A dissection of the effects of ethylene, H2O2 and high irradiance on antioxidants and several genes associated with stress and senescence in tobacco leaves.

Ethylene and hydrogen peroxide are involved in the modulation of stress responses in plants, but their interrelation is not well understood. This work was designed to find differences between the actions of ethylene and H2O2 on antioxidants and senescence markers. Leaves of Nicotiana tabacum were sprayed with H2O2 or with ethephon (precursor of ethylene). To find the possible modulation of responses to acute abiotic stress, ethephon- and H2O2-sprayed leaves were further subjected to high irradiance (HL). The application of H2O2 strongly stimulated ethylene synthesis (ACC). Ethylene and H2O2, as single factors, stimulated the trolox equivalent antioxidant capacity (TEAC) and the activity of catalase (CAT), in contrast to HL alone (stimulation of nonspecific peroxidases and the total glutathione pool). However, after combined treatments (ethylene+HL and H2O2+HL), the stimulatory action of H2O2 was related to TEAC and CAT activity, while the application of ethylene stimulated the total glutathione pool. Hydrogen peroxide enhanced the expression of the three CAT genes (Cat1, Cat2 and Cat3), in contrast to ethylene (Cat2 and Cat3) and HL (Cat1). In regard to the markers of senescence and pathogenesis the most pronounced difference between the actions of ethylene and H2O2, as single factors, was related to NPR1, whereas when leaf spraying was combined with HL, differences were found at WRKY53 and PR1a. HL reversed the stimulatory effects of H2O2/ethylene-driven enhancements of the expression of several genes (Cat1, Cat2, NPR1, WRKY53). These results show that multiple stressors, as usually encountered by plants in nature, may largely change those expression patterns of genes determined in a single factor analysis. Moreover, the actions of HL (often considered the internal H2O2 trigger) and of exogenous H2O2 on gene expression are clearly different.