Plant mediated, microbial and microbial-assistedphytoremediation
A comprehensive approach towards bioremediation of heavy metals is the utilization of different plant species (both wild and transgenic), microorganisms or both of them in combination. Different plants have diverse eco-physiological and growth characteristics that suit them to fit in different environments. Considering the problem of heavy metal pollution, some plants are sensitive while some show a certain degree of tolerance to heavy metal stress. The later offer opportunities for their utilization in phytoremediation processes. However, the only ability of plants to tolerate heavy metal stress does not make them ideal candidates for remediation, instead, their ability to extract heavy metals in larger quantities and biotransformation are some of the important features which can make the phytoremediation technique more suitable. Moreover, contaminated soil, types and concentration of heavy metals and plant species to be used in the remediation approach are influencing factors for a targeted phytoremediation method. In many empirical studies and reviews, the major steps involved in phytoremediation have been outlined below.
In phytoextraction, heavy metals are absorbed by plants, retained in their roots or translocated to above-ground parts (Gupta et al. 2019). Heavy metal absorption by roots and subsequent transport to other paits is influenced by many factors among which the role of specific transporters, enzymes, hormones, gene expression and metabolic modulation are significant (Dal Corso et al. 2019). Ideally, phytoextractant plants should possess extensive root-system, broad leaves, and hyper-accumulation potentials and should be short-lived. Several plants such as Brassicrapa, Cannabis sativa, Helianthus anmms and Zea mays (Meet s et al. 2005), Vertiveriazizanioides, Dianthus chinensis, and Rinnex spp. (Zhuang et al. 2007), Amaranthushypochondriacus, and Averrhoacarambola (Li et al. 2009,2012), Rosa multiflora and Sidahermaphrodita (Antonkiewicz et al. 2017), Cichoriumintybus, Ricinuscommunis, and Sesuviumportulacastrum (Ayyappan et al. 2016, Bursztyn Fuentes et al. 2018), and Salix spp. (Ishikawa et al. 2018) and several other plants have been successfitlly utilized in phytoextraction methods.
Like plants, microbial agents are also capable of extraction of heavy metals. Unlike plants, microbes have lower biomass and lack translocation organs, but they still cany out a significant proportion of heavy metals extraction from polluted soils. When used in combination with plants, they augment the phytoextraction process by facilitating roots to absorb efficiently, mobilize heavy metals and synergistically stimulate plant species to traffic them up. Microorganism whether bacteria, microalgae or fungi should be tolerant of heavy metal stress, and possess symbiotic potentials with plants. Rajkumar et al. (2010) reviewed the role of siderophore producing bacteria and concluded that these microbes facilitate heavy metal binding to plants’ roots. Ma et al. (2016b) suggested that endophytic bacteiia could help plants to tolerate heavy metal stress and efficiently remove heavy metals from soil. Khan and Banu (2019) highlighted that microbes in polluted soil detoxify the toxic heavy metals, make them mobile and available to plants for uptake, facilitate solubilization and by releasing chelating agents.
Phytostability defined as chemical stabilization of heavy metals in soil through complex formation, roots attachment, amendments application, allelopathic interactions, and phytochelation (Radziemska et al. 2017, Sliackira and Puthur 2019). Many plants and microorganisms have the potential to release allelochemicals, and exudates that can interact with heavy metals, leading to the formation of phytochemical-metal complexes. The complex formation depends on the nature of released phyto-microbial chemicals and the reactivity potential of heavy metals. The complex formation may either reduce the toxicity of heavy metals or their mobility in the soil which will correspond to their stabilization. Besides metal-complex formation, plant roots and microbes sequester heavy metals by accumulation and adsoiption, which reduces their bioavailability and results in the heavy metals’ stability (Yang et al. 2014). Several plants and microbes have been identified to phytostabilize heavy metal load in polluted soils by adsorption, accumulation, and through the release of phytochemicals into the rhizosphere. Daiy et al. (2010) revealed that Lupinusluteus assisted with plant growth- promoting bacteria (Bradyrhizobium sp. 750, Pseudomonas sp. and Ochrobactrumcytisi) proved effective remediation potentials by phytostabilizing heavy metals Cu, Cd and Pb. Daxy et al. (2010) revealed that plant growth-promoting bacteiia (Bradyrhizobium sp. 750, Pseudomonas sp. and Ochrobactrumcytisi) promotes plant growth (Lupinus luteus) by phytostabilizing heavy metals such as Cu, Cd and Pb. Similarly, Yang et al. (2014) documented some shrubs and grasses as good phytostabilizing agents for heavy metals (Pb, Zn, Mn, Cd) in mine tailing. Javaid (2011) highlighted the role of arbuscular mycorrhizal fiingi in heavy metal stabilization referring to their potential of releasing polyphosphate granules, chelation and adsorption. A significant reduction in the bioavailability of Pb and its stability was observed when Pb-polluted agricultural waste was treated with the compost of Phanerochaetechrysosporium (Huang et al. 2017).