Dorothea Ellinger

Elevated Early Callose Deposition Results in Complete Penetration Resistance to Powdery Mildew in Arabidopsis

Overexpressing callose synthase in Arabidopsis enlarges callose deposits during powdery mildew infection and gives complete penetration resistance to the fungi. A common response by plants to fungal attack is deposition of callose, a (1,3)-β-glucan polymer, in the form of cell wall thickenings called papillae, at site of wall penetration. While it has been generally believed that the papillae provide a structural barrier to slow fungal penetration, this idea has been challenged in recent studies of Arabidopsis (Arabidopsis thaliana), where fungal resistance was found to be independent of callose deposition. To the contrary, we show that callose can strongly support penetration resistance when deposited in elevated amounts at early time points of infection. We generated transgenic Arabidopsis lines that express POWDERY MILDEW RESISTANT4 (PMR4), which encodes a stress-induced callose synthase, under the control of the constitutive 35S promoter. In these lines, we detected callose synthase activity that was four times higher than that in wild-type plants 6 h post inoculation with the virulent powdery mildew Golovinomyces cichoracearum. The callose synthase activity was correlated with enlarged callose deposits and the focal accumulation of green fluorescent protein-tagged PMR4 at sites of attempted fungal penetration. We observed similar results from infection studies with the nonadapted powdery mildew Blumeria graminis f. sp. hordei. Haustoria formation was prevented in resistant transgenic lines during both types of powdery mildew infection, and neither the salicylic acid-dependent nor jasmonate-dependent pathways were induced. We present a schematic model that highlights the differences in callose deposition between the resistant transgenic lines and the susceptible wild-type plants during compatible and incompatible interactions between Arabidopsis and powdery mildew.

The Plasma Membrane of the Cyanobacterium Gloeobacter violaceus Contains Segregated Bioenergetic Domains

Gloeobacter violaceus, the most primordial of known organisms performing oxygenic photosynthesis, evolved before the evolutionary appearance of thylakoid membranes. This study demonstrates the presence of bioenergetic domains in its cytoplasmic membrane. The formation of these membrane domains might constitute evolutionary precursors of the thylakoid membrane. The light reactions of oxygenic photosynthesis almost invariably take place in the thylakoid membranes, a highly specialized internal membrane system located in the stroma of chloroplasts and the cytoplasm of cyanobacteria. The only known exception is the primordial cyanobacterium Gloeobacter violaceus, which evolved before the appearance of thylakoids and harbors the photosynthetic complexes in the plasma membrane. Thus, studies on G. violaceus not only shed light on the evolutionary origin and the functional advantages of thylakoid membranes but also might include insights regarding thylakoid formation during chloroplast differentiation. Based on biochemical isolation and direct in vivo characterization, we report here structural and functional domains in the cytoplasmic membrane of a cyanobacterium. Although G. violaceus has no internal membranes, it does have localized domains with apparently specialized functions in its plasma membrane, in which both the photosynthetic and the respiratory complexes are concentrated. These bioenergetic domains can be visualized by confocal microscopy, and they can be isolated by a simple procedure. Proteomic analysis of these domains indicates their physiological function and suggests a protein sorting mechanism via interaction with membrane-intrinsic terpenoids. Based on these results, we propose specialized domains in the plasma membrane as evolutionary precursors of thylakoids.

DONGLE and DEFECTIVE IN ANTHER DEHISCENCE1 Lipases Are Not Essential for Wound- and Pathogen-Induced Jasmonate Biosynthesis: Redundant Lipases Contribute to Jasmonate Formation

Lipases are involved in the generation of jasmonates, which regulate responses to biotic and abiotic stresses. Two sn-1-specific acyl hydrolases, DEFECTIVE IN ANTHER DEHISCENCE1 (DAD1) and DONGLE (DGL), have been reported to be localized in plastids and to be essential and sufficient for jasmonate biosynthesis in Arabidopsis (Arabidopsis thaliana) leaves. Here, we show that levels of 12-oxo-phytodienoic acid (OPDA) and jasmonic acid in three different DGL RNA interference lines and the dad1 mutant were similar to wild-type levels during the early wound response as well as after Pseudomonas infection. Due to the lack of sn-2 substrate specificity, synthesis of dinor OPDA was not expected and also not found to be affected in DGL knockdown and DGL-overexpressing lines. As reported, DAD1 participates in jasmonate formation only in the late wound response. In addition, DGL protein was found to be localized in lipid bodies and not in plastids. Furthermore, jasmonate levels in 16 additional mutants defective in the expression of lipases with predicted chloroplast localization did not show strong differences from wild-type levels after wounding, except for a phospholipase A (PLA) PLA-Iγ1 (At1g06800) mutant line that displayed diminished wound-induced dinor OPDA, OPDA, and jasmonic acid levels. A quadruple mutant defective in four DAD1-like lipases displayed similar jasmonate levels as the mutant line of PLA-Iγ1 after wounding. Hence, we identify PLA-Iγ1 as a novel target gene to manipulate jasmonate biosynthesis. Our results suggest that, in addition to DAD1 and PLA-Iγ1, still unidentified enzymes with sn-1 and sn-2 hydrolase activity are involved in wound- and pathogen-induced jasmonate formation, indicating functional redundancy within the lipase family.

Callose biosynthesis in Arabidopsis with a focus on pathogen response: what we have learned within the last decade.

BACKGROUND (1,3)-β-Glucan callose is a cell wall polymer that is involved in several fundamental biological processes, ranging from plant development to the response to abiotic and biotic stresses. Despite its importance in maintaining plant integrity and plant defence, knowledge about the regulation of callose biosynthesis at its diverse sites of action within the plant is still limited. The moderately sized family of GSL (GLUCAN SYNTHASE-LIKE) genes is predicted to encode callose synthases with a specific biological function and subcellular localization. Phosphorylation and directed translocation of callose synthases seem to be key post-translational mechanisms of enzymatic regulation, whereas transcriptional control of GSL genes might only have a minor function in response to biotic or abiotic stresses. SCOPE AND CONCLUSIONS Among the different sites of callose biosynthesis within the plant, particular attention has been focused on the formation of callose in response to pathogen attack. Here, callose is deposited between the plasma membrane and the cell wall to act as a physical barrier to stop or slow invading pathogens. Arabidopsis (Arabidopsis thaliana) is one of the best-studied models not only for general plant defence responses but also for the regulation of pathogen-induced callose biosynthesis. Callose synthase GSL5 (GLUCAN SYNTHASE-LIKE5) has been shown to be responsible for stress-induced callose deposition. Within the last decade of research into stress-induced callose, growing evidence has been found that the timing of callose deposition in the multilayered system of plant defence responses could be the key parameter for optimal effectiveness. This timing seems to be achieved through co-ordinated transport and formation of the callose synthase complex.

Interaction of the Arabidopsis GTPase RabA4c with Its Effector PMR4 Results in Complete Penetration Resistance to Powdery Mildew

Unexpectedly, an Arabidopsis RabA family GTPase directly interacted with a pathogen-induced, plasma membrane-bound callose synthase in unchallenged epidermal leaf cells as well as in response to powdery mildew. Overexpression of this Rab GTPase induced enhanced early callose deposition at powdery mildew infection sites and complete penetration resistance to this biotrophic fungus. The (1,3)-β-glucan callose is a major component of cell wall thickenings in response to pathogen attack in plants. GTPases have been suggested to regulate pathogen-induced callose biosynthesis. To elucidate the regulation of callose biosynthesis in Arabidopsis thaliana, we screened microarray data and identified transcriptional upregulation of the GTPase RabA4c after biotic stress. We studied the function of RabA4c in its native and dominant negative (dn) isoform in RabA4c overexpression lines. RabA4c overexpression caused complete penetration resistance to the virulent powdery mildew Golovinomyces cichoracearum due to enhanced callose deposition at early time points of infection, which prevented fungal ingress into epidermal cells. By contrast, RabA4c(dn) overexpression did not increase callose deposition or penetration resistance. A cross of the resistant line with the pmr4 disruption mutant lacking the stress-induced callose synthase PMR4 revealed that enhanced callose deposition and penetration resistance were PMR4-dependent. In live-cell imaging, tagged RabA4c was shown to localize at the plasma membrane prior to infection, which was broken in the pmr4 disruption mutant background, with callose deposits at the site of attempted fungal penetration. Together with our interactions studies including yeast two-hybrid, pull-down, and in planta fluorescence resonance energy transfer assays, we concluded that RabA4c directly interacts with PMR4, which can be seen as an effector of this GTPase.

Reduced susceptibility to Fusarium head blight in Brachypodium distachyon through priming with the Fusarium mycotoxin deoxynivalenol

The fungal cereal pathogen Fusarium graminearum produces deoxynivalenol (DON) during infection. The mycotoxin DON is associated with Fusarium head blight (FHB), a disease that can cause vast grain losses. Whilst investigating the suitability of Brachypodium distachyon as a model for spreading resistance to F. graminearum, we unexpectedly discovered that DON pretreatment of spikelets could reduce susceptibility to FHB in this model grass. We started to analyse the cell wall changes in spikelets after infection with F. graminearum wild-type and defined mutants: the DON-deficient Δtri5 mutant and the DON-producing lipase disruption mutant Δfgl1, both infecting only directly inoculated florets, and the mitogen-activated protein (MAP) kinase disruption mutant Δgpmk1, with strongly decreased virulence but intact DON production. At 14 days post-inoculation, the glucose amounts in the non-cellulosic cell wall fraction were only increased in spikelets infected with the DON-producing strains wild-type, Δfgl1 and Δgpmk1. Hence, we tested for DON-induced cell wall changes in B. distachyon, which were most prominent at DON concentrations ranging from 1 to 100 ppb. To test the involvement of DON in defence priming, we pretreated spikelets with DON at a concentration of 1 ppm prior to F. graminearum wild-type infection, which significantly reduced FHB disease symptoms. The analysis of cell wall composition and plant defence-related gene expression after DON pretreatment and fungal infection suggested that DON-induced priming of the spikelet tissue contributed to the reduced susceptibility to FHB.

Glucanocellulosic ethanol: the undiscovered biofuel potential in energy crops and marine biomass

Converting biomass to biofuels is a key strategy in substituting fossil fuels to mitigate climate change. Conventional strategies to convert lignocellulosic biomass to ethanol address the fermentation of cellulose-derived glucose. Here we used super-resolution fluorescence microscopy to uncover the nanoscale structure of cell walls in the energy crops maize and Miscanthus where the typical polymer cellulose forms an unconventional layered architecture with the atypical (1, 3)-β-glucan polymer callose. This raised the question about an unused potential of (1, 3)-β-glucan in the fermentation of lignocellulosic biomass. Engineering biomass conversion for optimized (1, 3)-β-glucan utilization, we increased the ethanol yield from both energy crops. The generation of transgenic Miscanthus lines with an elevated (1, 3)-β-glucan content further increased ethanol yield providing a new strategy in energy crop breeding. Applying the (1, 3)-β-glucan-optimized conversion method on marine biomass from brown macroalgae with a naturally high (1, 3)-β-glucan content, we not only substantially increased ethanol yield but also demonstrated an effective co-fermentation of plant and marine biomass. This opens new perspectives in combining different kinds of feedstock for sustainable and efficient biofuel production, especially in coastal regions.

Involvement of DAD1-like lipases in response to salt and osmotic stress in Arabidopsis thaliana

Acyl hydrolases remodel biological membranes and release signaling molecules in response to a variety of biotic and abiotic stresses. After wounding or pathogen treatment lipases are necessary to release fatty acids as substrate for jasmonate biosynthesis. In osmotic stressed tissue they maintain integrity and functionality of membranes and during senescence lipases destroy and recycle membranes. Recently the role of several acyl hydrolases including DEFECTIVE IN ANTHER DEHISCENCE (DAD1) and DAD1-like lipase, e.g. DONGLE (DGL) and the phospholipase A (PLA) PLA-Iγ1 in jasmonate biosynthesis after wounding were investigated and functional redundancy within this family has been stated. Here we report necessity of diverse DAD1-like lipases in response to salt and sorbitol treatment. The lipase PLA-Iγ1 and PLA-Iβ2, which were both impaired in wound response, were also affected in response to osmotic stress in seed germination assays. Based on our observations and interpretations of transcription analyses generated by AtGenExpress project we speculate about more general roles of the DAD1-like lipase in diverse biological processes.

Resistance of callose synthase activity to free fatty acid inhibition as an indicator of Fusarium head blight resistance in wheat

The fungal pathogen Fusarium graminearum is the causal agent of Fusarium head blight (FHB); a devastating crop disease resulting in heavy yield losses and grain contamination with mycotoxins. We recently showed that the secreted lipase FGL1, a virulence factor of F. graminearum, targets plant defense-related callose biosynthesis during wheat head infection. This effector-like function is based on a FGL1-mediated release of polyunsaturated free fatty acids (FFA) that can inhibit callose synthase activity. The importance of FGL1 in successful wheat head colonization was demonstrated in FGL1 disruption mutants (Δfgl1), where infection was restricted to directly inoculated spikelets and accompanied by strong callose deposition in the spikelet’s phloem. The application of polyunsaturated FFA to Δfgl1-infected spikelets prevented callose deposition in the phloem and partially restored wheat head colonization. The comparative analysis of 3 wheat cultivars revealed that the level of resistance to FHB correlated with resistance to FFA-dependent inhibition of callose biosynthesis. Therefore, resistance of callose biosynthesis to FFA inhibition might be used as marker and/or direct target in the breeding of FHB-resistant wheat cultivars.

Simple preparation of plant epidermal tissue for laser microdissection and downstream quantitative proteome and carbohydrate analysis

The outwardly directed cell wall and associated plasma membrane of epidermal cells represent the first layers of plant defense against intruding pathogens. Cell wall modifications and the formation of defense structures at sites of attempted pathogen penetration are decisive for plant defense. A precise isolation of these stress-induced structures would allow a specific analysis of regulatory mechanism and cell wall adaption. However, methods for large-scale epidermal tissue preparation from the model plant Arabidopsis thaliana, which would allow proteome and cell wall analysis of complete, laser-microdissected epidermal defense structures, have not been provided. We developed the adhesive tape – liquid cover glass technique (ACT) for simple leaf epidermis preparation from A. thaliana, which is also applicable on grass leaves. This method is compatible with subsequent staining techniques to visualize stress-related cell wall structures, which were precisely isolated from the epidermal tissue layer by laser microdissection (LM) coupled to laser pressure catapulting. We successfully demonstrated that these specific epidermal tissue samples could be used for quantitative downstream proteome and cell wall analysis. The development of the ACT for simple leaf epidermis preparation and the compatibility to LM and downstream quantitative analysis opens new possibilities in the precise examination of stress- and pathogen-related cell wall structures in epidermal cells. Because the developed tissue processing is also applicable on A. thaliana, well-established, model pathosystems that include the interaction with powdery mildews can be studied to determine principal regulatory mechanisms in plant–microbe interaction with their potential outreach into crop breeding.