Multifaceted Impacts of Plant-Beneficial Pseudomonas spp. in Managing Various Plant Diseases and Crop Yield Improvement

The modern agricultural system has issues with the reduction of agricultural productivity due to a wide range of abiotic and biotic stresses. It is also expected that in the future the entire world population may rapidly increase and will surely demand more food. Farmers now utilize a massive quantity of synthetic fertilizers and pesticides for disease management and to increase food production. These synthetic fertilizers badly affect the environment, the texture of the soil, plant productivity, and human health. However, agricultural safety and sustainability depend on an ecofriendly and inexpensive biological application. In contrast to synthetic fertilizers, soil inoculation with plant-growth-promoting rhizobacteria (PGPR) is one of the excellent alternative options. In this regard, we focused on the best PGPR genera, Pseudomonas, which exists in the rhizosphere as well as inside the plant’s body and plays a role in sustainable agriculture. Many Pseudomonas spp. control plant pathogens and play an effective role in disease management through direct and indirect mechanisms. Pseudomonas spp. fix the amount of atmospheric nitrogen, solubilize phosphorus and potassium, and also produce phytohormones, lytic enzymes, volatile organic compounds, antibiotics, and secondary metabolites during stress conditions. These compounds stimulate plant growth by inducing systemic resistance and by inhibiting the growth of pathogens. Furthermore, pseudomonads also protect plants during different stress conditions like heavy metal pollution, osmosis, temperature, oxidative stress, etc. Now, several Pseudomonas-based commercial biological control products have been promoted and marketed, but there are a few limitations that hinder the development of this technology for extensive usage in agricultural systems. The variability among the members of Pseudomonas spp. draws attention to the huge research interest in this genus. There is a need to explore the potential of native Pseudomonas spp. as biocontrol agents and to use them in biopesticide development to support sustainable agriculture.


INTRODUCTION
Agriculture is very important for animal and human food in the world, 1 but a large number of crop losses occur each year due to pathogen invasion, which involves a wide variety of pathogens ranging from viruses to prokaryotic bacteria, nematodes, eukaryotic fungi, and oomycetes. 2,3 On the other hand, it is predicted that the world's population will reach near 9 billion in 2050, which would generate more burden on food production, space, and the environment. 4 Since the beginning of the agricultural system, people also have been fighting against the various plant diseases, which significantly has led to the deployment and invention of synthetic fertilizers used to improve crop yield and control disease. These synthetic fertilizers pose a severe threat to the soil biota by disturbing environmental nutrient cycles, disrupting the biological communities existing in the environment, and having adverse health consequences. 5 The environmental protection and health issues of these harmful chemical materials have increased the necessity of searching for substitutions to control plant diseases and pests. 6 The application of biocontrol plantgrowth-promoting rhizobacteria (PGPR) has been proven to be ecofriendly and cost-effective and has a significant role in managing various diseases, as well as improving plant growth and productivity. 7 One group of very significant bacteria that has become a focal point of research on biological control of plant diseases is the genus Pseudomonas (ubiquitous, rod-shaped γ-proteobacteria and Gram-negative bacteria having polar flagella). 8 Using multilocus sequencing techniques, it has been estimated that the Pseudomonas genera is comprised of more than 100 species, including groups and subgroups. 9 The genome size of Pseudomonas spp. usually varies from 4.6−7.1 Mb and possesses 57.8−66.6% GC content with 4237−6396 expected genes. 10 Pseudomonas spp. has valuable applications in the biotechnology, biological control, bioremediation, and plant growth promotion (PGP). 11,12 The Pseudomonas genus is mostly used as a crop inoculant because of its abundant occurrence and versatile metabolic ability, which promote plant growth in many ways, including ACC deaminase activity, nutrient uptake, and antioxidant activities. 13 The particular strains of Pseudomonas f luorescens have been used as seed inoculants on various crop plants to stimulate growth parameters and enhance crop output. These bacterial agents quickly inhabited roots of potato, radish, and sugar beet, which significantly increased the plant yield. 14 P. f luorescens may assist in stimulating cabbage growth-related processes, especially by promoting seedlings' rapid growth and reducing transplant shock. 15 Among the nutrients, nitrogen (N) plays an important role in the growth and productivity of the plants. 16 Soil microbes transform atmospheric nitrogen into ammonia using enzyme nitrogenase. The members of Pseudomonas spp. have a reported nitrogen fixation ability. 17 Similarly, phosphorus is the second most important nutrient for plant development and growth, but it is present in an unavailable form. Microbes associated with rice, wheat, maize, and legume crops are mainly reported with a P-solubilization ability, while some others belong to Burkholderia, Enterobacter, Halolamina, Pantoea, Pseudomonas, Citrobacter, and Azotobacter are also described. 18 Along with this, potassium is also required for the development and establishment of plants. Various studies have highlighted the role of Pseudomonas spp. in solubilizing the complex soil minerals including potassium aluminum silicate. 19,20 P. f luorescens and Pseudomonas aeruginosa are the Gramnegative bacteria that produce hydrogen cyanide (HCN), a secondary metabolite involved in disease inhibition. 21 The antagonistic potential of P. f luorescens is exacerbated by the formation of HCN, which is promoted by iron availability in wet, O 2 -depleted soils. Iron is found in an abundant form but is not available to the plants 22 and microbes. To get iron for growth and development, some bacteria manufacture low-molecular-weight iron complexes known as siderophores, 27 which are helpful in limiting the phytopathogens. 23 In the rhizosphere, bacteria produce phytohormones such as auxin, gibberellins, abscisic acid, ethylene, and cytokinins that stimulate plant growth. 24 Similarly, antibiotics are smallmolecule toxins that may kill or resist the development of other microorganisms. 25 Several rhizospheric bacteria have also been shown to produce antibiotics as well as toxins 26 including amphisine, phenazine, 2−4-diacetylphloroglucinol, pioluteorina, pyrrolonitrin, hydrogen cyanide (HCN), oomycine, polymyxin, circulin, colistin, tensin, tropolone, and cyclic lipopeptides. 27 Pseudomonas spp. usually produce antibiotics that indirectly support plant growth stimulation. 28 Various studies have highlighted the active role of Pseudomonas spp. in suppressing various fungal pathogens, 29−31 inducing systemic resistance, 32 and producing gibberellic acid (GA) and jasmonic acid (JA) signals. 33 Pseudomonas spp. are extensively studied in disease management strategies. The most common compounds that take part in biocontrol mechanism include phenazine-1-carboxamide, lipopeptides, 2, 4-diacetylphloroglucinol (DAPG), pyrrolnitrin, pyoluteorin, and phenazine-1-carboxylic acid (PCA). 11 Pseudomonas strains survive well in stress conditions and are very beneficial in the control of several diseases triggered by fungal phytopathogens, 43 mainly damping-off disease caused by Pythium spp., 34 Fusarium solani causing okra root rot, 35 Rhizoctonia solani associated with Rhizoctonia root-rot, 36 and foliage blight disease caused by Phytophthora nicotianae. 37 Plants' ability to defend themselves is strengthened through priming as an adaptive technique. Activation of the induced defense mechanism is one of this phenomenon's characteristics. Pathogens, arthropods, and abiotic signals operate as warning signs that create a prime. The plant may experience changes at the physiological, transcriptional, metabolic, and epigenetic levels in response to the stimulation. Priming functions as a kind of immunological memory for the plant because it can persist a long time, be maintained throughout the plant's life cycle, and be passed down to next generations. 38 The genus Pseudomonas holds complex enzymatic system and produces hydrolytic enzymes that play a vital role in the suppression of various plant diseases. 11 Some important members of Pseudomonas, including P. aeruginosa, P. f luorescens, and Pseudomonas stutzeri, secrete chitinases and are considered as good biocontrol agents. Pseudomonas putida produces various mycolytic enzymes, including cellulase, lipase, protease, chitinase, and amylase, and develops innate resistance against Fusarium wilt disease in Solanum lycopersicum. 39 Pseudomonads also secrete specific biosurfactants, especially biological rhamnolipids (RLs). 40 RLs have antimicrobial activities against plant pathogens, control a variety of plant disease by stimulating plants defense response, 41 and also possess efficient remediation abilities. 42 As RLs breakdown the plasma membrane of fungal zoospores, the use of rhamnolipidproducing Pseudomonas spp. could help to manage damping-off disease in chilli and tomato plants. 43 They also stimulate the immune system and develop local resistance in grapeseed against Botrytis cinerea and Leptosphaeria maculans. 44 Pseudomonads aid plants growing under diverse abiotic pressure by producing exopolysaccharides. 45 For example, Pseudomonas azotoformans and Pseudomonas argentinensis are reported to reduce the saline stress in the Brassica juncea, improve yields, and could be used as a good biofertilizer. 46 Pseudomonas aeruginosa has been reported to enhance the growth of Vigna radiata (mung beans) plants under drought conditions, 47 while Pseudomonas poae and P. f luorescens species increased the number and plant biomass of petunia flower under drought and low-nutrient conditions. 48 Some Pseudomonas spp. have also been promoted as commercial products of plant growth promotion (PGP) and biological control, such as biopesticides and biofertilizers. 49 The Bio Save 10 LP and Bio Save 11 LP products obtained from Pseudomonas syringae strain ESC-10 are employed for the control of postharvest diseases of citrus fruit, apple, potatoes and stone fruits. Similarly, P. f luorescens A506 is used to make Blight Ban A506, which is registered as a biopesticide for the suppression of frost damage on pome fruits, peach, cherry, potato, strawberry, almond, and tomato. Blight Ban A506 is also used for the suppression of fire blight. 50 Thus, multifaceted impacts have created a need to explore the potential of locally occurring Pseudomonas spp. against broad range of phytopathogens and bacteria-based bioproduct development.

THE CURRENT SITUATION OF PLANT DISEASES
Plant pathology faces lots of ever-growing challenges. In the present era, the crop yield has become challenged because of threats from plant diseases and the huge application of synthetic fertilizers. Crop productivity declines due to high plant disease rates that are often between 21 to 30% worldwide in some of the most important crops. 50 Achieving the basic requirements of a rising population with little resources and without harming the atmosphere is biggest challenge for researchers and farmers. 51 Ray and co-workers reported that depleting arable lands and less available resources decrease the ability the agricultural yield to flourish. 52 Moreover, intensification, monocultures, and other high resource inputs (water, pesticides, and chemical fertilizer) in agriculture aim to attain maximum plant yield as the sole target, thus facilitating the progression of many plant diseases worldwide. 53 Synthetic pesticides do not decompose into simpler and harmless elements, and ultimately they persist in the soil as lethal remains often associated with health problems in humans. 54 Lucas suggested that several phytopathogens have developed resistance to chemical pesticides that have been in usage for a long period of time. 55 Therefore, few plant diseases of economic significance have become very problematic to handle due to depleting essential resources; 56 food market globalization and intensive crop production practices have worsened the condition. 57 The rapidly increasing demand for substitutions to chemical fertilizers has given chances for the use of biocontrol practices. 58 Providing adequate, nontoxic, and healthy food for society is always the most essential task of plant disease management. Therefore, plant disease management must ensure a good food supply by increasing the agricultural yield, decreasing food pollution, and also providing a variety of foods at reasonable prices. 59 Furthermore, raising the public awareness about environment and human health concerns associated with man-made toxic chemicals is also causing a shift toward the highly sustainable management applications that rely less on artificial fertilizers and have no bad effects on the environment and natural resources. 60 However, the utilization of agrarian products as phyto-stimulators, biocontrol agents, and biofertilizers with some suitable crop management practices is a fantastic choice in sustainable agricultural applications due to its environmentally safe nature. In this context, an effective and ecofriendly strategy is to use Pseudomonas spp. as one of the effective choices for the improvement of the plant growth and disease suppression to support sustainable agriculture.
with the passage of time; the newest molecular diversity analysis (MDA) represents 144 species and remains as the largest group of microbes. 64 However, the widespread occurrence of Pseudomonas spp. shows their adaptability through the physiological, environmental, and molecular diversity. 65 The phylogenetic relatedness of Pseudomonas strains within the genus can be determined through the sequence analysis of conserved genes. 66 According to Hofte and Altier, the biocontrol abilities of bacteria are straindependent and cannot link with phylogenetic variations. 67 They directly act on the physiology, growth, and nutritional status of the plant they inhabit. Pseudomonas spp. also has the potential to withstand the high temperature up to 35−38°C. 68 Numerous Pseudomonas spp. including P. aeruginosa, Pseudomonas chlororaphis, P. putida, and P. f luorescens are well-known for their capacity to improve plant growth and lessen a variety of plant diseases. 69 These are extensively found to be associated with the roots of plants and give advantage to plants by fending off plant pathogens. Several traits of Pseudomonas spp. enable them to serve as plant growth promoters and biocontrol agents. These traits include aggressive rhizosphere competence, quick colonization, formation of several root exudates, bioactive substances (such as vitamins, VOCs, siderophores, and antibiotics), and stress responses 70 (Figure 1). In case of P. f luorescens, genes of the tryptophan synthase-α chain (trpA) and tryptophan synthase-β chain (trpB) generate indole as an intermediate product, which via the expression of tryptophan 2-monooxygenase (iaaM) catalyzes indole-3-acetamide to IAA using iaaH, 71 increasing plant growth. Some plant hormones, such as indole acetic acid (IAA), cytokinins, gibberellins, and ethylene production inhibitors are produced by fluorescent Pseudomonas, which help to improve the plant roots capability to absorb water and certain nutrients. 72

PSEUDOMONAS SPP. AS PLANT-GROWTH-PROMOTING RHIZOBACTERIA
Plant-growth-promoting rhizobacteria (PGPR) are beneficial rhizospheric microbes that are free-living and engaged in promoting plant growth and development. PGPR inhabit the rhizosphere and roots of the plant, growing in the spaces between the root hairs and rhizodermal tissues. 73 PGPR are not only linked with the root to have advantageous impacts on plant growth and development but are also responsible for the obliteration of plant pathogenic microorganisms. These bacteria are being used at the field level and have long been well-known to increase agricultural yield in normal and stressed soils due to their ecofriendly nature. 74 They enhance the plant growth through establishing induced systemic resistance (ISR), competitive omission, and antibiosis and defending the plants against biotic agents. 75 They raise the level of antioxidant enzyme and protecting the plant cells from an oxidative stress. 76 Thus, PGPR act as one of the most significant ingredients in the formulation of biofertilizers. The rhizosphere is a soil zone under the influence of plant roots that is a challenging and heterogeneous environment shaped by plant rhizo-deposition 77 and colonized by many microorganisms, including plant-associated Pseudomonas spp. Pseudomonas spp. are very ubiquitous microbes in the rhizospheric zone that possess the characteristics to be wellsuited to thePGPR category ( Figure. 1). 78 Pseudomonas spp. have various PGPR qualities, including (i) fast in vitro growth chickpea produced siderophores and IAA and improved plant growth, nodulation, chlorophyll content, and leghemoglobin and the capacity for biomass production; (ii) colonizing and dominating in the spermosphere, rhizosphere, and inside the plant's body; (iii) the production of bioactive metabolites (antibiotics, growth-promoting substances, siderophores, and volatiles); (iv) quick use of seed and root exudates; (v) violent competition with other microorganisms; and (vi) an environmental stress resistance ability. Due to the production of the above stated compounds, they are responsible for the plant growth promotion and innate resistance of some soils to toxic pathogens. 79 It is well proven that the plant growth is enhanced through the application of these beneficial bacterial populations (Table  1). The P. fluorescence (FLPs) strain was used as an inoculant in the combination of microbial rich fertilizer to improve the growth parameters and yield of chickpea plants. 80 Similarly, P. aeruginosa isolated from rhizospheric soil, shoots, and roots of sugar cane provides a remarkable improvement in fresh and dry mass of sugar cane. 62 Specific strains of P. f luorescens and P. putida have been used as seed inoculants on crop plants to stimulate their growth and enhance crop yields. P. aeruginosa was utilized in along with B. subtilis and T. harzianum to combat Sclerotinia sclerotiorum, leading to plant health improvement by eliciting systemic resistance and proteome level changes. 81 P. azotoformans considerably enhanced plant nutrient (calcium, manganese and potassium) uptake in contrast to the mutant strain of P. azotoformans. 82

Biological Nitrogen Fixation.
The total nitrogen present in the atmosphere is 78%, but still it is not available to plants. One of the most efficient and ecofriendly ways to increase crop plant growth and productivity has proven to be the employment of N 2 -fixing microorganisms as biofertilizers. For sustainable agriculture and high yield, the use of biological nitrogen fixation is the best alternative to chemical fertilizers. Rhizobacteria are useful for fixing the nitrogen and make it obtainable to the plants. 107 However, an enzyme nitrogenase is used by PGPR to convert nitrogen into ammonia, which is the usable form for the plants. 108 The symbiotic bacteria, for example, rhizobia, and nonsymbiotic bacteria, for example, Azocarus (Anabaena, Nostoc), Gluconacetobacter diazotrophicus, Azotobacter, Azospirillum, cyanobacteria, etc., fix the atmospheric nitrogen into an available form. 109 In 1955, Anderson reported the nitrogen fixation by the genus Pseudomonas. 110 The structure of the gene that produced the nitrogenase enzyme and the optimal condition at which it is useful for nitrogen fixation were investigated using P. stutzeri, which was isolated from Chinese rice fields. 111 P. f luorescens, as well as fluorescent pseudomonads, were reported to stimulate nodulation in a chickpea plant. 112

Solubilization of P.
Phosphorus is the most important macronutrient after N for the growth and development of plants. A higher amount of P is present in an insoluble form in the soil but not available to plants. Monobasic and dibasic are the only forms in which P is available to plants. 108 P-based fertilizers are frequently used to overcome the deficiency of P and to enhance the crop production. 113 Microbes in the rhizosphere that convert the insoluble forms of P into available forms are usually known as orthophosphates. The use of these P-solubilizing bacteria (PSB) plays significant role in making P available to the plants. The P-solubilizing bacteria can dissolve the insoluble P by producing organic acids with low molecular weights. Several bacteria produced citric and gluconic acid. 153 Oteino reported that bacterial genera including Bacillus, Enterobacter, Rhodococcus, Mesorhizobium, Flavobacterium, Rhizobium, Pseudomonas, Erwinia, Burkholderia, Microbacterium, Beijerinckia, Serratia, and Arthrobacter can solubilize phosphates. 114 5.3. Production of HCN. HCN is a volatile compound that has antibacterial 115 as well as antifungal characteristics. 116 HCN is a very toxic compound used against phytopathogens. To inhibit phytopathogens and increase yield, most of the PGPR produced HCN. 117 HCN production is the trait of P. aeruginosa that is already well-known in disease inhibition. 118 A secondary metabolite, HCN produced by Pseudomonas entomophila, and entomopathogenic bacteria, has been linked to biological control and pathogenesis in many other bacteria. 119 In a recent study, HCN-producing fluorescent Pseudomonas and non-HCN-producing Bacillus licheniformis in a mixed culture were reported to improve the vegetative growth and photosynthetic parameters in a wheat crop. 120 5.4. Production of Siderophore. Iron is found in its native state as ferric ions and Fe +3 , which are slightly soluble; therefore, these forms are used neither by plants nor by microorganisms. Some extracellular compounds have been identified to be secreted by PGPR, usually known as siderophores. Kumar and co-researchers reported that siderophores are small iron-containing molecules with a high affinity for chelating chemicals released by the plant and microorganisms, acting as highly soluble Fe 3+ -binding chelating agents. 113 David et al. reported that siderophores producing Pseudomonas spp. are important for boosting plant growth and combating a variety of plant diseases. 121 Because of its function in both biological disease control and plant pathogen virulence, siderophore formation by plant-associated microbes is of much interest. The production of siderophores that takes place by bacteria like Pseudomonas produces pyoveridins; Agrobacterium tumefaciens, Erwinia chrysanthemi, and Enterobacteriacea generate catechols; Erwinia carotovora, Enterobacter cloacae, and other fungi make hydroxamates; and Rhizobium meliloti produces rhizobactin. 122 5.5. Phytohormone Production. Phytohormones are very essential for plant development. Most important hormones are gibberellins, indole-3-acetic acid (IAA), and cytokinins. The most prevalent and well-studied auxin, IAA, has a crucial role in the differentiation and development of tubers and seeds, as well as cell division. It also controls vegetative development and initiates the production of the lateral and adventitious root. 123 Auxin is produced by different bacterial genera like Pseudomonas, Xanthomonas, and Rhizobium, as well as Bacillus cereus, Enterobacter cloacae, Bradyrhizobium japonicum, Serratia marcescens, Burkholderia, Azotobacter, Alcaligenes faecalis, and Mycobacterium sp., etc., which help in plant growth and development. Similar to this, cytokinins play an essential role in cell enlargement, cell expansion, and cell division. 124 Gibberellin changes the morphology of plants, especially in stem tissues, and extension of cells takes place. Ethylene hormone is in gaseous form and is usually known as a wounding hormone because any physical or chemical perturbation is the reason for its production. The root growth is inhibited by the production of ethylene. Therefore, PGPR play an important in reducing ethylene production by mediating the phytohormones, maintaining ion homeostasis, and regulating the expression of stress-responsive genes. 125 5.6. Production of Antibiotics. Many distinct types of microbes produce toxic compounds that are hazardous to other pathogen microbes. However, it is advantageous to plant growth and development. These are antibacterial or microbial toxins that can be harmful to or eliminate other bacteria even in low quantities. 126 Kumar et al. evaluated that DAPG acts as an antibiotic toward fungus, bacteria, helminths, and other parasites because Pseudomonas spp. synthesize pyrrolnitrinis. 123 Pseudomonas spp. produce DAPG, which is involved in the inhibition of phytopathogens. A strong antibiotic phenazine of P. f luorescens has been utilized to combat all G. gramini infections of wheat. 127 Fluorescent Pseudomonas produces antibiotics and acts as a biocontrol agent. 157 Pioluteorine, phenazine-1-carboxylic acid 2,4-diacetylphloroglucinol (Phl), and pyocyanin are some of the potential antibiotics synthesized by P. fluorescens. 128 These antibiotics increase plant growth, suppress phytopathogens, and also enhance the soil fertility ( Figure 2).

INDIRECT MECHANISMS
6.1. Antifungal Activity. PGPR stimulate the plant development, which prevents the spread of phytopathogens. 129 Bacteria produce antifungal antibiotics, which help in the growth inhibition of pathogenic fungi. For example, P. f luorescens produces 2, 4-diacetylphloroglucinol. Some PGPR breakdown the fusaric acid produced by the Fusarium spp. associated with wilt disease and therefore prevent the pathogenesis. Pseudomonas stutzeri produces enzymes that can break down the cells of Fusarium solani. Recently, P. f luorescens was suggested as biocontrol agents because they protect plants against several fungal diseases, like black root-rot of tobacco, root-rot of wheat, and root-rot of a pea, etc. In a recent study, P. aeruginosa was characterized to produce certain metabolites, bioactive compounds, and antifungal compounds like 2,4-diacetylphloroglucinol (2,4-DAPG), pyoluteorin, and pyrrolnitrin that make this bacteria effective against a broad range of phytopathogenic fungal agents. 130 Similarly, P. stutzeri isolated from Withania somnifera seeds was found effective in the suppression of the mycelial growth of F. oxysporum var. ciceri and R. solani. 73 6.2. Induced Systemic Resistance (ISR). In the rhizosphere of plants, beneficial microbes induce systematic resistance as a defense in response to pathogenic attacks. 131 Microbes assist in structural changes that promote defense mechanisms via the accumulation of biochemical and phenolic compounds. 132 In a study, decreased susceptibility to wilt disease in carnation plants and foliar disease in cucumber due to the induction of systematic resistance was observed. 133  Pseudomonas strains also assist in inducing systematic resistance. Pseudomonas spp. like P. putida and P. f luorescens protect against pathogens in plants like tomato, sugar cane, and oak. 134 ISR induction on plants by PGPR result in substantial modifications in the plants structure and functions that lead to resistance against invading pathogens. 135 6.3. Rhizoremediation and Stress Control. Islam and co-researchers discussed that plants are frequently exposed to several stresses both abiotic and biotic that ultimately have an impact on plant development and survival. 136 Plants and microbes are potentially used in phytoremediation to eliminate harmful metals efficiently and economically from the polluted environment. 137 Heavy metals (HMs) are harmful to plant growth 138 and establishment, and certain microbes detoxify them. When the microbes are added to the contaminated soils, they lower the deleterious effects of metals on plants growth. A recent study has proved that the inoculation of Sedum alfredii with P. f luorescens improved the net photosynthetic rates, intercellular CO 2 concentration, transpiration rate, and stomatal conductance and upregulated the photosynthetic genes, which promote both the growth and Cd uptake ability of the plant. 139 Similarly, application of copper-tolerant siderophore-and ammonia-producing P. lurida on Helianthus annuus significantly improved the growth and phytoremediation ability of plants grown under Cu contaminated soil. 140 Pseudomonas spp. are also reported to enhance plant growth and improve the ability of the plants to survive under drought conditions, 141 heat shock, 142 and high salinity. 143 The direct and indirect modes of action of PGPR is presented in Figure 3.

Role of Pseudomonas Species in Plant Disease
Management. According to current estimates, a significant amount of food products and crops are lost each year due to various diseases caused by pathogens. These plant diseases are the major factor affecting food production and human societal development. 144 Generally, cereal crops are more commonly affected by soil-borne diseases. For plant disease control, the application of biological control agents has been recognized as an effective method. 145 The efficient control of diseases with the help of PGPR has been testified in many crop plants. 146 PGPR as biocontrol agents are reported to have a remarkable ability to increase crop production and tolerate abiotic and biotic stresses. 147 The term biocontrol is often employed for managing plant diseases that occur during the storage of food and at various plant growth stages. The studies on biocontrol by rhizobacteria are usually focused on controlling many pathogenic microbes. 148 Numerous bacterial isolates are known that act as biocontrol agents. Among several biocontrol agent, Pseudomonas spp. have been broadly studied as more dominant bacteria possessing the potential to defend plants against  (Table 2) 149 and promote plant growth under stress conditions. 150 Pseudomonas bacteria has ability to lessen the disease occurrence by developing inducible plant defense mechanisms, which make plants more resistant against invading pathogens. 151 Bacterial strains prevent pathogenic growth using various mechanisms such as by secreting the antibiotics and HCN; producing cell-walldegrading enzymes like β-1,3-glucanase, chitinase, toxins, biosurfactants, and important metabolites; and by competing for minerals. 152 The earliest known mechanism of biocontrol includes the production of beneficial compounds like siderophores, which proficiently sequester Fe (iron) and kill pathogens. 153 Disease control properties are also associated with various secondary metabolites coded by genes present in cores and flexible parts of the bacterial genome. 154 Through genome sequencing, it is predicted that approximately 6% of the genome of P. f luorescens Pf-5 secretes secondary metabolites, which indicates the direct link between secondary metabolite production and genetic elements 155 The biocontrol ability of Pseudomonas spp. has been reported in some important plants (tomato, wheat, and chickpea). 156 Rhizobacteria produce a variety of secondary metabolites, antibiotics, and bioactive compounds that make them excellent biocontrol agents. A research study has reported the antimicrobial traits of a siderophore-producing Pseudomonas strain against Rhizoctonia solani and Sclerotium rolfsii. 157 P. f luorescens produced 2,4-DAPG, which controls and manages take-all disease in Triticum aestivum. 158 Orozco-Mosqueda et al. showed that the trehalose production by the Pseudomonas UW4 played a significantly role in protecting tomato plants against salt stress. 159 Hydrogen cyanideproducing P. chlororaphis exhibits nematocidal activity against Meloidogyne hapla nematodes by decreasing the number of galls in tomato plants and by killing juveniles both in vitro and inside the plants. 160 A P. putida strain was revealed to decrease the damage instigated by Pythium ultimum on tomatoes and also increase the growth of the infected plants 161,199 . Many other recent studies have highlighted the beneficial impacts of Pseudomonas spp. on improving the plant growth by suppressing the colonization of phytopathogens. 43,162,163

PRODUCTION AND EFFECTS OF BACTERIAL VOLATILES
Low-molecular-weight volatile organic substances promote plant development. 190 The importance of volatile organic compounds (VOCs) in inducing resistance in host plants is comparable to the function they play in insect resistance. 43 Microorganisms interact with plants in distinctive ways. 191 Volatile natural compounds of microbial origin possess antimicrobial properties, promote plant growth, and serve as alerts by producing a variety of signaling molecules. 192 Most research on the consequences of bacterial fluctuations on flora have been accomplished thought the use of a bifurcated Petri dish, where in the two compartments are separated through a plastic rim. This permits the exchange of volatile compounds while also preventing the spread of nonvolatile metabolites via the medium. In addition to CO 2 , generation that probably contributes fairly to growth promotion was discovered in closed Petri dishes. 193 The use of P. chlororaphis showed that butanediol no longer only promotes plant development but also induces systemic resistance and drought tolerance 194 Intriguingly, beneficial microbes are well-known to produce a variety of dynamic volatiles that protect the plants from phytopathogens, and gas chromatography−mass spectrometry (GC-MS) has helped in profiling the microbial volatile organic compounds (VOCs). A recent study has highlighted the potential role of microbial volatile organic compounds (VOCs) produced by endophytic P. putida associated with black pepper in suppressing the mycelial growth of various fungal pathogens. 195 Similar to this, the volatile profile of P. chlororaphis subsp. Aureofaciens revealed that 3-methyl-1butanol, phenylethyl alcohol, and 2-methyl-1-butanol were the most abundant VOCs, which showed strong inhibition toward C. f imbriata associated with black rot disease in sweet potato. 196 SEM analysis has revealed that VOCs affect the morphological and structural features of B. cinerea and thus suppress fungal growth. 197

REGULATION OF SECONDARY METABOLISM AND BIOCONTROL
The two-component system GacS/GacA in Pseudomonads is essential for controlling how biological control components are expressed in a variety of pseudomonads. 38 The GacS sensor kinase is activated by unknown inputs, and the phosphorus relay then activates the GacA response regulator. When cells attain large population densities, positively activated GacA regulates the transcription of short regulatory RNAs. These short ribonucleic acids attach to distinct proteins, RsmA and RsmE, and by activating specific genes they relieve translational repression. The GacS/GacA system regulates 241 genes in P. aeruginosa, with nearly whole overlap with genes under the control of small regulatory RNA. 198 A competitive nonpathogenic root colonizer, P. chlororaphis strain O6, served as another example of the GacS/GacA signaling pathway's regulatory strength. As advantageous features, the O6 strain generates several secondary metabolites, like phenazines, formonitrile (HCN), siderophore, and pyrrolnitrin. Abiotic stresses including drought and salt as well as P. chlororaphis induced systemic resistance in roots through colonization, 199 the induction of triggered systemic resistance 200 via the upregulation of the volatile 2,3 butanediol, 160 and the expression of RpoS and, therefore, different genes structured in this opportunity σ-factor. A recent past study has revealed the pathways through which beneficial bacterial metabolites confer protection against pests and pathogens. During the root colonization, metabolites are produced that are controlled by the global regulatory Gac/Rsm signaling cascade, which is linked with the other regulatory pathways. The Gac/Rsm regulation aids in expressing the genes that encode these beneficial traits in P. chlororaphis. 201 It is interesting to note that 4-carbamoyl acetic acid, another systemic resistance inducer in plants, is not always under GacS regulation. 106 Proteomic analyzes of the characteristic of regulatory mutants were used to find out which pathways are coregulated. Recent studies on GacS variants revealed new proteins with as yet poorly defined functions in bioregulation. One of these proteins, PspB, is a serine protease that is connected to the DNA repair proteins that frame shifting and aggregation (a putative single-strand binding protein, Ssb, and recombinationrelated protein, RdgC); isoprenoid production including synthetic proteins, GATase1 ES1, and glucose 1 thymidylyl phosphate transferase; RmlA, probably concerned in hairrelated proteins; polyketide metabolism; and cell membrane organization (CpaC and LysM mandatory peptidoglycans), also in outer membrane protein (OprF). 202 These findings have highlighted the intricacy of the regulons involved in the appearance of the biological regulation phenotypes within a single agent. Other important regulators are also expected to exist in Pseudomonads spp. and other bacteria with efficient biological control traits. 203

PLANT METABOLISM INTERACTION INDUCES RESISTANCE AND PRIMING
Plants depend on immunogenicity to defend themselves against pathogenic attack. This acquired immunity completely depends on and encourages protective reactions. Preformed defenses are nonspecific and include structural barriers like the cell wall and cytoskeleton that keep viruses and parasites at bay, as well as chemicals with effects. 204 The pathogen's surface pathogen-associated molecular patterns (PAMPs) or proteins (effectors) that the pathogen has translocated inside the host cell are recognized to activate the induced defenses. 205 For example, in the case of microbial infection, plant immune response in the form of hypersensitive cell death is an important event to prevent the spread of attacking pathogens, 206 the production of free radicals, 207 plasma membrane defenses, 208 and phytoalexin synthesis. 209 On the other hand, the next opportunity in the protection response process is the transcription of pathogenesis response proteins 210 and a kind of planned cell death (also known as hypersensitivity reaction (HR), which restricts the microbial ability to spread). 211 For a long time, it is been counseled that the function of basic metabolism for the duration of plant pathogen relation is to aid the cell energy demand for plants. Because of the amount of gene production from various defense pathways, energy is required when carrying out the plant protection responses. 212 Additionally, defensive reactions seem to manipulate health value because, like in Arabidopsis, plants that integrate specific protection reactions are undersized and feature decreased fertility compared to mutant plants with deficient protective signaling pathways, which have higher fertility. 213 Anyhow, it seems that in order to create a strength level that is favorable to defense, the up-regulation of the associated protection pathways is compensated with the aid of using the down-regulation of the genes involved in different defense-associated pathways. Supporting this idea was the fact that genes concerned in photosynthesis and chlorophyll production are down -regulated in response to pathogenderived elicitors, both pathogenic and nonpathogenic infections, and both virulent and avirulent infections. 214 Utilizing fluorescence imaging of chlorophyll in special interactions between plant microbes, changes in photosynthesis have been reported regionally at the site of infection and in surrounding tissues and decreases in photosynthesis became quicker and greater after inoculation with an a virulent strain. 215 However, because light responses all through photosynthesis produce chloroplast ROS that may be utilized for protection responses, the overregulation of photosynthesis is unreasonable. 216 There is no experimental proof to explain why it occurs. However, two potential pathways have been put forward: (1) photosynthesis suppression brought on by pathogenic effectors 217 and (2) sugar signal moderation causes regulation.
In any way of the mechanism, it is likely that downregulating photosynthesis reduces the amount of energy required when other mechanisms that provide this energy are up-regulated. For instance, boosting the activity of cell wall invertase, respiratory metabolism, and glucose transporters can produce energy. 218 Formation of plant secondary metabolites like phytoalexins and the expression of protection-associated genes can both be further enhanced by this metabolic switch from supply to sink. 219 Protection tactics for dangerous surroundings have a reasonable cost. The epigenetic law of priming could permit plants to shield their offspring from recurrent biotic stress instead of undergoing costly irreversible genetic fixation of the trait and its associated costs. More information on this process will open possibilities to enhance sickness resistance in agricultural crops. In order to protect crops and reduce the use of pesticides and other harmful chemicals to treat diseases, it is vital to create sustainable methods of pest and disease control in the twenty-first century. 220

LYTIC ENZYMES AND PLANT DISEASE CONTROL
The one of the best biocontrol mechanisms used by Pseudomonas spp. to eradicate soil-borne pathogens is the formation of cell-wall-degrading enzymes (Table 3). 221 Hydrolytic enzymes that break down the cell wall include of β-1,3glucanase, protease, cellulose, and chitinase produced by biostimulant strains of PGPR, and they directly suppress the  222 Chitinase and β-1,3-glucanase activity have the potential to substantially destroy the chitin chain, which is an insoluble linear polymer of α-1, 4-Nacetylglucosamine. 223 Within the Pseudomonas genus, certain species secrete β-1,3 glucanases that contribute to the destruction of many pathogens: in P. cepacia, the β-1,3 glucanase reduces the disease incited by the toxic pathogens, e.g., S. rolfsii, P. ultimum, and R. solani. 223 Pseudomonasstrain K32 is HM-resistant, and its biological control ability is verified against six Oryza sativa fungal pathogens: Paecilomyce ssp., A. parasiticus, A. f lavu-s6Cladosporium herbarum6Alternaria alternate, and Rhizopus stolonifer. The inhibitory activity was ascribed to the production of β-1,3-glucanase, chitinase, and protease enzymes. 224 Apart from showing the formation of β-glucanases and chitinase enzymes, Pseudomonas spp. obstruct the proliferation of R. solani and P. capsici, which are the most dangerous crop pathogens across the globe. 225 In P. chlororaphis, quorum sensing (QS) regulates the exoenzyme production, antifungal substances, and biofilm formation. 226 QS is a cell-to-cell communication system that depends on cell density that regulates bacterial gene expression through signaling molecules (autoinducers), such as biofilm formation, bacterial virulence, and pigment production 227 Saritha et al. showed that P. putida discharges various important substances such as siderophores, hydrogen cyanide (HCN), chitinase, protease, urease and ACC-deaminase that impede mycelial growth of S. rolfsii6F. oxysporum, and C. f imbriata and as a result ensure the plant's good health. 228 Pseudomonas f luorescens strains also have aminocyclopropane-1-carboxylic acid-or ACC-deaminase activity 229 that significantly controls the quantity of plant ACC-deaminase left behind for the biosynthesis of ethylene. 128 Moreover, ACCdeaminase decreases undesirable effects of the stress brought by ethylene on plants by diminishing its quantity, 230 although it has been noted that a rapid decrease in ethylene levels can reduce seed emergence. 231 The expression of ACC deaminase by the endophyte, P. migulae 8R6 can make Madagascar periwinkle more resistant to phytoplasma infection. 232

PSEUDOMONAS SPECIES PRODUCING RHAMNOLIPIDS (BIOSURFACTANTS)
The rhamnolipids (RLs) are a special class of biosurfactants, and quorum sensing (QS) molecules regulate their secretion. Surfactants are very significant in QS molecules or cell-to-cell interaction, cellular differentiation, bacterial cell movement, and making of water canals, all which are properties of the Pseudomonas genus. These natural substances are secreted by the microbial cells 240 and have numerous benefits as compared to synthetic surfactants, having very little toxicity, excellent ecofriendly properties, biodegradability, acceptable surface activity at high pH, temperatures, and salinity levels, and the capability to be produced from the renewable feedstock. 241 They are extensively used in pharmaceuticals, agriculture, pesticide removal, the food industry, household cleaning, and improvement in oil recovery. 242 In agriculture, RLs effectively take part in disease management by diminishing the growth of phytopathogens. These biosurfactants also have antiviral, antifungal, and antibacterial potential. 243 The benefits and market potential of RLs have been extensively studied, 244,245 e.g., the virucidal effects of RLs were recalled when COVID-19 emerged as global issue. 246 RLs are also employed in the removal of HMs like As, Cr, Pb, Cd, and Ni from soils due to their anionic character and make the soil stress-free. 247 RLs are primarily formed by P. aeruginosa, although their secretion by some other Pseudomonas species has also been reported. 247 P. aeruginosa produces the majority of the glycolipid-type rhamnolipids, which are the most extensively studied biosurfactants. 248,249 This is due to their good surface activities, high yields after relatively shorter incubation periods, and ease microbe culviation. However, in P. aeruginosa, three QS systems exist including las, rhl, and pqs systems, which are responsible for regulating the RLs production. 250,251 The P. aeruginosa rhamnolipids illustrates a vast variety of the microbes such as Micrococcus luteus, Escherichia coli, S. epidermidis, A. faecalis, Mycobacterium phlei, and Serratia marcescens. 252 Moreover, P. aeruginosa RLs also exhibited antifungal potential against Penicillium chrysogenum, Aureobasidium pullulans, Aspergillus niger, and Chaetomium globosum. Similarly, RLs produced by a P. aeruginosa strain (DS9) were reported to exhibit antifungal activity against Fusarium sacchari instigating Pokkahboeng disease. 253 Similarly, in another research report, the antifungal property of RLs against seven plant pathogens was surveyed. The outcomes reveal the high ability of RLs derived from P. aeruginosa (ZJU211) to fight three Ascomycota, two Oomycetes, and two Mucor species. 254 The very first report on the insecticidal activity of RLs was reported by Kim. 255 They described that the rhamnolipid formed by Pseudomonas strain EP-3 has disruption ability against Myzus persicae (green peach aphid). The biofilm resistance against antimicrobial agents is becoming a worldwide problem, 256 whereas RLPs have prohibiting effect on the biofilm formation. 257

PLANT PROTECTION AGAINST ABIOTIC STRESS
PGPR belonging to various taxonomic groups colonize the plant rhizosphere and mediate stress responses by employing multiple mechanisms. These beneficial becteria facilitate the acquisition of essential nutrients or modulate the production of plant hormones and thus develop in plants the ability to withstand the abiotic stresses. 48 In addition to promoting tomato plant development in the absence of stress conditions, the endophytes P. brassicacearum exhibit YsS6 and P. migulae 8R6, both of which display ACC deaminase activity, and also caused biomass accumulation to be considerably colder, drier, and higher in chlorophyll and more blooms and shoots in tomato vegetation grown under treated under salt stress conditions. 258 Liu and his co-workers recently discovered that inoculating a tomato plant with ACC deaminase-producing P. azotoformans CHB 1107 improved the dry weight of the plant shoots and roots and significantly decreased the plant's reaction to salt stress by emitting less ethylene. These advantageous outcomes disappeared as P. azotoformans CHB 1107 M was used to vaccinate plants (acdS variant). Furthermore, it considerably improved plant uptake of potassium, manganese, and calcium in contrast to the P. azotoformans CHB 1107 acdS variant. 82 The ability of Pseudomonas spp. to express ACC deaminase and decrease the ethylene concentration in plants is associated with its capacity to minimize heavy metal quantity in certain plant tissues. 259 P. seudomonas also exhibited greater resistance to other abiotic stressors when foreign ACC deaminase genes were expressed. Similar to this, inoculation of freeze-stressed tomato plants with P. f rederiksbergensis OS211-acdS (carries the ACC deaminase and pRKACC plasmids) lowered ethylene emissions, reduced the ACC quantity, and decreased ACC oxidase activity. P. f rederiksbergensis promoted plant development more than the uncultivated strain without ACC deaminase, 260 evidencing the useful impact of ACC deaminase expression.
HM pollutants, salinity, drought, and low/excessive temperature are vital climate stressors that have an effect on regular plant physiological and biochemical processes, thereby significantly lowering plant development and productivity. The creation of numerous plant-useful metabolites may also amplify the impacts of environmental stress in plants, as well as to the useful activities of PBPs in plant development and biological manipulate activity. The sections that follow focus on the advantageous mechanisms of connection among PBPs and plants and how these relationships promote plant development and improvement in response to various forms of environmental stress. 261 Under drought conditions, the genes related to the synthesis of compatible solutes, exopolysaccharides, and secretion systems (type II, III, IV, and VI) were observed to be upregulated in P. f luorescens, which aided in alleviating plant stress. 141 The potential of Pseudomonas spp. could be effectively used to enhance the plants' ability to survive under abiotic stress conditions.

COMMERCIAL PRODUCTS BASED ON PSEUDOMONAS SPP.
With the rapidly increasing interest in ecofriendly biocontrol of soil-borne plant pathogens, many companies currently have developed biological control agents, biofertilizers, and biofungicides as commercial products under several trade names (Table.4). Several reports have explained the potential applications of bacterial inoculants for the maintenance of natural agriculture, for instance, biofertilization, biostimulant, phytostimulation, and bioremediation. 262 The basic aim of PGPR-based biocontrol products is to support sustainable approaches for the plant disease management. The two biggest potential markets for biocontrol products are now Europe and the United States, with South America coming in third. 263 Many rhizobacteria have been scrutinized at the laboratory level, and a few of them are successfully commercialized globally. 264,265 The applications of Pseudomonas inoculants as PGPR in Europe need authorization under the appropriate directives. These significant directives have been reviewed by many researchers. 265,266 Furthermore, plant protection products protect the plants against damage causing agents, have a direct impact on plants, and are used against undesirable plants such as herbicides. However, European legislatures have concerns with the usage and registration of plant protection commercial products that are addressed in Council Directive titled "The Plant Protection Directive", and its implementation is under the power of EFSA (European Food Safety Authority). This gives a complete framework for generating an official inventory of compounds that cause no hazard to the ecosystem. These beneficial compounds are authorized for utilization and subsequently can be marketed in any part of the world. In the 1990s, numerous Pseudomonas strains were approved as registered biopesticide products by the U.S. Environmental Protection Agency (US EPA). These Pseudomonas spp. are enlisted on USEPA biopesticide Web site and discussed in detail 267 (Table 4).

CONCLUSION
Rapidly growing plant pathogens and climate changes bring severe changes in food security and crops yield. In the present scenario, plant-growth-promoting rhizobacteria (PGPR) are some of the good suitable choices for the enhancement of plant growth and disease management over the consumption of chemical fertilizers and pesticides. Among many PGPR and biocontrol agents, Pseudomonas spp. play a crucial role in controlling the viability and crop-destruction activities of phytopathogens. Their biocontrol activity is directly associated with the production of antibiotics, such as DAPG, PHZs, PRN, PLT, and other hydrolytic enzymes. Advanced knowledge on plant protection features of P. f luorescens antagonists such as genes contributing to rhizosphere competence and suppressing plant diseases, along with factors affecting root colonization, is required for the management of phytopathogens.

FUTURE PROSPECTIVE
Sustainable agriculture is the basic requirement of the world in current times because of the harmful consequences of chemicals used in agriculture. However, a knowledge gap still exists regarding plant−microbial interactions under different stress circumstances, mainly biotic stress. Currently, various products in the market are advertised and sold as biofertilizers and biopesticides. Due to the need to decrease the use of chemical fertilizers, the world's demand for more of such products and organically grown food is increasing. The positive effects have already been mentioned, and the development and production of biofertilizers and biopesticides could be an appropriate solution for developing countries, which badly suffer from shortages of food. The great potential for the use of biological products in agriculture and food production is clear because it contributes to agricultural sustainability and gives solutions to issues regarding the environmental impact of existing agricultural practices. However, future research demands rhizo-engineering based on the favorable identification and partitioning of new biomolecules, which may make the distinctive settings for interactions between plants and microbes. The application and investigation of multistrain bacterial inoculants over a single strain could be an effective means of disease suppression and management. Moreover, genetic modifications for increasing the biocontrol efficiency of microbes can also be an emerging research field for future disease management. For example, the transformation of strains with augmented levels of antimicrobial activity and growth-increasing metabolites can be good options.