for Cd to 1000 mg·kg−1 for Cu, Co, Cr and Pb. These values exhibit a shoot-to-soil ratio of metal
concentration and the factor for bioaccumulation is higher than 1 [102].
Figure 2. Various processes involved in the phytoremediation of heavy metals.
Hyperaccumulators have been found to exhibit higher heavy metal tolerance and accumulating
abilities compared to other plants [103]. Many such plants like Arabidopsis halleri [104] and
Solanum nigrum L. [105] have been utilized for phytoremediation of cadmium. Table 2 summarizes the
list of different plants reported for remediation of heavy metals. However, the disadvantages that limit
the use of hyperaccumulators include difficulty in finding heavy metal hyperaccumulators, slow growth
and lower biomass yield. This makes the process quite time-consuming and therefore not feasible for
rapidly contaminated sites or sewage treatments [41]. However, different rhizospheric microorganisms
that may play important roles in plant growth and/or metal tolerance via different mechanisms are
known, and these can be beneficial for the design of a phytoremediation plan to select appropriate
multifunctional microbial combinations from the rhizosphere, which may include arbuscular
mycorrhizal fungi and plant growth-promoting rhizobacteria. It is suggested that the remediation role of
rhizosphere is the main part of phytoremediation and one of the main basic theories for removing
contaminants by the combined activity of plants and microorganisms [106]. The main reason for the
enhanced removal of metals in the rhizosphere is likely the increase in the number and metabolic
activities of microorganisms. In the rhizospheric degradation process, the metal toxicity to plants can be
reduced by the use of plant growth-promoting bacteria, free-living soil microorganisms that exert beneficial
effects on plant growth. In this process, plants can stimulate microbial activity about 10–100 times by
the secretion of exudates which contain carbohydrates, amino acids, flavonoids etc. [107]. In return, the
rhizosphere bacteria that contain ACC deaminase may act to insure that the ethylene level does not
impair root development and to facilitate the generation of larger roots which enhance seedling
survival [108]. It is reported that nickel-resistant soil bacterium Kluyvera ascorbata SUD 165 promoted
Phytoextraction