Biological control of crops pests using plant growth promoting rhizobacteria
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Kandan Aravindaram and Rajagopal Rangeshwaran
Contd from previous issue
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.
The writers are from ICAR-National Bureau of Agricultural Insect Resources, Hebbal, Bangalore, Karnataka
For further details contact: - Public Relations & Media Management Cell, CAU, Imphal. Email: [email protected]
Contd from previous issue
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.
The writers are from ICAR-National Bureau of Agricultural Insect Resources, Hebbal, Bangalore, Karnataka
For further details contact: - Public Relations & Media Management Cell, CAU, Imphal. Email: [email protected]