Systematic investigation of the tribe Lactuceae

In the course of the proposed project phytochemical and molecular- biologic investigation of members of the tribe Lactuceae (Asteraceae), with a focus on the subtribes Crepidinae, Hypochoerinae and Scorzonerinae will be performed. One aim of the project is the finding of new natural products or even classes of natural products, their isolation and structure elucidation by nuclear magnetic resonance spectroscopy and mass spectrometry. Furthermore, the occurrence of these new compounds as well as the occurrence of known compounds within the tribes mentioned above will be analyzed by high performance liquid chromatography. The obtained quantification data will be processed by multivariate data analysis. The relationships between the particular taxa found by that approach will be compared to the existing morphologically based system of this group of the Asteraceae. Since the Lactuceae are a phylogenetically young group, they possess only a limited number of differentiating morphological features. Therefore, differences between the phytochemically and morphologically based systems are to be expected. Besides a synthesis of the two systems, the phylogenesis of the Lactuceae will be investigated using molecular biologic techniques like gene sequencing and Amplified Fragment Length Polymorphism (AFLP). These techniques have the potential to identify genetical differences between related taxa and within populations, respectively. This combined morphological, phytochemical and molecular-biological approach will yield new insights into the phylogenesis of the Lactuceae as well as information about the chronological order of the differentiation of chemical and morphological features in the evolution of plant species. Systematic studies about the influence of different ecological factors like altitude, grazing, temperature or level of radiation on the secondary metabolite spectrum will give further clues about the applicability of the investigated substances as qualitative and/or quantitative chemosystematic markers. These investigations will also give first hints about optimal growing conditions for medicinal plants, which are cultivated for optimal yields of particular secondary metabolites.

The tribe Lactuceae comprises approximately 1500 species, some of them used as medicinal plants and some others as food plants. This wide usage is one of the reasons for the high number of publications on the Lactuceae, their chemistry, and taxonomy. In the course of the finished project, many new insights, especially into the chemistry of the Lactuceae, have been gained. A total of twelve papers in peer reviewed international journals were published from data and plant material acquired during the project. Especially noteworthy are the papers on tragoponic acid from Tragopogon porrifolius, on a chloro-sesquiterpene lactone glucoside from a Leontodon taxon, and the combined chemosystematic and molecular investigations of chemically differing populations of Leontodon autumnalis. Tragoponic acid is a new natural product, which represents biosynthetically a missing link between dihydrostilbenes and dihydroisocoumarins. Besides tragoponic acid, some other dihydrostilbenoids and dihydroisocoumarins were isolated from Tragopgon porrifolius. This is of special interest because Tragopogon porrifolius, white salsify, is an old vegetable species, which is still quite popular in the United Kingdom. The chloro-sesquiterpenoid glucoside from the genus Leontodon is the first of its kind and its occurrence proves that halogenated sesquiterpenoids occur as genuine natural products not only in marine organisms but also in higher land plants. The reasons why the plants actually synthesize this type of compound are still not resolved. However, there are to competing theories: a) the biosynthesis of chlorosesquiterpenoids is a means of detoxifying chloride ions, which are prevalent in salty soils; b) chlorosequiterpenoids are bioactive compounds and have protective functions for the plants producing them against herbivore and/or microbial attack. The combined phytochemical/molecular study on different chemotypes in Leontodon autumnalis is to be regarded as the first in a series of such investigations. Molecular studies have a predominant role in current investigations in systematic botany and allow formerly unimaginable insights into evolutionary history. On the other hand molecular studies often yield groupings, which are not or only poorly defined by morphologic characters. In such cases, phytochemical/chemosystematic data can help in phenetically characterizing these newly emerging groups. Moreover, the joint research efforts in the area of molecular biology have in addition to the mentioned paper resulted in a fruitful co-operation with various groups of the Leibniz Institute of Plant Genetics and Crop Plant Research Gatersleben. Currently co-operations exist with the gene bank, the herbarium, and the institute of experimental taxonomy.