Biocontrol agents
Biocontrol is the suppression of determined endeavors of one organism by one or more other organisms, often represented as natural antagonists. Biocontrol involves living organisms or natural substances to counteract or diminish the damage caused by destructive organisms (pests, weeds and microbial pathogens). They are predominantly used for the diminution of pest residents and generate yields that are free of any pest
(Molinari et al., 2019). More broadly, the term biocontrol has been applied to the use of natural products extracted from various sources of microbes. These formulations are the simple mixtures of natural ingredients with activities or complex mixtures with multiple effects on the host as well as the target pathogen
(Martins et al., 2018). In the present scenario, biological control has a significant role in reducing the effects of pests and diseases along with environmental effects. Biocontrol agents give protection to the crop through the crop period. They multiply easily in the soil and leave no residual problem
(De Silva et al., 2019). In the present review, we are confidently focusing on the growth promotion effects and antagonistic activities of
Pseudomonas on different commercial crops instead of chemical pesticides.
Pseudomonas
The genus
Pseudomonas is the most numerous among the cataloged genera of gram-negative bacteria
(Gomila et al., 2015). They are aerobic, non-spore-forming, which are straight or slightly curved about 0.5-1.0 µm by 1.5-5.0 µm and are motile employing one or more flagella, ubiquitous in agricultural soils and are well adapted to grow in the rhizosphere. It appears that they retain a very strict aerobic respiratory metabolism with oxygen
(Meyer et al., 2011). At the outset,
Pseudomonas is behaviourally versatile with free-living as well as parasitic forms capable of colonizing a wide variety of host organisms and ecological niches within hosts.
Pseudomonas retain many traits that make them well suited as biocontrol and growth-promoting agents
(Rajkumar et al., 2017). Furthermore, the typical
Pseudomonas in nature can exist in biofilms formats, attached to some surface or substrate, or in a planktonic form, as a unicellular organism and are actively swimming using its flagellum
(Gamalero et al., 2004). Classification of
Pseudomonas has been mentioned below in Table 1.
Diversity in Pseudomonas
The genus
Pseudomonas encompasses the most diverse and economically significant group of bacteria, currently containing more than 200 recognized species. Members of the genus are found in large numbers in all of the crucial natural environments and form intimate associations with plants. Members of the
Pseudomonas genus are highly adaptable as evidenced by the successful colonization of many peculiar environments and their immense deal of metabolic versatility and genetic plasticity
(Spiers et al., 2000). Substantially, they also comprise a metabolically versatile group of organisms that are known to engross numerous ecological niches including the rhizosphere and endosphere of many plants
(Alfano et al., 2000). On the contrary, the formation of spores occurs in some species, but these strains do not produce spores where there is the occurrence of retractile granules of reserve materials, which often look like spores. Henceforth many isolates are of interest as possible biocontrol agents
(Gomils et al., 2015).
Classic strains and novel concepts
Berkeley strains
On contemporary PGPR research can be traced, now nearly 4 decades old, began with the manifestation by the researchers. At the University of California, Berkeley certain isolates of
Pseudomonas that were applied to the seeds or seed pieces could colonize roots and helped to promote the growth of the potato, sugar beet and radish
(Joseph et al., 2012).
Dutch strains
A second lineage of contemporary
Pseudomonas biocontrol activity can be traced by bacterization studies with
fluorescent pseudomonads. These findings were followed by alike reports by Dutch researchers that
pseudomonads promoted the growth of potatoes
(Weller et al., 2012).
Antibiotic producers
The third lineage of contemporary
Pseudomonas biocontrol activity can be traced by bacterization studies with
fluorescent pseudomonads that produce antibiotics such as phenazine–1-carboxylic acid (PCA) and other derivatives such as 2,4- diacetylphloroglucinol (DAPG), pyrrolnitrin (Prn)
(Mann et al., 1986).
ISR pseudomonads
The fourth lineage of contemporary
Pseudomonas biocontrol activity can be traced by some
pseudomonads colonizing the roots protected plants from various pathogens by including systemic resistance
(Vilchez et al., 2000).
Need for biocontrol agents
Across the world, plant diseases are the major cause of yield loss. In certain, the use of chemical pesticides has led to numerous disadvantages which include toxic accretion of toxic residues in the environment. Hereby adaption of pathogens to such chemicals, in turn, lessen its efficiency and thus leads to a disagreeable effect on non-target organisms persuading in the same niche. Moreover, nowadays, consumers are more anxious about pesticide-free foods. This results in the emergence of eco-friendly strategies for plant disease management
(McSpadden et al., 2005).
Pseudomonas as biocontrol agents
Global interest in
Pseudomonas as biocontrol agents were evoked by research carried out at the University of California, Berkeley, during the late 1970s
(De La-Fuente et al., 2006). However, species of
Pseudomonas are efficient in employing a wide range of organic and inorganic compounds which in turn influences their capacity to live in varied environmental conditions
(McSpadden et al., 2005). Among various
Pseudomonas,
fluorescent Pseudomonas has particularly secured attention as a biocontrol agent of choice.
Pseudomonas formulated as products used as biological control agents has been mentioned in Table 2. Despite this,
Pseudomonas exerts its biocontrol activity through direct antagonism of phytopathogens and induction of disease resistance in the host plant
(Cartieaux et al., 2003).
PGPR of Pseudomonas
Most of the bacteria genera are represented as PGPR and many members of the genera
Pseudomonas implement beneficial effects on plants
(Vacheron et al., 2015). Whereas, many reports have assessed
Pseudomonas as PGPR and/or biological control agents that can affect plant fitness and are also relevant in biotechnological applications based on integrated plant-bacteria systems.
(Montesinos et al., 2002). Effect of
Pseudomonas on plant growth promotion have been mentioned in Table 4. On the other hand, different PGPR species have the mechanism of instantly enhancing plant development. Mechanism of PGPR have been mentioned in Fig 1. Strains of
Pseudomonas were able to trigger plant growth by different traits like nitrogen fixation, phosphate solubilization, production of organic acids and IAA (Indole Acetic Acid)
(Ahemad and Kibret 2014). Several researchers have selected strains of
Pseudomonas that generate gibberellin like substances in culture and reported that they are substantial to plant growth responses. These gibberellin like substances and other growth-promoting compounds were chiefly produced by
Pseudomonas (Kay et al., 2005).
Pseudomonas mediated induced systemic resistance in plants against pathogens have been mentioned in Table 3. Hence
Pseudomonas group possesses several species of rhizobacteria that have been used as strains for rhizobacteria colonization experiments. On the contrary, colonization potential is associated to hold up the nutritional balance from roots to microbes, as well as, the genes associated with rhizosphere colonization
(Compant et al., 2005) and certain genes are involved in rhizosphere colonization. Initially, a method called promoter trapping technology (IVET) accelerates the isolation of
Pseudomonas fluorescens genes which exhibited high levels of expression in the rhizosphere
(Siddiqui et al., 2006). Furthermore, this method identified 20 genes that were brought about during rhizosphere colonization and the models of expression were analyzed. As a result, fourteen genes showed significant homology to sequences in Genbank that are intricated in nutrient acquisition, stress response, or secretion
(Gamalero et al., 2004).
The current study mainly emphasizes the use of
Pseudomonas as biocontrol control agents in various crops and primarily focuses on
Pseudomonas-mediated induced systemic resistance in plant species against microbial pathogens. Microbial biological control agents are a mode of action that is used whenever microorganisms interact, communicate and regulate their co-existence between microbial cells and between microorganisms and plants. Biocontrol agents from
Pseudomonas fluorescens and also closely associated species have become eminent models for the analysis of plant protection mechanisms and secondary mechanisms. This has further resulted in a good understanding of their direct reaction to the pathogens. Likely, these biocontrol agents are also able to intrude with the functioning of the rest of the rhizosphere microbial community. Despite this, the
Pseudomonas inoculants may perform uncertainly from one field to another and also from one year to the next, as a consequence of variability in root colonization or expression of biocontrol traits. Henceforth, while the selection of particular strains superior root colonization and effectual functioning in the rhizosphere are key measures.
