Publikationen - Molekulare Signalverarbeitung

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.

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.

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