Glycine betaine enhances heavy metal phytoremediation via rhizosphere modulation and nitrogen metabolism in king grass-Serratia marcescens strain S27 symbiosis.

Microbe-Assisted Phytoremediation (MAP) is an eco-friendly method for remediating soil contaminated with heavy metals such as cadmium (Cd) and chromium (Cr). This study demonstrates the potential of a king grass-Serratia marcescens strain S27 (KS) co-symbiotic system to enhance heavy metal remediation. The KS symbiosis increased the biomass of king grass by 48 % and enhanced the accumulation of Cd and Cr in the whole plant by 2.75-fold and 1.82-fold, respectively. Root exudates like γ-aminobutyric acid (GABA), glycine betaine (GB), and allantoin (Alla) were tested for enhancing the KS symbiotic, with 0.75 mM GB (GB-KS) showing the highest removal efficiencies at 15.1 % for Cd and 14.2 % for Cr. Correlation analysis indicated a link between this enhancement and increased soil nitrogen content. Mechanistic studies revealed GB treatment altered the rhizosphere microbial community, promoting denitrifying bacteria and upregulating nitrogen transformation genes (nrfA) by over 7-fold. GB also enhanced nitrogen assimilation enzymes and antioxidant defenses in king grass, facilitating Cd and Cr transport and sequestration. X-ray fluorescence imaging and two-dimensional correlation spectroscopy showed GB promoted Cd and Cr accumulation in the xylem and phloem of king grass, with phenols and alcohols as key functional groups. The study highlights the potential of the GB-KS symbiotic system for effective soil remediation.