Final Activity Report Summary - DEFENCE SIGNAL (Mobility of Signals in Plant Resistance to Disease) Plants have evolved complex mechanisms to protect themselves from diseases caused by different pathogens, including bacteria, fungi, and viruses. In one such mechanism, plants recognize an attacker and induce death of the infected part of the plant which prevents the pathogen from spreading. At the same time a signal is sent from the dying tissue to healthy parts of the plant that causes the induction of systemic acquired resistance (SAR). SAR protects plants from a broad range of different pathogens for a long time after the initial localized infection. Importantly, the SAR state is maintained at such low energy costs for the plant that it does not significantly affect its growth or seed yield. Pathogen infection of crop plants results in enormous economic losses in agriculture and we set out to map SAR signals in plants to understand processes underlying plant systemic immunity and potentially use this knowledge to protect crops from disease. For our studies we use the model plant species Arabidopsis thaliana (thale cress). This weed is one of the most studied plants worldwide due to its short lifecycle and ease of manipulation. Many technological advances have been made with Arabidopsis which allow relatively fast analysis of fundamental mechanisms that can then be used to further our understanding of other plant species, including crops. Earlier research had shown that small proteins, or peptides, may be involved in SAR signalling. Therefore, we compared the protein content of extracts of Arabidopsis plants that had been induced for the generation of SAR signals to that of similar extracts of mutant Arabidopsis plants that no longer generate or transmit such signals. So far, we found seven proteins potentially involved in SAR, the most promising of which are three enzymes that process larger proteins into peptides. A mutant no longer making one of these so-called proteases had a lowered capacity to mount SAR in response to bacterial infection. In future, we will investigate if plants making more of this enzyme are more resistant to infection. If so, the peptide produced by this enzyme could be a tool to help protect plants in the farm field from disease.