INDUCTION, MODIFICATION, AND TRANSDUCTION OF THE SALICYLIC-ACID SIGNAL IN PLANT DEFENSE RESPONSES

Studies in our laboratory as well as others strongly suggest that sali cylic acid (SA) plays an important signaling role in plant defense aga inst pathogens, We have found that increases in endogenous SA levels c orrelates with both resistance of tobacco to infection with tobacco mo saic virus and induction of defense-related genes such as that encodin g pathogenesis-related protein 1 (PR-1), Some of this newly synthesize d SA was conjugated to glucose to form SA beta-glucoside, A cell wall- associated beta-glucosidase activity that releases SA from this glucos ide has been identified, suggesting that SA beta-glucoside serves as a n inactive storage form of SA, By purifying a soluble SA-binding prote in and isolating its encoding cDNA from tobacco, we have been able to further characterize the mechanism of SA signaling, This protein is a catalase, and binding of SA and its biologically active analogues inhi bited catalase's ability to convert H2O2 to O-2 and H2O. The resulting elevated levels of cellular H2O2 appeared to induce PR-1 gene express ion, perhaps by acting as a second messenger, Additionally, transgenic tobacco expressing an antisense copy of the catalase gene and exhibit ing depressed levels of catalase also showed constitutive expression o f PR-1 genes, To further dissect the SA signaling pathway, we have tes ted several abiotic inducers of PR gene expression and disease resista nce for their ability to stimulate SA production. Levels of SA and its glucoside rose following application of all of the inducers except 2, 6-dichloroisonicotinic acid. 2,6-Dichloroisonicotinic acid was found t o bind catalase directly and inhibit its enzymatic activity, Thus, it appears that many compounds that induce PR gene expression and disease resistance in plants inactivate catalases directly or indirectly.