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Available online at: https://jazindia.com 2677
Journal of Advanced Zoology
ISSN: 0253-7214
Volume 44 Issue S-5 Year 2023 Page 2677:2685
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Importance of PGPR in organic farming A Short Review
Shovana Pal1, Keshab Ghosh2, Deeti Das3, Aritri Laha4*
1Student of M.Sc., Department of Microbiology, School of Life Sciences, Swami Vivekananda University,
Barrackpore, 700012, West Bengal, India.
2 Student of M.Sc., Department of Microbiology, School of Life Sciences, Swami Vivekananda University,
Barrackpore, 700012, West Bengal, India.
3Student of M.Sc., Department of Biotechnology, School of Life Sciences, Swami Vivekananda University,
Barrackpore, 700012, West Bengal, India.
4*Assistant Professor, Department of Microbiology, School of Life Sciences, Swami Vivekananda University,
Barrackpore, 700012, West Bengal, India.
*Corresponding Author: Aritri Laha
*Assistant Professor, Department of Microbiology, School of Life Sciences, Swami Vivekananda University,
Barrackpore, 700012, West Bengal, India. E-mail: aritril@svu.ac.in
Article History
Received: 30/09/2023
Revised: 15/10/2023
Accepted: 30/10/2023
CC License
CC-BY-NC-SA 4.0
Abstract
Farmers' growing reliance on chemical fertilizers has enhanced agronomic
output, but it has also increased environmental contamination and put the
stability of the world's ecosystem in greater danger. By making abiotic
stresses more frequent, climate change has exacerbated the issue. Even if
agriculture is only permitted on 50% of the world's livable land, it is critically
necessary to ensure its sustainability and security. Boost crop yield and food
security while using little to no chemical fertilizers and pesticides is one of
contemporary agriculture's greatest problems. The vanguard of
environmentally friendly farming methods is rhizobacteria that promote plant
development (PGPR). They offer an advantageous and safe alternative to
chemical fertilizers as well as a suitable solution to less difficult situations.
Numerous bacterial species that function as PGPRs have significantly
enhanced plant growth, well-being, and production. The major subjects of
this review include the use of these rhizobacteria under various stress
circumstances, their significance in sustainable agriculture, and the
underlying mechanisms driving growth promotion.
Keywords: Beneficial bacteria; Biofertilizer; Colonization;
Phytopathogen; Abiotic stress; Agricultural Productivity and
Sustainability
Introduction:
By 2030, there will be an additional 8.5 billion people on the planet (Anonymous, 2015). This substantial rise
is said to be the outcome of unchecked and continued population growth in developing or poor nations. Hunger
and poverty are a direct result of this substantial increase. According to Anonymous (2015), India has the most
undernourished people in the world with a population of 194.6 million. Artificial fertilisers and pesticides are
now harmful to the environment, plant and soil health, and human well-being when used to increase
agricultural productivity. Another problem is climate change. Regular use of agrochemicals has an impact on
Journal of Advanced Zoology
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food texture and quality, but it also significantly impacts how the climate changes. As a safer, greener method
of reducing the effects of agrochemicals and climate change, sustainable agriculture employs organic farming.
In order to manage crops and livestock, organic agriculture often relies on natural processes rather than outside
inputs. Diversity is frequently used in inorganic farming as a management paradigm with the explicit intention
of fostering diversity. Ecosystems in the wild heavily rely on variety. In organic farming (OF), biofertilizers
including compost, vermicompost, and green manure are used to boost or maintain the fertility and nutritional
condition of the soil. It also uses biological management techniques including crop rotation, mixed cropping,
and the encouragement of insect predators to stop the spread and onslaught of pest illnesses. Due to their eco-
friendly and safe utilisation, several microbial species have also been utilised in OF throughout the past few
decades to increase agricultural productivity and in the biocontrol of phytopathogens. The finest living things
for maintaining ecological balance are microorganisms. They do the best ecological services. The beneficial
input known as plant growth-promoting rhizobacteria (PGPR) plays a role in the promotion of plant growth in
agricultural plants (Zhou et al. 2016). They have taken over the host's internal root system as well as the
rhizosphere and rhizoplane. These diagnostic indications are state-of-the-art. Additionally, certain Bacteroides
can penetrate the root, where they develop an endophytic population that eventually helps the plant survive
(Compant et al. 2005). Similarly to this, certain bacterial species can increase the amount of vital nutrients that
reach the roots of the plant, increasing plant productivity (Adesemoye and Kloepper 2009).
Rhizosphere: The region of soil known as the "rhizosphere" is where different biological and chemical
characteristics occur and have an impact on plant root secretions. According to Kumar et al. (2015), it is the
centre of strong interactions between the soil and the microflora, with effects that may be beneficial, negative,
or neutral. Their interactions have a profound influence on plant growth and production. Rhizobacteria in
particular and growth-promoting bacteria in general interact significantly in the rhizosphere. The plant
produces a variety of substances like root exudates that are rich in sugars, amino acids, organic acids,
flavonoids, proteins, and fatty acids in order to establish an optimal environment inside the rhizosphere.
Oftentimes, these root exudates contain tiny, non-metabolically produced chemicals. Depending on the
physiological condition, plant species, and microbiota, the exudates serve as signals to either reject certain
microbial diseases or to recruit helpful microorganisms (Ahmed et al., 2019). According to Lucini et al. (2019),
the exudates may also operate as rhizospheric messenger molecules between plant roots and rhizobacterial
organisms. They serve as critical growth triggers for soil microorganisms that are essential for accelerating
plant development and mobilising defence against phytopathogens. Plants exude lysates, which are released
from diseased cells during autolysis, chemicals that are produced metabolically or secreted, and mucilage, a
polysaccharide that is present in plants and is released in the rhizosphere, in addition to root exudates. Each of
these secretions acts as a chemoattractant for bacteria in the rhizosphere. A small number of PGPR that reach
the root might lead to endophytic interactions (Wozniak et al., 2019; Papik et al., 2020). Some of them mature
into endophytes, which eventually form the endodermis barrier over the root collar and vascular systems and
develop inside leaves, stems, and other organs. The degree to which these bacteria interact endophytically with
their host plants demonstrates their potential to adapt to different biological specialisations.
Role of PGPR in organic farming: Tesami and Maheshwari (2018) claim that plant growth-promoting
rhizobacteria (PGPR) are important players in agriculture. By fixing nitrogen, solubilizing phosphates,
lowering heavy metals, creating phytohormones like auxin, gibberellins, and cytokinins, mineralizing organic
substances in soil, rotting plant waste, stifling phytopathogens, and other processes (He et al., 2019), it
enhances crop health. By giving various distinctive genes and comprehending the biochemical enzymatic
pathways, antibiotics, and many other value-based biological substances, PGPR gives essential knowledge for
agricultural and environmental biotechnology (Backer et al., 2018). The age, species, and developmental stage
of the plant or crop, as well as the heterogeneity of the soil ecosystem, all have an impact on the efficacy of
PGPR. PGPR modifies the chemistry of the plant-soil system to encourage plant growth and health.
PGPR and abiotic stress: Abiotic stress is the adverse effect that non-living things have on living things in a
particular environment. Any unfavourable environmental circumstance that can have an impact on the
physicochemical characteristics of soil and the functional diversity of microorganisms might cause biological
stress. Because of the state of the ecosystem, abiotic stressors are increasingly common. The slowing pace of
plant growth has a substantial influence on global agriculture productivity. Crops including wheat, rice, maize,
and barley produce poorly when exposed to situations like high heat, drought, pH, and salt (Mickelbart et al.
2015). The extensive use of synthetic fertilisers continues to be the biggest obstacle to greater agricultural
output.
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One major environmental stressor that prevents fertile land from producing is soil salinity. By using less water
from the soil, plants can fight against the negative effects of salt, such as ion toxicity, nutritional deficiencies,
osmotic stress, and oxidative stress (Isayenkov and Maathuis 2019).
The drought issue, on the other hand, has become worse and is now even more harmful to the economy. The
mechanisms of salt stress and drought stress can overlap and be closely connected in some areas. The entire
physiology and mechanism of a plant are impacted (Ivanov 2015). In dry and semi-arid locations, drought has
an impact on agricultural development and profitability. By altering the amount of chlorophyll and bending
the photosynthetic process, drought conditions severely harm plants and prevent them from photosynthesis.
By 2050, crop-related issues (plant growth and development) are predicted to increase by 50% due to climate
change and drought stress (Vinocur and Altman 2005). In addition to the stress caused by sodium chloride,
crops are adversely affected by the combination of sodium hydrogen carbonate and sodium carbonate. By
preventing plants from absorbing phosphorus, manganese, zinc, iron, and copper, the building of these ions in
the soil raises the pH and alkalinity of the soil, which eventually causes osmotic stress and nutritional
deficiencies in plants. The plant's physiological health is therefore jeopardised (Chen et al. 2011, 2011). The
non-alkalophile rhizosphere residents' biological processes are inhibited by the high pH. Currently, heavy
metal contamination is a significant problem. Due to their toxicity and difficulty in removal, heavy metals
including lead, cadmium, copper, arsenic, and zinc remain in the environment for a very long period. Plants
allegedly collect heavy metals from the soil and release them into the food chain, according to Eteami and
Maheshwari (2018). Plants with hormonal irregularities are also far more susceptible to illness under other
conditions, such as nutrient deficits, phytopathogen attacks, etc.
These abiotic pressures (stresses that are not biological) are predicted to worsen throughout time as a result of
climate change. It is vital to put in place an environmentally friendly plan to decrease the occurrence and
consequences of abiotic stressors. So the best way to ensure sustainability in farming is to incorporate helpful
microbes. PGPRs can be used to increase a plant's ability to withstand biotic and abiotic environmental
challenges.
According to Grover et al. (2011), a number of taxa, including Paenibacillus, Rhizobium, Methylobacterium,
Bacillus, Achromobacter, and Variovorax, are thought to be involved in Burkholderia, Pantoea, Pseudomonas,
Azospirillum, and Enterobacter's capacity to tolerate abiotic stress.
According to Upadhyay et al. (2012), the Arthrobacter sp. SU18 and Bacillus subtilis SU47 strains can resist
up to 8% NaCl. Following the co-inoculation of these bacteria, wheat plants showed a rise in biomass, proline
content, sugar content, a decrease in salt content, and an increase in antioxidant enzyme activity. According to
Jha and Subramanian (2013), inoculating rice plants with Pseudomonas pseudoalcaligenes and Bacillus
pumilus increased germination rates by up to 16%, survival rates by up to 8%, dry weight by up to 27%, and
plant height by up to 31%. This reduced the impact of salt on developing plants.
Additionally, there were lower amounts of Na (71%) and Ca (36%), as well as higher concentrations of key
nutrients including N (26%), P (16%), and K (31%), as compared to uninoculated control plants. In the
rhizosphere of the desert-adapted plant Suaeda fruticosa, Bacillus licheniformis strain A2 was discovered by
Goswami et al. (2014). With the help of this isolation, groundnut plants expanded more swiftly, as evidenced
by a 43% increase in biomass and a 31% rise in plant height.
When 50 mM NaCl was added, the rise ranged between 24% and 28% in soil conditions. PGPRs can be used
to solve soil fertility issues in alkaline environments. The use of bacteria to increase nodule formation and
mitigate the effects of alkalinity on plants was successful. Nitrogenase activity was increased and mycorrhizal
dominance was fostered in the roots of the fava bean by the application of Rhizobium leguminosarum and
mycorrhizal fungi. With its co-inoculation, improvements in fava bean output and resilience to soil alkalinity
were also noted.
The rise in soil conditions was between 24% and 28% when 50 mM NaCl was introduced. In alkaline
conditions, PGPRs can be employed to address soil fertility problems. It was effective to utilise bacteria to
promote nodule development and lessen the negative effects of alkalinity on plants. Rhizobium leguminosarum
and mycorrhizal fungi were applied to the roots of the fava bean to improve nitrogenase activity and promote
mycorrhizal dominance. Improvements in fava bean yield and resistance to soil alkalinity were also reported
using its co-inoculation.
Salinity: Salinity issues affect the agricultural sector. According to Rengasamy (2002), the building of salts
caused by the prolonged use of agrichemicals is the main cause of the salinity problem. An uneven diet comes
from a change in plant homeostasis in salt-stressed soil locations. Since they are sessile, plants cannot escape
their environment; instead, they must strive to adapt to it. According to studies (Venkateswarlu et al., 2008),
PGPR is crucial for improving the development and production of salt-stressed plants.
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ROS Scavenging-based Halotolerance: Oxidative stress, also known as reactive oxygen species (ROS),
which is formed during salinity stress and consists of O2, O2, and H2O2, causes significant damage to cells.
By controlling the H2O2 level, an enzymatic and nonenzymatic defence antioxidant mechanism is produced
to reduce this toxicity (Bharti and Barnawal, 2019). The levels of ROS are routinely regulated by enzymes like
catalase and ascorbate peroxidase as well as non-enzymatic substances such as ascorbate (Kapoor et al., 2015).
Under salt stress, the plant releases ROS and dehydrates. The ability of PGPR to create both enzymatic and
non-enzymatic components helps the plant tolerate salt stress.
According to Miller et al. (2010), PGPR regulatory genes and ROS-responsive signalling are advantageous.
Certain PGPR decreases the increased soil salinity and increases photosynthetic efficiency by increasing the
antioxidant and polyamine content (Radhakrishnan and Baek, 2017). Catalase and other antioxidants are
produced by PGPR, which lessens the oxidative stress caused by ROS. For superoxide scavenging, plants
under salt stress that have undergone PGPR inoculation and have enhanced SOD activity are crucial.
Halotolerance by Reducing Ethylene Concentration: When the plant is stressed, the first peak appears, and
a few hours later, the second peak. 1-Aminocyclopropane-1-carboxylate (ACC) is a fast ethylene precursor.
ACC deaminase, which shields plants from ethylene stress, is produced by certain PGPR. The quantity of
ethylene is decreased by the ACC deaminase in plants, which converts ACC into -ketobutyrate and ammonia.
Plant ethylene levels are reduced by PGPR formulations with active ACC deaminase as ACC sinks (Glick,
2014). The amount of ethylene produced by plants under salt stress was decreased in seedlings treated with
ACC deaminase-containing bacteria as compared to the control without microbial inoculations, which allowed
for a partial reduction in the effects of salt stress (Barnawal et al., 2017).
Drought: Drought is the major barrier to the productivity of agriculture worldwide. It is believed to have
underestimated the nation's cereal production by 9–10%, according to Lesk et al. (2016). The capacity of a
plant to endure and flourish in dry conditions is known as drought resistance. In order for plants to grow and
meet food demands even when water resources are few, it is critical to identify techniques to increase their
abiotic stress tolerance (Mancosu et al., 2015). The active signalling genes DSM2, Os-NAP, and OsNAC5
improve the tolerance of abscisic acid (ABA) to drought. 2020 (Goswami and Suresh).
Making the Root System More Water Absorbent: According to Timmusk et al. (2014), bacterial changes to
the architecture of the root system increase root surface area, enhance nutrient and water absorption, and
promote overall plant development. To comprehend the fundamental process, extensive research is required.
In response to abiotic stress, a nanoformulation (SomRE) can lengthen roots without negatively affecting the
soil microbiota, claim Naik et al. (2020).
Promoting the Growth of Shoots: In response to drought stress, plants reduce the amount of available leaf
surface, which may impede shoot development and increase evaporative loss (Skirycz and Inzé, 2010).
Increased agricultural production results from plants infected with potent PGPR strains that sustain close to
average shoot growth under drought stress. These plants benefit from PGPR treatments that promote shoot
growth.
Relative Water Content: A more precise technique to assess a plant's water condition may be to look at the
relative water content (RWC) of its leaves, which is involved in tissue metabolism. Reduced RWC is a sign of
turgor insufficiency, which inhibits cellular proliferation and slows plant development (Ngumbi and Kloepper,
2016). RWC tuning might greatly improve drought resistance. According to reports, PGPR-treated plants
exhibited better RWC control than untreated ones. Dodd et al. (2010) assert that alterations to physiological
processes such as stomatal closure may result in improvements in RWC. Grover et al. (2014) observed a 24%
improvement in RWC in drought-stressed sorghum plants after injecting Bacillus sp. strain KB 129.
Tolerance to Drought by Osmotic Modification: A breakdown in the relationship between plants and water,
a reduction in CO2 uptake, an increase in cellular oxidative stress, membrane damage, etc. are only a few of
the impacts of drought on plants. According to Farooq et al. (2009), plants often employ osmotic adjustment
to combat drought stress. According to Kiani et al. (2007), the active accumulation of appropriate inorganic
and organic solutes during drought stress is referred to as osmotic adaptation. They maintain constant water
content, reduce water demand, and maintain cell turgor. In plants under drought stress, proline is an essential
osmolyte (Huang et al., 2014). By scavenging free radicals, buffering the capacity for cellular redox, and
scavenging free radicals, proline also aids in stabilising subcellular structures such as proteins and membranes
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(Hayat et al., 2012). Critical investigations have demonstrated that plants are more drought-tolerant when their
proline levels are greater (Lum et al., 2014). Proline concentrations are greater in plants that get PGPR
infusions. The proline concentration in cucumber leaves increased three to fourfold when three PGPR strains
(Bacillus cereus AR156, B. subtilis SM21, and Serratia sp. XY21) were mixed (Wange et al., 2012). Cucumber
plants were shielded from extremely dry circumstances by the higher proline content. Free amino acid and
soluble sugar contents in maize rose after the application of PGPR (Bano et al., 2013). Proline has to be
supported by additional free amino acids and soluble carbohydrates in order to withstand prolonged drought.
It has been claimed that rhizobacteria modify plant physiology in response to floods (Ravanbakhsh et al.,
2017). Studies reveal that the amount of ethylene regulation is influenced by the bacterial population in plant
roots. Floods restrict gas exchange, which quickly starts to collect inside the plant. Accumulated ethylene,
according to Sasidharan and Voesenek (2015), controls variables involved in flood adaptation. In their 2017
study, Ravanbakhsh et al. emphasised the contribution of R. palustris to the generation of ACC deaminase,
which decreased ethylene levels. Both the ACC synthase and ACC oxidase genes are responsible for the
increase in ACC levels that occurs during the flood (low oxygen situation). ACC deaminase helps ACC diffuse
out of the roots by lowering the high ACC concentration that has built up there. This technique aids in lowering
ethylene levels both before and after flooding. The responses to flood circumstances are greatly diminished by
any disruption of the ethylene signalling system (Ravanbakhsh et al., 2017).
Heavy Metals: Hazardous metals including Hg, As, Cd, and P hyper-aggregate in the soil, stressing plants and
abnormally reducing agricultural yield. Such metal particles quickly change the texture of the soil, which has
a detrimental impact on several biological processes and hinders crop development (Hamid et al., 2021). Most
microorganisms, especially heterotrophs, rely on their host plants for nourishment through a symbiotic
relationship. As a consequence, newly imported plants are less affected by pollutants (Zafar-ul-Hyeetal, 2013).
While PGPR promotes plant growth and production, it also enhances soil quality by employing a range of
methods to lessen the contamination caused by oil and metals. Certain metal-binding peptides are linked to
metal chelation or accumulation.
Metal Stress-Induced Systemic Tolerance (IST): Metals' ability to withstand stress is increased through
induced systemic resistance. The process' genetic underpinnings are well understood (Hobman and Crossman,
2015; Wheaton et al., 2015). Metal contamination in the rhizosphere slows down plant development by
preventing plants from absorbing nutrients (Lal et al., 2018). Immunising PGPR with a metal resistance feature
may help to reduce this. When PGPR is engaged, IST can successfully protect plants from abiotic stress. IAA
offers PGPR-enhanced nitrogen fixation and other growth conditions while lowering heavy metal stress (Guo
et al., 2020). According to Karthik and Arulselvi (2017), when under chromium (Cr6+) stress,
cellulosimicrobium allegedly prolonged the blooming period. In the rhizosphere of Phaseolus vulgaris, the
detected and defined Cr6+-resistant bacteria could also break down phosphate, create ammonia, and release
enzymes including lipase and amylase. There is evidence of a link between the root causes of induced systemic
resistance (ISR) and IST in host plants. Bacterial external membrane lipopolysaccharides, biosurfactants,
siderophores, volatile organic compounds, and other microbial metabolites are associated with ISR induction.
Studies have shown that gibberellic acid (GA) decreased Cd2+ absorption, which in turn decreased metal
toxicity (Zhu et al., 2012).
Rhizobacteria adapt their cell wall and membrane as part of their metal rejection process to shield the sensitive
cell components from heavy metals.
Siderophores' Metal Sequestration: By connecting to siderophores, which are produced by microbes,
bacteria and fungi may absorb iron from their surroundings. They result from a lack of iron and can stop iron
from turning into insoluble compounds. For a number of biological activities, including DNA synthesis and
respiration, iron) interacts with trace metals to create complexes. These chemical molecules with lower
molecular weight are highly attracted to iron. In times of iron shortage, microbes produce them. Siderophores
produced by microbes can effectively and noticeably withstand metal stress. For all living things to develop,
iron is a must. Soil-solubilized ferric ions are not readily available to soil microorganisms at neutral and
alkaline pH. By producing Fe3+ in an area proximal to the root, the siderophore produced by PGPRs may
prevent the development of harmful organisms (Wandersman and Delepelaire, 2004). Microbial siderophores
might access the restricted metal ion after making a link with it. Iron absorption was enhanced and Cd2+
retention was decreased by streptomycetes siderophores (Dimkpa et al., 2009a). Bacterial siderophores
decreased metal toxicity while promoting the growth of plant biomass (Dimkpa et al., 2009b).
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Biosurfactants in the Reduction of Metals: The majority of amphiphilic composite biosurfactants are found
on the surfaces of microorganisms. They facilitate soil metal removal and increase trace metal tolerance.
Biosurfactants continuously exhibit a link with trace metals due to their higher ties to harmful metals as a result
of their amphiphilic nature (Gupta and Kumar, 2017). Heavy metal levels in the soil are reduced as a result of
biosurfactants' capacity to bind to heavy metals (Lal et al., 2018).
Acids, both organic and inorganic and the reduction of heavy metals: According to Archana et al. (2012),
PGPR produces low-molecular-weight natural (organic) acids including citric and oxalic acids that greatly
lessen metal stress in agriculture. They assist plants to flourish in metal-polluted soils by forming less harmful
metal complexes. These complexes, like metallic oxalate crystals, increase resistance in plants by lessening
the cytological effects of native metal ions by inactivating them that are harmful to the plant(Gao et al., 2010).
The PGPR-precipitated inorganic acids may reduce mental stress. According to Gadd (2010), microbial organic
acid is anticipated to have a solids limit in order to successfully chelate heavy metals. By precipitating,
inorganic acids produced by PGPR may reduce mental stress. According to Gadd (2010), the chelation of
heavy metals by microbial organic acid may have a solids limit. Enzymatic activity, bacterial iron oxidation,
participation of microbial inorganic acids including H2S, H2CO3, and H3PO4, and circuitry are all capable of
instantly insolubilizing and immobilising heavy metals, according to Pagnanelli et al. (2010).
EPS from Bacteria in Metal Reduction: Staudt et al. (2004) defined extracellular polymeric substances
(EPS) as microbial polymers with heavy molecular weight homo- or heteropolysaccharides. It either sticks to
the bacterial cells' surfaces or secretes itself outside. Many anion-restraining sites found in the extracellular
polysaccharides produced by rhizospheric bacteria, such as the lipopolysaccharides, polysaccharides,
dissolvable peptides, and glycoproteins (Hassan et al., 2017), aid in the removal or recovery of heavy metals
from the rhizosphere through biosorption. Because it reduces the availability of heavy metals in soil and plant
systems, it is crucial for heavy metal decontamination (Rajkumar et al., 2012).
Bacteriocin: The peptide secretions known as bacteriocins are produced by bacteria and have a specific
antibacterial action. Both Gram-positive (like nisin) and Gram-negative (like colicin) bacteria have the ability
to create bacteriocins (Zimina et al., 2020). These poisons kill rival bacterial species by a highly focused
method of action (Rooney et al., 2020). In vitro studies using bacteriocins against tomato bacterial spot disease
have produced encouraging results (Prncipe et al., 2018).
Antibiosis: Due to their antibacterial, insecticidal, antiviral, phytotoxic, cytotoxic, and anthelminthic
characteristics, antibiotics generated by PGPR are more effective than conventional antibiotics (Fernando et
al., 2018). According to Ramadan et al. (2016), Pseudomonas species produce a wide range of antifungal
antibiotics, such as butyrolactones, rhamnolipids, 2,4-diacetyl phloroglucinol (2,4-DAPG), and N-
butylbenzene sulfonamide. The Bacillus species also secretes additional chemicals including antibiotics like
bacilysin, bacillaene, and mycobacillin. They also create a variety of lipopeptide biosurfactants, including the
antibiotic bacillomycin (Wang et al., 2015).
VOC Production: For certain bacteria and nematodes, the many volatile organic compounds (VOC) released
by PGPR are effective biocontrolling agents. Benzene, cyclohexane, tetradecane, and 2-(benzyloxy)-1-
ethanolamine are a few examples of VOCs. According to Kanchiswamy et al. (2015), HCN is one of the VOCs
(made by rhizospheric bacteria) that can biocontrol a number of phytopathogens. Numerous pathogenic
growths can be prevented by Pseudomonas sp. HCN (Hamid et al., 2021). Santoro et al. (2016) claim that
Bacillus spp. VOCs also effectively inhibit fungi. VOCs play a part in biological control and pollinator
recruitment by sending out signals (Liu and Brettell, 2019).
Lysis via Extracellular Enzyme: Stronger systems can be breached by infectious bacteria using the lytic
chemicals generated by PGPR. Chitinase and 1,3-glucanase, two extracellular rhizobacteria enzymes, are
connected to cell wall lysis (Goswami et al., 2016). Since chitin and 1,4-N-acetylglucosamine make up the
majority of the fungal cell wall, rhizobacteria's chitinase and 1,3-glucanase may break them down and have
potent antifungal effects. For instance, -glucanases and chitinases produced by P. fluorescens LPK2 and S.
fredii KCC5 aid in preventing the growth of wilts caused by Fusarium udum and F. oxysporum (Ramadan et
al., 2016). Protease, lipase, and chitinolytic activities of microbes have been linked to their capacity to kill
insects (Rakshiya et al., 2016). In addition, PGPR with ACC deaminase activity is crucial when all stressors,
including biocontrol, are combined.
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PGPR as Biofertilizer: Biofertilizers are living combinations of advantageous microorganisms that increase
the availability of nutrients and enhance the health of the soil and, as a result, the soil microflora. In other
words, plant growth-promoting microorganisms (PGPM) are the major ingredient in this biofertilizer. These
bacteria are intended to support plant growth and nutrient uptake. It has been acknowledged, nevertheless, that
PGPR has been used as a biofertilizer to improve soil quality and crop production all over the world.
Table: Classification of diverse types of biofertilizer and their groups.
Promotion of plant growth: In general, PGPRs boost plant growth indirectly by reducing the pathogenic
impact on plant growth and development by using bio-control mediators, environmental protectors, and
bacteria that colonise plant roots, according to Gupta et al. (2015). Providing nutrients (such as nitrogen,
potassium, and phosphorus) or adjusting plant hormone levels are two common ways to directly boost plant
development.
• N2 fixation: Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted to ammonia
(NH3) and subsequently made available to plants. This most likely entails the coexistence of bacteria and
plants in a symbiotic relationship, with the bacteria using the environment to change di-nitrogen into a form
that plants can absorb. The transformation of atmospheric di-nitrogen into ammonia is the second-most
important biological activity on land (Datta et al., 2015). It has been suggested that turning N2 into NH3 is
necessary for the N2 fixation process. 16 hydrogen- and ATP-mixed ATP molecules are used in this process,
which is carried out by a complex network of enzymes.
N2 + 8 H2 + 16 ATP 16 ADP + 16 Pi + 2 NH3 + 2 H2 +
• Phosphate solubilization: In addition to fixing nitrogen, the phosphate solubilization (PS) process
increases soil fertility. Phosphorus shortage seriously hinders crop output. According to Singh and Singh
(2018), the macronutrient phosphorus is essential for biological development. By giving plants access to the
insoluble inorganic P, microorganisms offer a biological method of solubilizing it. One crucial component of
PGPR that increases plant yields is the procedure by which various microorganisms considerably contribute
to turning insoluble phosphorus (P) into usable forms (i.e. orthophosphate). In balancing the expensive
inorganic resources of P fertilisers, phosphate solubilizers bacteria (PSB) play a more advantageous economic,
ecological, and agronomic function (Podile and Kishore, 2007).
• Phytohormones production: Phytohormones have two fundamental purposes: they promote responses
and promote plant growth. These are often generated hormone-carrying chemical messengers that accelerate
plant growth and output. Here are a few illustrations: (Ahmad et al., 2008) ABA, auxin, cytokines, gibberellins,
ethylene, indole-3-acetic acid, etc. Auxins found in nature, such as indole acetic acid (IAA), have a beneficial
effect on how roots develop in plants. Around 80% of soil rhizobacteria are capable of producing indole acetic
acid (IAA). IAA has the power to quicken cell division, activate plant hormones that help plants absorb
nutrients from the soil, and promote root development.
Conclusion:
Utilising alternative tactics, such as those that make use of PGPRs, is necessary to maintain soil fertility and
agricultural productivity. Unquestionably, PGPRs today play a multitude of roles that contribute to the
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sustainability of agriculture. A reduction in the global reliance on dangerous agrochemicals that imperil the
stability of agroecosystems has been brought about by increased production through the use of PGPRs in
various agroclimatic settings on a variety of crops. Even though different PGPR strains have continuously
demonstrated their value as biofertilizers, there are still knowledge gaps about the function and functioning of
PGPRs, and more study has to be done to fill these gaps. It's important to comprehend how PGPRs behave in
the rhizospheric environment of specific crops (Li et al. 2020). Ankati and Podile 2019; Xiong et al. 2020), the
role of PGPRs in the rhizosphere, and interactions between the various microbial communities in the
rhizosphere can all have a significant impact on the structure of the rhizobium. for rhizome-mediated
agricultural techniques that provide more stability and greater efficiency. For instance, have demonstrated that
it is possible to construct microbial consortia for use as biofertilizers using microbial taxa isolated from high-
yield plant varieties. A less-examined area is the assessment of enzymatic and non-enzymatic components in
preserving homeostasis under difficult circumstances including drought (Mishra et al. 2020), salt concentration
(Grossi et al. 2020), and heavy metals (Wu et al. 2019). To increase the lifespan, cell count, and performance
of PGPRs, stable bioinoculant formulations containing carrier materials such as biochar, compost, and crushed
maize cob have been created. Promote the inclusion of signal molecules that affect the interaction between
plants and microorganisms in biodegradable polymers to ensure their longer release and close contact with
plant roots in order to boost the marketability of bioinoculants (Cesari et al. 2019).
So it wouldn't be very ambitious to anticipate that biofertilizers will eventually entirely replace chemical
fertilisers. It can mature beyond its infancy if the impact of this technology on agriculture is given more
consideration. In a variety of agroecological situations, it can then significantly increase agricultural output
and sustainability.
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