XVI International Botanical Congess

Why plants with induced defense systems can nevertheless become more susceptible for biotic or abiotic stress and disease is currently an unresolved problem. For example, the air pollutant, ozone, can act as an inducer of signal substances (ethylene, salicylic acid), defense pathways (phytoalexins, PR-proteins, physical barriers, antioxidative systems) and additional plant stress response programs (heat-shock, UV-B, heavy metal ions). These multiple ozone effects have been demonstrated at the transcript, protein and metabolite levels. Ozone should therefore be a plant-strengthening agent. This has in fact been observed in the laboratory, e.g. for tobacco/tobacco mosaic virus and Arabidopsis/ Pseudomonas. However, in field experiments, ozone often led to enhanced abiotic or, upon infection, biotic disease symptoms. Increased susceptibility, or lack of protection, also occurred in certain cases when plant stress response networks were increased by other means, such as gene overexpression (e.g. PR-proteins) or xenobiotics (e.g., BION-). Such contradictory results may be related to the existence of strict spatial, temporal and network-regulatory requirements for active defence. New approaches to improved plant protection will be discussed.