Antioxidative response and detoxification in lettuce under combined stress of microcystins and anatoxins

Coexistence of microcystins and anatoxins is observed worldwide in water bodies due to algal blooms, posing a potential threat to water quality and agricultural output. However, hardly any information is available to evaluate the combined effect of microcystins and anatoxins on agricultural productivity and food safety through irrigation, hampering necessary risk assessment and crisis management. Therefore, this study used a simulated model with lettuce irrigated with water contaminated with only microcystins (5 or 100 μg L^-1) or anatoxins (5 or 100 μg L^-1) or their combination (5 + 5 or 100 + 100 μg L^-1) to study the regulation of ascorbate-glutathione (AsA-GSH) cycle and glutathione S-transferase (GST) system on growth and toxin accumulation in lettuce. The combination reduced the low-promoting and high-inhibiting effect of single toxin on lettuce growth, showing antagonistic effect during stress and recovery (without toxin exposure) periods. Antagonistic effect on lettuce growth was mainly related to H₂O₂ content regulated by activities and gene expression of ascorbate peroxidase, glutathione reductase and monodehydroascorbate in AsA-GSH cycle. High concentrations combination promoted higher GST activity by upregulating expression of LsGST to accelerate bioconjugation of toxin to GSH, being beneficial to decreasing toxin accumulation and restoring lettuce growth. Meanwhile coexistence of anatoxins (100 μg L^-1) decreased microcystins accumulation due to competitive uptake, but cannot eliminate health risk caused by 100 μg L^-1 microcystins. Therefore, enhancing GSH-involved regulation on plant tolerance could be helpful for alleviating threat of two toxins to crop growth and safety.