature enhance the physical, chemical and biological activities in soil–plant system, while precipitation and rainfall are known to accelerate plant growth and development. High soil moisture content regulates the movement of water-soluble trace elements during bioremediation. Soil properties, viz., pH, organic matter/organic carbon and cation exchange capacity (CEC) are the important factors that control the bioavailability of cations in soil. Soils with higher organic matter and high pH will form complex with heavy metals more firmly and become less available to plants for uptake and accumulation. Acidification of the rhizosphere is considered to increase the metal accumulation potential of plants raised on heavy metal contaminated soils. At acidic pH, heavy metals are found in free ionic forms and are more bioavailable, but at the basic pH metals form insoluble metal complexes with phosphates and carbonates (Sandarin and Hoffman 2007; Rensing and Maier 2003). Biological activities within the soil–plant system alter the bioavailability of metal elements. Microbes in the rhizosphere can produce chelating compounds, enhance the key nutrient uptake and also the availability of soil heavy metals (Rajkumar et al. 2012). Some plants secrete the organic components that form soluble complexes with heavy metal ions in soils. These soluble complex formations promote the mobility of heavy metals in soils. Yang et al. (2012) reported that root exudates include various organic acids and amino acids viz., oxalic acid, citric acid, tartaric acid, succinic acid, aspartic acid and glutamic acid, that form heavy metal soluble complexes and increase the mobility of Cd, Cu, Zn and Pb in soils (Fig. 4.2).

Fig. 4.2
A model diagram represents the H M Bioavailability, and it is classified by environmental factors, soil properties, and biological activities.

Bioavailability of the heavy metals (HM) to plants

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4.6.2 Plant Uptake

The movement of heavy metals in soils depends upon precipitation, redox potential, absorption/ adsorption and its complexation/methylation responses mediated by microbes along with plants (Kumar et al. 2017). The mechanism of plant metal uptake, rejection, translocation and sequestration is specific and highly variable within the plant varieties (Lone et al. 2008). Plants adopt two main strategies to combat heavy metal stress by either reduce metal uptake or increase vacuolar sequestration. The heavy metal is bioactivated by the root microbe’s interaction first which leads to root absorption and further compartmentalization.

  1. (i)

    Bioactivation of metals by root-microbe interaction

Several studies depicted the positive interaction of microorganisms with plant species in the rhizosphere (Dakora and Phillips 2002; Kuiper et al 2004). Plant growth promoting rhizobacteria increase the bioavailability of metal ions by dissolving them via changing the chemical properties (pH, redox state, organic matter) of soils in the rhizosphere and modify the heavy metal speciation in the root zone (Jing et al. 2007). They solubilize the ions like phosphate, siderophore and increase acid production (Kumar et al. 2017). During heavy metal stress, mycorrhizae release natural acids that enhance zinc solubility and its mobility, ultimately playing a significant role to strengthen plant survival rate (Giasson et al. 2008).

Bacterial endophytes are considered to be beneficial for host plants usually during stress conditions, because they regulate the plant growth promoting mechanisms like phytohormone production by activating enzymes viz., 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, ethylene and Indole acetic acid (IAA) (Hardoim et al. 2008; Rajkumar et al. 2012). Endophytes also known to enhance nitrogen fixation and phosphate availability in rhizosphere, hence helps to recover the plant during heavy metal (HM) stress conditions (Kuklinsky-Sobral et al. 2004).

  1. (ii)

    Root absorption and compartmentalization

The transport of nutrients and heavy metals from soils to plant roots occurs via symplastic and apoplastic transport. In symplastic transport heavy metals enter the root cells through the plasma membrane of the endodermis of the root. While in apoplastic transport, it enters the root apoplast via spacing within the cells. Generally, heavy metals and nutrient ions cross the membranes only with the aid of naturally occurring membrane transport proteins (Fig. 4.3). The abundance of these proteins depends upon tissue type and environmental conditions. If a small amount of nutrients is present in soils, then the plant requires high-affinity transporters for uptake; whereas if the nutrients in the soil are present in high concentrations (e.g. agricultural soils with fertilizers), then low-affinity transporters would be more useful for plant uptake (Cailliatte et al. 2010).

Fig. 4.3
A model diagram represents the heavy metals and nutrient ions crossing the membranes only with the aid of naturally occurring membrane transport proteins.

Uptake of heavy metal and its compartmentalization in various plant part through different transporter proteins

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Several transporter families have been reported in plants such as heavy metal ATPase (HMA), natural resistance and macrophage proteins (NRAMP), Zrt, Irt-like proteins (ZIP) etc. (Table 4.6). In the cytosol, toxic metals rapidly bind to chelators and are transferred to the vacuole for sequestration. Ingle et al. (2005) observed that histidine is involved in Ni-chelation in root cells and helps plant to tolerate Ni toxicity. Cr (III) in root cells is chelated with acetate and sequestered in the vacuole (Bluskov and Arocena 2005).

Table 4.6 Metal transporters involved in heavy metal uptake, transport and sequestration during phytoremediation (Bhargava et al. 2012)
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4.6.3 Translocation

Heavy metal transporters are required for translocation of metallic ions from root symplast to xylem apoplast due to endodermal barrier (casparian strips) in the root. The translocation of heavy metal ions depends on two factors: root pressure and leaf transpiration (Kumar et al. 2017).

  1. (i)

    Root symplast to apoplast through xylem tissues

Xylem loading of metals from root symplast is an important phenomenon making the plant to tolerate heavy metal toxicity instead of promoting its accumulation in root cells that would inactivate the enzymes involved in metabolic processes. Cation diffusion facilitator (CDF) type of proteins conveys a broad array of metal divalent ions from cytoplasm toward the outer cell parts and even within the subcellular compartments (Hanikenne et al. 2005). HMA2 proteins are energy-dependent transporters, despite having selective nature they also get activated by analogue metal ions. Hussain et al. (2004) isolated HMA2 and HMA4 transporters in Arabidopsis for Zn transportation within cellular compartments and homeostasis. Milner and Kochain (2008) deciphered the importance of HMA2 and HMA4 genes in metal loading into the xylem.

  1. (ii)

    Root apoplast to aerial (stem and leaves) tissues

Hyper accumulator plants rapidly translocate the absorbed metal ions from the root to the above-ground parts, while non-accumulators accumulate heavy metals only in their root portions. Heavy metals can be stored in root vacuoles. Due to the limited space and high heavy metal concentration in the soil matrix, it gets translocated to shoot tissue where sequestration and detoxification rate is comparatively high (Kumar et al. 2017). Generally, metals are stored in only chelated form but are transported from one cellular compartment to other in free ionic state according to the selectivity of transporter proteins (Ortiz et al. 1995). Research experiments showed that hyperaccumulator plants accumulate high concentration of heavy metals in stem and leaf vacuoles than the root tissues. In the leaf tissues, high amount of metals accumulates in epidermal tissues compared to the cortical and vascular tissues (Kupper et al. 2001; Kumar et al. 2017).

4.6.4 Sequestration/Detoxification

To cope up with heavy metal stress, plants adapt different survival strategies like compartmentalization, exclusion, complexation and synthesis of binding proteins (metallothioneins and phytochelatins). Heavy metal toxicity inside the plant cell gets detoxified by complex formation and compartmentalization to make them less available to metabolic active sites. Organic acids, glutathione precursor of phytochelatins and metallothiones play a significant role in detoxification/sequestration. Phytochelatins (PC) have an imperative role to detox cadmium in fungi and plants through conjugation. Glutathione enhances the PC synthesis and thus more PC-metal complex formation in the vacuole which ultimately enhances cadmium tolerance in plants (Lee et al. 2003).

In plants, different heavy metal ions (Cu, Hg, Zn, Pb, Cd) stimulate the enzyme, γ-glutamyl-cysteinyl dipeptidyl transpeptidase (PC synthase) for phytochelatin synthesis which results in glutathione conversion (GSH) to phytochelatin (Fig. 4.4). These phytochelatins are produced from glutathione (GSH) through oxidation and reduction reactions. The metal ion binds to cysteine sulfhydryl residues of phytochelatins and its sequestration occurs inside the vacuole (Zhu et al 2004; Kumar et al. 2017). In hyperaccumulator plants, toxic effects of Ni were overcome by enhancing GSH-dependent antioxidant mechanism that protects the plant from oxidative damage (Freeman et al. 2005). Metallothiones are metal-binding proteins that modulate the concentration of metals inside the cell by binding heavy metal ions to cysteine and thiol groups (Khan et al. 2004). Mn2+ metal detoxification involves uptake of ions from the plasma membrane, binding with malate and transportation through tonoplast to vacuole where Mn unbinds from malate and form complex with oxalate (Memon et al. 2001).

Fig. 4.4
An illustration represents the synthesis that results in glutathione conversion G S H to phytochelatin.

Sequestration and detoxification of heavy metals (HM) in plant cell

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Heavy metal toxicity hindered the functional group of important molecules that disrupt the metabolic enzyme activity and consequently inhibit or suppress photosynthetic rate, respiration rate and all physiological and biochemical processes of plants (Gupta et al. 2015; Ali et al. 2013). Naturally, plants develop various defense mechanisms against heavy metal stress inside the plant body which include compartmentalization reduction, suppression of high-affinity phosphate transport system, sequestration and translocation (Zhao et al. 2009). When metal ions cross enter into plant tissues by crossing these barriers then various cellular defense mechanisms (as a second line of defense viz., ROS production, antioxidants) are initiated to detox the adverse effect of noxious heavy metals (Silva and Matos 2016).

4.7 Conclusion and Future Prospects

The pollution due to heavy metals is of great concern because of its potential impact on human and animal health. It is imperative to protect the natural resources and biodiversity, by using cheaper and effective technologies. In phytoremediation, the plants have to retain the pollutant in their root or other parts by producing large biomass and microbes converting toxic forms of heavy metals to non-toxic forms. But till now no plant is known to fulfil both these criteria. At the heavily contaminated sites with both organic and inorganic pollutants, there is a limitation of plant growth and microbial activity, thus having reduced plant assisted bioremediation efficiency. Recent progress in molecular, biochemical and plant physiology fields provides a strong scientific base for achieving this goal. During the last decade, substantial efforts have been made by the researchers to identify plant hyperaccumulators, bioremediators for heavy metals and their mechanism of uptake, translocation. There is a huge genetic variation in different plant species, even among the cultivars of the same species. So, research must be carried out to study the mechanism of metal uptake, accumulation, exclusion, translocation and compartmentation for each species as they play a specific role in phytoremediation. Further, research is needed to study metal uptake at the cellular level including influx and efflux of different metals by different cell organelles and membranes.

  • There is a need of microbial profiling of rhizosphere under controlled and field conditions to examine the antagonistic and synergistic effects of different metal ions in soil and polluted waters.

  • Selected essential rhizosphere microorganisms and microbial strains able to degrade toxic pollutants can be studied in the natural habitat. Molecular techniques will further help in elucidating the fate and effect of these selected strains in the soil environment.

  • Standardization of the methods for heavy metal recovery from the hyperaccumulator plants will allow the detection of the new strains of the micro-organisms who can degrade or reduce the toxic metals to non-toxic metals as well as improve the fertility status of the soil.