Signal exchange at the plant cell interface under biotic and abiotic stress conditions.

Global changes in the atmosphere, together with substances of anthropogenic origin, result in increasing oxidising capacity of the environment that could give rise to so far unknown chemical and biochemical perturbations in plants, animals and man. Especially terrestrial plants are subjected to additional abiotic and biotic environmental stresses like high or low salinity, drought or metal poltutants: we are witnesses to endangered ecosystems and the reduction of arable lands. In plants, the mechanisms of defence and stress tolerance, and the signal - response relationship under complex stress situation, however, are not well understood and recquire urgent and detailed studies. .Though there is seemingly a great variability in research topics, the justification of this project relies on the observation of some common defense strategies under the different stress conditions (abiotic as well as biotic Recent research shows that drought and salt stress induce changes in typicat antioxidantia (e.g. ascorbic acid Vit. C; tocoferols - Vit. E) and the enzymes involved in the radicat-tempereing reactions (peroxidase superoxide dismutase ...) In general this stress condition is described as oxidative stress .The oxidative stress is characterized by an increased breakdown of nucleic acids, proteins and membranes as a consequence of the highly reactive radicats (hydrogen peroxide, superoxide) A thorough understanding on how these radicals are produced and activate the anti-oxidative metabolism is necessary to evaluate the stress conditions and tc develop remedies or improved defense responses.Stress tolerance can be regarded as the sum of traits of adaptive value expressed at different levels o organisation. Biochemical, physiological and structural features should all be considered, therefore, as determining factors. Between the appearance of the signal (e.g. decreased water potential, pathogenic attack ) and the physiological response, a cascade of events are involved: (1) perception of the signal by the plant cell, (2) the intracellutar transduction of the signal (3) the activation of defence mechanisms (4) transport of defence molecules.It can be stated that the first structure involved in the signal reception is the plasma membrane with it receptors and/or ion channels. The importance of the redox activity of plasma membrane (PM) has bee recognized in numerous functions of PM; e.g. iron uptake, cell defense against microbes, response to oxidative stress, etc. through production of oxygen radicals. However, the identification, purification, anc characterization of the chemical compounds responsible for the PM redox activity has been left for future works. Substrate specificity studies have shown that there is a redox system present in the plant PM which is different from the NADH-ferricyanide reductase presently purified and characterized (NFORase) from spinac} leaves. The present research is devoted to purify and characterize the redox enzyme left in the plant PM after separation of the NFORase mentioned above.Whereas for abiotic stress the signal can be of physical or mechanical nature, biotic stress is mainly recognised chemically trough elicitors (chemoperception). A large number of chemically characterised signal molecules are known, going from oligosaccharides to peptides and proteins. The signalling molecules are recognised by a specific receptor. Our knowledge about these receptors is still rudimentary. Yet, the response of elicitor molecules with membrane receptors induces the production of oxygen intermediates. Besides the release of oxygen radicals, the response to elicitors induces a change in the permeability of the plasma membrane to calcium ions, protons potassium and chloride ions. The elevated Ca2+ concentration triggers the calcium-calmodulin dependent cascad~ reactions as a second line in the signal tranduction mechanism, resulting in the activation of calcium and calrnoduli dependent enzymes like the Ca2+-ATPase and protein kinases. Phosphorylated proteins can play their role a messengers afterwards, carrying the signal towards the nucleus.Within the cell, one of the mechanisms of a plant defense against oxidative stress is an increased production of typical antioxidantia (ascorbic acid, tocoferols, glutathion... ) This variety of antioxidant systems in plants eliminates precursors of hydroxyl radicals (i.e. hydrogen peroxide, superoxide radical). One general antioxidant category which may be involved in abiotic stress tolerance indudes the enzymatic/peptidic scavenging systenns. These are catalase and peroxidase converting hydrogen peroxide into water and oxygen, superoxide dismutase thal catalyzes the conversion of the superoxide radical to hydrogen peroxide and oxygen and the antioxidative enzymes ascorbate peroxidase, monodehydroascorbate reductase and glutathione reductase. In the Halliwell-Asada activated oxygen detoxification pathway hydrogen peroxide is scavenged by the ascorbate-dependent actions of ascorbate peroxidase, NADPH-dependent monodehydroascorbate reductase and glutathione dehydroascorbate reductase.Every aspect of oxidative stress is the result of the integration of the different metabolic changes at the cellular level. Therefore it's almost inevitable to follow the effects of oxidative stress at the level of plant cells (cell culture, protoplasts)Wlthin the framework of (1) environment with the identification of stress (e.g. elicitors !), (2) the recognition of stress at the membrane level and (3) the transduction and metabolisation of stress induced components every partner in this project has'a specific and well integrated task to fillfil. The multidisciplinary approach and the complementary research activities of the different partners will guarantee further progress in elucidating plant stress physiology.