Hydrogen peroxide- and nitric oxide-induced systemic antioxidant prime-like activity under NaCl-stress and stress-free conditions in citrus plants

We tested whether pre-treatments of roots with H2O2 (10 mM for 8 h) or sodium nitroprusside (SNP; 100 μM for 48 h), a donor of radical dotNO, could induce prime antioxidant defense responses in the leaves of citrus plants grown in the absence or presence of 150 mM NaCl for 16 d. Both root pre-treatments increased leaf superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) activities, and induced related-isoform(s) expression under non-NaCl-stress conditions. When followed by salinity, certain enzymatic activities also exhibited an up-regulation in response to H2O2 or SNP pre-exposure. An NaCl-stress-provoked decrease in the ascorbate redox state was partially prevented by both pre-treatments, whereas the glutathione redox state under normal and NaCl-stress conditions was increased by SNP. Real-time imaging of radical dotNO production was found in vascular tissues and epidermal cells. Furthermore, NaCl-induced inhibition in radical dotOH scavenging activity and promotion of radical dotOH-mediated DNA strand cleavage was partially prevented by SNP. Moreover, NaCl-dependent protein oxidation (carbonylation) was totally reversed by both pre-treatments as revealed by quantitative assay and protein blotting analysis. These results provide strong evidence that H2O2 and radical dotNO elicit long-lasting systemic primer-like antioxidant activity in citrus plants under physiological and NaCl-stress conditions.