Sulfur as possible protection against plant pathogen attack

Sulfur is an essential macroelement for plant life and fulfills various biological functions in plant metabolism and defense. The decline in atmospheric sulfur deposition in the past decade has become a major nutritional problem for plant production. It has also contributed to a decreased capability of plants to fight against abiotic and especially biotic stress situations as sulfur containing components play a very important role in plant diseases resistance. One of the most important sulfur containing antioxidants is the tripeptide glutathione ( -glutamyl- cysteinyl-glycine). Glutathione protects plants against reactive oxygen species (ROS), either directly through scavenging them or through the ascorbate-glutathione cycle. ROS which are formed during various abiotic and biotic stress situations would otherwise lead to the destruction of biological membranes, proteins, RNA and DNA leading to mutation, cancer and eventually cell death. Especially during pathogen attack a strong correlation between elevated glutathione contents and increased stress tolerance against the oxidative stress induced by pathogen attack can be observed. Additionally, a strong activation of the ascorbate-glutathione cycle can be commonly observed during incompatible and also compatible plant pathogen infection. Therefore it seems that elevated glutathione contents and the activation of the ascorbate-glutathione cycle are directly involved in the development of resistance during plant microbe interactions. As cysteine, the initial product of sulfur assimilation, is usually the rate limiting factor of glutathione synthesis in plants, we will test the hypothesis whether the treatment with sulfur will lead to enhanced stress tolerance during pathogen attack due to elevated levels of glutathione and due to the activation of the most important components (ascorbate, glutathione, cysteine, glycine, glutamate, glutathione reductase and glutathione-S-transferase) of the ascorbate-glutathione cycle. The studies will be performed on transgenic and non-transgenic model plants (Arabidopsis thaliana infected with Pseudomonas syringae and Nicotiana tabacum infected with Tobacco mosaic virus) with enhanced antioxidative defense mechanisms, which will enable us to compare the data from our studies with studies made on those model plants by other research teams. Additionally, field experiments will be carried out in collaborative studies with Brassica napus (oilseed rape) plants naturally infected with the fungal pathogen Pyrenopeziza brassica. Because of its high demand for sulfur, it is particularly sensitive to sulfur deficiency and therefore an ideal widely used model crop to study the effects of sulfur fertilization in correlation with pathogen attack. Changes of the above mentioned metabolites have mostly been measured in whole organs rather than in single cells and organelles. Therefore their subcellular localization is still unclear. However, such data are essential to understand antioxidative defense strategies of plants on the subcellular level and to develop new defense strategies against pathogen attack by modifying levels of antioxidative metabolites. Therefore histo- and cytohistochemical and biochemical investigations (confocal laser scanning and transmission electron microscopy, high performance liquid chromatography) will be used to study changes of these antioxidative defense molecules in single cells and organelles and on the whole tissue level. The results obtained on the subcellular level will be supplemented on selected samples with biochemical investigations based on the whole organ level. Differences between antioxidative defense mechanisms in plants treated with sulfur and untreated ones should extend the knowledge about the role of sulfur in plant disease resistance during pathogen attack and could help to develop new defense strategies for agricultural use in the future.

The decline in atmospheric sulphur deposition in the past decade has become a major nutritional problem for plant production. The aim of this project was to evaluate the importance of sulphur fertilization on plant health for the defence against pathogen attack. The results of the project proved the hypothesis that sulphur fertilization induces increased stress tolerance of tobacco plants against tobacco mosaic virus (TMV). Nicotiana tabacum cv. Samsun nn (susceptible to TMV) and Nicotiana tabacum cv. Samsun NN (resistant against TMV) treated with sulphur showed less symptoms and necrotic lesions, respectively, than plants starved of sulphur. These results could be correlated with decreased amounts of TMV particles and coat protein in leaves. The stronger stress tolerance against TMV in sulphur treated Nicotiana plants was additionally correlated with a stronger activation of the ascorbate-glutathione cycle and plant defense genes. Thus, these results indicate that sulphur enhanced defense/sulphur induced resistance in plants against pathogens is induced through the stronger activation of defense genes in connection with higher antioxidative defense capacity in sulphur treated plants. These results are of great relevance for agriculture as they demonstrate that an adequate sulphur fertilization can improve plant health in situations of biotic stress.