Lipoic acid confers osmotic stress tolerance to maize seedlings by upregulating the enzymes of antioxidant defense and glyoxalase systems

Exogenous application of lipoic acid (LA), which is a special antioxidant substance, alleviates abiotic stress damage in plants. However, the mechanism of action of LA in stress tolerance is still not fully understood. Here, the effect of exogenous LA on the coordination of antioxidant defense and glyoxalase systems to alleviate osmotic stress damage was investigated. LA (12µM) was applied to the roots of 21-day-old seedlings in Hoagland nutrient solution for 8 hours and then the seedlings were exposed to 10% polyethylene glycol (PEG6000) for 3 days. Seedlings grown in Hoagland nutrient solution for 28 days were used as the control group. Exogenous LA under osmotic stress was found to increase the fresh and dry weights of the leaves, leaf relative water content, and non-enzymatic compounds such as ascorbate and glutathione (GSH) while significantly decreasing the contents of hydrogen peroxide (H2O2) and methylglyoxal (MG). LA application also increased some antioxidant enzyme activities such as ascorbate peroxidase (1.2-fold), glutathione peroxidase (1.3-fold), glutathione reductase (1.4-fold), and monodehydroascorbate reductase (1.8-fold). LA significantly induced the relative expression levels of the genes coding the antioxidant enzymes. Furthermore, LA stimulated the enzyme activities of the glyoxalase system (glyoxalase I (Gly I) (1.3-fold) and glyoxalase II (Gly II) (1.1-fold). Additionally, the relative expression levels of the Gly I and Gly II genes were consistent with the findings of the Gly I and Gly II activities. Moreover, exogenous LA induced the expression level of Gly II (1.4-fold) more than that of Gly I (1.3-fold). As a result, LA mitigates osmotic stress damage in maize by enhancing the activity of antioxidant and glyoxalase systems, enabling the rapid removal of reactive oxygen species and the toxic compound MG, thereby providing a protective mechanism. Further investigation of the effects of LA on crops exposed to abiotic stresses will contribute to improve the stress tolerance and increase agricultural yields.