Phosphorus (P) is a primary macronutrient that is crucial for the development and growth of plants
[64][37]. P-solubilization is defined as the mobility of bound inorganic P (P
i) through the desorption of P and dissolution of P-containing minerals, such as apatite (the group of phosphate minerals)
[65][38]. Alori, Glick and Babalola
[64][37] reported the excessive utilization of synthetic P fertilizers to uplift agricultural production to fulfill ever-increasing global food demand has the potential to pollute surface and groundwater, eutrophication of waterways, and deplete soil fertility. A variety of soil microbes are capable of solubilizing organic P into P
i, which can then be used by plants. These microbes enhance the growth as well as production of a broad range of crops.
5.1.3. Potassium Solubilization
Potassium is the third fundamental plant macronutrient, following nitrogen and phosphorus
[67][39]. This element takes part in several physiological and metabolic processes of the plants, including photosynthesis, stomata regulation, proper seed development, and promoting crop growth and yield
[68][40]. In soils, K-containing minerals that discharge K through weathering are feldspar, muscovite, biotite, alkali, and illite
[69][41]. However, a major dilemma is the unavailability of these minerals (for plants), but with the assistance of K solubilizing microorganisms inclusive of bacteria, actinomycetes, and molds, improved potassium dissolution can be achieved.
5.1.4. Phytohormones Production
Chemical messengers that are mediated in biochemical and physiological processes of higher plants that are active at very low concentrations refer as phytohormones
[72][42]. Charles Darwin was the first person who suggested that certain chemical compounds capable of stimulating growth in crops are latterly known as phytohormones. Microorganisms can stimulate growth and enhance the resistance of plants by synthesizing phytohormones
[73][43]. Plant roots are heavily surrounded by microbes because of root exudates that are rich in nutrient components
[74][44]. Classical bacterial-phytohormones are ethylene, cytokinins, auxins, abscisic acid, and gibberellins
[75][45].
5.2. Indirect Mechanism of PGPR
5.2.1. Siderophore Production by Bacillus spp.
Siderophores are low molecular weight, metal-chelating compounds that are produced under iron-limited conditions by some microbes and plants
[97][46]. Iron (Fe) acts as a key element in various kinds of biological processes, e.g., metabolism of oxygen, synthesis of DNA and RNA, transfer of electrons, and enzymatic processes. Siderophores have the capability to lessen the accessibility of Fe for pathogens
[98][47]. By functioning as biocontrol agents, microbes that create siderophores can restrict the spread of diseases and promote the growth of the plant
[11]. Out of various
Bacillus spp.,
B. licheniformis, B. anthracis, B. velezensis, B. thuringiensis, B. cereus, B. halodenitrificans, B. atrophaeus, B. mojavensis, B. pumilus, and
B. subtilis are the well-known for siderophore production
[99][48]. Siderophore is induced by numerous species of
Bacillus, and these species actively participate in the reduction of different plant diseases. For instance,
B. subtilis produced a siderophore, which was involved in the reduction of
Fusarium wilt and increased the pepper yield
[100][49].
5.2.2. Induced Systemic Resistance—ISR
Non-pathogenic rhizobacteria have the capacity to lessen diseases in plants by mediating a plant defense process called “Induced Systemic Resistance” (ISR)
[98][47]. It takes a combination of biotic and abiotic stimuli for plants to start developing the ISR (mechanism of resistance). Non-pathogenic rhizobacteria participate in mediating ISR and usually rely on the ethylene (ET) or jasmonate (JA) signaling pathways
[103][50], while Systemic Acquired Resistance (SAR) is promoted through the help of salicylic acid (SA). SAR is responsible for the stimulation of a particular group of defense-related genes, while ISR is not involved in the triggering of any certain kinds of defense-related genes
[104][51]. PGPR induces ISR in plants by releasing various metabolites, e.g., antibiotics, siderophores, volatile organic compounds (VOCs), etc. Through the release of these compounds, PGPR can trigger the mechanism of ISR in plants.
Bacillus spp. can initiate ISR by the production of antioxidant defense enzymes. Different defense-related enzymes, e.g., polyphenol oxidase (PPO), superoxide dismutase (SOD), peroxidase (POX), and phenylalanine ammonia-lyase (PAL), are induced by
B. subtilis. In tomato seedlings, the prolonged formation of antioxidant defense enzymes induces the mechanism of ISR against early and late blight diseases
[105][52].
5.2.3. Production of Lytic Enzymes
Lytic enzyme production is an intrinsic characteristic of biocontrol agents in the prevention of disease-causing microbes
[109][53]. The activity of lytic enzymes disrupts the cell walls of targeted pathogens by changing the structural stability and integrity
[110][54]. Chitin is a major constituent of the cell walls of fungi, among other composition molecules
[111][55]. Some bacterial strains (PGPR) can degrade fungal cell walls by producing hydrolytic enzymes, including chitinases, dehydrogenases, exo- and endo-polygalacturonases, lipases, phosphatases, proteases, β-glucanases, hydrolases, pectinolyases, and cellulases. Another study reported that the synthesis of lytic enzymes might also be helpful for bacteria to penetrate plant tissues and grow as endophytes
[112][56].
6. Plant Protection Activity Stimulated by Bacillus spp.
Strains of the
Bacillus spp. are used as biological control agents (BCAs) to protect plants from pathogenic diseases. Chemical pesticides are being replaced by BCAs, which is a viable option. As a result, various researchers are focusing on exploring their interactions with pests, plants, and pathogenic and beneficial microbes, as well as their environmental impact and human implications. Important characteristics, including efficacy, formulation, stability, and viability, were all thoroughly investigated in many studies.
6.1. Quorum Quenching
Communication inside the bacterial population is feasible with the help of quorum sensing molecules, N-acyl homoserine lactone (AHL). Such indicating molecules are the main reason for boosting the infectious diseases in the pathogenic microbes. Those microorganisms which release AHL lactonase enzyme behave as a biocontrol agent. AHL lactonase is an enzyme that hinders bacterial communication systems by breaking down the quorum-sensing signaling molecule. Quorum quenching was noticed in different
Bacillus spp., including
B. cereus,
B. thuringiensis, and
B. licheniformis [114][57].
6.2. Production of Volatile Organic Compounds (VOCs)
Lower molecular weight lipophilic compounds with high vapor pressure and low boiling point are released by microbial metabolic processes. VOCs function as signal molecules both over short and long distances in the rhizosphere
[115][58]. Additionally, 2,3-butanediol is a volatile organic compound produced by
B. subtilis engaged in the mechanisms of plant defense. Phytopathogens were challenged by using the root exudates from peppers inoculated with
B. subtilis.
6.3. Antibiotic Compounds
Antibiotic production by beneficial microorganisms
[86][59] is the most effective biological control method for controlling plant diseases. Such chemicals are secreted by
Bacillus spp. during sporulation and the stationary development stages
[86][59]. Bacitracin, Kanosamine, fengycin or plipastatin, surfactins, zwittermicin A, kurstakin, gramicidin, and iturins are important antibiotic compounds produced by
Bacillus spp. Bacitracin is another kind of antibiotic compound that has strong bactericidal activity. Different
Bacillus spp., including
B. subtilis and
B. licheniformis, have been found to synthesize bacitracin
[114][57].
6.4. Biofilm Formation by Bacillus spp.
In the past, induction of systemic resistance and synthesis of antimicrobial compounds were two reported methods that biocontrol agents utilize to combat phytopathogens. However, current research in the field of biocontrol has focused on biofilm formation and root colonization as defense mechanisms against biocontrol activity. Several
Bacillus spp. including
B. velezensis,
B. atrophaeus, and
B. subtilis have been reported to colonize roots and create biofilms as a biocontrol strategy. In many
Bacillus species, plant root exudates and various lipopeptides, including bacillomycin and surfactin, play a vital role in the formation of
biofilm [123][60].
7. Multifaceted Role of Bacillus thuringiensis as a Biocontrol Agent
Bacillus thuringiensis (
Bt) is an entomopathogenic bacteria that create parasporal crystal proteins (δ-endotoxins). These δ-endotoxins are poisonous to Lepidoptera, Coleoptera, and Diptera, among other insect pests
[124][61]. Throughout the previous century,
Bt has been regarded as the most effective bioinsecticide
[125][62]. Because
Bt is a rapid-acting and host-specific bioinsecticide, it has few side effects on non-target organisms. Furthermore, its production and use are simple and inexpensive
[126][63]. To generate transgenic crops that are resistant to pests, plant genetic engineering has successfully used
Bt as a source of Cry genes
[127][64].
The production of bacteriocins is the main antimicrobial activity of the Bt strain
[125][62]. To strengthen the defense against different microorganisms, prokaryotes frequently produce a variety of antimicrobial peptides. Bacteriocins are tiny, thermotolerant antimicrobial peptides produced by ribosome synthesis in the stationary phase, with molecular weights ranging from 3 to 12 kDa.
Various Bt strains can compete with plant pathogenic bacteria through the production of various bacteriocins and AHL-degrading enzymes. AHL-degrading enzyme (AiiA), released by some Bt strains, can reduce the virulence of pathogenic bacteria like
Erwinia carotovora, which causes soft rot in the roots of
Capsicum annuum [129][65]. Furthermore, the inclusion of vegetative cells of Bt in combination with other bacterial (
Streptomyces avermitilis and
Citrobacter farmeri) and fungal (
T. viride,
T. parareesei, and
Paecilomyces variotii) antagonists significantly increased their effectiveness to suppress
Ralstonia solanacearum in
Capsicum chinense [130][66] and
S. lycopersicum [131][67].
Bacillus thuringiensis (Bt) produces crystal proteins (Cry), also called δ-endotoxins.
Bacillus produces the most prominent group of insecticidal proteins, which are known as cry toxins. According to the nomenclature committee of Bt toxin, 78 distinct Cry toxins have been identified to date, with Cry1 being the most common
[132][68]. A wide range of
B. thuringiensis subspecies produces a variety of Cry toxins.
B. thuringiensis var kurstaki produces 31 distinct forms of Cry proteins, the most common of which are Cry1Aa and Cry1Ac.
B. thuringiensis israelensis is the main producer of Cry4, Cry10, and Cry11 toxins. Cry1 toxins are mostly active against Dipterans, Lepidopterans, and Coleopterans, whereas Cry2 toxins are mostly poisonous to Dipterans, Lepidopterans, and Hemipterans.
8. Conclusions
Pesticides have been proven to be a promising agent to fulfill the food demand of the growing population. However, these hazardous pesticides have caused human health problems, development of pest resistance, narrowing of biodiversity, and environmental challenges, raising concerns about the pesticides’ safety. Thus, the need to reduce reliance on these synthetic pesticides is pertinent. The application of PGPR is an auspicious solution for eco-friendly agriculture.
Bacillus spp. have been elucidated as growth promoters in sustainable agriculture through both direct and indirect mechanisms. The N
2-fixation, P and K Solubilization, phytohormones production by
Bacillus strains, moreover synthesis of antibiotics, production of lytic enzymes, and ISR are direct and indirect mechanisms, respectively, and all these action mechanisms of
Bacillus are supportive in the growth promotion of plants, pest resistance, and circumventing of disease. Some of the
Bacillus spp. have been documented as promising biocontrol agents. Food production and its accessibility always are an overwhelming priority to feed the world’s population. So, the best route is to be cautious about chemical-based pesticides. Biopesticides have long been attracting global attraction due to their safer strategy than conventional pesticides. Considering the importance of sustainable agriculture
[173[69][70][71][72],
174,175,176], Bacillus spp.-based bioproducts could be a promising addition to sustainable agriculture as there is a limited product range available. There is a dire need to explore the potential of
Bacillus spp. in combination with other compatible microbial agents to increase PGP activity and quality food production.