The environment’s suitability for human life, including the air, water, and soil, is a devastating global problem. Among them, soils are important parts of the domain. In addition to producing food and fibre, soil supports the global environment. Any changes to the soil can potentially affect the pedosphere, atmosphere, hydrosphere, & biosphere.¹ Lately, soil pollution with heavy metals (HMs) has been a critical worldwide ecological trouble due to the anthropogenic impacts of the absence of adequate handling. This issue has detrimental repercussions on the ecosystem and adverse implications on the species’ health, economy, and community.

Normally, Essential and non-essential HMs are two categories of HMs.² Biomes require essential elements in minimal amounts for necessary biochemical and physiological actions.³ Molybdenum, cobalt, selenium, copper, zinc, magnesium, manganese, nickel, and chromium are among their elements. An inadequate supply and an abundance of these micronutrients reveal many deficiency illnesses.⁴ Biomes do not require HMs that are toxic or non-essential for physiologic and metabolic activities.³

They consist of arsenic, beryllium, gallium, bismuth, germanium, cadmium, barium, gold, indium, antinomy, lead, lithium, aluminum, silver, mercury, uranium, strontium, platinum, tellurium, titanium, thallium, vanadium, and tin.⁵

Pb is a severe problem in addition to the HMs polluted in the soils for several reasons:

1. Humans have extensively used Pb for a long time, becoming the greatest pollutant on earth.
2. Pb is poisonous to every biota, even at small levels.⁶
3. Pb does not naturally degrade biologically or over an extended period (150–5000 years) in soils.⁵

Pb compounds remain essential to modern human existence.⁷ It’s important to take the soil’s lead pollution seriously. These encouraged us to look for a viable Pb remediation technique. Once Pb has contaminated the soil, it is hard to remove.^1,7 Although physiochemical techniques can clean up soils, almost are expensive, laborious, and difficult to operate. As a result, valuable soil components are degraded, the properties, structure, and fertility of the soils are altered, local soil microorganisms’ biological processes are disturbed, and secondary pollution hazards are created.⁸

Attention has been drawn to using plants connected with microorganisms as an environmentally beneficial method.⁹ Since bacterial metabolites greatly affect ambient metal speciation and transport and are biodegradable and less harmful.¹⁰ Plant-promoting bacteria (PGPB) are microbes that can assist plants in removing heavy metals from contaminated environments.¹¹ Plant-promoting rhizospheric bacteria (PGPR) and plant growth-promoting endophytic bacteria are two subgroups of PGPB (PGPE). While PGPE is found inside plant cells, PGPR surrounds the roots of herbs.⁵ PGPR has been widely researched in phytoremediation studies for a long time. PGPE has drawn more and more attention.¹² Since PGPE has more benefits than PGPR, including eliminating competitive issues,¹³ methods for enhancing phytoremediation are mostly comparable. They can, for instance, withstand extremely high HM concentrations. In addition to making siderophore and inorganic phosphate, it solubilizes and mobilizes HMs.¹⁴

Therefore, PGPE might be preferable for this study’s estimation of Pb phytoremediation. The selection of the host plant affects endophyte metabolic outputs & the isolation of endophytic bacteria.¹⁵ The roots of metallophytes with the maximum Pb level are believed to contain the beneficial Pb-resistant endophytic bacteria.¹⁶ In general, metallophytes that accumulate a significant amount of HMs can provide endophytes with a particular niche (HMs-stressed ecology) where they can exist strategies to withstand the toxicity of HMs.¹⁷ Pb metallophytes are typically detected in Pb-polluted environments.

Song Tho was picked as one of these Pb mines because little was recognized about the endophytic microbes produced by the nearby Pb-storing herbs. As indicated, increasing the Pb phytoremediation capability is difficult by inoculating the effective PGPE into fast-growing trees as its recent host. PGPE must colonize plant tissues successfully. The ability of Pb phytoremediation in trees connected with PGPE must be assessed using hydroponic and pot studies. A hydroponic examination is typically a quick and affordable operation.¹

Since it lowers the phase of plant development and period of the subject, it also lowers the area required for studies and differences because of the ecological effect.¹⁸ However, their absorption ability varies from those in soil because of photo availability. These influences are not found in the hydroponic exam. Thus, the hopeful studies correlated with effective PGPE require being cultivated in a polluted Pb environment to have the proper knowledge on Pb absorption impacted by soil chemical and microbial reactions.

Lead (Pb)
Pb is a non-essential HMs that are tremendously toxic and has adverse effects on most human and animal organ systems, leading to multisystem illness (Table 1). It is seen as a potential environmental threat on a global scale. Within the food chain, it experiences biomagnification. Humans exposed to lead in the environment or at work may experience long-term health impacts such as musculoskeletal, neurological, cardiovascular, hepatic, renal, pulmonary, and reproductive dysfunctions. The International Agency for Research on Cancer (IARC) recognized lead as a human carcinogen. Although lead toxicity and its reduction have been extensively studied and published, full control and avoidance of lead exposure still seem to be a long way off. The various sources of Pb are seen in Figures 1, 2, and 3.

Table (1):
Hazard effect of lead toxicity among different living creatures.

Occurrence in nature
Pb is generally found in the soil with a luxury of 14.8 mg/kg.¹⁹ Pb levels in surface soils range from 10 to 67 mg/kg globally, with the median value being 32 mg/kg .²⁰ Pb content ranges from 10 to 25 mg/kg in igneous rocks and 14 to 40 mg/kg in argillaceous sediments. It rarely occurs in 0.1-8.0 and 3.0-10 mg/kg rates in calcareous sediments and ultramafic rocks.²¹ Pb could be detected in sedimentary rocks such as sandstones and shales (22 mg/kg) (10 mg/kg).¹⁷ Galena is the Pb mineral that is most common (PbS). Anglesite, mimetite, minium, pyromorphite, and cerussite are some of their minerals.²¹

Pb contamination and behavior in soils
HMs can be found in five different states: fraction 1 is a soluble fraction, consisting of HMs in the soil solution as free ions & complexes; fraction 2 is exchangeable, consisting of HMs uptake on inorganic soil components & ion-exchange sites; fraction 3 is organic, consisting of HMs bound to organic matter; fraction 4 is insoluble, consisting primarily of HMs concentrated as oxides, carbonates.²² These can be further broken down into overall and reachable HM levels.

The total concentration is unavailable to plants for rapid uptake.¹⁹ The concentration in fractions 1 or 2 is available for plant absorption. However, different soil amendments can liberate fractions 3 and 4, but metals in fraction five may not be available.²² Pb can generally be found in soils in different forms: Pb can exist as a free metal ion and form complexes with inorganic substances, including HCO₃^–, CO₃₂^–, SO₄₂^–, and Cl^–. (4) Pb may be adsorbed onto particle surfaces like Fe-oxides, organic matter, biological material, & clay particles. (3) Pb may be present as organic ligands like amino acids, humic acids, & fulvic acids.¹⁷ Pb is considered to have the lowest bioavailability.¹⁷

Pb harmfulness to microorganisms
Pb is lethal to bacterial cells, even at small levels.⁵ For instance, at three mg/L of Pb (II) chloride, no discernible Desulfovibrio desulfuricans G20 growth was found (PbCl₂).⁵ Pb enters microbial cells through the absorption pathways for necessary divalent metals like Mn²⁺ and Zn²⁺. Pb poisoning results through changes in protein and nucleic acid structure, inactivation of enzyme action, disturbance of membrane functions, oxidative phosphorylation, and modifications in the osmotic balance, among other factors. Additionally, Pb²⁺ exhibits a higher affinity for thiol & oxygen groups than other necessary metals like Ca and Zn.⁵

Pb also interferes with bacterial survival & multiplication by destroying DNA, proteins, and lipids and replacing vital metal ions like Zn, Ca, and Fe in enzymes.²³ Thermus thermophilus strain Samu-SA1 was recently affected by Pb²⁺ at 200 and 300 mg/L. Pb²⁺ decreases protein content and -glucosidase and -maltosidase enzyme activity at 100 mg/L. Additionally, Pb influences the secretion of various proteins from T. thermophilus that range in molecular weight from 15 to 236 kDa.²⁴

Remediation strategies
HMs must be remedied to protect the ecosystem from their harmful effects and preserve its wonderful condition for future human generations.²⁵ There are typically three methods, which are separated into physical, chemical, and biological mechanisms:

Physical Approach
This strategy involves excavation and subsequent disposal at a landfill site or on-site management.²⁶ Thermal desorption techniques and soil substitution comprise on-site management. To lower pollution, pure soil is substituted for contaminated soil. The second method is soil spading. In a nutshell, dilution and normal degradation are achieved by deeply excavating the contaminated soil, which causes the toxins to disseminate into deep places.

The new soil is supplied and then covered at the surface using soil capping. This method is effective for soil with small regions and severe pollution but is also quite expensive and labor-intensive. The only HMs for which the thermal desorption method is best are volatile HMs. In a nutshell, volatile pollutants are produced by heating contaminated soil with steam, microwaves, or infrared radiation. As a result, they are collected using carrier gas or negative vacuum pressure. The ease of usage and reuse of the remedied soil are two benefits of this approach. However, the price of this approach is high, and the desorption period is lengthy.²⁷ In the case of soil removal and landfill placement, this moves the pollution problem elsewhere and creates additional risks due to the transit & migration of the pollutants to surrounding biological components.²⁶

Chemical method
This strategy includes knowledge vitrification, electro-kinetic remediation, chemical leaching, and chemical fixing. By rinsing the contaminated soil with freshwater, chemicals, or gas, chemical leaching can remove the contaminant from the soil by ion exchange, concentration, adsorption, & chelation. As a result, HMs in soil were moved from the liquid stage to the leachate, where they were collected.¹⁷

This method is expensive, results in metal-rich byproducts that need future processing, & typically renders the land unfit for the development of plants because it eliminates any biological effects.²⁶ To create insoluble with HMs, chemical fixing adds the reagents to the polluted soil. This procedure achieves soil remediation while reducing the transport of HMs to water, plants, and other ecological media. However, this method can clean up the soil with low pollution levels. Additionally, the bioavailability of fixed HMs could change due to ecological change.

The impact of the soil’s microbes and the makeup of the soil could be changed by the use of chemical agents to some extent. Electrokinetic remediation is a novel method. It primarily applies voltage to the two sides of the soil, creating an electric field gradient. By electromigration, electroosmotic flow, or electrophoresis, the contaminant is transported to the two-pole treatment room, where it will subsequently be handled.²⁷ Additionally, using chemicals increases expenses, the possibility of secondary contamination, and the volume of sludge produced.¹⁷

Biological method
This strategy is more appropriate because it is a typical ecological process with no negative environmental effects.²⁶ It includes phytoremediation and bioremediation, utilizing microbiomes like bacteria, algae, yeast, fungi, and plants).²⁸ Various techniques are available for bioremediation, including composting, land formation, venting, bioreactors, biofilters, bio-stimulation, and bioaugmentation.²⁵ Microbes can affect migration & transformation even though they cannot break down and destroy HMs because they change their physical and chemical properties.

Precipitation, extracellular complexation, oxidation-reduction reaction, and intracellular precipitation are all steps in the repair process.²⁷ Because they maintain the typical soil characteristics, are inexpensive, and enjoy widespread acceptability, the techniques used in this methodology are advantageous over chemical & physical processes.²⁵ Earthworms, a lower animal, exhibit the qualities of adsorbing, decomposing, and migrating the HMs, eliminating and inactivating HMs toxicity at this time.²⁷ Using plants to clean up a contaminated environment is known as phytoremediation.²⁹ In this instance, naturally occurring and genetically modified plants are possible.³⁰ Additionally, phytoremediation refers to using herbs & related soil bacteria to lower or eliminate the toxic impacts of toxins in the ecosystem’s soil, water, & air.¹⁷

Microbial remediation of lead
Engineering repair, physical and chemical restoration, or a mix of these procedures are common conventional heavy metal pollutant remediation techniques; however, no one technique guarantees total heavy metal degradation. Many of these techniques are economically and environmentally unsustainable.³¹ Bioremediation uses living organisms, mainly plants and bacteria, to clean up contaminated soils and water.³² These bacteria create and use various detoxifying methods, including biomineralization, biotransformation, biosorption, and bioaccumulation. It is a widely used remediation technique since it happens spontaneously and is reasonably priced.³³

Using bacteria or enzymes, bioremediation involves changing hazardous heavy metals and other chemicals into less damaging forms.^{20, 34-35} This method for revitalizing the environment is economical and kind to the environment.^36-37 The nature of the cell wall constituents and functional groups of microorganisms involved in the cleanup of heavy metals is the primary cell component.³⁷

Bacteria have developed defenses to withstand metal ion uptake in stressful settings. These mechanisms include the accumulation of metal ions in a less toxic state, biosorption to cell walls and entrapment in the extracellular capsule, precipitation of the efflux of metal ions outside the cell, and chemisorption of metal ions inside the cell.^36-37 It has been discovered that numerous bacteria, fungi, and algae can bind lead in different quantities. The function of bacteria in lead removal and recovery by biosorption is covered in detail in this review.

Bioremediation of lead by bacterial strains
The pb-tolerant bacterial strains from various sources are included in Table 2 and their mode of action for pb cleanup. Many bacterial strains have been identified and studied for their capacity to decrease lead in soil and liquid medium. The most effective way for bacterial strains to remove metals from an aqueous medium is by bioaccumulation, which depends on the metabolic activity of the bacterial cell.³⁶ The numerous methods established by bacteria in response to metal toxicity have been depicted using bacterial strains in several studies for lead breakdown.³⁵ Novel catabolic enzymes enable bacterial strains to thrive in ecological settings.³⁷

Table (2):
Pb-tolerant bacteria, sources and their mechanisms.

According to Murali et al.,³² after five days, bacteria recovered from soil samples contaminated with tannery effluent reduced by 3 mg/L Pb. These bacteria included Bacillus subtilis, Bacillus megaterium, Aspergillus niger, and Penicillium sp. Heavy metals are absorbed by bacteria by a variety of mechanisms, including biosorption, encapsulation in extracellular capsules, complexation, oxidation-reduction processes, precipitation, and transport through cell membranes. According to Dai et al.,³⁶ Lactobacillus brevis biosorbed Pb²⁺ (53.632 mg/g), and the ideal circumstances for Pb²⁺ ions adsorption were pH 6.0, 120 rpm/min, 3 g/L of bacterial concentration, 40°C, and a 12-hour contact period. The authors of this study also said that temperature had a significant impact on the biosorption of Pb²⁺ ions and that as the temperature rose, so did the biosorption capacity. The biosorption capacity reached a maximum of 42.35 mg/g at 40°C.

Similar to how pH dramatically altered biosorption capacity, biosorption was exceedingly poor when pH was raised to 3, and 3.6 mg/g of biosorption was attained at pH 2. The biosorption rose with pH and reached its maximum lead binding (44.4 mg/g) at pH 6. However, the biosorption amount fell as pH levels rose above 6 and peaked at 7.

Another Lactobacillus species was isolated and identified by Yi et al.,³⁷ and after 30 minutes of incubation, this strain’s maximum adsorption capacity was determined to be 60.6 mg Pb/g. 52 of the 96 LAB strains isolated for this study from kimchi bought at a local market in Iksan, South Korea, showed Pb resistance. Pre-cultured lactic acid bacteria (LAB) were incubated in 160 g of Man Rogosa Sharp broth media (MRS) broth medium at 37°C for 18 hours to remove Pb using the chosen lactic acid bacteria (LAB).

SEM-EDS analysis extensively studied the surface morphology of strain L-96 before and after Pb biosorption. The findings showed that Pb was mostly associated with bacterial cell surfaces and that EPS secretion was responsible for alterations in surface shape. Elsanhoty et al.,³⁸ subcultured various Lactobacillus species utilizing Man Rogosa Sharp broth media, including Lactobacillus acidophilus, Lactobacillus rhamnosus, Lactobacillus plantrium, and Streptococcus thermophiles (MRS broth). This study assessed the impact of bacterial concentration and pH on the ability to remove lead. The data demonstrated that removal increased approximately linearly with pH, with pH 7 producing the maximum removal of Pb (71.1%). Pb cations’ interaction with protons could result from pH effects for negatively charged binding sites.

Increased bacterial population helped with Pb elimination, especially when L. plantarum was used alone or in conjunction with L. acidophilus. Maximum Pb elimination was seen at pH 7 and high bacterial concentration. According to George et al.,³⁹ Bifidobacterium longum 46, Lactobacillus fermentum ME3, and Bifidobacterium lactis Bb12 all removed lead, with B. longum 46 having the highest capacity of 175.7 mg/g dry biomass. In this investigation, the effects of pH (2–6), bacterial content (0.5–1.5 g/l), and temperature (4, 22, and 37°C) were also examined. When the starting metal concentration was 1000 g/L, pH was 6, and the temperature was 37°C, 97 % of the lead was removed from the solutions.

In Tamil Nadu, India’s Tiruchchirappalli District, an effluent sample from the electroplating sector revealed that Micrococcus sp. had removed 84.27 percent of Pb. Bacterial isolates that perform best at various temperatures, pH levels, biomass levels, and Pb concentrations do so because these variables impact biosorption.⁴⁰

Tak Gene Zist Company sold Lactobacillus acidophilus ATCC 4356. The bacteria were injected into 10 ml MRS broth and then cultivated for 48 hours at 37°C. The isotherm models developed by Langmuir and Freundlich were used to investigate the capacity of the bacterial cells to absorb Pb at a particular time. The efficiency of Pb bio elimination was optimized across the selected contact times (1-4 days) using L. acidophilus concentrations (between 10¹⁰ and 10¹³ CFU), which the authors additionally looked at about contact time.⁴¹

Jin et al.,⁴² recovered Alcaligenes sp., BApb.1 from a mining site in Heilongjiang Province, China. For the maximum biosorption rate of 85.2 % and the maximum capacity of 56.8 mg/g, the ideal pH range, adsorbent dosage, initial Pb²⁺ concentration, contact duration, and temperature were reported to be 5, 1.5 mg/g, 100 mg/L, and 30 min.

Ten lactic acid bacteria strains to see how well they could tolerate and absorb lead. With a rate of lead absorption as high as 99.9% and a minimum inhibitory concentration of lead on L. plantarum reported as being higher than 1000 mg/L, Lactobacillus plantarum YW11 was discovered to have the strongest ability of lead absorbing and tolerance.⁴³ According to George et al.,⁴⁴ firmicutes—mostly lactic acid bacteria like Lactobacillus spp., with some Lactococcus, Pediococcus, and Carnobacterium representatives—actinobacteria, and protozoa—were effective at removing the potentially dangerous trace element lead. Between 50 and 90 % of the 99 distinct LAB strains tested for their capacity to remove Pb²⁺ salts at 25 PPM were able to immobilize the metal in the solution. According to the authors, the majority of the strains of Gram-negative bacilli were found to have Pb²⁺ removal capabilities between 45 and 65 %. In contrast, the ability of the Enterobacterales strains to eliminate Pb was consistently and somewhat low (54.14 6.7 %). Only two E. coli strains and one Hafnia alvei strain could absorb more than 75% of the lead in a solution with a 25 PPM concentration.

Numerous other researchers have also reported that certain bacterial strains, including Leuconostoc, mesenteroides,⁴⁵ and Bacillus sp, can degrade lead. AS2 ⁴⁶, Pseudomonas sp. W6.⁴⁷ Pardo et al., ⁴⁹ evaluated the biosorption potential of inactive freeze-dried Pseudomonas Putida biomass, and they found that 80 percent of the Pb was removed within 10 minutes of contact. The pH significantly impacts metal biosorption, and a range of 6.0 to 6.5 was the ideal pH. Another bacterial isolate, Serratia marcescens, could live at a lead concentration of 0.025 mg/L and reduce lead to a level between 0.0133 and 0.213 g/g in under 120 minutes. This strain’s pigment biosynthesis was also examined, and results showed that pigment production significantly decreased up to 80 minutes before ceasing.⁵⁰

In a different study, Jalilvand et al.,⁵¹ evaluated the biosorption potential of ureolytic bacteria isolated from calcareous mine soils in Iran. They found that Stenotrophomonas rhizophila, Variovorax boronicumulans, and Stenotrophomonas rhizophila could remove lead from the environment in under 72 hours. According to the authors of this study, these only bacterial strains could biomineralize. Based on morphological, biochemical, and 16S rDNA sequencing analysis, Ren et al.,⁵² determined that Bacillus sp. PZ-1 is a Pb²⁺-resistant bacterium. The researchers found that the biosorption rate was 93.01 percent, the adsorption capacity was 9.30 mg/g, and the ideal values for the initial Pb (II) concentration, pH, contact time, biomass concentration, and temperature were 400 mg/L, 5.0, 20 min, 40 g/L, and 15°C, respectively.

In a related experiment, a team of scientists isolated the lead-tolerant bacteria Arthrobacter sp. 25. In this study, the theoretical anticipated value (9.6 mg/g) of lead absorption was obtained using the response surface methodology (RSM). The maximum adsorption capacity (9.6 mg/g) was achieved under ideal conditions with initial lead ion concentration, pH value, and biosorbent dosage as 108.79 mg/L, 5.75 g/L, and 9.9 g/L, respectively.

SEM, energy-dispersive X-ray spectroscopy, atomic force microscopy (AFM), X-ray diffraction, and Fourier transform infrared spectroscopy (FTIR) were used to establish the biosorption mechanisms.⁵³ Bacillus gibsonii S-2 waste biomass was utilized as an inexpensive biosorbent to remove Pb²⁺ from an aqueous solution.⁵⁴ This study examined the maximal capacity of a particular bacterial strain to remove lead under ideal pH and temperature circumstances. The results revealed that the biosorption was best at a pH of 4.0 at three different temperatures (20, 30, and 40°C). The scientists found that Bacillus gibsonii S-2 may lower 333.3 PPM of lead. The mechanisms of Pb²⁺ biosorption, ion exchange, and complexation with the functional groups involved in Pb²⁺ adsorption have been studied using Fourier-transform infrared spectroscopy (FTIR) and Energy-dispersive X-ray spectroscopy (EDX).

Geobacillus thermodenitrificans, isolated from the Damodar River in India, was tested for its biosorption capacity using two different kinds of aqueous solutions by Chatterjee et al.⁵⁵ After being seeded in nutritious broth, the samples for this investigation were incubated at 65°C for 48 hours (Hi-media, Mumbai, India). Based on morphological traits, 30 colonies were divided and kept in slant cultures at 4°C. To establish the optimal pH for maximum Pb removal from an aqueous media, the pH range of 3.0-9.0 was examined. According to the data, pH was a factor in the metal adsorption. Dead biomass from G. thermodenitrificans is particularly sensitive to pH, and at pH 4.5, Pb²⁺ adsorption is at its highest level. The dead biomass of G. thermodenitrificans was reportedly reduced by 18.22 % and 36.86 %, respectively, of Pb²⁺ in industrial wastewater and synthetic metal solutions.

The biosorption capability of isolated industrial wastewater containing Staphylococcus saprophyticus was examined by Ilhan et al. in 2004. The research team also investigated the ideal biosorption conditions to attain maximal values. It looked into the biosorption process at various pH, temperature, and metal ion starting concentrations. A pH of 4.5 was found to be ideal, while a metal content of 100 mg/L was shown to be ideal for maximal biosorption. According to the authors, Staphylococcus saprophyticus may eliminate Pb²⁺ under these circumstances. Qiao et al. discovered a lead-resistant bacteria from the lead-contaminated soil of a lead mine in Nanjing, Jiangsu Province, China (2019). The biosorbent (Bacillus subtilis capacity)’s for lead adsorption grew as the pH rose, eventually reaching a maximum of 192.05 mg/g at pH 4.0. Bacillus subtilis X3’s adsorption rate rose with the number of contact hours and was measured at 260 mg/g after 10 minutes.

Endophytic bacteria-assisted phytoremediation
Definition & classification
The PGPE can support plant development.¹⁷ Therefore, endophytic bacteria could be identified as microorganisms that colonize interior plant cells without showing symptoms of infection or harming the plant.⁵⁶ Additionally, they may be collected from inside herbs or retrieved from surface-disinfected plants.⁵⁷ Because of the variety of hosts’ living environments and the intricate relationships between endophytes and their hosts, it is still unclear where endophytes came from. There are two theories, though. First, endophytes share the same genetic ancestry as the host because they originated from mitochondria & chloroplast in plant cells. Second, endophytes enter the host from outside the plant through the surface, a wound in the root, or stimulated channels.⁵⁸

Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Firmicutes Actinobacteria, and Bacteroidetes were split into 82 genera to represent endophytic bacteria. Most fall under the third first group.⁵⁹ They represent a wide spectrum of Gram-negative & Gram-positive microbes. The most cultivatable endophytic spp., found on various hosts, including woody trees, herbaceous plants, and grass spp., are Pseudomonaceae, Burkholderiaceae, and Enterobacteriaceae.⁶⁰

Procedures for altering heavy metal phytotoxicity
Plants produce ethylene in response to heavy metal stress, but excess ethylene harms the herbs. Endophytic microbe could create 1- aminocyclopropane-1-carboxylic (ACC) deaminase, which can control ethylene concentrations, enhance plant development, & ultimately lower poisonousness.⁶¹ Additionally, endophytic microbe could lower the poisonousness of Cd by elevating the plant’s absorption of a trace element like Zn or Fe.

Additionally, endophytic microbes create siderophores that promote plant growth in iron-deficient environments, lower the toxicity of HMs in HM-contaminated environments, & promote plant development in iron-deficient environments.¹⁷ In other words, endophytic bacteria diminish phytotoxicity induced by HMs levels, resulting in a healthy plant that can withstand HMs and extend the contract period. These factors raise the possibility of phytoremediation (Figure 4).

Procedures for modifying HMs bioavailability
The presence of HMs in the non-soluble form in the soil, particularly Pb, makes it difficult for plants to absorb them. By creating and releasing the bacterial siderophore and organic acids, PGPE can convert the form of heavy metal to a bioavailable form. Bacterial siderophore functions as a solubilizing factor by creating a combination with other metals such as Al, Cu, Cd, Pb, Ga, In, & Zn. As a result, HMs are released and dissolved from soil particles, boosting their bioavailability and absorption.¹⁷ Significantly, siderophores are seen to regulate and lower HMs absorption.⁶² To elevate the solubility of HMs, PGPE can also generate & produce low-molecular-mass organic acids like 5-ketogluconic acid, formic acid, citric acid, & oxalic acid.⁶³

Procedures of Pb resistance in bacteria
According to Lenart and Wolny-Koladka,⁶⁴ heavy metals severely impair microbial activity, soil fertility, and plant growth. In fact, significant alterations in a genetic and physiological trait of the soil microbial population may be seen at high Pb concentrations.⁶⁵ As a result, heavy metal-tolerant species persist while sensitive species with no beneficial effects are eliminated, leading to decreased functionality and species variety.⁶⁶ The microbial population was greatly impacted by how the bacteria altered root secretions, food cycles, and carbon transport from autotrophic plants. Associated bacteria reduce root development, respiration, and carbon transfer to the root. This carbon flux is the primary and crucial regulator of the soil’s microbial community.⁶⁷

The bacteria may withstand metal stress in various ways, including forming an extracellular barrier, actively transporting metal ions (efflux), sequestering metal ions inside and outside cells, and reducing metal ions.⁶⁸ Different strains of bacteria belonging to the genus “Pseudomonas” are disinfectants resistant to heavy metals, antibiotics, and detergents. Pseudomonas could be environmental pollution indicators.^69-70 Pseudomonas can withstand heavy metal stress in various ways, including P. aeruginosa-specific active extrusion of metal ions from cells and cellular mechanisms that reduce metal ions to less harmful forms.⁷¹ In general, the rehabilitation of Pb-polluted soils can benefit from appropriate bacterial inoculation.

By altering the synthesis of chlorophyll contents, enzymatic activities, and soil respiration, as well as lowering the Pb concentration in plants, bacterial inoculation may reduce Pb toxicity and its accumulation in dill plants. Compared to other strains and the control, strain P159 had the greatest influence on the parameters under study. This makes Pseudomonas strain P159 possibly ideal for plant development in Pb-affected soils and cleaner dill production, which can be further grown in contaminated soils without any potential dangers of metal toxicity, especially in low concentrations of Pb. Pb intrusion into other environmental compartments can be minimized, and Pb-contaminated soils can be covered.

Benefits and limitations of applying endophytic bacteria
Applying endophytic bacteria in phytoremediation has numerous advantages. The efficiency of the remediation techniques can be assessed by quantitative gene expression of the catabolic genes of contaminating bacteria. As harmful contaminants ingested by plants may be destroyed within the plant by endophytic degraders, genetic engineering of bacterial catabolic pathways is simpler to modify than that of plant catabolic pathways. This minimizes the toxicity of pollutants in ecological soil on flora & fauna.¹⁷ Additionally, there are a lot of drawbacks, including the possibility that the plant’s selection will limit its potency to certain seasons. Pollutant effects on phytotoxicity go together with it. If pollutants are not fully detoxified, and the local wildlife consumes the plants, pollutants or their metabolites may also enter the food chain.⁷²

Investigation on bacteria-assisted phytoremediation
A wide spectrum of PGPE is now known to successfully support phytoremediation by promoting plant growth, lowering phytotoxicity, accumulating metals, improving plants’ ability to withstand metals, altering metal bioavailability in soil, and translocating metal in plants.¹² It has been discovered that some endophytic bacteria promote plant development. Depending on the initial Cd content, S. nigrum, a Cd hyperaccumulator, PGPE extracted from it, boosted plant growth. Enterobacter sp on soil with a low Cd contamination level (12.1 mg/kg). LSE04 increased S. nigrum shoot length (13.7%), fresh weight (28.2%), & dry weight (41.4%).⁵ Briefed the lead bioremediation by bacteria (Figure 5).

Chen et al., ⁷³ found that Acinetobacter sp. LSE06 was demonstrated as the most significant development influence at the intermediate Cd concentration (about 63.7 %), with increases of up to 10.9 % for shoot length, 15.7 % for fresh weight, & 23.1 % for dry weight. Serratia nematodiphila LRE07 was the most effective at high Cd content (116.5 %). The plant’s shoot length, fresh weight, and dry weight rose by 18.9%, 23.1%, and 19.8%, respectively, compared to uninjected plants. When plants were injected with the growth-stimulating endophytic bacterium Pseudomonas sp A3R3, the fresh & dry weights of B. juncea were significantly increased compared to plants that weren’t injected, by 50% and 45%, respectively. Additionally, Psychrobacter sp. SRS8 may promote the growth of crops used for energy.⁷⁴

Ricinus communis has a percentage increase of 32% for fresh weight & 38% for dry weight, while Helianthus annuus has a percentage increase of 39% and 41%, respectively. Compared to the uninjected control, the endophytic Bacillus sp. MN3-4 boosted the root elongation of B. napus seedlings by 46.25 %.⁷⁵ From a metal-tolerant plant (C. communis) cultivated on Pb/Zn mine tailing, Pb-resistant with ACC deaminase-producing endophytic bacteria (Acinetobacter sp. Q2BJ2) Bacillus sp. Q2BG1) were identified. Compared to the uninjected controls, they can enhance the dry weight of B. napus aboveground cells raised on quartz sand, including 100 mg/kg of Pb (39 to 71 %) and roots (35 to 123 %).⁷⁶

In comparison to uninjected controls, which were 10, 27, 26, 38, and 52 % in soils containing Cd, 22 %, 17 %, 27 %, 31 %, and 34 % in soils containing Pb, and 24 %, 28 %, 19 %, 26 %, and 41 % in soil containing Zn, B. napus injected with the Rahnella sp. JN6 showed a marked elevation in chlorophyll content, plant height, and root length.⁷⁷ By altering the solubility and availability of HMs, PGPE could effectively increase phytoremediation. In contrast to uninjected controls (381 g/L), Pb-resistant endophytic bacteria P. fluorescens G10 & Microbacterium sp. G16 can considerably improve the water solubility of Pb post 60 hours of injection.⁷⁸

Different Zn resources, like zinc carbonate, zinc phosphate, or zinc oxide, were digested by Gluconacetobacter diazotrophicus, generating 5-ketogluconic acid and forming Zn available for plant absorption. By producing metal-mobilizing metabolites, the endophyte actinobacterium could mobilize zinc and copper.⁶³ Cd, Pb, & Zn solubilization in the metal-amended soil was significantly (p 0.05) increased by Rahnella sp. JN6 was 1.46, 1.25, and 1.30 times greater than that in the uninjected soil.⁷⁷ Metal precipitation is the major factor affecting how effective phytoremediation is. Some endophytic bacteria that are HMs resistant and promote plant growth can increase metal uptake and precipitation in plants.¹⁷ In contrast to uninjected plants, Psuedomonas sp. A3R3 considerably enhanced the Ni content in the shoot of A. serpyllifolium & B. napus by 10% & 15%, respectively, when cultivated in soil treated with 450 mg/kg of Ni.⁷⁴ P. fluorescens G10 and Microbacterium sp. G16 injection into B. napus increased Pb absorption in a shoot from 76 to 131 % by P. fluorescens and 59 to 80 percent by Microbacterium sp., contrasted to the uninjected control group.⁷⁸

Additionally, compared to the uninjected control, Acinetobacter sp. Q2BJ2 & Bacillus sp. Q2BG1 markedly (p 0.05) increased total Pb concentration in B. napus shoots (3.4-fold to 5.6-fold) and roots (2.1-fold to 3.5-fold).⁷⁶ Rahnella sp. JN6, a strain isolated from an Mn hyperaccumulator (P. pubescens), considerably elevated the metal levels in B. napus cells. Metal (Cd, Pb, & Zn) levels were 49 %, 47 %, and 28 % in aboveground tissues of uninjected control plants, compared to 106 %, 97 %, and 62 % in injected plants.

The concentrations of those metals in the root tissues of the uninjected control plants were 58 %, 46 %, and 33 %. In comparison, they were 140 %, 95 %, and 89 %, respectively, in the injected plants.⁷⁷ Similar to this, compared to uninjected plants, Bacillus pumilus E2S2 dramatically increased plant Cd absorption, root, & shoot length, and fresh & dry biomass. Several endophytic bacteria, like Herbaspiillum seropedicae, & Bulkholderia capacia may increase nickel precipitation in the yellow lupin roots, increasing the plant’s capacity for phytoremediation.⁶¹ Lately, Pseudomonas sp. LK9 dramatically boosted metal absorption when introduced into Cd hyperaccumulator (S. nigrum) growing in multi-HMs polluted soil.⁷⁹ Similar to this, the multi-metal resistant endophytic bacterium Bacillus sp. L14 recovered from S. nigrum improved the absorption of Cd²⁺, Pb²⁺, and Cu²⁺, respectively, 76%, 80%, and 21%.⁸⁰

On the contrary, metal-resistant endophytic microbes reduced HM absorption by plants, increasing plant biomass.^81-82 discovered that the rice tissues-derived endophytic bacteria Methylobacterium oryzae and Burkholderia sp. reduced the levels of Ni & Cd in the roots & shoots of Lycopersicon esculentum Mill. The translocation factor was computed to evaluate the effectiveness of PGPB injection on the translocation of HMs from roots to shoots.

Ma et al.,⁷⁵ PGPB Sanguibacter sp., reduced the TF of Zn in Nicotiana tabacum L., a plant cultivated in soil enriched in Cd & Zn. Ma et al.,¹² found that Cu-resistant endophytic bacteria increased the amount of Cu that B. napus transported from its roots to its shoots, increasing its capacity for phytoextraction. Ma et al.,⁷⁵ observed that when the amount of multi-metal polluted serpentine soil increased, the injection of PGPB strain (Pseudomonas sp. A3R3) marginally elevated the TF of Ni in B. juncea & R. communis. This shows that the PGPB injection significantly impacts Ni precipitation in plant shoots. The TF of Zn in both herb sp. was thereby drastically reduced by Pseudomonas sp. A3R3. Phytotoxicity can prevent phytoremediation from occurring.

Various endophytic microbes can lessen the toxicity of HMs. For example, by raising the protein and chlorophyll concentrations in leaf tissue, Psychrobacter sp. SRS8 could shield the plants (H. annuus & R. communis) from the suppressing impact of Ni.⁷⁴ Methylobacterium oryzae & Burkholderia sp. recovered from rice lowered the poisonousness of Ni & Cd in tomatoes and promoted plant growth in pot testing. Additionally, Shin et al.,⁸³ found that Bacillus sp. recovered from the hyperaccumulator A. firma’s roots can significantly lessen HMs phytotoxicity & improve Pb precipitation in A. firma. As demonstrated by the situations above, phyto-bacterial procedures are substantially more effective than individual phytoremediation methods for eliminating HMs. As a result, PGPB plays a crucial role in boosting HMs phytoremediation.