Greenhouse Microbes That Unlock Locked Phosphorus Emerge From Massive Bacterial Screen

In the humid, carefully calibrated world of greenhouse production, one of the most stubborn problems is invisible to the naked eye. Phosphorus, the macronutrient that drives root development, vegetative growth, and flower initiation, has a habit of vanishing from the reach of plants even when growers apply it generously. In peat-based soilless substrates, soluble fertilizer phosphorus quickly precipitates into insoluble complexes with metal cations, and iron phosphate in particular accumulates as a locked pool that plants cannot access. A new study published in Plant and Soil by researchers at The Ohio State University has now systematically hunted for bacteria capable of breaking that iron-phosphorus bond, screening more than a thousand bacterial isolates and validating the best candidates in live greenhouse trials with French marigolds.

The research team, led by Sachin Naik and Michelle Jones, drew on a curated collection of 1,044 rhizobacterial isolates originally recovered from ornamental greenhouse crops grown in peat-based substrates. Rather than relying on the traditional tricalcium phosphate assays that dominate the phosphate-solubilizing bacteria literature, the team targeted iron phosphate directly, reasoning that the calcium-focused screens used in most prior studies may miss the strains most relevant to soilless production. Peat substrates contain significant intrinsic iron, much of it bound to organic matter, and under the mildly acidic to near-neutral conditions typical of greenhouse containers, phosphorus is progressively sequestered into stable iron phosphate precipitates that resist conventional fertilizer corrections.

To find bacteria that could liberate this trapped nutrient, the researchers deployed a high-throughput malachite green colorimetric assay. Each isolate was grown in triplicate in minimal medium containing iron phosphate dihydrate as the sole phosphorus source, and after four days of incubation the soluble phosphorus released into the culture supernatant was quantified spectrophotometrically. The results revealed a strikingly stratified microbial landscape: nearly 70 percent of the isolates showed minimal solubilization below 8 percent, about 28 percent displayed moderate activity, and only 24 isolates, a mere 2.3 percent of the collection, exceeded the 25 percent threshold set for high performers. Notably, Bacillus velezensis, the phosphate-solubilizing strain included as a commercial reference from the biostimulant Lalrise Vita, showed almost no activity against iron phosphate, solubilizing less than 3 percent.

Whole-genome sequencing of the top performers revealed a taxonomically diverse guild dominated by the genera Pantoea, Bacillus, Priestia, Paenibacillus, Pseudomonas, and Stenotrophomonas. Average nucleotide identity analysis confirmed species-level assignments for most strains, and the isolates proved remarkably cosmopolitan in origin, having been recovered from zinnia, vinca, petunia, geranium, and coleus. This broad host distribution suggests these bacteria are not tightly host-specific but instead occupy shared rhizosphere niches across taxonomically diverse plant families, a property that could make them useful in the mixed plantings and crop rotations common in commercial greenhouse operations.

The critical question, of course, was whether laboratory solubilization would translate into real plant benefit. Twenty-seven candidate strains were tested in a high-throughput greenhouse experiment using French marigold ‘Durango Yellow’ grown in peat-perlite substrate adjusted to pH 7.0, a deliberately severe phosphorus-limiting condition. After seedling establishment, insoluble iron phosphate served as the sole phosphorus source, and bacterial treatments were applied as weekly substrate drenches for four weeks. A laser-based digital phenotyping platform measured plant biomass, leaf area, and spectral reflectance indices that track chlorophyll content, canopy greenness, and senescence. Five strains emerged as clear winners: Pantoea communis C3A8, Pantoea formicae C8D10, Pseudomonas sp. C6E7, Pseudomonas sp. C9D1, and Priestia megaterium C3F10.

Intriguingly, two of those five, strains C3F10 and C9D1, had shown only about 3 percent solubilization in the in vitro assay, yet they ranked among the top performers in the greenhouse screen. This disconnect between biochemical activity in a flask and plant growth promotion in a pot carries an important lesson for biofertilizer development: in vitro solubilization percentage alone does not reliably predict in planta performance. The researchers deliberately included low and mid-range solubilizers in their greenhouse trials to test this very question, and the answer was unambiguous. Success in the rhizosphere depends on far more than raw solubilizing chemistry, encompassing colonization ability, stress tolerance, and the competitive exploitation of root-derived carbon.

A second, more rigorous validation experiment with 30 replicate blocks confirmed the greenhouse results at pH 6.5. Pantoea communis strain C3A8 delivered the strongest overall response, significantly increasing shoot digital biomass and 3D leaf area while boosting the green leaf index and shifting canopy color composition toward healthier green hues. Most tellingly, plants inoculated with C3A8 accumulated significantly more phosphorus in their shoot tissue than controls. Because iron phosphate was the only phosphorus supplied during the treatment period, this tissue analysis provides direct evidence that the bacterium released plant-available phosphorus from the insoluble iron phosphate pool. The authors believe this is the first report of phosphate solubilization ability in P. communis, expanding the known functional diversity of a genus previously represented mainly by P. agglomerans in this literature.

The genomic analysis explains why these strains differ so markedly in performance. Pantoea communis C3A8 and Pantoea formicae C8D10 each carry three copies of the gcd gene, which encodes glucose dehydrogenase, the central enzyme in gluconic acid-mediated inorganic phosphate solubilization, along with the pqqF and pqqL genes needed to synthesize its pyrroloquinoline quinone cofactor. Both also harbor extensive arsenals of phosphatase genes for mineralizing organic phosphorus, complete phosphate transport systems, and hundreds of genes for iron acquisition and root colonization. The Pseudomonas strains took a different genomic route, pairing single gcd copies with multiple pqqL copies, three copies of phoD and ppx in C6E7, and elevated counts of nitrogen acquisition and volatile synthesis genes. Priestia megaterium C3F10, remarkably, lacks gcd entirely and may instead rely on intracellular polyphosphate accumulation and controlled release, driven by five copies of the phosphoenolpyruvate synthase gene and multiple phoR regulatory copies.

Rhizosphere competence genes added another layer of ecological insight. The Pantoea strains carry multiple gene clusters for trehalose and mannitol catabolism, pathways linked to osmoprotection and persistence under the fluctuating moisture conditions of container substrates, while C3A8 uniquely encodes an inositol catabolic cluster, a trait shown in other bacteria to be essential for rhizosphere colonization. Priestia megaterium C3F10 carries gene clusters for sucrose and levan metabolism, which in other systems support biofilm formation and root surface attachment, along with the 3-oxoadipate pathway for degrading aromatic compounds derived from lignin and phenolic root exudates. The authors are careful to note that these genomic signatures generate hypotheses about mechanism rather than demonstrated links, and that direct substrate-level measurements of water-extractable phosphorus and rhizosphere enzyme activity will be needed to confirm which solubilization pathways operate in situ.

The practical implications extend well beyond ornamental horticulture. Greenhouse production depends almost entirely on fertigation, and the limited substrate volume and low cation exchange capacity of soilless containers routinely drive fertilizer runoff, phosphorus leaching, and environmental contamination. Growers frequently over-apply phosphorus to compensate for its rapid immobilization, a practice that wastes resources, disrupts micronutrient balance, and can even induce iron deficiency in crops. The five validated strains, with their complementary mechanisms spanning gluconic acid-mediated dissolution, organic phosphorus mineralization, nitrogen acquisition, and polyphosphate dynamics, suggest that microbial consortia could deliver synergistic benefits exceeding those of any single inoculant. If follow-up work confirms performance across diverse ornamental species and substrate types, these iron phosphate solubilizers could form the basis of biofertilizer formulations that let growers cut synthetic phosphorus inputs while maintaining crop quality, turning a stubborn chemistry problem into a biological opportunity.

Subject of Research: Iron phosphate solubilizing bacteria for sustainable phosphorus management in soilless greenhouse crop production

Article Title: High-throughput screening and genomic characterization of iron phosphate solubilizing bacteria for sustainable greenhouse production

Article References: Naik, S., Quijia-Pillajo, J., Chapin, L. J., & Jones, M. L. (2026). High-throughput screening and genomic characterization of iron phosphate solubilizing bacteria for sustainable greenhouse production. Plant and Soil. https://doi.org/10.1007/s11104-026-09134-x

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09134-x

Keywords: phosphate solubilizing bacteria, iron phosphate, greenhouse production, soilless substrate, plant growth promoting rhizobacteria, Pantoea communis, Pseudomonas, Priestia megaterium, whole genome sequencing, phosphorus availability, biofertilizer, controlled environment agriculture