From health to sickness; the metabolomics transition associated with plant disease and defense.

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As plants are firmly rooted to the ground, they are open to attack by a wide variety of nasties such as insects, fungi, bacteria and viruses. Fascinatingly, despite this constant invasion of privacy, disease is an exception due to the deployment of an extraordinary complex and highly effective network of synergistic defensive strategies -collectively known as basal defense or non-host resistance. Many of these defensive barriers are the combination of actions of one or more metabolites - small chemicals. Metabolites are synthesised by the plant and act in a multitude of ways - as signals, as sedatives or even as toxins - to individually or cooperatively prevent pathogen ingress. When pathogens, such as bacteria, do successfully invade a plant, they themselves employ a variety of strategies to further suppress or evade host defenses and manipulate host metabolism to provide nutrients for their multiplication. In sum, they turn their intercellular environment into a cosy apartment with food and nutrients online. They achieve this 'success' by actively injecting proteins into the plant cell via what is basically a syringe-like structure known as a type III secretion system. These 'effector' proteins manipulate host transcription (expression of genes which are the template for protein synthesis) and by definition protein expression, to orchestrate a complex network of signalling events. Occasionally, one or more of these effector proteins is recognised and triggers an alarm signal that both locally and systemically induces immunity, leading to disinfection of the plant. Our studies underpinning this proposal have used a technique known as transcriptional profiling to look at how the expression of all plant genes is modified by various pathogen challenges. Analysis of the very complex expression patterns revealed families of genes specifically involved in basal defense and disease. In particular, we found the successful pathogens induced gene expression patterns modified from those seen in unsuccessful attempts to infect. These data represent a significant milestone in our understanding of plant defense as many of these genes encode proteins which themselves produce or modify metabolites that might enhance or interfere with plant immunity. Through a combination of modern technologies we now wish to discover the chemicals that coordinate defense. We will attempt to look at all the small molecules / a procedure known as metabolomics or metabolite profiling / and identify which ones change in quantity following specific treatments. Profiling can be targeted (to identify known compounds) or non-targeted, measuring the pattern of changes. New technologies allow us to undertake large scale metabolic profiling to identify differences between plant tissues treated in different ways or undergoing different reactions. This way we can identify metabolites that differ, even though we may not actually be able to identify the metabolite from the preliminary screen. By using this technology in combination with mutants which cannot fully activate specific defense pathways, we will obtain clues to how the important compounds protect or destablise resistance and maybe even identity the signal molecules that start the whole process of defense. In short, we aim to identify metabolite 'signatures' specifically associated with various aspects of defense. The long-term outcome of this work will be better understanding of the small molecules (i) recruited for plant defense and (ii) those metabolites associated with successful infections. These results will inform future strategies aimed toward manipulating plant responses to develop broad spectrum immunity to pathogens.