Soybean Roots Recruit Phosphate-Solubilizing Bacteria Through a Starvation Signal

Phosphorus is one of the most stubborn nutrients in agriculture. Although soils may hold vast total reserves of the element, most of it is locked into insoluble mineral forms that plant roots cannot absorb, forcing farmers to apply phosphate fertilizers that are increasingly expensive and derived from finite rock deposits. A study published in the journal Microbiome by a team at the Root Biology Center of Fujian Agriculture and Forestry University, working with colleagues at South China Agricultural University, now reveals an elegant strategy that soybean plants use to solve this problem themselves: when starved of phosphorus, they switch on a conserved internal signaling pathway that actively recruits soil bacteria capable of unlocking insoluble phosphate. The finding, reported by Xiaoqian Wang, Jianfeng Lei, Kefei Zhang and senior authors Hong Liao and Yongjia Zhong, provides one of the most complete mechanistic pictures to date of how a plant’s nutritional status shapes its own microbiome.

The central player in the new work is the phosphate starvation response, or PSR, pathway, a transcriptional signaling cascade that plants deploy when phosphorus supplies run low. In model plants such as Arabidopsis, the PSR pathway is orchestrated by a family of MYB-like transcription factors known as PHR proteins, which bind to specific DNA motifs and activate hundreds of genes involved in phosphorus scavenging, including high-affinity phosphate transporters, purple acid phosphatases and genes that remodel root architecture. Soybean carries its own suite of PHR genes, and the researchers focused on one member, GmPHR33, as a potential hub connecting internal phosphorus status to the composition of the microbial communities living on and around the roots. What makes this study distinctive is that it connects this well-characterized internal signaling system to an external ecological outcome: the selective enrichment of particular bacterial partners in the rhizosphere.

The investigation began in the field rather than in a growth chamber. The team grew soybean under high-phosphorus and low-phosphorus field conditions and used 16S rRNA gene amplicon sequencing, processed through a QIIME2 pipeline, to profile the bacterial communities inhabiting the bulk soil, the rhizosphere, the root surface and the root interior, including nodules. Across these root-associated compartments, phosphorus deficiency produced a consistent and striking pattern: the relative abundance of Burkholderia, a bacterial genus famous for its ability to solubilize mineral phosphate, rose significantly under low-phosphorus conditions. This was not a laboratory artifact confined to sterile conditions; it was a reproducible shift in the field, suggesting that the plant itself was doing something to favor these phosphate-solubilizing bacteria when the nutrient became scarce.

To test whether the PSR pathway was genuinely responsible for this recruitment, the researchers manipulated the pathway genetically in both directions. When they overexpressed GmPHR33, creating soybean lines in which the phosphate starvation response was constitutively active regardless of external phosphorus supply, the roots of these plants became markedly more enriched in Burkholderia. Conversely, when they knocked out GmPHR33 using CRISPR-Cas9 gene editing, a vector for which was kindly provided by Yuefeng Guan, the enrichment of Burkholderia was repressed. The dose-response logic of this experiment is compelling: turning the signal up increases bacterial recruitment, and turning it down reduces recruitment, which is exactly what one would expect if the PSR pathway were the causal driver rather than a passive correlate of phosphorus status.

With the genetic switch identified, the next question was how a transcription factor inside root cells communicates with bacteria living outside in the soil. The answer, according to the study, lies in root exudation, the process by which roots release a cocktail of organic compounds into the rhizosphere. The team found that the PSR pathway regulates the exudation of malate, a simple organic acid that many phosphate-solubilizing bacteria can metabolize and that also happens to chelate metal cations, thereby helping to release phosphate from insoluble calcium and iron complexes. Malate thus serves a double function: it is both a chemical attractant and a carbon reward for Burkholderia, and simultaneously a direct agent of phosphate mobilization in the soil solution.

The molecular wiring of this exudation control was traced to a specific module. Among the members of the GmALMT family of aluminum-activated malate transporters, whose expression the researchers examined under phosphorus deficiency, one gene, GmALMT29, emerged as the mediator of malate release. When the team knocked out GmALMT29, the enrichment of a labeled phosphate-solubilizing Burkholderia strain on soybean hairy roots dropped significantly, confirming that this transporter is a functional link in the chain connecting PSR activation to bacterial recruitment. In effect, the plant runs a signaling circuit that reads phosphorus levels, and when levels fall, the circuit opens a gate that pours malate into the rhizosphere, drawing in bacterial partners that repay the favor by solubilizing phosphate.

The researchers did not stop at correlation and gene expression; they isolated and identified actual phosphate-solubilizing Burkholderia strains from the soybean rhizosphere, including a strain designated RBC-PS25, and labeled it so they could visualize and quantify its enrichment on roots. In a particularly telling experiment, they knocked out CheW, a gene essential for bacterial chemotaxis, in RBC-PS25. The chemotaxis-defective mutant was no longer efficiently enriched on soybean roots, and plants inoculated with it showed reduced phosphorus uptake compared with plants colonized by the wild-type strain. This result demonstrates that the active movement of Burkholderia toward the root, guided by chemical gradients of exuded compounds, is what converts the plant’s signal into a tangible nutritional benefit.

The study also hints at broader relevance beyond soybean. In supplementary analyses, the team observed that peanut, another legume, similarly enriches Burkholderia to cope with phosphorus deficiency, suggesting that this recruitment mechanism may be a shared strategy among legumes rather than a soybean-specific quirk. Given that legumes already host nitrogen-fixing rhizobia in specialized nodules, the new findings add a second, less formalized symbiosis to their repertoire: a looser, exudate-mediated partnership with free-living phosphate solubilizers that does not require nodule organogenesis but delivers a critical nutrient all the same. The rhizosphere, in this view, functions as an extension of the plant’s own phosphorus acquisition machinery, recruited on demand.

The technical rigor of the work deserves note. The authors validated their community profiles with careful comparisons between bulk soil and rhizosphere samples, confirmed that phosphorus treatment influenced nodule bacteria and root microbes in defined ways, generated transgenic GmPHR33-overexpression and Gmphr33 knockout lines and verified their growth performance and phosphorus concentrations, and made their QIIME2 pipelines and R analysis scripts available in the supplementary materials for reproducibility. Field trials were supported by the Root Biology Center, and the work was funded by the National Key Research and Development Program of China, the National Natural Science Foundation of China and provincial science foundations, reflecting the strategic priority that China places on nutrient-efficient cropping systems.

The practical implications could be significant. Phosphate rock reserves are concentrated in a handful of countries and are a non-renewable resource, while phosphorus runoff from over-fertilized fields drives eutrophication of waterways. If breeders can select or engineer soybean and other crops for stronger PSR pathway activity, or for higher GmALMT29-mediated malate exudation, they could effectively farm the soil microbiome as a living phosphorus fertilizer, reducing external inputs. The study also suggests that inoculant formulations containing phosphate-solubilizing Burkholderia would work best in combination with plant genotypes that actively signal and feed these bacteria. More fundamentally, the research reframes the plant microbiome not as a passive accident of soil chemistry but as a managed resource, shaped by an internal nutrient-sensing circuit that evolution has tuned over millions of years. As the authors conclude, legumes possess a novel mechanism to enrich functional microbes that help them cope with phosphorus deficiency, and learning to speak that chemical language fluently may become one of the most important tools in sustainable agriculture.

Subject of Research: How the phosphate starvation response pathway in soybean enriches phosphate-solubilizing Burkholderia to improve phosphorus acquisition

Article Title: The PSR signaling pathway facilitates enrichment of P-solubilizing Burkholderia to enhance P acquisition in soybean

Article References: Wang, X., Lei, J., Zhang, K., Shi, J., Li, Y., Zhai, C., Zhao, X., Xu, H., Wang, C., Xu, R., Wang, X., Liao, H., & Zhong, Y. (2026). The PSR signaling pathway facilitates enrichment of P-solubilizing Burkholderia to enhance P acquisition in soybean. Microbiome. https://doi.org/10.1186/s40168-026-02555-x

Image Credits: AI Generated

DOI: 10.1186/s40168-026-02555-x

Keywords: phosphate starvation response, phosphate-solubilizing bacteria, Burkholderia, soybean, rhizosphere microbiome, malate exudation, GmPHR33, GmALMT29, plant-microbe interaction, phosphorus acquisition, root exudates, sustainable agriculture