Priestia sp. B6 enhances lead (Pb) transport into the root xylem via apoplastic pathway in Salix integra

Abstract Priestia sp. B6 (strain B6) enhances lead (Pb) translocation from the roots to the aerial parts in Salix integra, nearly doubling its Pb translocation capacity. This study aims to elucidate the mechanism by which strain B6 facilitates Pb entry into the root xylem via the apoplastic pathway. Priestia sp. Strain B6 was capable of colonizing the roots, branches, and leaves of S. integra. It migrated from the roots to the branches via the xylem, and subsequently moved to the epidermis of the branches and leaves through intercellular spaces. The deposition sites of strain B6 and Pb were primarily located in the cell walls and intercellular spaces. Inoculation with strain B6 resulted in a maximum 70.22% increase in Pb concentrations in the root cell walls, and this was associated with reduced pectin methylesterase (PME) activity and enhanced the number and migration activity of functional groups. Additionally, Pb desorption capacity was increased, allowing Pb to re-enter the intercellular spaces. Besides, abscisic acid (ABA), and gibberellin A3 (GA3) concentrations and phenylalanine ammonia-lyase (PAL) activity were reduced by 40.48%, 52.78%, and 62.23%, respectively. Consequently, the Casparian strip formed further from the root tip, and both the Casparian strip and suberin lamellae developed incompletely, which facilitated Pb entry into the root xylem via the apoplastic pathway. Simultaneously, the Pb detoxification capacity of S. integra was enhanced by reducing the H2O2, OH- and increasing the concentrations of chelating agents glutathione (GSH) and metallothionein (MT), as well as the activities of superoxide dismutase (SOD) and peroxidase (POD). These findings indicate that strain B6 enhances Pb translocation through the apoplastic pathway while promoting Pb detoxification in the roots of S. integra.