Cultural techniques capture diverse phosphate-solubilizing bacteria in rock phosphate-enriched habitats

Abstract

Phosphorus (P) deficiency is a common problem in croplands where phosphate-based fertilizers are regularly used to maintain bioavailable P for plants. However, due to their limited mobility in the soil, there has been an increased interest in microorganisms that can convert insoluble P into a bioavailable form, and their use to develop phosphate-solubilizing bioinoculants as an alternative to the conventional use of P fertilizers. In this study, we proposed two independent experiments and explored two entirely different habitats to trap phosphate-solubilizing bacteria (PSBs). In the first experiment, PSBs were isolated from the rhizoplane of native plant species grown in a rock-phosphate (RP) mining area. A subset of 24 bacterial isolates from 210 rhizoplane morphotypes was selected for the inorganic phosphate solubilizing activities using tricalcium phosphate (TCP) as the sole P source. In the second experiment, we proposed an innovative experimental setup to select mycohyphospheric bacteria associated to arbuscular mycorrhizal fungal hyphae, indigenous of soils where agronomic plant have been grown and trapped in membrane bag filled with RP. A subset of 25 bacterial isolates from 44 mycohyphospheric morphotypes was tested for P solubilizing activities. These two bacterial subsets were then screened for additional plant growth-promoting (PGP) traits, and 16S rDNA sequencing was performed for their identification. Overall, the two isolation experiments resulted in diverse phylogenetic affiliations of the PSB collection, showing only 4 genera (24%) and 5 species (17%) shared between the two communities, thus underlining the value of the two protocols, including the innovative mycohyphospheric isolate selection method, for selecting a greater biodiversity of cultivable PSB. All the rhizoplane and mycohyphospheric PSB were positive for ammonia production. Indol-3-acetic acid (IAA) production was observed for 13 and 20 isolates, respectively among rhizoplane and mycohyphospheric PSB, ranging, respectively, from 32.52 to 330.27 μg mL−1 and from 41.4 to 963.9 μg mL−1. Only five rhizoplane and 12 mycohyphospheric isolates were positively screened for N2 fixation. Four rhizoplane PSB were identified as siderophore producers, while none of the mycohyphospheric isolates were. The phenotype of one PSB rhizoplane isolate, assigned to Pseudomonas, showed four additive PGP activities. Some bacterial strains belonging to the dominant genera Bacillus and Pseudomonas could be considered potential candidates for further formulation of biofertilizer in order to develop bioinoculant consortia that promote plant P nutrition and growth in RP-enriched soils.

Rhizospheric Bacteria of Cover Legumes from Acidic Soils Are Capable of Solubilizing Different Inorganic Phosphates

“…The high solubilization potential of the bacteria identified in this study is attributed to their ability to produce and release substances that facilitate the conversion of insoluble phosphates into soluble forms. According to [ 14 ], this process can be carried out through various mechanisms such as the production of organic acids (citric acid, acetic acid, lactic acid, and gluconic acid) that lower the pH of the immediate environment, causing the dissolution of insoluble phosphate compounds such as iron and aluminum phosphates, converting them into soluble phosphate forms. Another process involves the release of enzymes such as phosphatases and phytases, which catalyze the release of phosphorus from organic and inorganic compounds, making it available for plant uptake [ 12 , 13 ].…”

“…These bacteria can also produce chelating compounds (siderophores) that bind to cations such as iron and aluminum. This reduces the formation of insoluble phosphates and releases phosphorus in soluble forms [ 14 ]. Additionally, phosphate-solubilizing bacteria can participate in other biochemical processes, such as the reduction of Fe(III) to Fe(II), which can also contribute to phosphorus solubilization [ 6 ].…”

“…Furthermore, some rhizospheric bacteria, including those from the genera Pseudomonas sp., Bacillus sp., Brevibacillus sp., Paenibacillus sp., and Brevibacterium sp. [ 14 ], have the ability to secrete organic acids such as lactic acid, acetic acid, succinic acid, and palmitic acid, which can acidify the surrounding environment, thus solubilizing aluminum and iron phosphates which are typically more insoluble in acidic soils [ 15 ]. Likewise, through increasing the accessibility of phosphorus for plants, these bacteria can reduce the need for phosphate fertilizers.…”

Evaluating Rhizobacterial Antagonists for Controlling Cercospora beticola and Promoting Growth in Beta vulgaris

“…Both bacterial and fungal agents have demonstrated their potential as BCAs [23,24], addressing a wide spectrum of diseases, notably including the management of C. beticola [25,26]. Furthermore, rhizospheric bacteria are recognized for their abilities to promote plant growth, including traits such as alleviating soil nutrient deficiencies (e.g., nitrogen fixation, phosphorus solubilization, and improving iron uptake), synthesizing plant growthpromoting hormones, inhibiting ethylene production through 1-aminocyclopropane-1carboxylate deaminase activity [27,28], and stimulating overall plant development [29,30], which extends to sugar beet cultivation [31,32].…”