Microscopic processes ruling Zn bioavailability to roots of Euphorbia pithyusa L. pioneer plant.

Euphorbia pithyusa L. was used in a plant-growth-promoting assisted field trial experiment. In order to unravel the microscopic processes at the interface, thin slices of E. pithyusa roots were investigated by micro-X-ray fluorescence mapping (?-XRF). Roots and rhizosphere materials were examined by X-ray absorption spectroscopy (XAS) at the Zn K-edge, by X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). Results indicate some features common to all the investigated samples: i) in the rhizosphere of E. pithyusa Zn was found to occur in different phases; ii) Si and Al are mainly concentrated in the epidermis of the roots forming a rim; iii) Zn is mostly stored in root epidermis, and does not appear coordinated to organic molecules, but mainly occurring in mineral phases such as Zn-silicates. We interpreted that roots of E. pithyusa significantly promote mineral evolution in the rhizosphere. Concomitantly, the plant uses Si and Al extracted by soil minerals to build a biomineralization rim, which is able to capture Zn. This Zn-silicate biomineralization has relevant implication for phytoremediation techniques and for further biotechnology development, which can be better designed and developed after acquiring specific knowledge of molecular processes ruling mineral evolution and biomineralization processes.