Abstract
Effective translation of research findings from laboratory to agricultural fields is essential for the success of biocontrol or growth promotion trials employing beneficial microorganisms. The rhizosphere is to be viewed holistically as a dynamic ecological niche comprising of diverse microorganisms including competitors and noxious antagonists to the bio-inoculant. This study was undertaken to assess the effects due to the soil application of an endophytic bacterium with multiple pathogen antagonistic potential on native bacterial community and its sustenance in agricultural soil. Pseudomonas aeruginosa was employed as a model system considering its frequent isolation as an endophyte, wide antagonistic effects reported against different phytopathogens and soil pests, and that the species is a known human pathogen which makes its usage in agriculture precarious. Employing the strain ‘GNS.13.2a’ from banana, its survival in field soil and the effects upon soil inoculation were investigated by monitoring total culturable bacterial fraction as the representative indicator of soil microbial community. Serial dilution plating of uninoculated control versus P. aeruginosa inoculated soil from banana rhizosphere indicated a significant reduction in native bacterial cfu soon after inoculation compared with control soil as assessed on cetrimide- nalidixic acid selective medium against nutrient agar. Sampling on day-4 showed a significant reduction in P. aeruginosa cfu in inoculated soil and a continuous dip thereafter registering >99% reduction within 1 week while the native bacterial population resurged with cfu restoration on par with control. This was validated in contained trials with banana plants. Conversely, P. aeruginosa showed static cfu or proliferation in axenic-soil. Lateral introduction of soil microbiome in P. aeruginosa established soil under axenic conditions or its co-incubation with soil microbiota in suspension indicated significant adverse effects by native microbial community. Direct agar-plate challenge assays with individual environmental bacterial isolates displayed varying interactive or antagonistic effects. In effect, the application of P. aeruginosa in rhizospheric soil did not serve any net benefit in terms of sustained survival. Conversely, it caused a disturbance to the native soil bacterial community. The findings highlight the need for monitoring the bio-inoculant(s) in field-soil and assessing the interactive effects with native microbial community before commercial recommendation. varying interactive or antagonistic effects. In effect, the application of P. aeruginosa in rhizospheric soil did not serve any net benefit in terms of sustained survival. Conversely, it caused a disturbance to the native soil bacterial community. The findings highlight the need for monitoring the bio-inoculant(s) in field-soil and assessing the interactive effects with native microbial community before commercial recommendation.
Highlights
With increasing awareness about the hazardous effects of agrochemicals employed in crop husbandry, there is an impetus on the usage of safe and effective microorganisms in agriculture toward protection against biotic and abiotic stresses and in crop production (Zarb et al, 2005; Thomas and Upreti, 2015)
It was quite striking that Kan+TTC:nutrient agar (NA) supported the growth of diverse microorganisms including fungi from soil while NA normally did not support fungal growth
Pseudomonas aeruginosa proved to be a poor survivor in agricultural soil with a quick decline in the cfu of the inoculant within a week while it showed survival and proliferation under axenic conditions
Summary
With increasing awareness about the hazardous effects of agrochemicals employed in crop husbandry, there is an impetus on the usage of safe and effective microorganisms in agriculture toward protection against biotic and abiotic stresses and in crop production (Zarb et al, 2005; Thomas and Upreti, 2015). The conditions in the field are different influenced by soil, water, and edaphic factors as well as the native microbial community (van Veen et al, 1997; Bakker et al, 2013; Tyc et al, 2014). The successful performance of a bio-inoculant in the agricultural field is governed by its ability to survive in field soil and the interactive effects with the native microbiome. The introduced organisms in soil are vulnerable to physical stresses and antagonistic effects by other microorganisms (Acea et al, 1988; van Veen et al, 1997). It is important that the candidate bio-inoculant shall not cause undue biological perturbation in the native soil microbial community. Effects due to the introduced organisms on resident soil microcosm is a topic of much interest to the microbiologists (Trabelsi and Mhamdi, 2013; Tyc et al, 2014)
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