Publikationen - Molekulare Signalverarbeitung

Glucosinolates are a class of secondary metabolites with important roles in plant defense and human nutrition. To uncover regulatory mechanisms of glucosinolate production, we screened Arabidopsis thaliana T‐DNA activation‐tagged lines and identified a high‐glucosinolate mutant caused by overexpression of IQD1 (At3g09710). A series of gain‐ and loss‐of‐function IQD1 alleles in different accessions correlates with increased and decreased glucosinolate levels, respectively. IQD1 encodes a novel protein that contains putative nuclear localization signals and several motifs known to mediate calmodulin binding, which are arranged in a plant‐specific segment of 67 amino acids, called the IQ67 domain. We demonstrate that an IQD1‐GFP fusion protein is targeted to the cell nucleus and that recombinant IQD1 binds to calmodulin in a Ca2+‐dependent fashion. Analysis of steady‐state messenger RNA levels of glucosinolate pathway genes indicates that IQD1 affects expression of multiple genes with roles in glucosinolate metabolism. Histochemical analysis of tissue‐specific IQD1 ::GUS expression reveals IQD1 promoter activity mainly in vascular tissues of all organs, consistent with the expression patterns of several glucosinolate‐related genes. Interestingly, overexpression of IQD1 reduces insect herbivory, which we demonstrated in dual‐choice assays with the generalist phloem‐feeding green peach aphid (Myzus persicae ), and in weight‐gain assays with the cabbage looper (Trichoplusia ni ), a generalist‐chewing lepidopteran. As IQD1 is induced by mechanical stimuli, we propose IQD1 to be novel nuclear factor that integrates intracellular Ca2+ signals to fine‐tune glucosinolate accumulation in response to biotic challenge.

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.

Immature scutella of barley were transformed with cDNA coding for a 13-lipoxygenase of barley (LOX-100) via particle bombardment. Regenerated plants were tested by PAT-assay, Western-analysis and PCR-screening. Immunocytochemical assay of T0 plants showed expression of the LOX cDNA both in the chloroplasts and in the cytosol, depending on the presence of the chloroplast signal peptide sequences in the cDNA. A few transgenic plants containing higher amounts of LOX-derived products have been found. These are the candidates for further analysis concerning pathogen resistance.

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