Abstract
Rock phosphate (RP) is a natural source of phosphorus for agriculture, with the advantage of lower cost and less impact on the environment when compared to synthetic fertilizers. However, the release of phosphorus (P) from RP occurs slowly, which may limit its short-term availability to crops. Hence, the use of P-solubilizing microorganisms to improve the availability of P from this P source is an interesting approach, as microorganisms often perform other functions that assist plant growth, besides solubilizing P. Here, we describe the characterization of 101 bacterial isolates obtained from the rhizosphere and endosphere of maize plants for their P solubilizing activity in vitro, their growth-promoting activity on millet plants cultivated in soil amended with RP, and their gene content especially associated with phosphate solubilization. For the in vitro solubilization assays, two mineral P sources were used: rock phosphate from Araxá (Brazil) mine (AP) and iron phosphate (Fe-P). The amounts of P released from Fe–P in the solubilization assays were lower than those released from AP, and the endophytic bacteria outperformed the rhizospheric ones in the solubilization of both P sources. Six selected strains were evaluated for their ability to promote the growth of millet in soil fertilized with a commercial rock phosphate (cRP). Two of them, namely Bacillus megaterium UFMG50 and Ochrobactrum pseudogrignonense CNPMS2088, performed better than the others in the cRP assays, improving at least six physiological traits of millet or P content in the soil. Genomic analysis of these bacteria revealed the presence of genes related to P uptake and metabolism, and to organic acid synthesis. Using this approach, we identified six potential candidates as bioinoculants, which are promising for use under field conditions, as they have both the genetic potential and the experimentally demonstrated in vivo ability to improve rock phosphate solubilization and promote plant growth.
Highlights
Phosphorus (P) is an important macronutrient for plant development due to its structural role in the synthesis of nucleic acids and membranes
In this study we aimed to: (1) obtain bacterial isolates from the rhizosphere of maize plants in soil fertilized with rock phosphate (RP) and soil without P fertilization; (2) determine the in vitro potential of these isolates, together with that of other rhizospheric and endophytic bacteria previously isolated from maize plants of the same EMS farm, for the solubilization of P; (3) evaluate their ability to stimulate millet growth in non-sterile soil fertilized with RP in a greenhouse environment; and (4) investigate the gene pool related to phosphorus solubilization and uptake of six selected strains, in order to better understand the relation considered positive and were stored at −80◦C in Tryptone Soy between genotype, phenotype and fitness
101 bacteria were used, 32 of which were isolated from maize rhizosphere by enrichment technique using the Araxá Phosphate (AP) as P source, described in this study, 2 were isolated from maize rhizosphere and obtained from the Multifunctional Microorganisms Collection from Embrapa Maize and Sorghum, Brazil (CMMF-EMS), 57 endophytes were previously isolated from maize and belong to the UFMG Applied Microbiology Laboratory (LMA-UFMG) culture collection, and 10 endophytes were isolated from maize and kindly provided by CMMF-EMS (Supplementary Table S1)
Summary
Phosphorus (P) is an important macronutrient for plant development due to its structural role in the synthesis of nucleic acids and membranes It plays an essential role in practically all processes involving energy transfer, such as photosynthesis; an adequate P supply is required for plant growth, and its bioavailability in soil can influence crop productivity (Bisson et al, 2017). When P is added to soil in the form of soluble fertilizers, it rapidly forms insoluble complexes with calcium in alkaline soils, and with iron, aluminum and silicates in tropical acidic soils (Novais and Smyth, 1999) This process makes soil P less available to plants, thereby affecting their productivity (Devau et al, 2009). In order to compensate for this drawback, several authors have evaluated the simultaneous application of RP and solubilizing microorganisms to the soil, as a way to increase the availability of P from this less soluble source (Ribeiro et al, 2018; Yagi et al, 2020)
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