We are studying two signal transduction cascades that help plants cope with biological and environmental stress conditions: phospholipid-based cellular signalling and volatile-based external communication. The main character in the first pathway is the lipid second messenger, phosphatidic acid (PA), which is produced via activation of phospholipase D (PLD), hydrolyzing structural phospholipids such as phosphatidylcholine, or via the combined action of phospholipase C (PLC) and diacylglycerol kinase (DGK). By manipulating the activity of individual PLC, DGK and PLD genes in tomato and Arabidopsis plants we aim to elucidate their role in stress signalling and development. How PA exerts its effects is still unknown, mainly due to the lack of characterised PA targets. We are using PA-coated Sepharose beads together with mass spectrometry to isolate and identify such targets. As it is mostly unknown where lipid signalling events take place sub-cellularly, we are using fusions of specific lipid-binding domains with YFP as fluorescent "biosensors" that visualize changes in phospholipid pools during stress treatments. In our second theme, we are using a tritrophic system comprising tomato (Lycopersicon esculentum), spider mites (Tetranychus urticae) and predatory mites (Phytoseiulus persimilis) to study the molecular basis of the direct and indirect defence mechanisms of plants against herbivores. Both a micro-array based approach and a metabolomics approach are applied to analyse this system. We have adopted Arabidopsis as a model system for the genetic analysis of volatile perception and aim to characterize those genes that are important for the response to volatile signals. In addition, we are studying the release of floral scent (benzenoids) in Petunia with the aim of understanding how the biosynthesis of pollinator-attracting volatiles is regulated at the molecular level.