IS HYDROGEN-PEROXIDE A 2ND-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 c atalase activity. Based on this finding elevated levels of hydrogen pe roxide (H2O2) were postulated to act as a second messenger of SA in th e SAR signal transduction pathway. A series of experiments have been c arried out to clarify the role of H2O2 in SAR signaling. No increase o f H2O2 was found during the onset of SAR. Induction of the SAR gene, P R-1, by H2O2 and H2O2. inducing chemicals is strongly suppressed in tr ansgenic tobacco plants that express the bacterial salicylate hydroxyl ase gene, indicating that H2O2 induction of SAR genes is dependent on SA accumulation. Following treatment of plants with increasing concent rations of H2O2, a dose-dependent accumulation of total SA species was found, suggesting that H2O2 may induce PR-1 gene expression through S A accumulation. While the results do not support a role for H2O2 in SA R signaling, it is suggested that SA inhibition of catalase activity m ay be important in tissues undergoing a hypersensitive response.