Pseudomonas species or pseudomonads are known for their metabolic and ubiquitous diversity which enables them to survive in extreme conditions such as in marine and terrestrial environments as well as in association with flora and fauna. The sequenced genomes of many strains of Pseudomonas spp. show their vast repertoire of biotechnological applicability potential with respect to their genetic makeup and also exhibit industrially important applications due to physicochemical tolerances to extreme conditions (such as temperature, pH, and toxic chemicals and solvents). The best studied species include opportunistic human and plant pathogens, soil bacteria, and the plant growth-promoting pseudomonads. Pseudomonas species also are plant-commensals known for exhibiting effective antimicrobial activities and enabling plants to retrieve key nutrients. Hence, studying Pseudomonas with respect to its various characteristics in response to plant interactions is a far more important subject to be studied for their effective applications. In this review, the Pseudomonads have been analyzed extensively for their genome; biomolecules produced and plant beneficial activities. Thus, the present work helps future endeavors for Pseudomonad research by streamlining the areas.
Shaikh S, Yadav N, Markande AR. Interactive potential of Pseudomonas species with plants. J App Biol Biotech. 2020;8(6):101-111.
Understanding of the beneficial activity and diversity of plant-bacterial association is important for maintenance and sustainable agriculture in agricultural-ecosystems [1-3]. The plant growth promoting rhizobacteria colonize the rhizosphere rapidly, suppress soil-borne pathogen at the root surface, and stimulate plant growth [4-7].
Since the dawn of agriculture, humans have been battling against plant diseases and pests which were greatly helped by the invention and deployment of chemical pesticides use which enhanced crop production increasing the need for persistent disease management strategies in 20th century. Although chemicals are available for plant-pest and disease-management, no alternatives are available for pest resistance for pesticides. The environmental, health, and safety concerns of these chemical products have increased the need for search for alternatives to control plant diseases and pests [8,10].
One group of bacteria that have become a focal point for research on biocontrol of plant diseases is the genus Pseudomonas (ubiquitous Gram-negative rod shaped gamma-proteobacteria possessing polar flagella) [13,51]. Their physiological and ecological diversity reported globally (which can extended to genetic level) are known for their secondary metabolite production which are applicable for integrated biocontrol of plant diseases [18-20]. The genome size of Pseudomonas species typically varies from 4.6 to 7.1 Mb having G+C content of 57.8 to 66.6% with 4237–6396 predicted genes . Using multi-locus sequencing techniques, genus Pseudomonas, currently, possesses more than 100 species with further groups and sub-groups [3,22,24,26]. The details of pseudomonads whose genome has been elucidated are discussed in Table 1.
Table 1: Genome studies of Pseudomonads
|Strain||Source of isolation||References|
|P. aeruginosa 2192||Chronically-infected cystic fibrosis patient|
|P. aeruginosa C3719||“Manchester epidemic strain” isolated from cystic fibrosis patient|
|P. aeruginosa LESB58||“Liverpool epidemic strain”isolated from cystic fibrosis patient||Winstanley et al.,(2009) |
|P. aeruginosa PA14||Wound, from culture collection at University of California at Berkeley||Lee et al., (2006) |
|P. aeruginosa PAO1||Burn wound||Stover et al., (2000) |
|P. entomophila L48||Fruit or fruit fly||Vodovar et al., |
|P. fluorescens Pf-5||Soil||Paulsen et al., (2005) |
|P. fluorescens PfO-1||Soil||Silby et al., (2009) |
|P. fluorescens SBW25||Leaf of sugar beet|
|P. putida KT2440||Cured strain lacking the TOL plasmid||Nelson et al., (2002) |
|P. putida W619||Endophytic strain isolated from poplar||Taghavi et al., (2009) |
|P. stutzeri A1501||Rice paddy soils||Yan et al., (2008) |
|P. syringae pv. oryzae||Rice||Reinhardt et al., (2009) |
|P. syringae pv. phaseolica 1448A||Bean||Joardar et al., (2005) |
|P. syringae pv. syringae B728a||Leaf of bean||Feil et al., (2005) |
|P. syringae pv. tomato DC3000||Tomato||Buell et al., (2003) |
|P. syringae pv. tomato T1||Tomato||Almeida et al., |
|P. protegens CHA0||Developmental stages of cabbage root fly||Flury et al. (2019) |
|P. syringae pv. syringae B728||Common bean||Helmann et al. (2019) |
|P. syringae pv. actinidiae (Psa)||Kiwi fruit plant||Donati et al. (2020) |
|P. chlororaphis subsp. aurantiaca ARS-38||Cotton||Mehnaz et al. (2020) |
|P. aeruginosa AJD 2||Monocropic cotton rhizosphere||Joshi and Chitanand (2020) |
Genus Pseudomonas are ecologically significant and the most heterogeneous group termed as pseudomonads are characterized by the presence of a complex enzymatic system and elevated metabolic versatility. With simple nutritional requirements Pseudomonas spp. are found in nature, from industrial equipment, oils-pills, aseptic solutions, cosmetics, medical products, and clinical instruments . Certain they are known to be pathogens or carriers for plants and human infections, while others strains and species exhibit biocontrol and bioremediation activities [14,18]. They have also been reported for their ability to produce effective biosurfactants  and efficient remediation capabilities [33,35].
Fluorescent pseudomonads are visually distinguishable than other species in the genus due to their water-soluble fluorescent pigment production. Root associated pseudomonads in rhizosphere (which is a zone of high microbial activity) enhances and acts in the management of plant health [20,24,25]. They are known to act as both plant pathogens (Pseudomonas syringae) and plant growth promoters (fluorescent pseudomonads) and fight the phytopathogens [22,26,27]. They are known to be rapid root colonizers directly or indirectly acting as plant growth promoter or plant pathogen [16,43].
The mechanism of action of the plant growth promoting pseudomonads by synthesis of various metabolites such as antimicrobial biomolecules (antibiotics) and volatiles such as hydrogen cyanide (HCN) , production of siderophore-nutrient competition (even iron), niche exclusion , and systemic resistance induction (ISR) [48,50,51]. The present review covers different aspects of plant growth promoting bacterial (fluorescent pseudomonad) interaction with plants and rhizospheric microorganisms enhancing the disease resistance with agricultural and horticultural uses.
At present, the plant pathogenic Pseudomonas species are taxonomically restricted to specific group of organisms using rRNA gene analysis and about 21 plant associations are reported with 50 different pathogenic strains (termed pathovars or pv- a confusing concept of taxonomy) of P. syringae. The complete genomes of three important pathovars belonging to P. syringae have been sequenced. Host reactions to pathogen infections have been studied extensively because P. syringae pv. tomato could infect other model plants such as Arabidopsis. The common symptoms of plant pathogenic pseudomonads that cause various important diseases show cankers, galls, blights, soft rots, leaf, and stem spots. Other important virulence factors causing pathogenicity include type III secretion system, production of secondary metabolites (hormones, phytotoxins, pectolytic enzymes, exopolysaccharides, etc.), and ice nucleation activity. No single strategy has been effective against plant pathogenic pseudomonads and may need polyphasic strategies (combinations of physical, chemical, and biological strategies) . For specific detection of plant pathogenic pseudomonads, tools of molecular biology are increasingly gaining diagnostic importance [20,27,52].
A variety of P. syringae pv. syringae symptoms on woody plants are dependent on the microbial strain, plant part infected, and the environment . Disease symptoms of P. syringae infections are due to plant-microbe interactions involving molecular interactions modulated by biotic and abiotic conditions and hence the concept in P. syringae having continuum of potential pathogenesis with evolutionary significance . Multiple symptoms can occur simultaneously on a single plant, such as (1) turning of brown to black coloration of flowers and/or flower buds, (2) buds-dead and dormant, (3) necrotic leaf spots, (4) discolored and/or blackened leaf veins and petioles (due to systemic invasion and infection), (5) fruit spots and blisters, (6) shoot-tip die-back (appearing as dead and blackened twig tissue extending to some distance from the tip), and (7) stem cankers: Bark depressions which darken with age [53,55]. The bacterium was found to have the ability to cause specific symptomology of each organ of the host plant and can form association with plant opening (such as stomata or trichomes, and wounds) resulting in apoplast infection. The pathogen could survive environmental in winter, wind dispersal, or even rain . Causative agent of bacterial canker of Kiwi fruit – P. syringae pv. actinidiae (Psa) encodes three LuxR solos and no cognate LuxI. Thus, displaying a non-canonical quorum sensing system, Psa could perceive the acyl-homoserine lactones (AHLs) of neighboring bacteria and regulate its virulence in the host .
Many strains of pseudomonads are fluorescent made up of seven subgroups and around 50 studied strains and species. They are common surface-microflora of virtually all plant tissues, along with many other natural habitats. Some of these plant associated strains are excellent promoters of plant growth and antagonists of plant pathogens. These strains have been enhanced as commercial products of plant growth promotion and agricultural bio-control agents (biofertilizers and biopesticides) [3,26].
Fluorescent pseudomonads produce phytohormones – indole acetic acid (IAA), gibberlins, cytokinins, and ethylene production inhibitors, helping in increasing the plant root absorptive surface for nutrient and water uptake . They can act directly on the nutritional status and thus growth and physiology of plant they colonize. P. fluorescens having aminocyclopropane-1-carboxylic acid- or ACC-deaminase activity [18,58] is important as it controls the quantity of plant ACC deaminase left for ethylene biosynthesis . When checked using in vitro plate assays of the fluorescent strain, P. aeruginosa PJHU15 was found to be positive for the production of IAA and phosphate solubilization [60,61]. This fluorescent P. aeruginosa was used in consortium with Trichoderma harzianum and Bacillus subtilis leading to improvement of plant health inducing the systemic resistance and proteome-level changes when challenged with Sclerotinia sclerotiorum [3,48,59,62,63]. The consortium was also modulated for nutritional and antioxidant quality of pericarps of pea seeds . Mutualistic, host associated bacteria were checked on model plant Arabidopsis against P. fluorescens and here, commensal and pathogenic lifestyles of these host-associated bacteria convergently lost and gained in multi-lineage homologous recombination further constituting the early step of bacterial differentiation into pathogenic and commensal lifestyles .
A significant group of players in crop growth, yield, and maintenance having capability, as phosphate solubilizing and biocontrol agents are termed as plant growth promoting rhizobacteria (PGPR). Pseudomonads possess many PGPR traits such as (i) rapid growth in vitro and provision for mass production; (ii) utilize seed and root exudates rapidly; (iii) colonize and dominate the rhizosphere, spermosphere, and even in the interior of plants; (iv) bioactive metabolite production (volatiles, siderophores, growth promoting substances, and antibiotics); (v) environmental stress resistance, and (vi) compete aggressively with other microorganisms. They are also responsible for the innate suppressiveness of some soils to pathogens [3,53,61,66-68]. The pseudomonads exhibited spatial separation from the pathogen on the above ground plant parts, either in the stem  or on the leaf surface . Pseudomonads which have been reported to possess PGPR traits are discussed in Table 2.
Table 2: Pseudomonads as PGPR and plant responses
|Bacteria||Enzymatic activity||Host plant||Plant responses||References|
|P. polymyxa||Indole-3-acetic acid||Wheat grass||Increased growth over uninoculated Control||Holl et al., (1988) |
|P. putida||ACC deaminase||Tomato||Inoculated tomato seed increased plant resistance in 55 days to nine consecutive days of flooding and increased resistance to salinity||Grichko and Glick, (2001) |
|P. asplenii||ACC deaminase||Rape seeds||Significant increase in fresh and dry weight and biomass yield||Reed and Glick, (2005) |
|P. putida||Indole-3-acetic acid||Canola||Two–threefold increases in the length of seedling roots||Ahmad, Ahmad and Khan, (2005) |
|P. fluorescens||ACC deaminase||Maize||Increased root length and fresh weight under saline conditions||Kausar and Shahzad, (2006) |
|P. fragi||Hydrogen cyanide||Wheat seedlings||Significantly increases the germination percentage, germination rate, plant biomass and nutrient uptake||Selvakumar et al., (2008) |
|P. fluorescens||ACC deaminase||Groundnut plants||Improved the saline resistance and yield|
|P. putida UW4||Indole-3-acetic acid and ACC deaminase||Canola||Under saline conditions, protected the seedling of canola from growth inhibition|
|P. aeruginosa||Hydrogen cyanide||Wheat||Control fungus diseases and enhance defense against phyto-pathogen||Rana et al., (2011) |
|P. chlororaphis||Siderophore production||Maize||Increased root shoot biomass and seed germination rate||Hayat, Ahmed and Sheirdil, (2012) |
|P. fluorescens Psd||Tryptophan Monooxygenase||Sorghum (var. Sudex chari)||Increased root shoot biomass and seed germination rate||Kochar, Upadhyay and Srivastava, (2011) |
|P. fluorescens EBC191||Indoleacetonitrilase, Nitrile hydrolase||NA||NA||Kiziak et al., (2005) |
|Pseudomonas sp. K-9||Phenylacetaldoxime Dehydratase||NA||NA||Kato and Asano, (2005) |
|P. putida WCS358||Cell envelope components Flagella||Arabidopsis||Inducement of systemic resistance||Meziane et al., (2005) |
|P. fluorescens WCS374||Lipopolysaccharides||Radish||Inducement of systemic resistance||Leeman et al., (1995) |
|P. fluorescens WCS417||Lipopolysaccharides||Arabidopsis Carnation Radish||Inducement of systemic resistance||Van Peer and Schippers, (1992); Leeman et al., (1995); Van Wees et al., (1997) [105–107]|
|P. putida WCS358||LPS or pseudobactin||Arabidopsis Bean Tomato||Inducement of systemic resistance||Meziane et al., (2005) |
|P. putida BTP1||Iron-regulated metabolites N-alkylated benzylamine Derivative||Bean||Inducement of systemic resistance||Ongena et al., (2005) |
|P. fluorescens CHA0 P. fluorescens WCS374 P. putida WCS358 P. putida WCS358 P. putida WCS358 P. putida WCS358||Pseudobactin siderophore||Tobacco Radish Arabidopsis Bean Eucalyptus Tomato||Inducement of systemic resistance||Maurhofer et al., (1994); Leeman et al., (1996); Meziane et al., (2005); Ran et al., (2005) [57,63,84,109]|
|P. aeruginosa 7NSK2 P.aeruginosa 7NSK2 P. fluorescens P3 pchBA||Salicylic acid||Bean Tobacco Tobacco||Inducement of systemic resistance and root exudates||Maurhofer et al., (1994); De Meyer and Höfte, (1997); De Meyer, Audenaert and Höfte, (1999) [84,110,111]|
|P. putida WCS358||Cell envelope components Flagella||Arabidopsis||Inducement of systemic resistance||Meziane et al., (2005) |
|P. fluorescens WCS374||Lipopolysaccharides||Radish||Inducement of systemic resistance||Leeman et al., (1995) |
|P. fluorescens WCS417||Lipopolysaccharides||Arabidopsis Carnation||Inducement of systemic resistance||Van Peer and Schippers, (1992) Leeman et al., (1996); Van Wees et al., (1997) [106,107,109]|
|P. aeruginosa 7NSK2||Pyocyanin and pyochelin (and/or salicylic acid)||Tomato||Inducement of systemic resistance||Audenaert et al., (2002) |
|P. fluorescens WCS374||Unknown||Radish||Inducement of systemic resistance||Leeman et al., (1996) |
|P. fluorescens WCS417||Unknown||Radish||Inducement of systemic resistance||Iavicoli et al., (2003) |
|P. fluorescens CHA0||Antibiotics 2,4-Diacetyl- phloroglucinol||Arabidopsis Tomato||Disease suppression||Siddiqui and Shaukat, (2003) |
|P. fluorescens Q2-87||2,4-Diacetyl phloroglucinol||Arabidopsis||Inducement of systemic resistance||Weller et al., (2004) |
|P. putida KT2440||Indole compounds, Siderophore synthesis, and phosphate solubilization||Soybean and Corn||Induced systemic resistance in response to certain foliar pathogens||Costa-Gutierrez et al. (2020) |
|P. chlororaphis subsp., aurantiaca ARS-38||Production of indole acetic acid, hydrogen cyanide, lahorenoic acid, phenazines (lipopeptide), and a hydroxamate-type siderophore||Wheat||Increased root and shoot dry weights in wheat seedling growth||Mehnaz et al. (2020) |
|P. psychrotolerans CS51||Auxin biosynthesis, nitrate and nitrite ammonification, phosphate-specific transport system, and the sulfate transport system||Cucumber||Endogenous indole-3-acetic acid (IAA) and gibberellins (GAs) resulting in enhanced cucumber growth (root shoot length) and increased the heavy metal tolerance||Kang et al. (2020) |
|P. syringae pv. tomato (Pst) DC3000||Pathogencity||Arabidopsi thaliana||Cation-Chloride Co-Transporter 1 (CCC1)||Han et al. (2020) |
Pseudomonas brassicacearum, a harmless commensal and a member of Pseudomonas fluorescence group containing more than 51 species, is known for its plant growth promotion (PGP) and biocontrol activities. It is also closely related to P. corrugata, an opportunistic phytopathogen .
Fluorescent pseudomonads are known producers of variety of antibiotics and act as biocontrol agents [3,17,24,46,71]. The biocontrol agents produced by fluorescent pseudomonads include pyocyanin , pyrrolnitrin , phenazine-1-carboxylic acid , 2,4-diacetylphloroglucinol (Phl), and pyoluteorin . P. fluorescens SF4c produces more than one functional bacteriocin (such as S-type bacteriocin and phage-tail-like bacteriocin-Tailocins) by their regulator PrtR gene . Flury et al. (2019) showed the role of insects such as cabbage root fly, Delia radicum in different developmental stages by harboring persistant root-colonizing P. protegens CHA0 as dispersal agents to new host plants .
Fluorescent pseudomonads have been long known for their production of HCN in disease suppression [77,78]. The rate of HCN production has been reported to be relative to the plant species and its root exudates which show in reciprocation, beneficial effect on the growth of plant . Some of these act as biocontrol by producing HCN have also been reported for their ability to induce plant resistance against phytopathogenic fungal diseases [80,81], for example, in wheat . Ferramola et al. (2020) used the Larrea divaricata Cav. (jarilla) proteins to induce the antibody production and used the cross-reactivity of antibodies produced against nosocomial pathogen Pseudomonas aeruginosa .
Plant exudates dictate the plant dependent rhizosphere microflora. The surface surrounding rhizosphere acts as carbon sink [3,49,83] providing various nutrients (including important elements, water, and other secondary metabolites such as antimicrobials, enzymes, vitamins, mucilage, and plant growth regulators). Thus resulting in influx of diversity of micro- and macro-organisms (pathogens included) at the rhizosphere site and resulting in competition for nutrients and consequently at this niche. The fast adaptability of fluorescent pseudomonads and other beneficial microorganisms (PGPR) to such condition make them effective competitors against pathogens. Most of these PGPR are flagellated and respond with chemotactic responses for plant exudates reaching root surfaces faster than others [26,84].
Many pseudomonads such as P. psychrotolerans CS51 and P. aeruginosa AJD 2 have been studied recently for their genome-wide ability to encode PGP traits [86,87]. Singh et al. (2019) have compiled an excellent overview of different PGPR strains of Pseudomonas spp. and their mediated tolerance responses for different heavy metals .
Cellular communication or quorum sensing (QS) within the spatially structured P. fluorescens rhizospheres communities was found to be possible. QS signaling is dependent on cell density, their spatial distribution and mass transfer . N-acyl homoserine lactone (AHL) based QS signaling molecules is predominantly seen among Gram-negative bacteria. Pseudomonad motility on semi-solid surfaces is mediated by type IV pili and peritrichous flagella [53,93] and Pyoverdine seems to be playing a major role in this locomotion as the mutations in pvdQ (which codes for stages of pyoverdine biosynthesis) resulted in bacterial motility loss .
The rhamnolipids are a group of biosurfactants and their production is regulated by the QS molecules. These biosurfactants (Rhamnolipids) have some extreme properties such as antimicrobial properties (antibacterial, antifungal, and antiviral) [72,90,91]. These surfactants are important in cell-to-cell interaction (or Quorum sensing), bacterial cell motility, cellular differentiation, and water channels formation these are the characteristics of the Pseudomonas biofilm. In comparison to the chemical surfactants, biological biosurfactants are more valuable for the environment and different industrial uses [97,101,103]. Rhamnolipids are widely used in agriculture, pharmaceutical, pesticide removal, improvement in oil recovery, household cleaning, and food industry. P. aeruginosa rhamnolipid shows the wide range of the bacteria such as A. faecalis, E. coli, Micrococcus luteus, Mycobacterium phlei, Serratia Marcescens, and S. epidermidis. P. aeruginosa rhamnolipids are also show the antifungal activity against the Aspergillus niger, Aureobasidium pullulans, Chaetomium globosum, and Penicillium chrysogenum [76,78,93,118]. Rhamnolipids are also shows the antiviral activity against TMT and Potato X virus .
Among the various rhizobacteria, Pseudomonas spp., are aggressive rhizospheres and rhizoplane colonizers of different crop plants  with broad spectrum of antagonistic activity against plant pathogens [79,80,82,119,120]. The primary biocontrol mechanism of many pseudomonads includes the production of metabolites such as HCN, antibiotics, and siderophores . The beneficial effect on dry mass of plant shoot was more evident with HCN producing Pseudomonas strains , especially P. aeruginosa, a PGPR has been found to be an effective biocontrol agent of root pathogens [15,81]. Many pseudomonads have been reported for such abilities such as control of damping off of cotton seedlings caused by R. solani using the antibiotic produced by the P. fluorescens  and Septoria tritici (Mycosphaerella graminicola) suppressed by P. aeruginosa strain leci . The siderophore producing pseudomonads with mixtures of Bradyrhizobium japonicum strain USDA 110 improved nodulation [3,87,124]. P. fluorescens CHAO, isolated in Switzerland, has been the most highly studied pseudomonad capable of producing different bioactive compounds (such as IAA, siderophores, antibiotics, and HCN) making it the best PGPR so far [89,125].
Plants can be protected from various pests and diseases by the strains of pseudomonads which induce systemic resistance or ISR [3,24,115-117]. The enhancement of plant defensive capacity due to specific chemical and biotic stimuli is called induced resistance . It was found that PGPR especially fluorescent pseudomonads induced systemic resistance (ISR) leading to plant disease suppression [68,69]. Pseudomonads beneficial to plants are studied in Table 3.
Table 3: Plant beneficial pseudomonads.
|Strain||Origin/ Plant protection||Plant-beneficial traits documented||Reference|
|DR54||Sugarbeet||Viscosinamide, chitinase||Sanguin et al., (2008) |
|F113||Sugarbeet||DAPG, HCN, pyoverdine, ACC deaminase, T3SS||Moënne-Loccoz et al., (1998) |
|KD||Wheat||T3SS, HCN, pyoverdine||Rezzonico et al., (2007) |
|Pf29A||Wheat||Pathogen growth inhibition, ISR||Barret et al.,(2009) (Barret et al. 2009) |
|Q2-87||Wheat||DAPG, HCN, ACC deaminase||Weller, (2007) |
|Q8r1-96||Wheat||DAPG||Mavrodi et al., (2006) |
|SBW25||Sugar beet||T3SS, competition, pyoverdine||Sanguin et al., (2008) |
|WCS365||Potato||ISR, siderophore, competition, T3SS||de Weert et al., (2002) |
|WCS374||Potato||ISR, pseudoverdine, pseudomonine, salicylate, T3SS||Pieterse et al., (2003) |
|Pseudomonas spp.||Arabidopsis and Potato||Phenazine-production and Rhizosphere colonization||Zboralski et al. (2020) |
|2P24||Wheat||DAPG, HCN, pyoverdine||Sanguin et al., (2008) |
|2-79||Wheat||Phenazine-1-carboxylate, pyoverdine, anthranalate, T3SS||Cook et al., (1995) |
|CHA0||Tobacco||DAPG, HCN, ISR, pyoluteorin, pyoverdine, salicylate, pyrrolnitrin, ISR||Haas and Défago, (2005) |
|Pf-5||Cotton||Pyoluteorin, pyrrolnitrin, DAPG, HCN, pyoverdine||Loper, Kobayashi and Paulsen, (2007) |
|LBUM677||Soybean, Canola and Corn gromwell||Increase in plant biomass, total oil content and lipid composition||Jiménez et al. (2020) |
|KT2440||Soybean and Corn||Seed germination, root and stem length increment under saline conditions||Costa-Gutierrez et al. (2020) |
Fluorescent pseudomonads with biocontrol capability (Production of DAPG: 2,4-diacetylphloroglucinol, ACC: 1-aminocyclopropane-1-carboxylate, ISR: Induced systemic resistance, HCN: hydrogen)
Costa-Gutierrez et al. (2020) showed the ISR of soybean and corn against certain foliar pathogens due to root-colonizing non-pathogenic P. putida KT2440 . Gislason and de Kievit (2020) studied all the 21 sequenced genomes of P. brassicacearum and P. corrugata clade for PGP, biocontrol activities, and pathogenicity. They reported the extreme similarity among these two groups of beneficial and harmful bacteria. The bacterial ability to manipulate plant immune system to form harmful/harmless associations, the physiological and genotypic state of the host plant, and other stressors (biotic/abiotic) contribute to the plant-microbe interactions and results . The strain P. putida KT2440 was found to be excellent root colonizer of agronomical important crops with ability to activate the ISR against certain plant pathogens .
Azelaic acid, a dicarboxylic acid is shown to play the Arabidopsis plant signaling specifically promoting the resistance priming by salicylic acid (SA) as a part of plant immunity against Pseudomonas nitroreducens DSM 9128 .
Many reports suggested that pathogenic bacterial cell surface components such as the lipopolysaccharides can induce resistance (ISR) as reported in P. fluorescens inducement of carnation plants against Fusarium oxysporum f. sp. dianthi infections . The LPS of P. fluorescens strains was demonstrated to be of important in ISR against wilt of radish caused by F. oxysporum f. sp. raphanin . However, redundancy of ISR triggering traits in P. fluorescens strains was reported for the suppression of Fusarium wilt in radish . In A. thaliana, application of isolated LPS of P. fluorescens and P. putida has been reported to be involved in ISR against P. syringae pv. tomato, triggering ISR which was further found to be having redundancy in ISR triggering traits in these strains [12,58,122]. A mutant of P. fluorescens strains lacking the 0-antigen no longer triggered ISR and the iron-regulated elicitor of ISR in BTP1 (an N alkylatedbenzylamine derivative) in bean and tomato . Pseudobactin mediated ISR was effective against Tobacco necrosis virus in tobacco with reduction in numbers of viral lesions and lesion diameter in comparison to pseudobactin-negative mutant of P. fluorescens CHA0 .
Under iron limiting conditions, most aerobic and facultative anaerobic microorganisms (including fluorescent Pseudomonads) produce siderophores (low-molecular weight Fe3+ specific chelators). The siderophores sequester ferric ions and form ferrated siderophores which are, in turn, taken up by microbial cells through surface mediated uptake . Siderophores have also been implicated in ISR in several systems like bacterial wilt suppression caused by Ralstonia solanacearum in Eucalyptus urophylla (due to P. putida siderophores) . It was observed that bacterial SA production was not involved in ISR by PGPR as SA production was suppressed in the rhizosphere probably due to SA being a precursor of SA-containing siderophores such as “pseudomonine” in P. fluorescens  and “pyochelin” in P. aeruginosa  thus being utilized to extinction. SA has been reported for its complex ISR activity in tobacco, tomato and bean but was predominantly seen in mutant that can synthesize it but unable to incorporate it in pyochelin [110,127,144]. Many antibiotics are produced by the Pseudomonas spp. strains including 2,4-diacetylphloroglucinol (DAPG) and its role in ISR was recently demonstrated in Arabidopsis. Here, DAPG produced by P. fluorescens CHA0 elicited ISR against Peronospora parasitica . In tomato, P. fluorescens CHA0 induced DAPG mediated ISR against the root-knot nematode Meloidogyne javanica (as DAPG-negative mutant was ineffective and restoration of effectiveness on mutant complementation) . DAPG produced by P. fluorescens in Arabidopsis was also found to be effective for the ISR against P. syringae pv. Tomato . The phenazine antibiotic pyocyanin produced by P. aeruginosa was found to be involved in ISR against B. cinerea in tomato .
The Pseudomonas spp. ability to induce plant responses are summarized in Figure 1. There is a need for model designing for understanding the microbe . The ability of the bacterial genus in effectively establishing itself as plant pathogen and growth promoter has increased the value of studies in this regard. The future of this research relies on the development of effective microbial combinations and consortia providing a stable community which could work effectively against plant pathogens and improve the plant growth [111,129,146].
|Figure 1: Pseudomonas sp. - Plant interactions. The orange section shows the inducement of systemic resistance in plants and blue sections indicate the cation solubilisations|
[Click here to view]
For understanding of this complex microorganism Pseudomonas, it is imperative to understand the mechanisms involved in plant growth promotion and different aspects of these interactions. The rhizospheric competence is a prerequisite for effective biocontrol applications, root-microbe, cell-to-cell and microbe-to-microbe interaction, while genetic and environmental factors affecting growth will help in elucidation of the mechanisms should be adopted. Thus, there is a need for designing different strategic approaches and constructing models to improve the efficiency of this bacterium. The discovery of strains from diverse ecological niches and targeting biosynthetic genes specifically may result with the identification of biomolecules and metabolites, detection of their mechanisms involved and may further increase the knowledge of the topic. Basic genetic engineering methods can be employed coupled with multiple modes of action. Exploration of molecular tools and techniques to study the interactions of fluorescent Pseudomonads with –plants and – pathogens by studying genome expression and proteome level changes during interactions can clarify the complex rhizosphere biodiversity. Thus, further studies should focus on the identification of genes and gene-products in Pseudomonads and plants that decide improved biocontrol and efficient colonization of rhizospheres. Further studies into ISR in fluorescent pseudomonads can open new horizons for research in signaling network and related mechanisms involved.
The authors would like to thank the Provost, Uka Tarsadia University and HOD and Principal, PD Patel Institute of Applied Sciences, Charotar University of Science and Technology, respectively, for all the help and encouragement.
SS and NY collected data and prepared the basic manuscript (given first and second authorship according to contribution) under the guidance of corresponding author AM who planned, corrected, and structured the manuscript.
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