Influence of soil reaction on diversity and antifungal activity of fluorescent pseudomonads in crop rhizospheres
Introduction
Pseudomonas spp. make up a diverse group of bacteria that are generally found in all geographical conditions. Fluorescent pseudomonads are ubiquitous bacteria that are common inhabitants of rhizospheres, and are the most studied group within the genus Pseudomonas. These fluorescent pseudomonads stimulate plant growth by facilitating either uptake of nutrients from soil or producing certain plant growth promoting substances. Besides, these bacteria also prevent proliferation of phytopathogens and thereby support plant growth (Weller, 1988). However, their efficacy often varies since the populations are not distributed at random. Apart from host plants, soil factors such as composition, organic matter, pH, water and oxygen availability play significant role in selection of natural flora (Ross et al., 2000). Survival of fluorescent pseudomonads upon introduction varies widely in soils of different pH and salinity especially in rhizospheres (Rangarajan et al., 2001). The prevailing soil and/or climatic conditions greatly influence the performance of an introduced effective strain isolated elsewhere (Capper and Higgins, 1993). It is therefore essential to isolate indigenous organisms that can be used in same ecological niche. For these, the structure and diversity of bacterial community in relation to environmental factors and ecosystem functions are to be understood (Torsvik et al., 1996). The present study was, therefore, aimed to assess the effect of soil reaction on diversity and antifungal activity of rhizosphere populations of fluorescent pseudomonads associated with the host plants, tea, gladiolus, carnation and black gram grown in acidic soils under similar climatic conditions.
Section snippets
Site description and sample collection
Soil samples were collected from seven different locations (32°6′N, 76°3′E, 1300 m above msl) representing rhizospheres of mainly tea (Camellia sinensis Linn.), gladiolus (Gladiolus hortulanus L.H. Bailey), carnation (Dianthus caryophyllus Linn.) and black gram (Vigna mungo Linn.), around Palampur, the site of the institute. The soil texture was silty clay loam (30% clay, 53% silt and 17% sand) with an average pH of 5.2 and the climate is wet temperate with an average annual rainfall of 2491 mm.
Isolation and identification of fluorescent pseudomonads
Twenty-eight fluorescent pseudomonads were isolated from the rhizosphere soil samples. PCR amplification of the ITS (16S-23S) region from all 28 isolates gave a single amplicon of 560 bp size, confirming the identity of these isolates as P. fluorescens.
Biochemical characterisation
Substrate utilisation pattern showed that the isolates from all four rhizospheres were gram-negative, positive for catalase, oxidase, casein, urea and gelatin hydrolysis, indole formation and citrate utilisation with varied patterns of carbon
Discussion
Plant rhizosphere is a versatile and dynamic ecological environment of intense microbe–plant interactions for harnessing essential micro- and macro-nutrients from a limited nutrient pool (Jeffries et al., 2003). Soil has been known to play dominant role in influencing rhizosphere populations than the host plant. Endophytic populations were greatly influenced by plants rather than rhizosphere ones (Latour et al., 1996). Screening of natural populations could provide valuable isolates, with
Conclusion
The forgoing studies concluded that the bacterial populations in rhizospheric soil and their secondary metabolite production were greatly influenced by soil reaction besides providing valuable stress tolerant antagonistic isolates. However, the variation in secondary metabolite production implied that a range of other factors in natural environment might have their role as well, which are to be studied in detail.
Acknowledgements
The authors are thankful to the Director, IHBT (CSIR), Palampur for support and encouragement during the course of this investigation. This work was supported by the Council of Scientific and Industrial Research, Government of India (30-(2050)/SMM 02/2003-dated 19.1.2004) through co-ordinated network programme. IHBT Publication Number: 0588.
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