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

Abstract

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 (H 2 O 2 ) 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 H 2 O 2 in SAR‐signaling. No increase of H 2 O 2 was found during the onset of SAR. Induction of the SAR gene, PR‐1, by H 2 O 2 and H 2 O 2 ‐inducing chemicals is strongly suppressed in transgenic tobacco plants that express the bacterial salicylate hydroxylase gene, indicating that H 2 O 2 induction of SAR genes is dependent on SA accumulation. Following treatment of plants with increasing concentrations of H 2 O 2 , a dose‐dependent accumulation of total SA species was found, suggesting that H 2 O 2 may induce PR‐1 gene expression through SA accumulation. While the results do not support a role for H 2 O 2 in SAR signaling, it is suggested that SA inhibition of catalase activity may be important in tissues undergoing a hypersensitive response.