Plant Growth-Promoting Rhizobacteria in Management of Biotic and Abiotic Stresses

Abstract

Plant Growth-Promoting Rhizobacteria (PGPR) modifies the activity of the relevant genes to affect the physiological traits, metabolites, pathways, and proteins of plants. Traditional methods for creating salt-tolerant crops are expensive, laborious, and occasionally difficult to adopt. It has been proposed that using microorganisms that encourage plant growth is a suitable and economical method of raising plant tolerance. These evocative microbes can act as a mediator between plants and their morphological, physiological, and molecular responses. Extensive research has been done on the signaling pathways used by hormones, plant receptors, and microbial signals to stimulate PGPR in plants. This chapter aims to increase comprehension of the convergence mechanisms used by these signaling molecules as well as the ambiguities of signaling activities that occur in the host as a result of interactions with PGPR under demanding environmental situations. In order to address biotic and abiotic stressors in agricultural areas and hence raise global food production, the use of rhizobacteria inoculants is a viable strategy.

1. Introduction

Stress, as a term, refers to an external factor that exerts an impact on the vegetative development of the plant. The environmental conditions that trigger stress impede the plants’ growth and progression, thereby curbing their capacity to propagate and transmit their genetic traits to the succeeding generations. In the natural milieu, stress can be attributed to both biotic and abiotic factors that operate in a concerted manner [1]. The ramifications of climate change on the outcomes of abiotic stress are manifold, imperiling the durability and efficacy of agricultural systems [2]. Abiotic and biotic stresses are significant ecological menaces that drastically diminish agricultural output. Abiotic stressors include a variety of environmental elements like heat, cold, salinity, and drought. But biotic stressors involve a wide variety of living things, including fungi, bacteria, viruses, nematodes, and insects [3].

Plants, in order to counteract stressful conditions, elicit particular responses that result in a reconfiguration at various levels, such as genetic and molecular, among others, as a means of safeguarding themselves from these stressors. At the cellular level, stress provokes alterations in cell division and cell cycle, alongside modifications in the endomembrane system, cellular vacuolization, and structural changes in the cell wall. Furthermore, plants adjust their metabolisms to adapt to diverse environmental stressors on a biochemical level. In recent times, numerous investigations have explored the relationship between stress response and the genetic makeup of plants [4]. Defense mechanisms in plants are often supported by microbial communities. The transition of plants from water to land established a crucial function for microorganisms, which encompassed safeguarding plants against various stressful conditions [5]. The soils in proximity to the roots have been identified as being hotspots for microbial activity. Plants have the ability to generate signals that promote the development of specific microbial communities and subsequently regulate their genetic and biochemical activity [6]. The act of housing bacterial communities, also known as PGPR, within the rhizosphere is a crucial aspect of plant growth and development. These rhizobacteria serve to provide aid to plants in mitigating stress, as well as offering overall assistance in plant vitality [7]. The utilization of plant growth-promoting rhizobacteria (PGPR) in the many signaling pathways of plant morphological, physiological, and molecular responses is examined in this chapter, along with the function that hormones, plant receptors, and microbial signals play. The convergence methods of these signaling molecules are also expounded upon, alongside an analysis of the uncertainties associated with host signaling activities. Consequently, an exploration of the interaction between PGPR and plants in stress-inducing environments is presented.

2. Stress in plants: Biotic and abiotic

Abiotic stress refers to a variety of environmental conditions, including heat, UV rays, salt, floods, droughts, and heavy metals. This array of stressors has caused significant loss of essential crop plants on a global scale [8]. On the other hand, biotic stress denotes the impairment brought about by various organisms, including insects, herbivores, nematodes, fungi, bacteria, and weeds [9]. The primary biotic and abiotic stressors on plants are depicted in Figure 1.

Figure 1.

Principal biotic and abiotic stressors that affect plants (it belongs to the authors study).

2.1 Stress on plants: negative effects of biotic and abiotic

The effects of the biotic variables are wide-ranging and include the death of the entire plant or its organs, a reduction in the bulk of the root, stem, leaf, or inflorescence, complete defoliation, holes, and cavities on plant parts, as well as other signs of eating [10]. The growth and development of plants are seriously threatened by abiotic stressors, which can include but are not limited to extremely hot or low temperatures, insufficient or excessive water availability, high saline levels, the presence of heavy metals, and exposure to UV radiation. The outcome is a significant decline in worldwide crop yields [11].

2.2 Plant stress management: Biotic and abiotic

Enhancing plant resilience necessitates emphasizing the impact of stressed conditions on these interactions over an extended duration, with a primary focus on the growth of crop species under varying pedoclimatic situations, consequent to diverse approaches pertaining to plant, soil, and water management. The consequences of the interplay between these overlapping factors are influenced by environmental factors. The effects of these interactions may be constructive or detrimental, contingent upon the extant circumstances, often resulting in them being inadequately stable for pragmatic implementation. To fully utilize these microorganisms in practical field settings, it is necessary to enhance our understanding of their interactions with the environment and plants. This can be accomplished by furnishing knowledge regarding the intricate feedback mechanisms operative between plants and microbes in the aftermath of stress events, primarily [12].

3. Rhizobacteria

Rhizobacteria, also referred to as root-associated bacteria, can have a parasitic, helpful, or neutral effect on a plant’s growth. The term “rhiza” originates from the Greek language, where it denotes “root.” Typically, rhizobacteria establish symbiotic relationships with numerous plants, thereby engendering mutualism. In scientific literature, rhizobacteria are frequently referred to as plant growth-promoting rhizobacteria (PGPRs). Joseph W. Kloepper introduced the term “PGPRs” for the first time in the later part of the 1970s [13].

PGPR typically comprises 2–5% of the rhizosphere microorganisms. The use of biofertilizer, which supplies a significant amount of the nitrogen supply to crops globally, makes this group of microbes extremely important. The interaction of PGPRs with host plants differs depending on the species. There are two important types of relationships: rhizospheric and endophytic. Rhizospheric connections entail PGPRs colonizing the host plant’s root surface or superficial intercellular spaces, which frequently results in the development of root nodules [1].

3.1 Opportunity and attributes of plant growth-promoting rhizobacteria (PGRP)

Plant growth-promoting rhizobacteria (PGPR) are microorganisms that reside in plant roots and aid in plant growth by supplying increased mineral nutrition, producing plant hormones or other molecules that stimulate plant growth and strengthen the plant’s defenses against biotic and abiotic stresses, or shielding plants from pathogens by reducing the survivability of pathogenic microorganisms [14].

3.2 Plant stress tolerance mediated by rhizobacteria

Through nutrient solubilization, nitrogen fixation, the production of phytohormones, siderophores, and biomolecules with pathogen-repelling properties, the group of microbes known as rhizobacteria can enhance plant growth and health [15]. To lessen plant stress, PGPR employs both direct and indirect mechanisms. The complex direct action of PGPR against plant abiotic stresses involves a number of features, including improved mineral acquisition, increased nutrient availability, improved water absorption, and facilitation of exopolysaccharide, biofilm, and the development of numerous organic solutes, including sugars, organic acids, amino acids, and polyamines [16]. Considerations for how PGPR can indirectly lessen plant stress include chemotaxis, phytohormone synthesis, and modulation, changes in their levels, activation of antioxidant defense mechanisms, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, and the control of stress-responsive genes in plants. Together, these activities control a range of biological plant processes and aid in the establishment of local and remote rhizobacterial-induced abiotic stress tolerance in plants [14].

3.3 Rhizophore: signaling between plants and microbes

One of the key elements influencing the rhizomicrobiome is the cellular response shown by the microorganisms or plants that leads to metabolic transformation, catabolism, and chemical resistance [17]. Plants exhibit their capacity for adaptation by releasing root exudates into the rhizosphere, which enables them to choose the appropriate microbial communities while simultaneously discouraging the growth of detrimental communities. This selection process, also known as “niche colonization,” enables the plants to create a healthy microbiome [18]. The directionality of communication allows us to distinguish between two different signaling groups in the rhizosphere: inter- and intraspecies microbial signaling and interkingdom signaling between microorganisms and plants. Acyl-homoserine lactones (AHLs or AI-1) and autoinducer peptides (AIPs), both of which are found in both Gram-positive and -negative bacteria, are grouped as two distinct kinds of quorum sensing signals in bacteria. Additionally, autoinducer type 2 (AI-2), a molecule with characteristics similar to both AHLs and AIPs, is present in both Gram-positive and -negative bacteria. Antibiotics, which have been found to encourage intra- or interspecies communication, are one of the numerous QS bacterial signal types [19].

3.3.1 Microorganisms

3.3.1.1 Inter- or intraspecies signaling

Microorganisms within the rhizomicrobiome engage in mutual communication via the synthesis of signaling molecules, which enables them to regulate their genetic expression [20]. Inter- or intraspecies communication among microorganisms is facilitated through the quorum sensing mechanism, which entails coordination that is dependent on cell density. Quorum sensing, a cellular interaction mechanism, involves the generation, emission, and identification of chemical signals, also referred to as autoinducers (AIs) [21]. These AIs control the gene expression of specific bacterial processes, such as biofilm formation, adhesion, and motility, upon detection by the receiver, propagation, virulence, metabolism, and symbiotic aggregation [22]. Acyl-homoserine lactone (AHLs or AI-1) and autoinducer peptides (AIPs) are two families used to categorize quorum sensing signals in bacteria, the former being present in Gram-negative bacteria and the latter in Gram-positive bacteria. AHLs and AIPs share characteristics with autoinducer type 2 (AI-2), which is present in both Gram-positive and Gram-negative bacteria [23].

Volatile organic compounds (VOCs) constitute a crucial category of signaling molecules. They are synthesized and released by microbes for the purpose of long-range communication within a microbial community, as well as in microbe–plant interaction [23]. Volatile organic compounds (VOCs) are a set of small molecular weight lipophilic compounds, with a range of 100–500 Da. Through unique metabolic processes that depend on the genotype of the relevant species, several bacterial and fungal species create these compounds [24]. According to reports, volatile organic compounds (VOCs) generated by bacteria include sulfury, terpenoids, alkanes, ketones, and alkenes [25]. Studies have shown that VOCs operate as antimicrobial QS signaling molecules and have an important impact on microbial activity, including but not limited to virulence, stress resistance, and biofilm formation [26]. In addition to these functions, published research has shown that VOC signals also regulate plant growth, notably root architecture and hormone signaling, as well as plant immunity to biotic and abiotic stresses [27]. Additional signaling routes involving microbial VOCs will become clearer with more study. Therefore, these intricate signaling pathways among rhizosphere microorganisms play a crucial role in the formation of the rhizomicrobiome by attracting specific bacteria via inter- or intraspecies communication.

3.3.1.2 Interkingdom

The communication between microbial and botanical species, known as interkingdom signaling, which either induces or suppresses gene expression, has a profound impact on plant growth. Interkingdom signaling can be categorized into two groups: microbe–plant signaling and plant-microbe signaling, depending on the direction of the stimulus.

3.3.1.2.1 Microbe: plant signaling

Microorganisms produce and release signals in the field of microbe–plant signaling that cause symbiotic interactions with the plant. Specific modifications in the plant transcriptome can be induced by rhizosphere-originating signals from the microbial population. Similar to how plants manufacture phytohormones, PGPR also creates these signaling molecules. The signals that stimulate the growth of plants have the ability to govern their developmental procedures, and furthermore, they can bestow plants with the capacity to withstand abiotic and biotic stressors.

The majority of the work now in existence on the interaction between microorganisms and plants has focused on interactions that have positive effects, like fostering plant development and reinforcing plant defenses against biotic and abiotic challenges. The microorganisms in the rhizosphere that interact favorably with plants are mycorrhiza, rhizobia, plant growth-promoting bacteria, and fungi (PGPR or PGPF) [17]. Microbe-associated molecular patterns (MAMPs), such as flagellin, chitin, and lipopolysaccharides, are usually referred to as microbial signals and can be detected by plants. Pattern recognition receptors (PRRs) are activated throughout the perception process, and this activation triggers a local defense response by way of a hormone signaling network. The resulting immune responses are produced as a consequence of this intricate mechanism [28]. Interkingdom communication has been reported to involve quorum sensing signals. Bacterial QS signals like N-butanoyl homoserine lactone and N-hexanoyl homoserine lactone (AHLs), which are recognized by plants, encourage the formation of a symbiotic interaction between the bacteria and the plants. Additionally, changes in hormone levels in plants promote root growth. Previous investigations back up these results [29]. DSF, yet another bacterial QS molecule, is responsible for triggering innate immunity in a variety of plants by the recognition of dangerous microorganisms via PRRs [30]. Bacterial QS molecules not only cause physiological changes in plants, but they also cause the release of chemicals that are similar to the pathogenic microorganisms’ QS molecules [31].

3.3.1.2.2 Signaling compounds

Microbial populations that include parasitic, commensal, and mutualistic bacteria find a home in plants. The plant’s reaction to these microbial signals is the secretion of chemical compounds known as “root exudates.” Both high- and low-molecular-weight compounds, including proteins and mucilage, as well as organic acids, sugar, aliphatic acids, fatty acids, amino acids, flavonoids, and secondary metabolites, are present in these exudates [32]. The biology of the rhizosphere is significantly influenced by the interaction between plants and microorganisms, which is made possible by the production of phytochemicals [33]. The released compounds draw rhizospheric microbes to the plant roots, where they engage in either harmful or symbiotic relationships. Of all the plant-microbe interactions that have been examined, the relationship between legumes and nitrogen-fixing bacteria has been the most extensively studied. The establishment of a symbiotic association is defined by a sequence of signals that culminate in the formation of root nodules. These nodules function as a nutrient source for rhizospheric bacteria, who reciprocate by providing the plant with a readily available form of nitrogen. Over the past decade, there has been significant research into the signals that plants emit to promote nodule formation [34].

VOCs, many plant organs, including leaves, flowers, fruits, and roots, emit volatile organic compounds [35]. About 1% of all secondary metabolites found in plants are classified as volatile organic compounds (VOCs), which include terpenoids, fatty acids, phenylpropanoids, and amino acids. The propensity of VOCs to permeate plant membranes with ease and subsequently disperse into the surrounding air or soil is the mechanism by which this occurs. They facilitate the attraction of pathogenic root colonizers toward the soil while simultaneously hindering their proliferation [36]. The dispersion of signaling compounds, comprising of volatiles, root exudates, and strigulates, emanating from plants, serves as a wide-ranging mechanism to stimulate bacterial receptor proteins, which, in turn, triggers a microbial response that regulates the gene expression. It is a well-established fact that the discerned chemical signals have a significant impact on the growth and defense mechanisms of plants. The intricate and specific nature of these phytochemicals leads to modifications or alterations within the rhizomicrobiome.

3.4 Hormone signaling pathway and rhizobacteria

Hormones, also referred to as plant growth regulators, are chemical substances that have a significant impact on the development and differentiation of plant cells, tissues, and organs. Additionally, these substances act as essential chemical messengers that enable communication between various cellular units [37]. The preponderance of scholarly inquiry has assessed the phytohormonal production of flora to facilitate their resilience in the face of non-living environmental pressures. Presently, it is incontrovertible that the rhizomicrobiome of plants, which encompasses rhizobacteria that stimulate plant growth, also engenders phytohormones which control how quickly plants develop [38]. Their various mechanisms of action have been divided into two categories: direct mechanisms, which include nitrogen fixation, phosphate solubilization, siderophore production, and phytohormone production, and indirect mechanisms, which include parasitism, induced systemic resistance, antibiosis, induced competition for nutrients, and production of a variety of metabolites [39].

3.5 Plant hormones and rhizobacteria as signaling molecules

Plants and rhizobacteria both have the ability to make phytohormones, which operate as essential bioregulators for a variety of cellular mechanisms and signaling pathways in plants. Numerous phytohormones, including auxins (AUX), cytokinin (CK), abscisic acid (ABA), ethylene (ET), salicylic acid (SA), jasmonic acid (JA), gibberellic acids (GA), strigulates (SL), and brassinosteroids (BRs), are included in this group [40].

3.6 Rhizobacterial hormone signaling and interaction with plant hormones

It has become clear that phytohormones, which are produced by both plants and plant growth-promoting rhizobacteria (PGPR), have a role in the resilience of plants to abiotic stresses in addition to serving other important roles. The plant growth-promoting rhizobacteria (PGPR) can control how well plants tolerate abiotic stress by modulating plant hormones and physiological responses. Studies have been conducted concerning the hormonal signaling of rhizobacteria and its interaction with phytohormones, which serve as mediators of the plant’s capacity to endure abiotic stress [41]. The complex interaction of numerous signaling molecules including ABA, ET, SA, and JA along with AUX and GA has a significant impact on the genetic pathways that control plants’ capacity to endure stress [42]. In order to coordinate the actions of several genes and the regulators that control them in response to stressful stimuli, hormonal intercommunication is essential. Exogenous phytohormones have the same ability to change the hormone levels in plants as phytohormones produced by microbes. Plant hormonal balance can be altered by microbes by either generating growth regulators or inducing their production within the plant [43]. The differences between signaling molecules synthesized by PGPR and plants are shown in Figure 2.

Figure 2.

To establish a mutually advantageous association in the rhizosphere, the presence of signaling molecules produced by both PGPR and plants is imperative. The PGPR is responsible for producing a variety of hormones such as indole acetic acid (IAA), GA, zeatin, and ABA. Additionally, it produces VOCs including 2-heptanol, 2-endecanone, and pentadecane, in addition to ACC deaminase. Additionally, it generates acyl-homoserine lactones (AHLs), such as 3-oxo-C6HL and 3-oxo-C8HL, and cyclopeptides (CDPs). These compounds cause the activation of plant signaling cascades, which eventually boost plant development and increase its ability to withstand stress. The creation of signaling molecules is induced by PGPR, which is comparable to how growth hormones like SA, JA, CK, and IAA modulate plant signaling and stress responses. This relationship promotes plant growth by providing essential minerals through nitrogen fixation, ion uptake of necessary components including Fe, Zn, and micronutrients, as well as the solubilization of phosphate (it belongs to the authors study).

4. Stress management in plants using rhizobacteria

4.1 Stress management mechanism facilitated by PGPR

Plants are commonly subjected to various environmental challenges, including but not limited to salinity, heavy metals, and water scarcity. To adapt to these unfavorable conditions, plants commonly modify their root morphology. It has been suggested that phytohormones like auxin and ethylene significantly alter the root architecture. Indole acetic acid (IAA), a phytohormone that is essential for increasing cellular elongation and controlling root growth in the lower sections of the plant, is unquestionably produced mostly in the aerial regions of the plant body, particularly the shoot [44]. Given that these bacterial strains can produce Indole-3-acetic acid (IAA) at a level that promotes root growth and the production of lateral root hairs, it is not surprising that plants that are associated with Plant Growth Promoting Rhizobacteria (PGPR) may exhibit similar patterns of root growth [45]. The process of encouraging root growth comprises an increase in surface area, which benefits the plant organism by facilitating increased absorption of water and nutrients [46]. In order to control plant development under stress, endophytic rhizobacteria must produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase [47]. It is interesting that ACC deaminase is used by rhizobacteria to speed up the breakdown of ACC (1-aminocyclopropane-1-carboxylic acid), producing ammonia as a nitrogen source. For the creation of ethylene, ACC acts as a precursor molecule. Because of this, the root system is improved by the hydrolysis of ACC, which also lowers ethylene production and increases auxin (IAA) levels [48].

The activity of a number of enzymes is required for the regulation of plant body under broad-spectrum environmental stress. These enzymes are ascorbate peroxidase (APX), catalase (CAT), proteinase inhibitors, phenylalanine ammonia lyase (PAL), polyphenol oxidase (PPO), peroxidase (PO), superoxide dismutase (SOD), and lipoxygenase (LOX). According to reports, all of these enzymes have been linked to induced acquired resistance (ISR) [49]. Phytoalexins and phenolic compounds can be produced when certain kinds of enzymes are activated, providing immediate defense against pathogenic interactions. Volatile organic compounds (VOCs), Flagellin, LCOs, cyclodipeptides (CDPs), and lipopolysaccharides (LPS) are only a few examples of the signal molecules that rhizobacteria make that are capable of being recognized by plant receptors (PRs). Prior exposure to the ethylene response mutant (ethylene receptor1/ER1) and jasmonic acid response mutant (jasmonic acid receptor1/JAR1) causes an increase in sensitivity to ethylene and jasmonic acid, respectively, which results in the production of several defense compounds. The natural buildup of salicylic acid (SA) and its related signaling pathway are crucial in controlling the emergence of systemic acquired resistance (SAR). A crucial issue to take into account is the existence of pathogen-related proteins (PR proteins), which have a significant impact on the formation of SAR. The downstream effects of the ISR and SAR pathways differ when a regulatory mutation known as non-expressor pathogen-related genes 1 (NPR1) is present despite being regulated by various substances [50]. NPR1 is capable of expertly controlling the production of proteins that are crucial for resisting pathogens via the SAR pathway, the JA and ethylene-mediated ISR pathway, and other pathways that are solely dependent on SA for their expression.

4.1.1 Biotic stress management by rhizobacteria

Similarly, in response to PGPR, plants produce signaling molecules such as plant growth hormones (SA, JA, CK, and IAA), which support their signaling and stress response. Through nitrogen fixation, ion absorption (Fe, Zn, and micronutrients), and phosphate solubilization, the associated PGPR increases plant development [51]. The PGPR produces hydrolytic enzymes, siderophores, and antibiotics and modulates plant ethylene levels as defense strategies against plant diseases [52]. Through the mechanism of cell wall alteration via lignin deposition, the PGPR displays a defense response [53]. The fact that plants activate some signaling pathways in response to PGPR and pathogenic microorganisms suggests that these pathways are regulated, balanced, and overlap with one another. These pathways, which interact with one another, are regulated by hormones. In recent years, it has been possible to identify a large number of regulatory molecules involved in the interaction between the pathways leading to systemic acquired resistance (SAR) and induced systemic resistance (ISR).

4.1.2 Abiotic stress management by rhizobacteria

The Plant Growth-Promoting Rhizobacteria (PGPR) strain KT 2440 of Pseudomonas putida is a highly significant PGPR with major application in agriculture. Because of its in-depth analysis, a wide range of defenses against non-living elements have been developed. The magnificent exopolysaccharide (EPS) is a remarkable defensive barrier that emerges on the root surface, warding off the malevolent Na + ions that attempt to invade the root cells. This awe-inspiring EPS is the result of the tireless efforts of the plant growth-promoting rhizobacteria (PGPR). This exceptional creation not only protects the root but also imparts an incredible tolerance to salt stress. The enzyme EptA’s synthesis of EPS is governed by a gene, which is however not the only determinant. The paramount regulatory factors, namely PmrA and PmrB, work in tandem to stimulate the expression of the eptA gene through a dual-component signaling pathway. This intricate mechanism thus ensures the proper regulation of EPS production. PmrA, a remarkable cytoplasmic response regulator, stands at the forefront of this regulatory signaling cascade. PmrB, on the other hand, stands guard as a sensor kinase embedded in the membrane, ever vigilant and ready to perceive any changes in its surroundings [54]. The activation of this enzyme is believed to be triggered by Vanadate and various other signals, including high metal ions, low pH, and PmrB kinase. This results in the phosphorylation of PmrA and the activation of the eptA gene and eventually leads to the synthesis of EPS [55]. The minuscule microorganisms generate EPS as a reaction to a plethora of environmental factors, imparting the bacterium with safeguarding traits against that particular duress. Inquisitive scrutiny has established that the ABA and SA communication routes function as pivotal arbiters in response to both non-living and living adversities. The pathways of plant signaling are diverse and versatile, encompassing mitogen-activated protein kinase (MAPK) as well as calcium signaling. These pathways are capable of initiating numerous other plant signaling pathways, while also influencing the expression of various stress response genes [56].

5. Convergence of signaling molecules

Lipo-Chito oligosaccharides, sometimes referred to as nodulation factors, are exquisite signaling substances that plant growth promoting rhizobacteria (PGPR) can produce with skill. The presence of these compounds instigates the flourishing of nodules and plays a pivotal role in fostering the harmonious coexistence between the roots of flora and rhizobacteria [57]. The growth of lateral roots in host plants can be significantly influenced by a variety of factors, among which is LCO. The manner in which stressful conditions impact this phenomenon can be directly attributed to these factors. Particularly noteworthy are the LCOs produced by bacterial associations, such as those containing oligosaccharide-binding LysM sites in lysin motif receptor-like kinases (LysM-RLKs), which have the remarkable ability to be detected by plant receptors. This interaction triggers the initiation of signal transduction pathways that ultimately lead to the activation of nodulation [58]. The amalgamation of LCOs and chitiologosaccharides (Cos) has the potential to augment the efficacy of particular pathways. This action is attained by triggering the inflow of calcium (Ca²⁺) through the plasma membrane, producing reactive oxygen species (ROS), and activating MAPK (mitogen-activated protein kinase) pathways. Plant defense systems can be activated by this interaction between chitiologosaccharides (Cos) and LYK/CERK1 receptors [59].

The pathway of MAPK assumes a pivotal role in the conveyance of signals during symbiotic interactions, as demonstrated by the earlier-mentioned discoveries. Numerous additional studies have also demonstrated that flora containing LCOs enhance the genes that bind to calmodulin. This supports the combination of LCO and CaMB and their conceivable involvement in the calcium signaling pathway in host plants. The proteins binding to calmodulin (CaMB) are exquisitely perceptive to calcium (Ca²⁺), and they possess the capability to oversee a diverse range of target proteins. Furthermore, the emergence of MYB44, a result of the amalgamation of acyl-homoserine lactones (AHLs) and G-protein coupled receptor (GPCR), has the potential to stimulate and spur the growth of roots. The culmination of these discoveries alludes to an unwavering alignment amidst PGPR and plant communication pathways. This alignment will undeniably be pivotal in safeguarding plant fitness and amplifying fortification mechanisms against phytopathogens and environmental adversities. The depiction of the crossing point between rhizobia and plant signaling pathways is vividly portrayed in Figure 3. Nitric oxide (NO) acts as a connecting agent between the PGPR and the host plant’s signaling, according to a number of thorough research studies. In the presence of oxygen, bacterial nitric oxide synthase (bNOS), which is found in certain rhizobacteria, can catalyze the conversion of l-arginine to l-citrulline, producing NO [60]. Anaerobic denitrification, which is carried out by bacteria that are free to move around, is another route for nitric oxide to be produced. The nitrate reductase, nitrite reductase, NO reductase, and N2O reductase enzymes work together to convert nitrate (NO3) to nitrogen [60]. The process of denitrification can also take place in aerobic conditions by utilizing periplasmic nitrate reductase (Nap) rather than the usual membrane-bound respiratory nitrate reductase (Nar). Heterotrophic nitrification is a distinct method of bacterial NO production that converts ammonia into hydroxylamine (NH₂OH), NO₂, and NO₃. NO, a lipophilic diffusible bioactive molecule, has the capability to participate in numerous signaling pathways, including those that are elicited by stress and those that are implicated in development [60].

Figure 3.

The interplay between rhizobacteria and the host plant organism gives rise to an array of advantageous physiological pathways via intricate signaling mechanisms (it belongs to the authors study).

6. Conclusion

The utilization of PGPR denotes a collection of bacterial strains that are employed to alleviate stress reactions in plants. Novel research on hormones, functioning as signaling molecules in plant-rhizobacteria interactions and overseeing biotic and abiotic stress, unlocks fresh pathways for investigating the advantageous associations in plants. Future investigations hold the potential to unravel a plethora of knowledge regarding the intricate molecular pathways that are identified by the realms of molecular biology and proteomic analyses concerning the perception of plant hormones. These investigations may shed light on the significant influence of phytohormones on plants’ reactions to biotic and abiotic stresses. In the realm of environmental agriculture, there exist numerous genetic modification techniques with recognizable efficacy. The CRISPR-Cas genome editing, for instance, is a viable option for crops and commercial biotech goods pertinent to PGPR, albeit facing several challenges. However, by fostering mutually advantageous collaboration between industry and research, one can leverage omics-based genetic tools to enhance PGPR and effectively meet the pressing need for sustainable biofertilizers.