Biostimulation of Mesembryanthemum crystallinum L. (The Common Ice Plant) by Plant Growth-Promoting Rhizobacteria: Implication for Cadmium Phytoremediation

Plant growth-promoting rhizobacteria (PGPR) employ various mechanisms to enhance plant development and growth as well as to mitigate environmental stress, including heavy metal contamination. Cadmium is a particularly severe stressor, toxic to both plants and soil microbiota. Mesembryanthemum crystallinum L. (the common ice plant), a fast-growing semi-halophyte, was previously investigated for phytoremediation potential towards saline environments and toxic metals, especially cadmium and chromium. The study was aimed at assessing whether bacteria isolated from the rhizosphere of M. crystallinum treated with Cd reveal growth-promoting traits and if the plant tolerance to Cd results from a synergistic action of the Cd/salt-resistant strains. The isolates demonstrated PGP characteristics, including nitrogen fixation, phosphate solubilization, and production of ammonia, indolyl-3-acetic acid (IAA), and siderophores. A microbial consortium consisting of these strains was developed and applied to pots with M. crystallinum. After a 14-day experiment, plant growth and Cd-accumulation potential were evaluated upon treatment with 1 mM or 10 mM Cd, either in the presence or absence of NaCl. Plant inoculation with the consortium stimulated Cd accumulation both by roots and shoots at 10 mM Cd under saline conditions. The results suggest that bioaugmentation of M. crystallinum with the bacterial community can be used as an effective, sustainable phytoremediation method for cadmium-contaminated soils.