Chitosan-induced enhanced expression and activation of alternative oxidase confer tolerance to salt stress in maize seedlings

This study aimed to investigate the possible alleviating effect of chitosan on salt-induced growth retardation and oxidative stress and to elucidate whether this effect is linked to activation of mitochondrial respiration on the basis of alternative respiration in maize seedlings. Salt stress significantly reduced root length and plant height in comparison to the control, whereas foliar application of chitosan ameliorated the adverse effect of salinity to a certain degree. Moreover, chitosan resulted in plant growth promotion as compared to unstressed seedlings. The separate applications of chitosan and salt had a stimulatory effect on the activities of antioxidant enzymes; however, combined application of chitosan and salt were more effective than that of chitosan or salt alone. Similarly, mitochondrial total respiration rate (V-t) and alternative respiration capacity (V-alt) were increased by separate applications of chitosan and salt; however, the combination of chitosan and salt gave the highest values for these parameters. The highest values of V-alt/V-t was recorded at seedlings treated with salt plus chitosan. Similarly, cytochrome respiration capacity was also increased by chitosan in both stress-free and stressed conditions. In addition, AOX1, encoding alternative oxidase, was significantly upregulated by chitosan and/or salt. The maximum transcript level was recorded at seedlings treated with salt plus chitosan. Chitosan also significantly decreased superoxide anion and hydrogen peroxide contents and lipid peroxidation level under normal and the stressed conditions. These results suggest that the mitigating effect of chitosan on salt stress is linked to activation of alternative respiration at biochemical and molecular level.