Effect of exogenous ethylene on physiological metabolism of Zea mays seedlings under cadmium stress

There has been increasing heavy metalsespecially cadmium (Cd) pollution in farmlands in China. Studies have identified the crucial role of exogenous ethylene in the reversal of Cd stress in plants such as Arabidopsis thaliana mustard. However, few studies have been done on maize (Zea mays), which is the second largest staple crop in China. To investigate the potential process by which exogenous ethylene alleviates Cd stress in maize, hydroponic experiments were conducted. The experiments included a treatment that served as a blank control and others that were Cd and (NH₄)₂SO₄ treatments. Changes in physiological indexes of maize seedling leaf along with subcellular distribution of Cd in leaves and roots of the plant were determined under Cd treatment, exogenous ethylene treatment and exogenous sulphur treatment. The results suggested that H₂O₂ and malondialdehyde (MDA) contents of maize seedling leaf decreased under exogenous ethylene and exogenous (NH₄)₂SO₄ treatments, comparing with Cd treatment alone. Also, the rate of net photosynthesis was promoted by 1.23 times and 1.22 times respectively under exogenous ethylene and exogenous (NH₄)₂SO₄ treatments. The activity of antioxidant enzymessuperoxide diamutase (SOD), catalase (CAT) significantly decreased, while the contents of antioxidantsascorbic acid (AsA) and glutathione (GSH) significantly increased under exogenous ethylene or exogenous (NH₄)₂SO₄ treatments with Cd stress. The results suggested that exogenous ethylene reduced Cd-induced oxidative stress and the degree of lipid peroxidation by enhancing non-enzymatic antioxidant reaction. However, it did not affect enzymatic antioxidant reaction, but then promoted photosynthetic processes. Compared with Cd treatment alone, the activities of ATP sulfurylase and glutathione reductase (GR), and the contents of cysteine and GSH in maize seedlings increased respectively by 54.43%, 27.93%, 50.77%, and 49.85% with exogenous ethylene treatment. However, there was no significant change in non-protein thio (NPT) and phytochelatins (PCs) contents. The results showed that ethylene potentiated GSH biosynthesis to resist Cd conditions. To show this that was the case, a GSH biosynthetic inhibitor-buthionine sulfoximine (BSO) - was applied on maize seedlings under Cd and exogenous ethylene conditions. Compared with Cd plus exogenous ethylene treatment, BSO significantly decreased GSH content, increased H₂O₂ content and reduced net photosynthesis rate. Furthermore, Cd content in roots significantly increased while it decreased in leaves after treatment with exogenous ethylene under Cd stress. Further analysis showed that Cd content in cell wall and vacuole of roots was enhanced with exogenous ethylene treatment. Totally, exogenous ethylene reversal of the effect of Cd stress on maize was a complex process involving the promotion of GSH and AsA contents and Cd distribution in roots. On the one hand, exogenous ethylene treatment enhanced non-enzymatic antioxidant capacity by increasing the contents of GSH and AsA, and not by improving the activities of antioxidant enzymes nor chelating NPT and PC in maize leaf. On the other hand, translocation of Cd from maize root to leaf was reduced by enhancing Cd sequestration in cell walls and vacuoles of maize root. The results provided the fundamental information for the application of ethylene in the reversal of heavy metal stress.