A novel method for in situ imaging of root exudates and labile elements reveals phosphorus deficiency-induced mobilization of rare earth elements in the rhizosphere of Phytolacca americana

Aims: Phosphorus (P) deficiency-induced mobilization of rare earth elements (REEs) in the rhizosphere contributes to REE accumulation in the hyperaccumulator Phytolacca americana, but a lack of in situ methods for visualization of the root-soil interface limits our understanding of the underlying processes. Methods: Diffusive gradients in thin-films (DGT) devices were used for probing root exudates, REEs and P in the P. americana rhizosphere. Desorption electrospray ionization mass spectrometry and laser-ablation inductively coupled mass spectrometry were used for in situ imaging of root exudates, REEs and P sorbed on the DGT. Results: The novel approach demonstrated here is capable of synchronously and quantitatively characterizing the distribution of root exudates and labile elements in the rhizosphere. The secretion fluxes of citrate and oxalate in the rhizosphere under P deficiency were three times higher than under P sufficient condition; and the lanthanum (La) fluxes in the rhizosphere under P deficiency were ten times greater than at P sufficiency condition. The enrichment of P and La under P deficiency and depletion under P sufficient conditions in the rhizosphere suggests that P deficiency-induced organic acid secretion is crucial for the mobilization of soil REEs and subsequent REE accumulation in P. americana. Conclusion: The combination of DGT devices with mass spectrometry imaging is technically feasible for in situ synchronous imaging of root exudates, REEs and labile elements at the root-soil interface. Our study shed light on processes of mobilization of mineral elements in the rhizosphere induced as a side-effect of the P acquisition mechanism.