Systemic Acquired Resistance (SAR) can be conditioned in Arabidopsis by chemical and biological activators. This acquired resistance to a subsequent infection manifests itself as a phenocopy of the Hypersensitive Reaction (HR) and is correlated with an increased expression of so-called PR genes, which code for „pathogenesis related“ proteins. SAR is also accompanied by a local and systemic accumulation of salicylic acid (SA). Treatment of plants with SA can also induce SAR against a broad spectrum of pathogens. SA serves as an important local signal substance which is essential for the conditioning of the SAR state. Further factors which might contribute to signalling in establishing the SAR state are either unknown or their role is controversial. The latter applies particular to the role of reactive oxygen species (ROS) in local and systemic signals. The work described in this thesis addresses the question of the role that various ROS play in establishing biologically and chemically induced SAR. In the first phase various test systems for the induction of SAR in Arabidopsis were established. SA accumulation as well as PR gene expression and the course of challenge inoculation were used as markers for the SAR state. If ROS accumulate either locally or systemically during SAR induction it could be possible to detect them. Therefore, various ROS were tested for. H2O2 accumulated locally but not systemically after inoculation with PmsDC3000/avrRpm1. O2.- was only detected locally after wounding. If ROS produced at the site of inoculation are involved in conditioning SAR, then treatment with ROS or ROS-generating systems might lead to the induction of resistance in systemic leaves, and / or the accumulation of molecular markers for SAR (PR mRNA, SA). To test this possibility, leaves were treated with H2O2, paraquat (in light -O2.-), H2O2-generating, O2.--generating and NO-generating enzyme systems. However, none of the treat-ments gave results comparable to biological or chemically induced SAR in the controls. If ROS produced at the site of inoculation are involved in conditioning SAR, then the application of inhibitors or treatments which affect ROS metabolism might lead to measurable changes in the SAR response. Candidates for ROS synthesis are the plasmamembrane NAD(P)H oxidase, cell wall peroxidases or xanthine oxidase (XO). If O2.- produced by NAD(P)H oxidase is involved in the SAR, its inhibition by DPI might be expected to lead to a reduction in SAR or its molecular markers. Similar, if O2.- is produced via XO activity, allopurinol (a XO inhibitor) treatment might be expected to lead to a reduction SAR marker accumulation and / or SAR itself. If there is H2O2 involved in conditioning SAR, then its destruction via infiltrated catalase might be expected to lead to a reduction in SAR markers. Additionally, the effects of the NO scavenger Carboxy-PTIO-potassium on SAR induction was investigated. In summary it can be said that, neither DPI nor catalase, allopurinol and Carboxy-PTIO-potassium led to a significant reduction in SA or PR transcript accumulation in either the inoculated or the systemic leaves. No significant effect on the conditioning of SAR was detected as the histological level either (challenge inoculation). The possible role of intracellular O2.- production was investigated using transgenic plants over- or under expressing (sense / antisense) a cytosolic superoxide dismutase (SOD). However, no SAR relevant phenotypes were observed. Independent of the nature of the systemic signal which leads to the conditioning of SAR throughout the plant, the pathway of signal transport was investigated. To address this aspect the pattern of assimilate transport out of an inoculated leaf into the rest of the plant was compared to the pattern induction of SAR and SAR molecular markers. 
The results are discussed in terms of the possible role of ROS in local and systemic signalling in Arabidopsis during the establishment of SAR.