Biochemical and Molecular Changes in Downy Mildew‐infected Sunflower Triggered by Resistance Inducers

Abstract

AbstractIn the present study, the mechanism of resistance to Plasmopara halstedii, the downy mildew pathogen of sunflower, triggered by three resistance‐inducing chemicals, benzo (1,2,3) thiadiazole‐7‐carbothioic acid S‐methyl ester (BTH), DL‐β‐amino butyric acid (BABA) and 2,6‐dichloroisonicotinic acid (INA), was investigated in susceptible, completely resistant and partially resistant sunflower genotypes. By applying P. halstedii‐specific primers, no detectable pathogen marker transcript accumulation was found in the infected but completely resistant sunflower hypocotyls; however, pretreatments with either of the three resistance inducers decreased the transcript accumulation in both the infected susceptible and the partially resistant lines. Benzothiadiazole pretreatment before inoculation considerably enhanced enzyme activities in the infected susceptible and the completely resistant genotypes but not in partially resistant plants. Pretreatment with resistance inducers before inoculation increased glutathione S‐transferase, defensin and catalase transcript levels in the susceptible but decreased in the partially resistant plants. Our results indicate that the resistance‐inducing chemicals can improve resistance in all of the sunflower genotypes to downy mildew and increase enzyme activities of peroxidase and polyphenol oxidase, as well as accumulation of mRNAs of glutathione S‐transferase, defensin and catalase. However, it is important to emphasize that activation of these defence‐related proteins did not correlate with the degree of resistance, but rather with the amount of necrotic tissues.