Gene prospection and mechanisms of phosphate solubilization in selected bacterial and fungal strains

Abstract

Phosphorus (P) is one of the most required elements by organisms. In agriculture, P is mainly supplied through mineral phosphate fertilization, in forms readily available to plants. In soil, P dynamics are complex and only a small fraction of the phosphate fertilizer may be available for plants and microorganisms, while a large part of P can be strongly retained in the mineral fraction of the soil or incorporated into microbial biomass. Microorganisms play an important role in the P cycling and bacteria and fungi are the main contributors to the cycling of this element between the organic and mineral fractions. In this sense, there are numerous reports of phosphate solubilization by bacteria and fungi and the some mechanisms, despite already known, still need to be better understood to reach biotechnological potential. The objectives of this work were to evaluate the potential and mechanisms of phosphate-solubilizing bacterial and fungal strains in the solubilization of calcium and iron phosphates, evaluate the potential of bacterial strains to promote maize growth and identify genes related to organic acid production in the bacterial strains. In the first work, in vitro solubilization experiments with bacterial strains were carried out in order to compare the potentials and mechanisms of these organisms. Then, genomic DNA was sequenced to identify the species and search for genes related to the production of organic acids. The strains were inoculated into maize plants and the plant's growth promotion potential was evaluated as a function of the strains' solubilization capacity. Paraburkholderia fungorum UFLA 04-21 and Pseudomonas anuradhapurensis UFPI B5-8A solubilized more than 60% of phosphate in vitro and produced large amounts of citric/maleic and gluconic/tartaric acids, respectively. All 11 organic acids studied were identified, although most strains did not produced all of them. Most of the genes and enzymes related to the production of organic acids were found in the genomes. Plants inoculated with UFPI B5-6, UFPI B5-8A, and UFLA 03-10 strains accumulated more biomass than plants fertilized with Bayóvar phosphate alone. Strains UFLA 03-10 and UFPI B5-8A increased the accumulation of most macronutrients, including P, and some micronutrients. In the second work, bacterial and fungal strains were compared regarding the potentials and mechanisms of calcium and iron phosphate solubilization. Acinetobacter pittii UFLA 03-09 and Penicillium flavigenum E24 were the microorganisms that solubilized most of the calcium phosphate. Iron phosphate was solubilized to a considerably lesser extent due to phosphate characteristics and the most promising microorganisms were Brukholdeia cepacia LMG 1222 and Penicillium flavigenum E24. Bacteria and fungi had significant correlations between soluble P and organic acids when grown in calcium phosphate medium, but only fungi had a significant correlation for iron phosphate. All the low molecular weight organic acids studied were identified in the strains and the genomic analysis identified several genes linked to the production of organic acids. The results contribute to the understanding of phosphate solubilization processes by microorganisms, especially considering bacterial and fungal strains with high potential for plant growth promotion.