Publikationen - Molekulare Signalverarbeitung

The peroxidation of polyunsaturated fatty acids, common to all eukaryotes, is mostly catalyzed by members of the lipoxygenase enzyme family of non-heme iron containing dioxygenases. Lipoxygenase products can be metabolized further in the oxylipin pathway by several groups of CYP74 enzymes. One prominent oxylipin is jasmonic acid (JA), a product of the 13-allene oxide synthase branch of the pathway and known as signaling substance that plays a role in vegetative and propagative plant development as well as in plant responses to wounding and pathogen attack. In barley roots, JA level increases upon colonization by arbuscular mycorrhizal fungi. Apart from this first result regarding JA, no information is available on the relevance of lipidperoxide metabolism in arbuscular mycorrhizal symbiosis. Thus we analyzed fatty acid and lipidperoxide patterns in roots of Medicago truncatula during mycorrhizal colonization. Levels of fungus-specific fatty acids as well as palmitic acid (16:0) and oleic acid (18:1 n − 9) were increased in mycorrhizal roots. Thus the degree of arbuscular mycorrhizal colonization of roots can be estimated via analysis of fungal specific esterified fatty acids. Otherwise, no significant changes were found in the profiles of esterified and free fatty acids. The 9- and 13-LOX products of linoleic and α-linolenic acid were present in all root samples, but did not show significant differences between mycorrhizal and non-mycorrhizal roots, except JA which showed elevated levels in mycorrhizal roots. In both types of roots levels of 13-LOX products were higher than those of 9-LOX products. In addition, three cDNAs encoding CYP74 enzymes, two 9/13-hydroperoxide lyases and a 13-allene oxide synthase, were isolated and characterized. The transcript accumulation of these three genes, however, was not increased in mycorrhizal roots of M. truncatula.

Autoregulatory mechanisms have been reported in the rhizobial and the mycorrhizal symbiosis. Autoregulation means that already existing nodules or an existing root colonization by an arbuscular mycorrhizal fungus systemically suppress subsequent nodule formation/root colonization in other parts of the root system. Mutants of some legumes lost their ability to autoregulate the nodule number and thus display a supernodulating phenotype. On studying the effect of pre-inoculation of one side of a split-root system with an arbuscular mycorrhizal fungus on subsequent mycorrhization in the second side of the split-root system of a wild-type soybean (Glycine max L.) cv. Bragg and its supernodulating mutant nts1007, we observed a clear suppressional effect in the wild-type, whereas further root colonization in the split-root system of the mutant nts1007 was not suppressed. These data strongly indicate that the mechanisms involved in supernodulation also affect mycorrhization and support the hypothesis that the autoregulation in the rhizobial and the mycorrhizal symbiosis is controlled in a similar manner. The accumulation patterns of the plant hormones IAA, ABA and Jasmonic acid (JA) in non-inoculated control plants and split-root systems of inoculated plants with one mycorrhizal side of the split-root system and one non-mycorrhizal side, indicate an involvement of IAA in the autoregulation of mycorrhization. Mycorrhizal colonization of soybeans also resulted in a strong induction of ABA and JA levels, but on the basis of our data the role of these two phytohormones in mycorrhizal autoregulation is questionable.

During the symbiotic interaction between Medicago truncatula and the arbuscular mycorrhizal (AM) fungus Glomus intraradices, an endogenous increase in jasmonic acid (JA) occurs. Two full-length cDNAs coding for the JA-biosynthetic enzyme allene oxide cyclase (AOC) from M. truncatula, designated as MtAOC1 and MtAOC2, were cloned and characterized. The AOC protein was localized in plastids and found to occur constitutively in all vascular tissues of M. truncatula. In leaves and roots, MtAOCs are expressed upon JA application. Enhanced expression was also observed during mycorrhization with G. intraradices. A partial suppression of MtAOC expression was achieved in roots following transformation with Agrobacterium rhizogenes harboring the MtAOC1 cDNA in the antisense direction under control of the cauliflower mosaic virus 35S promoter. In comparison to samples transformed with 35S∷uidA, roots with suppressed MtAOC1 expression exhibited lower JA levels and a remarkable delay in the process of colonization with G. intraradices. Both the mycorrhization rate, quantified by fungal rRNA, and the arbuscule formation, analyzed by the expression level of the AM-specific gene MtPT4, were affected. Staining of fungal material in roots with suppressed MtAOC1 revealed a decreased number of arbuscules, but these did not exhibit an altered structure. Our results indicate a crucial role for JA in the establishment of AM symbiosis.