Rhizosphere Soil Aggregation and Plant Growth Promotion of Sunflowers by an Exopolysaccharide-Producing Rhizobium sp. Strain Isolated from Sunflower Roots

Abstract

Root-adhering soil (RAS) forms the immediate environment where plants take up water and nutrients for their growth. We report the effect of an exopolysaccharide (EPS)-producing rhizobacterium (strain YAS34) on the physical properties of sunflower (Helianthus annuus L.) RAS, associated with plant growth promotion, under both water stress and normal water supply conditions. Strain YAS34 was isolated as a major EPSproducing bacterium from the rhizoplane of sunflowers grown in a French dystric cambisol. Strain YAS34 was assigned to the Rhizobium genus by 16S ribosomal DNA gene sequencing. Inoculation of sunflower seeds and soil with strain YAS34 caused a significant increase in RAS per root dry mass (dm) (up to 100%) and a significant increase in soil macropore volume (12 to 60 m in diameter). The effect of inoculation on sunflower shoot dm (up to ؉50%) and root dm (up to ؉70%) was significant under both normal and water stress conditions. Inoculation with strain YAS34 modified soil structure around the root system, counteracting the negative effect of water deficit on growth. Using [15 N]nitrate, we showed that inoculation made the use of fertilizer more effective by increasing nitrogen uptake by sunflower plantlets.Soil structure has a strong impact on a range of processes influencing crop yield. The basic units of soil structure, named aggregates, comprise solid material and pores. These aggregates determine the mechanical and physical properties of soil such as retention and movement of water, aeration, and temperature (16). Aggregate formation is an important factor controlling germination and root growth (17).Several studies have shown that formation of stable aggregates strongly depends on both the nature and the content of organic matter (10,12,14,18,29). Unstable aggregates generally have a lower content of organic matter than do stable ones (24). Plant roots contribute to soil organic material, and thereby to soil aggregate stability, directly through the root material itself (36) and indirectly through stimulation of microbial activity in the rhizosphere (4). It is generally believed that microbial action on soil aggregation is due to the production of exopolysaccharides (EPS) (25). This is supported by experimental observations demonstrating that the amendment of soil with microbial EPS results in an increased soil aggregation (14, 26).The influence of microbes on aggregate stability has largely been studied in bulk soil (15,25,34). Relatively little attention has been paid to the influence of microorganisms, particularly EPS-producing rhizobacteria, on the aggregation of root-adhering soil (RAS) (3,36). Understanding the effects of microorganisms on RAS aggregation is important because RAS forms the immediate environment where plants take up water and nutrients for their growth. Factors liable to change the physical properties of RAS can be expected to modify absorption of water and minerals by plants. In previous work, we found that inoculation of wheat with Paenibacillus polymyxa (selected for its nit...

EPS Production, PHB Accumulation and Abiotic Stress Endurance of Plant Growth Promoting Methylobacterium Strains Grown in a High Carbon Concentration

“…Our results correspond with findings for EPS production under conditions of nitrogen limitation. Data suggesting the involvement of EPS in aggregation of numerous PGPR strains have been published earlier (Alami et al, 2000;Bahat-samet et al, 2004;Burdman et al, 2000). Similar findings could also be observed in our present study in which strains CBMB20, CBMB27, and CBMB35, showing better EPS production also yielded higher flocs.…”

Effect of exo-polysaccharides producing bacterial inoculation on growth of roots of wheat (Triticum aestivum L.) plants grown in a salt-affected soil

“…inoculation, an improved biomass yield of shoot portion of the inoculated wheat plants resulted in a higher uptake of water from the soil which decreased the negative effects of the salts stress on growth of the plants. No doubt an improved RS permeability as a result of a higher soil aggregation around roots (Alami, et al 2000), a decreased soluble salts content of the rhizosphere soil resulted from binding of the excessive soluble cations into the rhizosheath-root complex through EPS-cation-soil bridging (Marshall, 1975;Morel, et al, 1991) and a positive water potential of the bacterial EPS (Miller, et al, 1996) content of the RS could all support to the plants to draw a greater quantity of water from the rhizosphere soil. However, as the increase in growth and biomass yield of the plants grown in the stressed environments follow the decrease in stress on growth (Munns, 2002), a higher soil water uptake by a well grown shoot portion of the plants could not be solely responsible for improved growth of the wheat plants inoculated with the EPS-producing bacterial isolates.…”

“…A higher insoluble soil saccharides content of RS of the inoculated than un-inoculated wheat plants was thus indication of an enhanced EPS synthesis activity in the rootzone. An increased mass of soil aggregated around roots of the inoculated than un-inoculated wheat plants and a highly significant positive correlation (r=0.866, p<0.01) of water insoluble soil saccharides with RS/root ratio signified the implication of the bacterial EPS in aggregating soil around roots over other soil microbial secretions or plant mucilages (Watt, et al, 1993;Alami, et al, 2000;Bezzate, et al, 2000). Origin of the EPS, however, could not be ascribed specifically to the inoculated EPSproducing bacterial isolates since the chemical structure and quantity of the individual component of the soil saccharides were not determined.…”

Isolation of Fluorescent Pseudomonas spp. from Diverse Agro-Ecosystems of India and Characterization of their PGPR Traits

“…All the strains differed in synthesizing the EPS (Table 3) which could be due to their metabolic diversity. Plant growth promoting effect of EPS producing bacteria has been well established (Alami et al, 2000). Role of EPS producing Rhizobium and its impact on wheat growth and soil structure was demonstrated by Kaci et al (2005).…”