Gamma-Aminobutyric Acid Enhances Cadmium Phytoextraction by Coreopsis grandiflora by Remodeling the Rhizospheric Environment

Gamma-aminobutyric acid (GABA) significantly affects plant responses to heavy metals in hydroponics or culture media, but its corresponding effects in plant–soil systems remain unknown. In this study, different GABA dosages (0–8 g kg−1) were added to the rhizosphere of Coreopsis grandiflora grown in Cd-contaminated soils. Cd accumulation in the shoots of C. grandiflora was enhanced by 38.9–159.5% by GABA in a dose-dependent approach because of accelerated Cd absorption and transport. The increase in exchangeable Cd transformed from Fe-Mn oxide and carbonate-bound Cd, which may be mainly driven by decreased soil pH rather than GABA itself, could be a determining factor responsible for this phenomenon. The N, P, and K availability was affected by multiple factors under GABA treatment, which may regulate Cd accommodation and accumulation in C. grandiflora. The rhizospheric environment dynamics remodeled the bacterial community composition, resulting in a decline in overall bacterial diversity and richness. However, several important plant growth-promoting rhizobacteria, especially Pseudomonas and Sphingomonas, were recruited under GABA treatment to assist Cd phytoextraction in C. grandiflora. This study reveals that GABA as a soil amendment remodels the rhizospheric environment (e.g., soil pH and rhizobacteria) to enhance Cd phytoextraction in plant–soil systems.