Publications - Cell and Metabolic Biology

Oxygen limitation (hypoxia), arising as a key stress factor due to flooding, negatively affects plant development. Consequently, maintaining root growth under such stress is crucial for plant survival, yet we know little about the root system\'s adaptions to low‐oxygen conditions and its regulation by phytohormones. In this study, we examine the impact of hypoxia and, herein, the regulatory role of group VII ETHYLENE‐RESPONSE FACTOR (ERFVII) transcription factors on root growth in Arabidopsis. We found lateral root (LR) elongation to be actively maintained by hypoxia via ERFVII factors, as erfVII seedlings possess hypersensitivity towards hypoxia regarding their LR growth. Pharmacological inhibition of abscisic acid (ABA) biosynthesis revealed ERFVII‐driven counteraction of hypoxia‐induced inhibition of LR formation in an ABA‐dependent manner. However, postemergence LR growth under hypoxia mediated by ERFVIIs was independent of ABA. In roots, ERFVIIs mediate, among others, the induction of ABA‐degrading ABA 8′‐hydroxylases CYP707A1 expression. RAP2.12 could activate the pCYC707A1:LUC reporter gene, indicating, combined with single mutant analyses, that this transcription factor regulates ABA levels through corresponding transcript upregulation. Collectively, hypoxia‐induced adaptation of the Arabidopsis root system is shaped by developmental reprogramming, whereby ERFVII‐dependent promotion of LR emergence, but not elongation, is partly executed through regulation of ABA degradation.

Arbuscular mycorrhizal (AM) symbiosis modulates plant‐herbivore interactions. Still, how it shapes the overall plant defence strategy and the mechanisms involved remain unclear. We investigated how AM symbiosis simultaneously modulates plant resistance and tolerance to a shoot herbivore, and explored the underlying mechanisms. Bioassays with Medicago truncatula plants were used to study the effect of the AM fungus Rhizophagus irregularis on plant resistance and tolerance to Spodoptera exigua herbivory. By performing molecular and chemical analyses, we assessed the impact of AM symbiosis on herbivore‐triggered phosphate (Pi)‐ and jasmonate (JA)‐related responses. Upon herbivory, AM symbiosis led to an increased leaf Pi content by boosting the mycorrhizal Pi‐uptake pathway. This enhanced both plant tolerance and herbivore performance. AM symbiosis counteracted the herbivore‐triggered JA burst, reducing plant resistance. To disentangle the role of the mycorrhizal Pi‐uptake pathway in the plant\'s response to herbivory, we used the mutant line ha1‐2, impaired in the H+‐ATPase gene HA1, which is essential for Pi‐uptake via the mycorrhizal pathway. We found that mycorrhiza‐triggered enhancement of herbivore performance was compromised in ha1‐2 plants. AM symbiosis thus affects the defence pattern of M. truncatula by altering resistance and tolerance simultaneously. We propose that the mycorrhizal Pi‐uptake pathway is involved in the modulation of the plant defence strategy.

The identification of metabolites by mass spectrometry constitutes a major bottleneck which considerably limits the throughput of metabolomics studies in biomedical or plant research. Here, we present a novel approach to analyze metabolomics data from untargeted, data-independent LC-MS/MS measurements. By integrated analysis of MS1 abundances and MS/MS spectra, the identification of regulated metabolite families is achieved. This approach offers a global view on metabolic regulation in comparative metabolomics. We implemented our approach in the web application “MetFamily”, which is freely available at http://msbi.ipb-halle.de/MetFamily/. MetFamily provides a dynamic link between the patterns based on MS1-signal intensity and the corresponding structural similarity at the MS/MS level. Structurally related metabolites are annotated as metabolite families based on a hierarchical cluster analysis of measured MS/MS spectra. Joint examination with principal component analysis of MS1 patterns, where this annotation is preserved in the loadings, facilitates the interpretation of comparative metabolomics data at the level of metabolite families. As a proof of concept, we identified two trichome-specific metabolite families from wild-type tomato Solanum habrochaites LA1777 in a fully unsupervised manner and validated our findings based on earlier publications and with NMR.

In nature, plants are subject to various stresses that are often accompanied by wounding of the aboveground tissues. As wounding affects plants locally and systemically, we investigated the impact of leaf wounding on interactions of Medicago truncatula with root‐colonizing microorganisms, such as the arbuscular mycorrhizal (AM) fungus Glomus intraradices, the pathogenic oomycete Aphanomyces euteiches and the nitrogen‐fixing bacterium Sinorhizobium meliloti. To obtain a long‐lasting wound response, repeated wounding was performed and resulted in locally and systemically increased jasmonic acid (JA) levels accompanied by the expression of jasmonate‐induced genes, among them the genes encoding allene oxide cyclase 1 (MtAOC1) and a putative cell wall‐bound invertase (cwINV). After repeated wounding, colonization with the AM fungus was increased, suggesting a role of jasmonates as positive regulators of mycorrhization, whereas the interaction with the rhizobacterium was not affected. In contrast, wounded plants appeared to be less susceptible to pathogens which might be caused by JA‐induced defence mechanisms. The effects of wounding on mycorrhization and pathogen infection could be partially mimicked by foliar application of JA. In addition to JA itself, the positive effect on mycorrhization might be mediated by systemically induced cwINV, which was previously shown to exhibit a regulatory function on interaction with AM fungi.

Mesembryanthemum crystallinum L. (Aizoaceae) is a drought‐ and salt‐tolerant halophyte that is able to endure harsh environmental conditions. Upon irradiation with high light irradiance (1200–1500 µ mol m−2 s−1) it displays a rapid cell‐specific accumulation of plant secondary metabolites in the upper leaf epidermis; a phenomenon that is not detectable with salt or drought treatment. The accumulation of these compounds, the betacyanins and acylated flavonol glycosides, increases if the plants are exposed to polychromatic radiation with a progressively decreasing short‐wave cut‐off in the ultraviolet range. The response is localized in the epidermal bladder cells on the tips of young leaves and epidermal layers of fully expanded leaves. It is demonstrated that the accumulation of flavonols and betacyanins can be described by a weakly sigmoid dose function in combination with an exponential decrease of the response function of the plant with increasing wavelength.

A mutant of Synechocystis sp. strain PCC6803 was obtained by random cartridge mutagenesis, which could not grow at low sodium concentrations. Genetic analyses revealed that partial deletion of the sll 0273 gene, encoding a putative Na+ /H+ exchanger, was responsible for this defect. Physiological characterization indicated that the sll 0273 mutant exhibited an increased sensitivity towards K+ , even at low concentrations, which was compensated for by enhanced concentrations of Na+ . This enhanced Na+ demand could also be met by Li+ . Furthermore, addition of monensin, an ionophore mediating electroneutral Na+ /H+ exchange, supported growth of the mutant at unfavourable Na+ /K+ ratios. Measurement of internal Na+ and K+ contents of wild‐type and mutant cells revealed a decreased Na+ /K+ ratio in mutant cells pre‐incubated at a low external Na+ /K+ ratio, while it remained at the level of the wild type after pre‐incubation at a high external Na+ /K+ ratio. We conclude that the Sll0273 protein is required for Na+ influx, especially at low external Na+ concentrations or low Na+ /K+ ratios. This system may be part of a sodium cycle and may permit re‐entry of Na+ into the cells, if nutrient/Na+ symporters are not functional or operating.

In barley leaves, there is a dramatic alteration of gene expression upon treatment with jasmonates leading to the accumulation of newly formed proteins, designated as jasmonate‐inducible proteins (JIPs). In the present study, a new jasmonate‐inducible cDNA, designated pHvJS37, has been isolated by differential screening of a γgt10 cDNA library constructed from mRNA of jasmonate‐treated barley leaf segments. The open reading frame (ORF) encodes a 39‐9 kDa polypeptide which cross‐reacts with antibodies raised against the in vivo JIP‐37. The hydropathic plot suggests that the protein is mainly hydrophilic, containing two hydrophilic domains near the C‐terminus. Database searches did not show any sequence homology of pHv.JS37 to known sequences. Southern analysis revealed at least two genes coding for JIP‐37 which map to the distal portion of the long arm of chromosome 3 and are closely related to genes coding for JIP‐23. The expression pattern of the JIP‐37 genes over time shows differential responses to jasmonate, abscisic acid (ABA), osmotic stress (such as sorbitol treatment) and desiccation stress. No expression was found under salt stress. From experiments using an inhibitor and intermediates of jasmonate synthesis such as α‐linolenic acid and 12‐oxophytodienoic acid, we hypothesize that there is a stress‐induced lipid‐based signalling pathway in which an endogenous rise of jasmonate switches on JIP‐37 gene expression. Using immunocytochemical techniques, JIP‐37 was found to be simultaneously located in the nucleus, the cytoplasm and the vacuoles.