Expression of the ptxD gene confers soybean effective metabolism of phosphite, which modulates growth, phosphate signaling, and defense mechanisms in a phosphate-dependent manner

Optimizing phosphorus fertilizer usage while effectively controlling weeds are two major needs in agriculture. The Phi-ptxD system, based on the expression of the ptxD bacterial gene, which encodes a phosphite oxidoreductase that enables plants to utilize phosphite (Phi) as the sole phosphorus (P) source, has provided promising outcomes in both model plants and crops. Additionally, Phi has gained popularity as a plant biostimulant when used in well-fertilized plants, but the mechanisms through which it promotes plant growth and activates defense responses are just beginning to be unraveled. Therefore, providing further benefits to the Phi-ptxD cultivation system. In this study, we demonstrated that the expression of a codon-optimized ptxD sequence in soybean confers plants the capacity to effectively metabolize Phi as the only P source, which becomes highly important in low-P environments. We conducted a comprehensive characterization of the transgenic plants and the non-transformed counterpart using transcriptomics, metabolomics, and high-throughput plant phenotyping technologies to understand in detail the effects of Phi when used as fertilizer in the context of optimal and deficient P fertilization. Our findings demonstrate that Phi significantly enhances the growth, metabolism, and stress responses of soybean, with its effects highly dependent on Pi availability. In Wt plants, for instance, Phi enhances growth under optimal P-fertilization conditions, suppresses the responses to Pi starvation at short time frames, and induces phytotoxicity in the long term. Phi triggered the upregulation of genes related to ABA and JA signaling, suggesting that the Phi treatment enhances plant defense and stress adaptation, which correlated with the metabolomic analysis data that shows increased levels of osmoprotectants such as proline, sorbitol, and mannitol, as well as phenylpropanoids and coumarins associated with antioxidant defenses and biotic stress responses. Hyperspectral imaging provided additional understanding, revealing distinct spectral shifts in response to Phi and Pi treatments, suggesting alterations in pigment composition.This study shows the multiple interactions between Phi and Pi signaling and stress responses, providing data that can be used in further research to inform a possible mechanistic explanation of how Phi acts as a beneficial molecule and growth enhancer. The ability of ptxD-expressing plants to activate defense responses highlights its use not only as it enables the plants to utilize Phi effectively, but also as a stress-resilience tool in agricultural systems.