Phosphate (P)-solubilizing microorganisms as a group form an important part of the microorganisms, which benefit plant growth and development. Growth promotion and increased uptake of phosphate are not the only mechanisms by which these microorganisms exert a positive effect on plants. Microbially mediated solubilization of insoluble phosphates through release of organic acids is often combined with production of other metabolites, which take part in biological control against soilborne phytopathogens. In vitro studies show the potential of P-solubilizing microorganisms for the simultaneous synthesis and release of pathogen-suppressing metabolites, mainly siderophores, phytohormones, and lytic enzymes. Further trends in this field are discussed, suggesting a number of biotechnological approaches through physiological and biochemical studies using various microorganisms.
One of the most studied approaches in solubilization of insoluble phosphates is the biological treatment of rock phosphates. In recent years, various techniques for rock phosphate solubilization have been proposed, with increasing emphasis on application of P-solubilizing microorganisms. The P-solubilizing activity is determined by the microbial biochemical ability to produce and release metabolites with metal-chelating functions. In a number of studies, we have shown that agro-industrial wastes can be efficiently used as substrates in solubilization of phosphate rocks. These processes were carried out employing various technologies including solid-state and submerged fermentations including immobilized cells. The review paper deals critically with several novel trends in exploring various properties of the above microbial/agro-wastes/rock phosphate systems. The major idea is to describe how a single P-solubilizing microorganism manifests wide range of metabolic abilities in different environments. In fermentation conditions, P-solubilizing microorganisms were found to produce various enzymes, siderophores, and plant hormones. Further introduction of the resulting biotechnological products into soil-plant systems resulted in significantly higher plant growth, enhanced soil properties, and biological (including biocontrol) activity. Application of these bio-products in bioremediation of disturbed (heavy metal contaminated and desertified) soils is based on another important part of their multifunctional properties.
Arbuscular mycorrhizal fungi is an important group of soil microorganisms which form beneficial symbiotic associations with roots with a wide range of plants thus improving plant growth, nutrition and health. This paper reviews the current status of preparation and formulation of mycorrhizal inoculum applying polymer materials with determined characteristics. The most widely used methods are based on the entrapment of fungal materials in natural polysaccharide gels. The potential of such inoculant preparations is illustrated by various studies which include immobilization of mycorrhized root pieces, vesicles and spores, in some cases co-entrapped with other plant beneficial microorganisms. Suggestions for further research in this field are also discussed.
Bacillus thuringiensis in entrapped in k-carrageenan was capable of producing indole-3-acetic acid (IAA) in the presence of rock phosphate ore (RPO). After five repeated batch fermentation experiments, an average of 6.9 mg=L IAA was recorded in the presence of 1.5 g=L RPO and 4.7 mg=L IAA in the RPO-free control. The bacterium simultaneously solubilized phosphates with a maximum soluble P of 115 mg=L and 150 mg=L after the fourth batch cycle in 1.5 and 3.0 g=L RPO amendment, respectively. Addition of tryptophan to the media increased IAA production to 13.9 and 20.7 mg=L per batch on RPO-free and 1.5 g=L RPO-supplemented media with poor bacterial growth and RPO solubilization.The solubilization of Phosphorus (P) in the rhizosphere is the most common mode of action of PGPR (plant growth-promoting bacteria) that increases nutrient availability to plants (Rodriguez and Fraga, 1999). Reports are available on RPO (rock phosphate ore) solubilization by microbes in flask cultures and soil (Whitelaw, 2000;Vassilev and Vassileva, 2003). RPO solubilization was attributed to the excretion of microbial metabolites such as organic acids or proton extrusion associated with ammonium assimilation in liquid media (Seshadri et al., 2004).The advantages of applying gel-entrapped microorganisms in RPO solubilization in repeated-batch fermentations and soil-plant systems have been demonstrated earlier (see review in Vassilev et al., 2001). In an earlier study, we have reported increases in growth and P in plants co-inoculated with gel-entrapped Bacillus thuringiensis (B.t.) and mycorrhizae that could be attributed to possible P-solubilization and phytohormone production (Marulanda et al., 2002). This is not surprising, since plant growth-promoting rhizobacteria have long been known to engage in both the solubilization of RPO and production of phytohormones (Vessey, 2003). However, there is a lack of in vitro studies on simultaneous RPO solubilization and IAA Address correspondence to Nikolay Vassilev, 441 production. We made an attempt to prove the simultaneous production of indole-3-acetic acid (IAA) and RPO solubilization using gel-entrapped B.t. under in vitro conditions. B.t. isolated from a local desertified soil from the Alicante province (Spain), (Marulanda et al., 2002) was used in this study. Bacterial cells (1 Â 10 6 colony forming units=mL) were inoculated to accumulate biomass, which was further used for encapsulation. Cells harvested in the exponential phase were entrapped in k-carrageenan as described earlier (Vassileva et al., 1998). About 10 mL gel beads (3.10 4 CFU=250 beads), rinsed with sterile distilled water, were transferred to 100 mL of cultivation medium (Oxoid nutrient broth) in 250 mL Erlenmeyer flasks. RPO (Morocco fluorapatite; 1 mm mesh; 12.8% P) was added to the medium at 1.5 and 3.0 g=L concentrations. To study the impact of tryptophan on IAA production, another set of experiments was carried out with the addition of 0.01% tryptophan to the medium containing 1.5 g=L RPO. Repeated batch fermentation (five...
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