Transcriptomic and physiological analyses unravel the effect and mechanism of halosulfuron-methyl on the symbiosis between rhizobium and soybean

Halosulfuron-methyl (HSM) is a new and highly effective sulfonylurea herbicide widely used in weed control, but its residue in the environment poses a potential risk to soybean. Soybean-rhizobium symbiotic nitrogen fixation is crucial for sustainable agricultural development and ecological environment health. However, the impact of HSM on the symbiosis between soybean and rhizobium is unclear. In this study, the effects of HSM on the soybean-rhizobium symbiotic process and nitrogen fixation were investigated by means of transcriptomic and physiological analyses. Treatment with a concentration of HSM less than 0.5 mg L^-1 had no effect on rhizobium growth, but significantly reduced nodules number, the biomass of soybean nodules, and nitrogenase activity in root nodules (P < 0.05). Transcriptomic analysis showed that differentially expressed genes (DEGs) involved in NH₄⁺ assimilation were significantly downregulated (P < 0.05). In addition, the activities of NH₄⁺ assimilation enzymes were markedly reduced. This result was further confirmed by the accumulation of NH₄⁺ in root nodules, indicating that the inhibition of nitrogen fixation by HSM may be caused by excessive NH₄⁺ accumulation in root nodules. Furthermore, DEGs involved in flavonoid synthesis, phytohormone biosynthesis, and phytohormone signaling transduction were significantly downregulated (P < 0.05), which was consistent with the decrease in flavonoid and phytohormone contents determined in this study. These results suggested that HSM may inhibit soybean nodulation by inhibiting flavonoid synthesis in soybean roots, disrupting the balance of plant endogenous hormones in roots during symbiosis, and blocking the transmission of hormone signals during the symbiosis. Our findings provide new insights into the effects of HSM on the legume-rhizobium nodule symbiotic process.

UV-irradiation and BDD-based photoelectrolysis for the treatment of halosulfuron-methyl herbicide

This paper reports the development of a novel photoelectrochemical (PEC) oxidation technique based on UV-C irradiation and boron-doped diamond (BDD) anode and its application for the effective removal of the commercial herbicide halosulfuron-methyl (HSM). The study evaluated the influence of the following key operating variables in the photoelectrochemical process: current density, pH, temperature, and initial HSM concentration. With regard to HSM degradation/mineralization, the application of high current densities was found to be more advantageous once it promoted a more rapid degradation and mineralization, with 96% of total organic carbon removal, though the process became more energy-demanding over time. The initial concentration of HSM did not modify the relative degradation rate, though the degradation process became more efficient as expected in a mass-transfer controlled process. The use of acidic pH (pH 3) was found to be more suitable than neutral conditions; this is probably because an anionic resonant form of HSM may be formed in neutral conditions. The temperature level was also found to affect the rate of HSM removal and the degradation efficiency. Finally, the substitution of Na₂SO₄ by NaCl promoted a more rapid and effective degradation; this is attributed to high production of powerful oxidants. However, only 70% mineralization was reached after 3 h of treatment; this is probably related to the formation of recalcitrant chlorinated sub-products.

Integrated proteomics, metabolomics and physiological analyses for dissecting the toxic effects of halosulfuron-methyl on soybean seedlings (Glycine max merr.)

Halosulfuron methyl (HSM) is a herbicide widely used to control sedge and broad-leaved weeds during crop production, but its environmental residue may damage non-target crops. Here, proteomics and metabolomics methods were used to explore the phytotoxicity mechanisms of HSM against soybean (Glycine max Merr.). Soybean seedlings were exposed to 0.01, 0.05 and 0.5 mg/L HSM for 8 d. The HSM applications significantly reduced chlorophyll and carotenoid contents in HSM-treated seedlings. Additionally, chlorophyll a fluorescence was seriously affected. The glutathione, hydrogen peroxide and malondialdehyde contents, as well as antioxidant enzyme activities, significantly increased in seedlings exposed to HSM. Furthermore, five enzymes involved in the tricarboxylic acid (TCA) cycle, α-ketoglutarate dehydrogenase, isocitrate dehydrogenase, aconitase, malic dehydrogenase and succinate dehydrogenase, were inhibited to varying degrees in HSM-treated seedlings compared with controls. Proteomics results showed multiple differentially abundant proteins involved in chlorophyll synthesis, photosystem processes and chloroplast ATP synthetase were down-regulated. Metabolomics analyses revealed that metabolites involved in the TCA cycle decreased significantly. Moreover, metabolites and proteins related to reactive oxygen species detoxification accumulated. In conclusion, the phytotoxicity mechanisms of HSM against soybean mainly act by damaging the photosynthetic machinery, inhibiting chlorophyll synthesis, interrupting the TCA cycle and causing oxidative stress. These results provide new insights into the toxicity mechanisms of sulfonylurea herbicides against non-target crops.

Degradation Dynamics of Halosulfuron-methyl in Two Textured Soils

A laboratory experiment was conducted to study the degradation dynamics of halosulfuron-methyl residues in sandy loam and clay loam soil. The herbicide formulation was applied at 0.034 and 0.068 mg kg^- 1 equivalent to field application dose of 67.5 and 135 g a.i. ha^- 1 as single and double dose respectively. Soil samples were collected on 0 (1 h), 1, 3, 7, 10, 15, 30 and 45 days after treatments. Extraction was done using modified QuEChERS method. Residues were estimated by UPLC coupled with quadrupole Dalton mass detector. Average recoveries ranged from 85.5% to 94.5% for both soils at different fortification levels of 0.005 to 0.1 mg kg^- 1 with limit of detection (LOD) and limit of quantification (LOQ) as 0.001 and 0.005 mg kg^- 1, respectively. Dissipation followed first order kinetics with half-life of 8.4 to 10.7 days in both soil at two doses. The residues reached below LOQ of 0.005 mg kg^- 1 after 45 days of herbicide application.

Interactions between salicylic acid and antioxidant enzymes tilting the balance of H2O2 from photorespiration in non-target crops under halosulfuron-methyl stress

Halosulfuron-methyl (HSM) is a safe, selective and effective sulfonylurea herbicide (SU) for the control of sedge and broadleaf weeds in sugarcane, corn, tomato, and other crops. The primary site of action is acetolactate synthase (ALS), a key enzyme of branched chain amino acids (BCAAs) synthesis. In addition to ALS inhibition, BCAAs deficiencies and oxidative damage may be involved in toxic effects of SUs. However, secondary targets of HSM relevant to plant physiological responses are unclear. In the present study, comparative growth inhibition and peroxidization injury between sensitive and tolerance crops were observed at biochemical and physiological levels suggesting involvement of H2O2, ethylene, salicylic acid (SA) in the oxidative stress responses to HSM. HSM caused accumulation of H2O2, stimulated photorespiration and consequent accumulation of SA that worsened the peroxidization injury to the sensitive C3 plant soybean (Glycine max). The growth inhibition at low concentrations of HSM could be lessened by supplementary BCAAs, reactive oxygen species scavengers or ethylene inducers, whereas the oxidation damage at high concentrations of HSM could not be reversed and ultimately lead to plant death. H2O2 at a low level stimulated the antioxidase system including glutathione S-transferase activities in the HSM-tolerant C4 maize (Zea mays), which contributes to HSM tolerance. H2O2 plays an important role on HSM stress responses in both HSM-sensitive and HSM-tolerant soybean and maize.