Purpose: In recent years, considerable research has been conducted on the use of plant growth regulators (PGRs) to improve crop yields. Large amounts of PGRs are applied to regulate crop growth. However, few studies have assessed the impact of PGRs, which leave soil residues, on soil microorganisms, especially rhizobia. Methods: In this study, the influence of inoculation of soybean roots with Sinorhizobium fredii and Bradyrhizobium japonicum exposed to gibberellin A3 (GA₃) and mepiquat chloride (MC) on nodule number, nitrogenase activity, and a symbiotic nitrogen fixation gene (fixA) expression was studied. The nitrate content, free amino acids, and nitrate reductase activity of the roots, and seed yield and quality of soybean were measured. Result: Nodulation was promoted to some extent, whereas nitrogenase activity and fixA expression were inhibited to some extent by treatment with GA₃; however, these effects were reversed by treatment with MC. In a pot experiment to study phenotypic characteristics, rhizobia treatment in combination with GA₃ or MC significantly improved soybean yield and seed nitrogen content, and increased the root nitrate content, free amino acid content, and nitrate reductase activity. Conclusion: The results indicated that PGRs, which leave soil residues, have significant positive effects on the growth and activity of soybean and rhizobia.

In recent years, quinoid redox mediators (QRMs) have attracted increasing attention because of their key role in wastewater biotreatment. Previous studies have shown that the anaerobic respiration of many bacteria could be coupled to the reduction and reoxidation of quinone groups. Thus, QRMs are widely involved in the microbial transformation of various organic and inorganic substances. To date, few reviews have focused on the involvement of quinoid compounds in environmental pollution biotreatment processes. In this paper, we review the different types of QRMs that interact closely with microorganisms, the characteristics of those QRMs, the involvement of QRMs during the biotransformation of recalcitrant organic pollutants, heavy metal ions and metallic oxides, and their enhancement on microbial fuel cells. Finally, the future research focus and application prospects with regard to different types of QRMs are proposed. This study can improve our understanding of QRM-mediated environmental pollution biotreatment processes and provide fundamental guidance on what kinds of QRMs are practical for engineering applications.