Role of salicylic acid and oxidative burst in expression of resistance to Alternaria blight

The accumulation of salicylic acid and H₂O₂ during pathogenic infection of mustard plants with Alternaria brassicae spores was investigated to understand the role of these two defense compounds in the expression of resistance. Comparisons were made between a susceptible Brassica juncea variety RH30 and a Brassica carinata variety HC1, which is known to be resistant. An oxidative burst was detected as in situ accumulation of H₂O₂, in both the Brassica spp. after pathogen application. However, H₂O₂ generation was extracellular in the resistant variety and both extra- and intracellular in the susceptible variety. Endogenous levels of SA increased over 2.5-fold in the resistant variety HC1 in response to pathogen application and this increase was observed only in conjugated SA levels. Pathogen application also led to an increase in the antioxidant enzymes, guaiacol-dependent peroxidase (GDP) and superoxide dismutase (SOD) in HC1. Exogenous SA application to leaves led to over threefold increase in the free and conjugated SA levels in both varieties. Pathogen application to the SA pretreated plants led to over 10-fold increase in endogenous SA levels in both varieties as compared to the levels in controls and this correlated with a decrease in disease symptoms in both species. SA appeared to regulate defense responses in Brassica spp. in a concentration-dependent manner. While 2.7-fold increase in endogenous SA levels (as seen in HC1) led to an induction of antioxidant enzymes, over 10-fold increases in endogenous SA levels (as seen after exogenous SA application in both varieties) brought about no induction of the antioxidant enzymes, probably because SA itself served as an antioxidant.