Biological control of crops pests using plant growth promoting rhizobacteria

Kandan Aravindaram / Rajagopal Rangeshwaran *

The Pseudomonas fluorescens group of bacteria is very diverse and comprises members that make part of the beneficial rhizospheremicrobiota that cooperates with the plant (Mendes et al., 2013; Venturi and Keel, 2016), include stimulation of plant growth and defense, mobilization of soil nutrients and suppression of phytopathogenic fungi, protists and bacteria via antimicrobial compounds (Haas and Defago, 2005).

Recent research identified a phylogenetically distinct P.fluo-rescens subgroup, specified by strains of Pseudomonas protegens and Pseudomonas chlororaphis which exhibits potent insecticidal activities as an extra (Ruffner et al., 2015; Fluryet al., 2016).

In a typical course of infection, ingested entomopathogenic pseudomonads colonize the gut, breach the intestinal epithelial barrier, invade the hemocoel, proliferate, and eventually kill the insect (Kupferschmied et al., 2013), the virulence factors and mechanisms contributing to their capacity to invade and kill insects currently are largely unknown.

A major insect virulence factor identified so far is the Fit toxin, which typically is produced by strains of P.protegens and P.chloro-raphis and is significant for their pathogenicity towards Lepidopteran larvae (Flury et al., 2016). Mutants lacking the Fit toxin retain substantial toxicity, stressing that insect pathogenicity of these bacteria is multifactorial.

Loper at al. (2016) report on the identification of additional insect pathogenicity factors of a representative insecticidal pseudomonad, i.e. P.protegens Pf-5, in an oral infection model. Their approach was inspired by the observation that P.prote-gens mutants defective for the global regulator GacA are severely impaired in oral toxicity to Dipteran and Lepidopteran insects (Ruffner et al., 2013; Flury et al., 2016).

Loper et al. (2016) identified rhizoxin as a major factor in oral toxicity of P.protegens Pf-5 towards Drosophila. The macrolide molecule interferes with mitosis in eukaryotic cells, which enables broad antifungal, cytotoxic, and phytotoxic activities, and it has demonstrated function in plant pathogen suppression (Loper et al., 2008).

Insect toxicity now emerges as an additional feature of rhizo-xin, illustrating that P.protegens can deploy certain toxic secondary metabolites for plant-beneficial as well as for insect-pathogenic activities.

Remarkably, the rhizoxin gene cluster is only present in a subset of P.protegens strains and linked to the Fit toxin gene cluster and both clusters are part of a dynamic genomic region, which likely evolved via events involving horizontal transfer.

MECHANISMS OF PGPR

Fluorescent Pseudomonad exhibits multiple numbers of mechanisms to promote plant growth and to serve as potential biocontrol agents. Generally, PGPR traits associated with the biocontrol of plant pests include:
1. Synthesizing va- rious phytohormones inclu-ding auxins and cytokinins.
2. Providing mechanisms for the solubilization of mi-nerals such as phosphorus.
3. Antibiotic synthesis.
4. Secretion of iron binding siderophores to obtain soluble iron from the soil and provide it to a plant thereby deprive fungal pathogens in the vicinity, of soluble iron.
5. Production of low molecular weight metabolites such as hydrogen cyanide with antifungal activity.
6. Production of enzymes including chitinase, b-1-3-glucanase, protease and lipase which can lyse some fungal cells. 7. Production of oxidative stress enzymes such as catalases, super-oxide dismu-tases, peroxidase and poly- phenol oxidases for scaven-ging active oxygen species.
8. Out-competing phytopathogens for nutrients and occyping niches on the root surface.
9. Lowering the production of stress ethylene in plants with the enzyme ACC deaminase.

Regarding the genus Pseudomonas, for example, the insect association found for Pseudomonas entomo-phila, which is related to the soil bacterium Pseudomonas putida, appeared to be unique (Vodovar et al., 2006). Although, genome wide comparisons of members from the genus Pseudomonas revealed several loci encoding potential virulence factors with insecticidal properties.

Studies have shown that the efficacy of rhizobacteria as biological control agents against foliar pathogens is enhanced by mixing individual strains (Liu et al., 2016). Similarly, several reports demonstrated that PGPR can affect phyllo-plane-feeding insects.

Earlier Radjacommare et al. (2006) reported that P.fluorescens very effectively reduce the rice leaf folder population and enhanced the attraction of plant predators in rice field conditions. Saravanakumar et al. (2008) showed that treatment of rice plants with a combination of three P.fluorescens strains (Pf1, TDK1, and PY15) led to higher activity of polyphenol oxidase (PPO) and lipoxygenase (LOX) compared to plants treated with individual strains, chemical or untreated controls.

This increase in PPO and LOX correlated with malformation of adult leaf folder,Cnaphalocrocismedinalis in rice plant (Saravana-kumar et al., 2008). Kloepper et al. (2013) reported that cotton plants treated with Blend-9 emitted higher amounts of plant volatiles following Helio-thisvirescens larvae infesta- tion. Gadhave and Gange (2016) showed that both individual strains of the tested Bacillus spp. (i.e., Bacillus cereus, Bacillus subtilis, and Bacillus amyloliquefaciens) and their mixtures suppressed growth and development of Brevi-corynebrassicae.

Zebelo et al. (2016) reported that mixtures of bacilli strains (Blend-8 and Blend-9) induced cotton resistance and reduced growth and development of S.exigua via increased level of gossypol.

Elicitation of vigorous plant growth is a main characteristic of some rhizobacteria in addition to their biological control potential. Several studies have shown that with some rhizobacteria strains, early plant growth is often related to induced systemic resistance and improved marketable yields (Liu et al., 2016).

The presence of both IAA and PAA metabolic pathways in all the strains from the P.chlororaphis group suggests that modification of plant hormone levels could be one of the main mechanisms for plant-bacteria interactions common to all P.chloro-raphis strains.

CONCLUSION

Biocontrol of insect pests by PGPR is multifactorial and important mechanisms include antagonism between the biocontrol agents and the pests, degradation of virulence factors and induction of systemic resistance in the plant.

Not surprisingly, both the biocontrol agents and the pests have developed defense mechanisms to minimize the impact of antagonism and some of these strategies have been shown to be active in the rhizosphere. Under field conditions, the application of biocontrol PGPR strains has given promising results in cereals, vegetables, fruit and ornamental plant production.

Mixtures of PGPR strains combining antibiosis and ISR might be most effective in practice. Under field conditions, the efficacy and consistency of biocontrol PGPR still needs to be improved. The market for biocontrol PGPR is relatively small at present, but has the potential to grow as they provide an environmentally friendly means to control pathogens.

At present, there are many scientificchallenges for research in the field of biocontrolPGPR. It will be important to exploit molecular techniques to study the genome expression of plant-beneficial and plant pathogenic microorganisms in situ, and to obtain a fuller picture of rhizosphere biodiversity.

The discovery of many traits and genes that are involved in the beneficial effects of PGPR has resulted in a better understanding of the performance of bioinoculants in the field, and provides the opportunity to enhance the beneficial effects of PGPR strains by genetic modification.


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