Publikationen - Molekulare Signalverarbeitung

Small-molecule phytohormones exert control over plant growth, development, and stress responses by coordinating the patterns of gene expression within and between cells. Increasing evidence indicates that currently recognized plant hormones are part of a larger group of regulatory metabolites that have acquired signaling properties during the evolution of land plants. This rich assortment of chemical signals reflects the tremendous diversity of plant secondary metabolism, which offers evolutionary solutions to the daunting challenges of sessility and other unique aspects of plant biology. A major gap in our current understanding of plant regulatory metabolites is the lack of insight into the direct targets of these compounds. Here, we illustrate the blurred distinction between classical phytohormones and other bioactive metabolites by highlighting the major scientific advances that transformed the view of jasmonate from an interesting floral scent to a potent transcriptional regulator. Lessons from jasmonate research generally apply to other phytohormones and thus may help provide a broad understanding of regulatory metabolite–protein interactions. In providing a framework that links small-molecule diversity to transcriptional plasticity, we hope to stimulate future research to explore the evolution, functions, and mechanisms of perception of a broad range of plant regulatory metabolites.

Over the next decade, wheat grain production must increase to meet the demand of a fast growing human population. One strategy to meet this challenge is to raise wheat productivity by optimizing plant stature. The Reduced height 8 (Rht8) semi-dwarfing gene is one of the few, together with the Green Revolution genes, to reduce stature of wheat (Triticum aestivum L.), and improve lodging resistance, without compromising grain yield. Rht8 is widely used in dry environments such as Mediterranean countries where it increases plant adaptability. With recent climate change, its use could become increasingly important even in more northern latitudes. In the present study, the characterization of Rht8 was furthered. Morphological analyses show that the semi-dwarf phenotype of Rht8 lines is due to shorter internodal segments along the wheat culm, achieved through reduced cell elongation. Physiological experiments show that the reduced cell elongation is not due to defective gibberellin biosynthesis or signalling, but possibly to a reduced sensitivity to brassinosteroids. Using a fine-resolution mapping approach and screening 3104 F2 individuals of a newly developed mapping population, the Rht8 genetic interval was reduced from 20.5 cM to 1.29 cM. Comparative genomics with model genomes confined the Rht8 syntenic intervals to 3.3 Mb of the short arm of rice chromosome 4, and to 2 Mb of Brachypodium distachyon chromosome 5. The very high resolution potential of the plant material generated is crucial for the eventual cloning of Rht8.

Although RNase-based self-incompatibility (SI) is suspected to operate in a wide group of plant families, it has been characterized as the molecular genetic basis of SI in only three distantly related families, Solanaceae, Plantaginaceae, and Rosaceae, all described over a decade ago. Previous studies found that gametophytic SI, controlled by a multi-allelic S-locus, operates in the coffee family (Rubiaceae). The molecular genetic basis of this mechanism remains unknown, despite the immense importance of coffee as an agricultural commodity. Here, we isolated ten sequences with features of T2-S-type RNases from two Coffea species. While three of the sequences were identified in both species and clearly do not appear to be S-locus products, our data suggest that six sequences may be S-alleles in the self-incompatible C. canephora, and one may be a relict in the self-compatible C. arabica. We demonstrate that these sequences show style-specific expression, display polymorphism in C. canephora, and cluster with S-locus products in a phylogenetic analysis that includes other plant families with RNase-based SI. Although our results are not definitive, in part because the available plant materials were limited and data patterns relatively complex, our results strongly hint that RNase-based SI mechanism operates in the Rubiaceae family.

To elucidate the molecular mechanisms underlying pathogen‐associated molecular pattern (PAMP)‐induced defense responses in potato (Solanum tuberosum ), the role of the signaling compounds salicylic acid (SA) and jasmonic acid (JA) was analyzed. Pep‐13, a PAMP from Phytophthora , induces the accumulation of SA, JA and hydrogen peroxide, as well as the activation of defense genes and hypersensitive‐like cell death. We have previously shown that SA is required for Pep‐13‐induced defense responses. To assess the importance of JA, RNA interference constructs targeted at the JA biosynthetic genes, allene oxide cyclase and 12‐oxophytodienoic acid reductase, were expressed in transgenic potato plants. In addition, expression of the F‐box protein COI1 was reduced by RNA interference. Plants expressing the RNA interference constructs failed to accumulate the respective transcripts in response to wounding or Pep‐13 treatment, neither did they contain significant amounts of JA after elicitation. In response to infiltration of Pep‐13, the transgenic plants exhibited a highly reduced accumulation of reactive oxygen species as well as reduced hypersensitive cell death. The ability of the JA‐deficient plants to accumulate SA suggests that SA accumulation is independent or upstream of JA accumulation. These data show that PAMP responses in potato require both SA and JA and that, in contrast to Arabidopsis, these compounds act in the same signal transduction pathway. Despite their inability to fully respond to PAMP treatment, the transgenic RNA interference plants are not altered in their basal defense against Phytophthora infestans .

Oxygenated polyunsaturated fatty acids synthesized via the lipoxygenase pathway play a role in plant responses to pathogen attack. In solanaceous plants, the preferential stimulation of the 9-lipoxygenase pathway in response to pathogen infection leads to the formation of the divinyl ether-containing polyunsaturated fatty acids colneleic and colnelenic acid, as well as hydroxy and trihydroxy polyunsaturated fatty acids. To functionally assess the role of divinyl ethers, transgenic potato plants were generated which express an RNA interference construct directed against the pathogen-inducible 9-divinyl ether synthase. Efficient reduction of 9-divinyl ether synthase transcript accumulation correlated with reduced levels of colneleic and colnelenic acid. However, in response to infection with virulent Phytophthora infestans, the causal agent of late blight disease, no significant differences in pathogen biomass could be detected suggesting that the levels of antimicrobial divinyl ethers are not critical for defense against Phytophthora infestans in a compatible interaction.

Coffea arabica is susceptible to several pests and diseases, some of which affect the leaves and roots. Systemic acquired resistance (SAR) is the main defence mechanism activated in plants in response to pathogen attack. Here, we report the effects of benzo(1,2,3)thiadiazole-7-carbothioic acid-s-methyl ester (BTH), a SAR chemical inducer, on the expression profile of C. arabica. Two cDNA libraries were constructed from the mRNA isolated from leaves and embryonic roots to create 1587 nonredundant expressed sequence tags (ESTs). We developed a cDNA microarray containing 1506 ESTs from the leaves and embryonic roots, and 48 NBS-LRR (nucleotide-binding site leucine-rich repeat) gene fragments derived from 2 specific genomic libraries. Competitive hybridization between untreated and BTH-treated leaves resulted in 55 genes that were significantly overexpressed and 16 genes that were significantly underexpressed. In the roots, 37 and 42 genes were over and underexpressed, respectively. A general shift in metabolism from housekeeping to defence occurred in the leaves and roots after BTH treatment. We observed a systemic increase in pathogenesis-related protein synthesis, in the oxidative burst, and in the cell wall strengthening processes. Moreover, responses in the roots and leaves varied significantly.