Lipoic Acid Can Maintain Stimulation of the Antioxidant System at Lower Reactive Oxygen Species, Ascorbate and Glutathione Levels in Osmotic Stressed Maize

Abstract

Lipoic acid (LA), a unique antioxidant compound, can stimulate the antioxidant defense system in plants subjected to abiotic stresses. We aimed to determine the role of LA in induction of the antioxidant system at lower reactive oxygen species (ROS), ascorbate (ASC) and glutathione (GSH) levels in osmotic-stressed maize (Zea mays L.). For this purpose, ROS, GSH and ASC contents were decreased using N,N'‑dimethylthiourea (DMTU), L-buthionine sulfoximine (BSO) and acriflavine (AF), respectively. Pots containing 21-day-old seedlings were divided into nine groups consisting of a non-stressed group; polyethylene glycol₆₀₀₀ (PEG)-induced osmotic stress (PEG) group, LA, DMTU, BSO and AF treatment groups; and DMTU, BSO, and AF-combined LA treatment groups under osmotic stress. ROS contents and membrane damage after the DMTU, BSO, and AF-combined LA treatments were lower than those after the DMTU, BSO, and AF treatments, respectively. Moreover, the LA treatments in combination with DMTU, BSO and AF increased dry weight, activities of antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase and dehydroascorbate reductase), and contents of GSH, ASC, and LA compared to the DMTU, BSO and AF treatments. Additionally, the relative expression levels of SUPEROXIDE DISMUTASE, CATALASE1 and ASCORBATE PEROXIDASE1 genes were consistent with the findings for their related antioxidant enzyme activities. These results indicated that LA could adjust ROS level and maintain stimulation of the antioxidant system at lower ROS, GSH, and ASC levels in osmotic stressed maize. Furthermore, LA may play a signaling role and assume the function of ASC and GSH in maize under PEG-induced osmotic stress.