Abstract

Challenge by a pathogen induces systemic acquired resistance (SAR) in plants, a state marked by the elevated expression of pathogenesis related (PR) genes and enhanced resistance to a broad spectrum of pathogens. SAR requires the endogenous accumulation of salicylic acid (SA), and can be induced by exogenous application of SA or related molecules such as benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH). All tested plants have a SAR response, but some important aspects of SAR differ between species. In this work, four (partial) PR-1 cDNAs were cloned from papaya. One of these, PR-1d, was shown to be induced by BTH. This data, together with previous data showing the induction of SAR related enzymes and enhanced tolerance to a pathogen in response to BTH, demonstrates that papaya has a SAR response and it is induced by BTH. With this knowledge, global profiling of papaya genes induced by BTH was carried out by suppression subtractive hybridization. 25 unique expressed sequence tags (ESTs) induced by BTH were identified, including homologs of numerous genes known to be defense related, and some genes previously unknown to have defense functions. A papaya homolog of NPR1, shown to be required for SAR signal transduction in Arabidopsis, was isolated and found to contain all three structural domains required for activity in Arabidopsis. This data, together with the profile of BTH induced genes, and induction kinetics for some of these genes, shows that papaya SAR is similar in many important aspects to SAR in the model system Arabidopsis. Additionally, tobacco plants over-expressing Arabidopsis NPR1 were produced and found to produce elevated (compared to wild-type) levels of PR-1a mRNA in response to SA treatment. This iv demonstrates that even in a heterologous system, over-expression of NPR1 may confer an enhanced SAR response.