HYDROGEN-PEROXIDE DOES NOT FUNCTION DOWNSTREAM OF SALICYLIC-ACID IN THE INDUCTION OF PR PROTEIN EXPRESSION

The roles of salicylic acid (SA) and H2O2 in the induction of PR prote ins in tobacco have been examined. Studies were conducted on wild-type tobacco and plants engineered to express a bacterial salicylate hydro xylase capable of metabolizing SA to catechol (SH-L plants). Wildtype and PR la-GUS-transformed plants express PR-1a following challenge wit h Pseudomonas syringae pathovar syringae, SA or 2,6-dichloro-isonicoti nic acid (INA). In contrast, SH-L plants failed to respond to SA but d id express PR-1a following INA treatment. H2O2 and the irreversible ca talase inhibitor 3-amino-1,2,4-triazole (3-AT) were found to be weak i nducers 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 b een proposed suggesting that SA binds to and inhibits a catalase induc ing an increase in H2O2 leading to PR protein expression. Catalase act ivity has been measured in tobacco and no significant changes in activ ity following infection with P. syringae pv, syringae were detected. F urthermore, inhibition of catalase activity in vitro in plant extracts requires pre-incubation and only occurs at SA concentrations above 25 0 mu M. Leaf disks preincubated with 1 mM SA do accumulate SA to these levels and PR-1a is efficiently induced but there is no apparent inhi bition of catalase activity. It is also shown that a SA-responsive gen e, PR-1a, and a H2O2-sensitive gene, AoPR-1, are both relatively insen sitive to 3-AT suggesting that induction of these genes is unlikely to be due entirely to inhibition of an endogenous catalase.