Abstract
Exopolysaccharides (EPS) of microbial origin with novel functionality, reproducible physico-chemical properties, are important class of polymeric materials. EPS are believed to protect bacterial cells from dessication, produce biofilms, thus enhancing the cells chances of bacterial colonizing special ecological niches. In rhizosphere, EPS are known to be useful to improve the moisture-holding capacity. Three Bacillus spp. strains identified by 16s rDNA sequence analysis as B. amyloliquefaciens strain HYD-B17; B. licheniformis strain HYTAPB18; B. subtilis strain RMPB44 were studied for the ability to tolerate matric stress and produce EPS under different water potentials. EPS production in all the three Bacillus spp strains increased with increasing water stress indicating correlation between drought stress tolerance and EPS production. Among the isolates, strain HYD-17 showed highest production of EPS. The exopolysaccharide composition of the three strains was further analyzed by HPLC. Drought stress influenced the ratio of sugars in EPS and glucose was found as major sugar in strains HYTAPB18 and RMPB44 whereas raffinose was major sugar found in strain HYD-B17. Inoculation of EPS producing Bacillus spp. strains in soil resulted in good soil aggregation under drought stress conditions at different incubation periods. This study shows that exposure to water stress conditions affects the composition and ratios of sugars in EPS produced by Bacillus spp. strains HYD-B17, HYTAPB18 and RMPB44 influencing abiotic stress tolerance of the microorganisms.
Highlights
Dry land areas are stressed environments in terms of water availability and fluctuations in moisture content
This study shows that exposure to water stress conditions affects the composition and ratios of sugars in EPS produced by Bacillus spp. strains HYD-B17, HYTAPB18 and RMPB44 influencing abiotic stress tolerance of the microorganisms
Bacillus spp. strains namely, B. amyloliquefaciens HYD-B17; B. licheniformis HYTAPB18; B. subtilis RMPB44, used in the present study were isolated (Sandhya et al, 2009) from rhizosphere of crop plants grown under different semi arid zones and screened for drought tolerance in Trypticase soya agar (TSA) at different water potentials (-0.05, -0.15, -0.30, -0.49, -0.73 matric potentials (MPa)) using polyethylene glycol (PEG 6000) The 16S rDNA gene sequences of these strains have been submitted to GenBank under the accession numbers GU120097, GQ160905 and GQ160904 respectively
Summary
Dry land areas are stressed environments in terms of water availability and fluctuations in moisture content. Their components and content heavily depend on many factors, such as bacterial group, cultivation time, substrate and growth state (Read et al, 1987) They are rich in high molecular weight polysaccharides (10 to 30 kDa) and have wide range of chemical structures of homopolymeric or heteropolymeric type, made up of sugar and non-sugar components, and the range of monosaccharide combinations, together with non-carbohydrate substituents and varied linkage types, makes the EPS an excellent agent and attributes diversity in bacteria (Celik et al, 2008; Kumar et al, 2007). EPS from bacteria are complex mixture of high molecular weight polymers forming microbial aggregates that adhere to surfaces and help in cementing soil particles together In their native state, polysaccharides are degraded by other microorganisms but they appear to be protected from such degradation soon after they are firmly bound within an aggregate or incorporated into clay lattice. It is aimed to investigate effect of water stress on monosaccharide composition of EPS
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