Is hydrogen peroxide a second messenger of salicylic acid in systemic acquired resistance?

Elevated levels of salicylic acid (SA) are required for the induction of systemic acquired resistance (SAR) in plants. Recently, a salicylic acid-binding protein (SABP) isolated from tobacco was shown to have catalase activity. Based on this finding elevated levels of hydrogen peroxide (H2O2) were postulated to act as a second messenger of SA in the SAR signal transduction pathway. A series of experiments have been carried out to clarify the role of H2O2 in SAR signaling. No increase of H2O2 was found during the onset of SAR. Induction of the SAR gene, PR-1, by H2O2 and H2O2. inducing chemicals is strongly suppressed in transgenic tobacco plants that express the bacterial salicylate hydroxylase gene, indicating that H2O2 induction of SAR genes is dependent on SA accumulation. Following treatment of plants with increasing concentrations of H2O2, a dose-dependent accumulation of total SA species was found, suggesting that H2O2 may induce PR-1 gene expression through SA accumulation. While the results do not support a role for H2O2 in SAR signaling, it is suggested that SA inhibition of catalase activity may be important in tissues undergoing a hypersensitive response.