The roles of salicylic acid (SA) and H2O2 in the induction of PR proteins in tobacco have been examined. Studies were conducted on wild-type tobacco and plants engineered to express a bacterial salicylate hydroxylase capable of metabolizing SA to catechol (SH-L plants). Wild-type and PR-1a-GUS-transformed plants express PR-1a following challenge with Pseudomonas syringae pathovar syringae, SA or 2,6-dichloro-isonicotinic acid (INA). In contrast, SH-L plants failed to respond to SA but did express PR-1a following INA treatment. H2O2 and the irreversible catalase inhibitor 3-amino-1,2,4-triazole (3-AT) were found to be weak inducers of PR-1a expression (relative to SA) in wild-type tobacco but were unable to induce PR-1a in SH-L plants, suggesting that the action of these compounds depends upon the accumulation of SA. A model has been proposed suggesting that SA binds to and inhibits a catalase inducing an increase in H2O2 leading to PR protein expression. Catalase activity has been measured in tobacco and no significant changes in activity following infection with P. syringae pv. syringae were detected. Furthermore, inhibition of catalase activity in vitro in plant extracts requires pre-incubation and only occurs at SA concentrations above 250 microM. Leaf disks preincubated with 1 mM SA do accumulate SA to these levels and PR-1a is efficiently induced but there is no apparent inhibition of catalase activity. It is also shown that a SA-responsive gene, PR-1a, and a H2O2-sensitive gene, AoPR-1, are both relatively insensitive to 3-AT suggesting that induction of these genes is unlikely to be due entirely to inhibition of an endogenous catalase.