Ameliorative Effects of Brassinosteroid on Excess Manganese-Induced Oxidative Stress in Zea mays L. Leaves

Manganese (Mn) is becoming an important factor limiting crop growth and yields especially on acid soils.The present study was designed to explore the hypothesis that brassinosteroid application can enhance the tolerance of maize(Zea mays L.)to Mn stress and if so,whether or not the mechanism underlying involves regulation of antioxidative metabolism in leaves.The effects of 24-epibrassinosteroid(EBR)on the growth,photosynthesis,water status,lipid peroxidation,accumulation of reactive oxygen species,and activities or contents of antioxidant defense system in maize plants under Mn stress were investigated by a pot experiment.At supplemented Mn concentrations of 150-750 mg kg-1 soil,the growth of plants was inhibited in a concentration-dependent manner.The semi-lethal concentration was 550 mg Mn kg-1 soil.Foliage application with 0.1 mg L-1 EBR significantly reduced the decrease in dry mass,chlorophyll content,photosynthetic rate,leaf water content,and water potential of plants grown in the soil spiked with 550 mg kg-1 Mn.The oxidative stress caused by excess Mn,as reflected by the increase in malondialdehyde (MDA) content and lipoxygenase (LOX,EC 1.13.11.12) activity,accumulation of superoxide radical and H2O2,was greatly decreased by EBR treatment.Further investigations revealed that EBR application enhanced the activities ofsuperoxide dismutase (SOD,EC 1.15.1.1),peroxidase (POD,EC 1.11.1.7),catalase (EC 1.11.1.6),ascorbate peroxidase (APX,EC 1.11.1.11),dehydroascorbate reductase (DHAR,EC1.8.5.1),and glutathione reductase (GR,EC 1.6.4.2),and the contents of reduced ascorbate and glutathione,compared with the plants without EBR treatment.It iS concluded that the ameliorative effects of EBR on Mn toxicity are due to the upregulation of antioxidative capacity in maize under Mn stress.