Possible roles for mannitol and mannitol dehydrogenase in the biotrophic plant pathogen Uromyces fabae

The haustorial transcriptomes of Uromyces appendiculatus and Phakopsora pachyrhizi and their candidate effector families

Haustoria of biotrophic rust fungi are responsible for the uptake of nutrients from their hosts and for the production of secreted proteins, known as effectors, which modulate the host immune system. The identification of the transcriptome of haustoria and an understanding of the functions of expressed genes therefore hold essential keys for the elucidation of fungus-plant interactions and the development of novel fungal control strategies. Here, we purified haustoria from infected leaves and used 454 sequencing to examine the haustorial transcriptomes of Phakopsora pachyrhizi and Uromyces appendiculatus, the causal agents of soybean rust and common bean rust, respectively. These pathogens cause extensive yield losses in their respective legume crop hosts. A series of analyses were used to annotate expressed sequences, including transposable elements and viruses, to predict secreted proteins from the assembled sequences and to identify families of candidate effectors. This work provides a foundation for the comparative analysis of haustorial gene expression with further insights into physiology and effector evolution.

A pair of receptor-like kinases is responsible for natural variation in shoot growth response to mannitol treatment in Arabidopsis thaliana

Growth is a complex trait that adapts to the prevailing conditions by integrating many internal and external signals. Understanding the molecular origin of this variation remains a challenging issue. In this study, natural variation of shoot growth under mannitol-induced stress was analyzed by standard quantitative trait locus mapping methods in a recombinant inbred line population derived from a cross between the Col-0 and Cvi-0 Arabidopsis thaliana accessions. Cloning of a major QTL specific to mannitol-induced stress condition led to identification of EGM1 and EGM2, a pair of tandem-duplicated genes encoding receptor-like kinases that are potentially involved in signaling of mannitol-associated stress responses. Using various genetic approaches, we identified two non-synonymous mutations in the EGM2[Cvi] allele that are shared by at least ten accessions from various origins and are probably responsible for a specific tolerance to mannitol. We have shown that the enhanced shoot growth phenotype contributed by the Cvi allele is not linked to generic osmotic properties but instead to a specific chemical property of mannitol itself. This result raises the question of the function of such a gene in A. thaliana, a species that does not synthesize mannitol. Our findings suggest that the receptor-like kinases encoded by EGM genes may be activated by mannitol produced by pathogens such as fungi, and may contribute to plant defense responses whenever mannitol is present.

Biological activities of organic extracts of four Aureobasidium pullulans varieties isolated from extreme marine and terrestrial habitats

We report on the screening for biological activities of organic extracts from seven strains that represent four varieties of the fungus Aureobasidium pullulans, that is A. pullulans var. melanogenum, A. pullulans var. pullulans, A. pullulans var. subglaciale and A. pullulans var. namibiae. We monitored haemolysis, cytotoxicity, antioxidant capacity and growth inhibition against three bacterial species. The haemolytic activity of A. pullulans var. pullulans EXF-150 strain was due to five different haemolytically active fractions. Extracts from all of the other varieties contained at least one haemolytically active fraction. Short-term exposure of cell lines to these haemolytically active organic extracts resulted in more than 95% cytotoxicity. Strong antioxidant capacity, corresponding to 163.88 μg ascorbic acid equivalent per gram of total solid, was measured in the organic extract of the strain EXF-3382, obtained from A. pullulans var. melanogenum, isolated from the deep sea. Organic extracts from selected varieties of A. pullulans exhibited weak antibacterial activities.

Mannitol is essential for the development of stress-resistant ascospores in Neosartorya fischeri (Aspergillus fischeri)

The polyol mannitol is one of the main compatible solutes in Neosartorya fischeri and accumulates in conidia and ascospores. Here, it is shown that biosynthesis of mannitol in N. fischeri mainly depends on mannitol 1-phosphate dehydrogenase (MpdA). Reporter studies and qPCR analysis demonstrated that mpdA is moderately expressed in vegetative hyphae and conidiophores, while it is highly expressed during development of ascospores. Deletion of mpdA reduced mannitol in whole cultures as much as 85% of the wild type, while trehalose levels had increased more than 4-fold. Decreased mannitol accumulation had no effect on mycelial growth irrespective of heat- or oxidative stress. Notably, conidia of the ΔmpdA strain had higher mannitol and lower trehalose levels. They were more sensitive to heat stress. The most distinct phenotype of mpdA deletion was the absence of full development of ascospores. Formation of cleistothecia, and asci was not affected. The ascus cell wall, however, did not dissolve and asci contained incompletely formed or aborted ascospores. Addition of the Mpd inhibitor nitrophenide to the wild type strain also resulted in disturbed ascospore formation. Taken together, these results show that mannitol has a role in sexual development of N. fischeri and in stress resistance of conidia.

Horizontal gene transfer in eukaryotic plant pathogens

Gene transfer has been identified as a prevalent and pervasive phenomenon and an important source of genomic innovation in bacteria. The role of gene transfer in microbial eukaryotes seems to be of a reduced magnitude but in some cases can drive important evolutionary innovations, such as new functions that underpin the colonization of different niches. The aim of this review is to summarize published cases that support the hypothesis that horizontal gene transfer (HGT) has played a role in the evolution of phytopathogenic traits in fungi and oomycetes. Our survey of the literature identifies 46 proposed cases of transfer of genes that have a putative or experimentally demonstrable phytopathogenic function. When considering the life-cycle steps through which a pathogen must progress, the majority of the HGTs identified are associated with invading, degrading, and manipulating the host. Taken together, these data suggest HGT has played a role in shaping how fungi and oomycetes colonize plant hosts.

The ATF/CREB transcription factor Atf1 is essential for full virulence, deoxynivalenol production, and stress tolerance in the cereal pathogen Fusarium graminearum

Fusarium graminearum is a necrotrophic plant pathogen of cereals that produces mycotoxins such as deoxynivalenol (DON) and zearalenone (ZEA) in grains. The stress-activated mitogen-activated protein kinase FgOS-2 is a central regulator in F. graminearum and controls, among others, virulence and DON and ZEA production. Here, we characterized the ATF/CREB-activating transcription factor FgAtf1, a regulator that functions downstream of FgOS-2. We created deletion and overexpression mutants of Fgatf1, the latter being also in an FgOS-2 deletion mutant. FgAtf1 localizes to the nucleus and appears to interact with FgOS-2 under osmotic stress conditions. Deletion mutants in Fgatf1 (ΔFgatf1) are more sensitive to osmotic stress and less sensitive to oxidative stress compared with the wild type. Furthermore, sexual reproduction is delayed. ΔFgatf1 strains produced higher amounts of DON under in vitro induction conditions than that of the wild type. However, during wheat infection, DON production by ΔFgatf1 is strongly reduced. The ΔFgatf1 strains displayed strongly reduced virulence to wheat and maize. Interestingly, constitutive expression of Fgatf1 in the wild type led to hypervirulence on wheat, maize, and Brachypodium distachyon. Moreover, constitutive expression of Fgatf1 in the ΔFgOS-2 mutant background almost complements ΔFgOS-2-phenotypes. These data suggest that FgAtf1 may be the most important transcription factor regulated by FgOS-2.

TaEIL1, a wheat homologue of AtEIN3, acts as a negative regulator in the wheat-stripe rust fungus interaction

Transcription factors (TFs) play crucial roles in the transcriptional regulation of plant development and defence responses. Increasing evidence has implicated ETHYLENE INSENSITIVE3 (EIN3) in the plant defence response to pathogen infection and environmental stimuli. However, the role of EIN3 in wheat resistance to Puccinia striiformis f. sp. tritici (Pst) is not clear. Here, TaEIL1 was isolated by rapid amplification of cDNA ends (RACE) based on a sequence fragment from a wheat-Pst interaction cDNA library. The TaEIL1 protein contains a typical EIN3-binding domain, and transient expression analyses indicated that TaEIL1 is localized in the nucleus. Yeast one-hybrid assay revealed that TaEIL1 exhibits transcriptional activity, and its C-terminus is necessary for the activation of transcription. TaEIL1 transcripts were regulated by environmental stress stimuli and were decreased under salicylic acid (SA) treatment. When wheat leaves were challenged with Pst, the transcript level of TaEIL1 in the compatible interaction was approximately three times higher than that in the incompatible interaction. Knocking down TaEIL1 through the Barley stripe mosaic virus (BSMV) virus-induced gene silencing (VIGS) system attenuated the growth of Pst, with shortened hyphae and reduced hyphal branches, haustorial mother cells and colony size. Moreover, enhanced necrosis was triggered by the Pst avirulent race CYR23, indicating that the hypersensitive response was strengthened in TaEIL1-silenced wheat plants. Thus, the up-regulation of defence-related genes and increased sucrose abundance might contribute to the enhanced disease resistance of wheat to the virulent race CYR31. Taken together, the results suggested that the suppression of TaEIL1 transcripts could enhance the resistance of wheat to stripe rust fungus.

Metabolic effect of an exogenous gene on transgenic Beauveria bassiana using liquid chromatography-mass spectrometry-based metabolomics

Genetic modification of Beauveria bassiana with the scorpion neurotoxin aaIT gene can distinctly increase its insecticidal activity, whereas the effect of this exogenous gene on the metabolism of B. bassiana is unknown until now. Thus, we investigate the global metabolic profiling of mycelia and conidia of transgenic and wild-type B. bassiana by liquid chromatography-mass spectrometry (LC-MS). Principal component analysis (PCA) and orthogonal projection to latent structure discriminant analysis (OPLS-DA) reveal clear discrimination of wild-type mycelia and conidia from transgenic mycelia and conidia. The decrease of glycerophospholipids, carnitine, and fatty acids and the increase of oxylipins, glyoxylate, pyruvic acid, acetylcarnitine, fumarate, ergothioneine, and trehalose in transgenic mycelia indicate the enhanced oxidative reactions. In contrast, most metabolites related to oxidative stress are not altered significantly in conidia, which implies that there will be no significant oxidative stress reaction when the aaIT gene is quiescent in cells.

Role of mannitol metabolism in the pathogenicity of the necrotrophic fungus Alternaria brassicicola

In this study, the physiological functions of fungal mannitol metabolism in the pathogenicity and protection against environmental stresses were investigated in the necrotrophic fungus Alternaria brassicicola. Mannitol metabolism was examined during infection of Brassica oleracea leaves by sequential HPLC quantification of the major soluble carbohydrates and expression analysis of genes encoding two proteins of mannitol metabolism, i.e., a mannitol dehydrogenase (AbMdh), and a mannitol-1-phosphate dehydrogenase (AbMpd). Knockout mutants deficient for AbMdh or AbMpd and a double mutant lacking both enzyme activities were constructed. Their capacity to cope with various oxidative and drought stresses and their pathogenic behavior were evaluated. Metabolic and gene expression profiling indicated an increase in mannitol production during plant infection. Depending on the mutants, distinct pathogenic processes, such as leaf and silique colonization, sporulation, survival on seeds, were impaired by comparison to the wild-type. This pathogenic alteration could be partly explained by the differential susceptibilities of mutants to oxidative and drought stresses. These results highlight the importance of mannitol metabolism with respect to the ability of A. brassicicola to efficiently accomplish key steps of its pathogenic life cycle.

Transcriptional and metabolic changes associated to the infection by Fusarium verticillioides in maize inbreds with contrasting ear rot resistance

Fusarium verticillioides causes ear rot and grain mycotoxins in maize (Zea mays L.), which are harmful to human and animal health. Breeding and growing less susceptible plant genotypes is one alternative to reduce these detrimental effects. A better understanding of the resistance mechanisms would facilitate the implementation of strategic molecular agriculture to breeding of resistant germplasm. Our aim was to identify genes and metabolites that may be related to the Fusarium reaction in a resistant (L4637) and a susceptible (L4674) inbred. Gene expression data were obtained from microarray hybridizations in inoculated and non-inoculated kernels from both inbreds. Fungal inoculation did not produce considerable changes in gene expression and metabolites in L4637. Defense-related genes changed in L4674 kernels, responding specifically to the pathogen infection. These results indicate that L4637 resistance may be mainly due to constitutive defense mechanisms preventing fungal infection. These mechanisms seem to be poorly expressed in L4674; and despite the inoculation activate a defense response; this is not enough to prevent the disease progress in this susceptible line. Through this study, a global view of differential genes expressed and metabolites accumulated during resistance and susceptibility to F. verticillioides inoculation has been obtained, giving additional information about the mechanisms and pathways conferring resistance to this important disease in maize.

Bioinformatic inference of specific and general transcription factor binding sites in the plant pathogen Phytophthora infestans

Plant infection by oomycete pathogens is a complex process. It requires precise expression of a plethora of genes in the pathogen that contribute to a successful interaction with the host. Whereas much effort has been made to uncover the molecular systems underlying this infection process, mechanisms of transcriptional regulation of the genes involved remain largely unknown. We performed the first systematic de-novo DNA motif discovery analysis in Phytophthora. To this end, we utilized the genome sequence of the late blight pathogen Phytophthora infestans and two related Phytophthora species (P. ramorum and P. sojae), as well as genome-wide in planta gene expression data to systematically predict 19 conserved DNA motifs. This catalog describes common eukaryotic promoter elements whose functionality is supported by the presence of orthologs of known general transcription factors. Together with strong functional enrichment of the common promoter elements towards effector genes involved in pathogenicity, we obtained a new and expanded picture of the promoter structure in P. infestans. More intriguingly, we identified specific DNA motifs that are either highly abundant or whose presence is significantly correlated with gene expression levels during infection. Several of these motifs are observed upstream of genes encoding transporters, RXLR effectors, but also transcriptional regulators. Motifs that are observed upstream of known pathogenicity-related genes are potentially important binding sites for transcription factors. Our analyses add substantial knowledge to the as of yet virtually unexplored question regarding general and specific gene regulation in this important class of pathogens. We propose hypotheses on the effects of cis-regulatory motifs on the gene regulation of pathogenicity-related genes and pinpoint motifs that are prime targets for further experimental validation.

The Janus face of reactive oxygen species in resistance and susceptibility of plants to necrotrophic and biotrophic pathogens

Plant pathogens can be divided into biotrophs and necrotrophs according to their different life styles; biotrophs prefer living, while necrotrophs prefer dead cells for nutritional purposes. Therefore tissue necrosis caused by reactive oxygen species (ROS) during pathogen infection increases host susceptibility to necrotrophic, but resistance to biotrophic pathogen. Consequently, elevation of antioxidant capacity of plants enhances their tolerance to development of necroses caused by necrotrophic pathogens. Plant hormones can strongly influence induction of ROS and antioxidants, thereby influencing susceptibility or resistance of plants to pathogens. Pathogen-induced ROS themselves are considered as signaling molecules. Generally, salicylic acid (SA) signaling induces defense against biotrophic pathogens, whereas jasmonic acid (JA) against necrotrophic pathogens. Furthermore pathogens can modify plant's defense signaling network for their own benefit by changing phytohormone homeostasis. On the other hand, ROS are harmful also to the pathogens, consequently they try to defend themselves by elevating antioxidant activity and secreting ROS scavengers in the infected tissue. The Janus face nature of ROS and plant cell death on biotrophic and on necrotrophic pathogens is also supported by the experiments with BAX inhibitor-1 and the mlo mutation of Mlo gene in barley. It was found that ROS and elevated plant antioxidant activity play an important role in systemic acquired resistance (SAR) and induced systemic resistance (ISR), as well as in mycorrhiza induced abiotic and biotic stress tolerance of plants.

Comparison of salt stress resistance genes in transgenic Arabidopsis thaliana indicates that extent of transcriptomic change may not predict secondary phenotypic or fitness effects

Engineered abiotic stress resistance is an important target for increasing agricultural productivity. There are concerns, however, regarding possible ecological impacts of transgenic crops. In contrast to the first wave of transgenic crops, many abiotic stress resistance genes can initiate complex downstream changes. Transcriptome profiling has been suggested as a comprehensive non-targeted approach to examine the secondary effects. We compared phenotypic and transcriptomic effects of constitutive expression of genes intended to confer salt stress tolerance by three different mechanisms: a transcription factor, CBF3/DREB1a; a metabolic gene, M6PR, for mannitol biosynthesis; and the Na⁺/H⁺ antiporter, SOS1. Transgenic CBF3, M6PR and SOS1 Arabidopsis thaliana were grown together in the growth chamber, greenhouse and field. In the absence of salt, M6PR and SOS1 lines performed comparably with wild type; CBF3 lines exhibited dwarfing as reported previously. All three transgenes conferred fitness advantage when subjected to 100 mm NaCl in the growth chamber. CBF3 and M6PR affected transcription of numerous abiotic stress-related genes as measured by Affymetrix microarray analysis. M6PR additionally modified expression of biotic stress and oxidative stress genes. Transcriptional effects of SOS1 in the absence of salt were smaller and primarily limited to redox-related genes. The extent of transcriptome change, however, did not correlate with the effects on growth and reproduction. Thus, the magnitude of global transcriptome differences may not predict phenotypic differences upon which environment and selection act to influence fitness. These observations have implications for interpretation of transcriptome analyses in the context of risk assessment and emphasize the importance of evaluation within a phenotypic context.

Plant defense mechanisms are activated during biotrophic and necrotrophic development of Colletotricum graminicola in maize

Hemibiotrophic plant pathogens first establish a biotrophic interaction with the host plant and later switch to a destructive necrotrophic lifestyle. Studies of biotrophic pathogens have shown that they actively suppress plant defenses after an initial microbe-associated molecular pattern-triggered activation. In contrast, studies of the hemibiotrophs suggest that they do not suppress plant defenses during the biotrophic phase, indicating that while there are similarities between the biotrophic phase of hemibiotrophs and biotrophic pathogens, the two lifestyles are not analogous. We performed transcriptomic, histological, and biochemical studies of the early events during the infection of maize (Zea mays) with Colletotrichum graminicola, a model pathosystem for the study of hemibiotrophy. Time-course experiments revealed that mRNAs of several defense-related genes, reactive oxygen species, and antimicrobial compounds all begin to accumulate early in the infection process and continue to accumulate during the biotrophic stage. We also discovered the production of maize-derived vesicular bodies containing hydrogen peroxide targeting the fungal hyphae. We describe the fungal respiratory burst during host infection, paralleled by superoxide ion production in specific fungal cells during the transition from biotrophy to a necrotrophic lifestyle. We also identified several novel putative fungal effectors and studied their expression during anthracnose development in maize. Our results demonstrate a strong induction of defense mechanisms occurring in maize cells during C. graminicola infection, even during the biotrophic development of the pathogen. We hypothesize that the switch to necrotrophic growth enables the fungus to evade the effects of the plant immune system and allows for full fungal pathogenicity.

454-pyrosequencing of Coffea arabica leaves infected by the rust fungus Hemileia vastatrix reveals in planta-expressed pathogen-secreted proteins and plant functions in a late compatible plant-rust interaction

Coffee (Coffea arabica L.), one of the key export and cash crops in tropical and subtropical countries, suffers severe losses from the rust fungus Hemileia vastatrix. The transcriptome of H. vastatrix was analysed during a compatible interaction with coffee to obtain an exhaustive repertoire of the genes expressed during infection and to identify potential effector genes. Large-scale sequencing (454-GS-FLEX Titanium) of mixed coffee and rust cDNAs obtained from 21-day rust-infected leaves generated 352 146 sequences which assembled into 22 774 contigs. In the absence of any reference genomic sequences for Coffea or Hemileia, specific trinucleotide frequencies within expressed sequence tags (ESTs) and blast homology against a set of dicots and basidiomycete genomes were used to distinguish pathogen from plant sequences. About 30% (6763) of the contigs were assigned to H. vastatrix and 61% (13 951) to C. arabica. The majority (60%) of the rust sequences did not show homology to any genomic database, indicating that they were potential novel fungal genes. In silico analyses of the 6763 H. vastatrix contigs predicted 382 secreted proteins and identified homologues of the flax rust haustorially expressed secreted proteins (HESPs) and bean rust transferred protein 1 (RTP1). These rust candidate effectors showed conserved amino-acid domains and conserved patterns of cysteine positions suggestive of conserved functions during infection of host plants. Quantitative reverse transcription-polymerase chain reaction profiling of selected rust genes revealed dynamic expression patterns during the time course of infection of coffee leaves. This study provides the first valuable genomic resource for the agriculturally important plant pathogen H. vastatrix and the first comprehensive C. arabica EST dataset.

Mannitol transport and mannitol dehydrogenase activities are coordinated in Olea europaea under salt and osmotic stresses

The intracellular accumulation of organic compatible solutes functioning as osmoprotectants, such as polyols, is an important response mechanism of several plants to drought and salinity. In Olea europaea a mannitol transport system (OeMaT1) was previously characterized as a key player in plant response to salinity. In the present study, heterotrophic sink models, such as olive cell suspensions and fruit tissues, and source leaves were used for analytical, biochemical and molecular studies. The kinetic parameters of mannitol dehydrogenase (MTD) determined in cells growing in mannitol, at 25°C and pH 9.0, were as follows: K(m), 54.5 mM mannitol; and V(max), 0.47 μmol h⁻¹ mg⁻¹ protein. The corresponding cDNA was cloned and named OeMTD1. OeMTD1 expression was correlated with MTD activity, OeMaT1 expression and carrier-mediated mannitol transport in mannitol- and sucrose-grown cells. Furthermore, sucrose-grown cells displayed only residual OeMTD activity, even though high levels of OeMTD1 transcription were observed. There is evidence that OeMTD is regulated at both transcriptional and post-transcriptional levels. MTD activity and OeMTD1 expression were repressed after Na+, K+ and polyethylene glycol (PEG) treatments, in both mannitol- and sucrose-grown cells. In contrast, salt and drought significantly increased mannitol transport activity and OeMaT1 expression. Taken together, these studies support that olive trees cope with salinity and drought by coordinating mannitol transport with intracellular metabolism.

Enzymes of mannitol metabolism in the human pathogenic fungus Aspergillus fumigatus--kinetic properties of mannitol-1-phosphate 5-dehydrogenase and mannitol 2-dehydrogenase, and their physiological implications

The human pathogenic fungus Aspergillus fumigatus accumulates large amounts of intracellular mannitol to enhance its resistance against defense strategies of the infected host. To explore their currently unknown roles in mannitol metabolism, we studied A. fumigatus mannitol-1-phosphate 5-dehydrogenase (AfM1PDH) and mannitol 2-dehydrogenase (AfM2DH), each recombinantly produced in Escherichia coli, and performed a detailed steady-state kinetic characterization of the two enzymes at 25 °C and pH 7.1. Primary kinetic isotope effects resulting from deuteration of alcohol substrate or NADH showed that, for AfM1PDH, binding of D-mannitol 1-phosphate and NAD(+) is random, whereas D-fructose 6-phosphate binds only after NADH has bound to the enzyme. Binding of substrate and NAD(H) by AfM2DH is random for both D-mannitol oxidation and D-fructose reduction. Hydride transfer is rate-determining for D-mannitol 1-phosphate oxidation by AfM1PDH (k(cat) = 10.6 s(-1)) as well as D-fructose reduction by AfM2DH (k(cat) = 94 s(-1)). Product release steps control the maximum rates in the other direction of the two enzymatic reactions. Free energy profiles for the enzymatic reaction under physiological boundary conditions suggest that AfM1PDH primarily functions as a D-fructose-6-phosphate reductase, whereas AfM2DH acts in D-mannitol oxidation, thus establishing distinct routes for production and mobilization of mannitol in A. fumigatus. ATP, ADP and AMP do not affect the activity of AfM1PDH, suggesting the absence of flux control by cellular energy charge at the level of D-fructose 6-phosphate reduction. AfM1PDH is remarkably resistant to inactivation by heat (half-life at 40 °C of 20 h), consistent with the idea that formation of mannitol is an essential component of the temperature stress response of A. fumigatus. Inhibition of AfM1PDH might be a useful target for therapy of A. fumigatus infections.

Reactive oxygen species in phytopathogenic fungi: signaling, development, and disease

Reactive oxygen species (ROS) play a major role in pathogen-plant interactions: recognition of a pathogen by the plant rapidly triggers the oxidative burst, which is necessary for further defense reactions. The specific role of ROS in pathogen defense is still unclear. Studies on the pathogen so far have focused on the importance of the oxidative stress response (OSR) systems to overcome the oxidative burst or of its avoidance by effectors. This review focuses on the role of ROS for fungal virulence and development. In the recent years, it has become obvious that (a) fungal OSR systems might not have the predicted crucial role in pathogenicity, (b) fungal pathogens, especially necrotrophs, can actively contribute to the ROS level in planta and even take advantage of the host's response, (c) fungi possess superoxide-generating NADPH oxidases similar to mammalian Nox complexes that are important for pathogenicity; however, recent data indicate that they are not directly involved in pathogen-host communication but in fungal differentiation processes that are necessary for virulence.

Characterization of mannitol in Curvularia protuberata hyphae by FTIR and Raman spectromicroscopy

FTIR and Raman spectromicroscopy were used to characterize the composition of Curvularia protuberata hyphae, and to compare a strain isolated from plants inhabiting geothermal soils with a non-geothermal isolate. Thermal IR source images of hyphae have been acquired with a 64 × 64 element focal plane array detector; single point IR spectra have been obtained with synchrotron source light. In some C. protuberata hyphae, we have discovered the spectral signature of crystalline mannitol, a fungal polyol with complex protective roles. With FTIR-FPA imaging, we have determined that the protein content in cells remains fairly constant throughout the length of a hypha, whereas the mannitol is found at discrete, irregular locations. This is the first direct observation of mannitol in intact fungal hyphae. Since the concentration of mannitol in cells varies with respect to position and is not present in all hyphae, this discovery may be related to habitat adaptation, fungal structure and growth stages.

Novel insights into mannitol metabolism in the fungal plant pathogen Botrytis cinerea

In order to redefine the mannitol pathway in the necrotrophic plant pathogen Botrytis cinerea, we used a targeted deletion strategy of genes encoding two proteins of mannitol metabolism, BcMTDH (B. cinerea mannitol dehydrogenase) and BcMPD (B. cinerea mannitol-1-phosphate dehydrogenase). Mobilization of mannitol and quantification of Bcmpd and Bcmtdh gene transcripts during development and osmotic stress confirmed a role for mannitol as a temporary and disposable carbon storage compound. In order to study metabolic fluxes, we followed conversion of labelled hexoses in wild-type and ΔBcmpd and ΔBcmtdh mutant strains by in vivo NMR spectroscopy. Our results revealed that glucose and fructose were metabolized via the BcMPD and BcMTDH pathways respectively. The existence of a novel mannitol phosphorylation pathway was also suggested by the NMR investigations. This last finding definitively challenged the existence of the originally postulated mannitol cycle in favour of two simultaneously expressed pathways. Finally, physiological and biochemical studies conducted on double deletion mutants (ΔBcmpdΔBcmtdh) showed that mannitol was still produced despite a complete alteration of both mannitol biosynthesis pathways. This strongly suggests that one or several additional undescribed pathways could participate in mannitol metabolism in B. cinerea.

Spatial and developmental differentiation of mannitol dehydrogenase and mannitol-1-phosphate dehydrogenase in Aspergillus niger

The presence of a mannitol cycle in fungi has been subject to discussion for many years. Recent studies have found no evidence for the presence of this cycle and its putative role in regenerating NADPH. However, all enzymes of the cycle could be measured in cultures of Aspergillus niger. In this study we have analyzed the localization of two enzymes from the pathway, mannitol dehydrogenase and mannitol-1-phosphate dehydrogenase, and the expression of their encoding genes in nonsporulating and sporulating cultures of A. niger. Northern analysis demonstrated that mpdA was expressed in both sporulating and nonsporulating mycelia, while expression of mtdA was expressed only in sporulating mycelium. More detailed studies using green fluorescent protein and dTomato fused to the promoters of mtdA and mpdA, respectively, demonstrated that expression of mpdA occurs in vegetative hyphae while mtdA expression occurs in conidiospores. Activity assays for MtdA and MpdA confirmed the expression data, indicating that streaming of these proteins is not likely to occur. These results confirm the absence of the putative mannitol cycle in A. niger as two of the enzymes of the cycle are not present in the same part of A. niger colonies. The results also demonstrate the existence of spore-specific genes and enzymes in A. niger.

Is there leaderless protein secretion in plants?

The sugar alcohol mannitol and it’s catabolic enzyme mannitol dehydrogenase (MTD), in addition to welldocumented roles in metabolism and osmoprotection, may play roles in hostpathogen interactions. Research suggests that in response to the mannitol that pathogenic fungi secrete to suppress reactive oxygen-mediated host defenses, plants make MTD to catabolize fungal mannitol. Yet previous work suggested that pathogen-secreted mannitol is extracellular, while in healthy plants MTD is cytoplasmic. We have presented results showing that the normally cytoplasmic MTD is exported into the cell wall or extracellular space in response to the endogenous inducer of plant defense responses salicylic acid (SA). This SA-induced secretion is insensitive to brefeldin A, an inhibitor of Golgimediated protein transport. Together with the absence of MTD in Golgi stacks and the lack of a documented extracellular targeting sequence in the MTD protein, this suggests MTD is secreted by a non-Golgi, pathogen-activated secretion mechanism in plants. Here we discuss the potential significance of non-Golgi secretion in response to stress.

Genes acquired by horizontal transfer are potentially involved in the evolution of phytopathogenicity in Moniliophthora perniciosa and Moniliophthora roreri, two of the major pathogens of cacao

Moniliophthora perniciosa and Moniliophthora roreri are phytopathogenic basidiomycete species that infect cacao causing two important diseases in this crop: "Witches' Broom" and "Frosty Pod Rot", respectively. The ability of species from this genus (Moniliophthora) to cause disease is exceptional in the family Marasmiaceae. Species in closely related genera including, Marasmius, Crinipellis, and Chaetocalathus, are mainly saprotrophs and are not known to cause disease. In this study, the possibility that this phytopathogenic lifestyle has been acquired by horizontal gene transfer (HGT) was investigated. A stringent genome comparison pipeline was used to identify potential genes that have been obtained by Moniliophthora through HGT. This search led to the identification of three genes: a metallo-dependent hydrolase (MDH), a mannitol phosphate dehydrogenase (MPDH), and a family of necrosis-inducing proteins (NEPs). Phylogenetic analysis of these genes suggests that Moniliophthora acquired NEPs from oomycetes, MDH from actinobacteria and MPDH from firmicutes. Based on the known gene functions and on previous studies of M. perniciosa infection and development, a correlation between gene acquisition and the evolution of the phytopathogenic genus Moniliophthora can be postulated.

Salicylic acid stimulates secretion of the normally symplastic enzyme mannitol dehydrogenase: a possible defense against mannitol-secreting fungal pathogens

The sugar alcohol mannitol is an important carbohydrate with well-documented roles in both metabolism and osmoprotection in many plants and fungi. In addition to these traditionally recognized roles, mannitol is reported to be an antioxidant and as such may play a role in host-pathogen interactions. Current research suggests that pathogenic fungi can secrete mannitol into the apoplast to suppress reactive oxygen-mediated host defenses. Immunoelectron microscopy, immunoblot, and biochemical data reported here show that the normally symplastic plant enzyme, mannitol dehydrogenase (MTD), is secreted into the apoplast after treatment with the endogenous inducer of plant defense responses salicylic acid (SA). In contrast, a cytoplasmic marker protein, hexokinase, remained cytoplasmic after SA-treatment. Secreted MTD retained activity after export to the apoplast. Given that MTD converts mannitol to the sugar mannose, MTD secretion may be an important component of plant defense against mannitol-secreting fungal pathogens such as Alternaria. After SA treatment, MTD was not detected in the Golgi apparatus, and its SA-induced secretion was resistant to brefeldin A, an inhibitor of Golgi-mediated protein transport. Together with the absence of a known extracellular targeting sequence on the MTD protein, these data suggest that a plant's response to pathogen challenge may include secretion of selected defensive proteins by as yet uncharacterized, non-Golgi mechanisms.

Salicylic acid stimulates secretion of the normally symplastic enzyme mannitol dehydrogenase: a possible defense against mannitol-secreting fungal pathogens

The sugar alcohol mannitol is an important carbohydrate with well-documented roles in both metabolism and osmoprotection in many plants and fungi. In addition to these traditionally recognized roles, mannitol is reported to be an antioxidant and as such may play a role in host-pathogen interactions. Current research suggests that pathogenic fungi can secrete mannitol into the apoplast to suppress reactive oxygen-mediated host defenses. Immunoelectron microscopy, immunoblot, and biochemical data reported here show that the normally symplastic plant enzyme, mannitol dehydrogenase (MTD), is secreted into the apoplast after treatment with the endogenous inducer of plant defense responses salicylic acid (SA). In contrast, a cytoplasmic marker protein, hexokinase, remained cytoplasmic after SA-treatment. Secreted MTD retained activity after export to the apoplast. Given that MTD converts mannitol to the sugar mannose, MTD secretion may be an important component of plant defense against mannitol-secreting fungal pathogens such as Alternaria. After SA treatment, MTD was not detected in the Golgi apparatus, and its SA-induced secretion was resistant to brefeldin A, an inhibitor of Golgi-mediated protein transport. Together with the absence of a known extracellular targeting sequence on the MTD protein, these data suggest that a plant's response to pathogen challenge may include secretion of selected defensive proteins by as yet uncharacterized, non-Golgi mechanisms.

Botrytis cinerea isolates collected from grapes present different requirements for conidia germination

Botrytis cinerea presents high variability in several biological traits, which can be explained by the high degree of genotypic diversity among isolates. Because this genetic variability might be related to phenotypic differences the requirements for conidia germination of three natural isolates (G1, G5 and G11) obtained from grapes and belonging to the same genetic group were analyzed. The results showed that contact with a solid surface was a common requisite for conidia germination of the isolates but they differed in their nutritional requirements to germinate. Isolate G11 was able to germinate in the absence of a carbon or nitrogen source. G1 and G5 required the presence of a carbon source such as glucose, fructose or sucrose. In G11 and G5 isolates a much higher rate of germination was obtained in the presence of sucrose. It was shown with a pharmacological approach that the cAMP stimulated the germination only in those isolates requiring a carbon source. Conidia germination of G1 and G5 was inhibited by EGTA, a calcium chelator. Isolate G11 germinated in the presence of this compound. On the other hand the germination of three B. cinerea isolates required protein synthesis and did not require RNA synthesis. To explain the ability of isolate G11 to germinate in water the content of total and reducing sugars, mannitol/L-arabitol, trehalose, and proteins in the nongerminated conidia of the three isolates was compared. The isolates presented similar amounts of total and reducing sugars. In the three isolates the amount of mannitol/L-arabitol was higher than that of trehalose. In isolate G11 total protein content was twice higher than in the other isolates.

Isomalt production by cloning, purifying and expressing of the MDH gene from Pseudomonas fluorescens DSM 50106 in different strains of E. coli

A NADH-dependent mannitol dehydrogenase gene (mtlD) was cloned from Pseudomonas fluorescens, subcloned into an expression vector (pDEST110) and entered into different strains of E. coli to compare their protein expression and the enzyme specific activity. Purifications were accomplished by Ni(2+)-NTA affinity chromatography. Using this approach, the efficiency of purification process significantly increased (up to 90%) so that the purified enzyme gave a sharp single band (55 kDa) in SDS-PAGE. The results showed that among the strains, BL21 (DE3) plysS exhibited the maximum expression level of MDH(mannitol dehydrogenase) (11 mg L(-1)). Results from activity assay with fructose as substrate also showed that in this strain the specific activity of 63 U mg(-1) protein monitored for the enzyme, the record not reported before. Resazurin staining also indicated that the enzyme reduced fructose, whereas oxidized other substrates including mannitol, sorbitol and arabitol under optimal assay condition. From HPLC analysis it was showed for the first time that the enzyme could convert substrate isomaltulose to the specific products, GPM and GPS. Interestingly, because of the high specificity of the enzyme for substrate, the method can be used as an alternative approach to substitute nonspecific conventional method of isomalt production.

Common themes in nutrient acquisition by plant symbiotic microbes, described by the Gene Ontology

A critical function for symbionts is the acquisition of nutrients from their host. Relationships between hosts and symbionts range from biotrophic mutualism to necrotrophic parasitism, with a corresponding range of structures to facilitate nutrient flow between host and symbiont. Here, we review common themes among the nutrient acquisition strategies of a range of plant symbiotic microorganisms, including mutualistic symbionts, biotrophic pathogens that feed from living tissue, necrotrophic pathogens that kill host tissue, and hemibiotrophic pathogens that switch from biotrophy to necrotrophy. We show how Gene Ontology (GO) terms developed by the Plant-Associated Microbe Gene Ontology (PAMGO) Consortium can be used for describing commonalities in nutrient acquisition among diverse plant symbionts. Where appropriate, parallels found among animal symbionts are also highlighted.

Dynamic carbon transfer during pathogenesis of sunflower by the necrotrophic fungus Botrytis cinerea: from plant hexoses to mannitol

The main steps for carbon acquisition and conversion by Botrytis cinerea during pathogenesis of sunflower cotyledon were investigated here. A sequential view of soluble carbon metabolites detected by NMR spectroscopy during infection is presented. Disappearance of plant hexoses and their conversion to fungal metabolites were investigated by expression analysis of an extended gene family of hexose transporters (Bchxts) and of the mannitol pathway, using quantitative PCR. In order to analyse the main fungal metabolic routes used by B. cinerea in real time, we performed, for the first time, in vivo NMR analyses during plant infection. During infection, B. cinerea converts plant hexoses into mannitol. Expression analysis of the sugar porter gene family suggested predominance for transcription induced upon low glucose conditions and regulated according to the developmental phase. Allocation of plant hexoses by the pathogen revealed a conversion to mannitol, trehalose and glycogen for glucose and a preponderant transformation of fructose to mannitol by a more efficient metabolic pathway. Uptake of plant hexoses by B. cinerea is based on a multigenic flexible hexose uptake system. Their conversion into mannitol, enabled by two simultaneously expressed pathways, generates a dynamic intracellular carbon pool.

Analysis of expressed sequence tags from Uromyces appendiculatus hyphae and haustoria and their comparison to sequences from other rust fungi

Hyphae, 2 to 8 days postinoculation (dpi), and haustoria, 5 dpi, were isolated from Uromyces appendiculatus infected bean leaves (Phaseolus vulgaris cv. Pinto 111) and a separate cDNA library prepared for each fungal preparation. Approximately 10,000 hyphae and 2,700 haustoria clones were sequenced from both the 5' and 3' ends. Assembly of all of the fungal sequences yielded 3,359 contigs and 927 singletons. The U. appendiculatus sequences were compared with sequence data for other rust fungi, Phakopsora pachyrhizi, Uromyces fabae, and Puccinia graminis. The U. appendiculatus haustoria library included a large number of genes with unknown cellular function; however, summation of sequences of known cellular function suggested that haustoria at 5 dpi had fewer transcripts linked to protein synthesis in favor of energy metabolism and nutrient uptake. In addition, open reading frames in the U. appendiculatus data set with an N-terminal signal peptide were identified and compared with other proteins putatively secreted from rust fungi. In this regard, a small family of putatively secreted RTP1-like proteins was identified in U. appendiculatus and P. graminis.

Mannitol biosynthesis is required for plant pathogenicity by Alternaria alternata

Mannitol has been hypothesized to play a role in antioxidant defense. In previous work, we confirmed the presence of the two mannitol biosynthetic enzymes, mannitol dehydrogenase (MtDH) and mannitol 1-phosphate 5-dehydrogenase (MPDH), in the fungus Alternaria alternata and created disruption mutants for both enzymes. These mutants were used to investigate the role of mannitol in pathogenicity of A. alternata on its host, tobacco. Conidia of all mutants were viable and germinated normally. GC-MS analysis demonstrated elevated levels of trehalose in the mutants, suggesting that trehalose may substitute for mannitol as a storage compound for germination. Tobacco inoculation showed no reduction in lesion severity caused by the MtDH mutant as compared with wild type; however, the MPDH mutant and a mutant in both enzymes caused significantly less disease. Microscopy analysis indicated that the double mutant was unaffected in the ability to germinate and produce appressoria on tobacco leaves and elicited a defense response from the host, indicating that it was able to penetrate and infect the host. We conclude that mannitol biosynthesis is required for pathogenesis of A. alternata on tobacco, but is not required for spore germination either in vitro or in planta or for initial infection.

Advanced data-mining strategies for the analysis of direct-infusion ion trap mass spectrometry data from the association of perennial ryegrass with its endophytic fungus, Neotyphodium lolii

Direct-infusion mass spectrometry (MS) was applied to study the metabolic effects of the symbiosis between the endophytic fungus Neotyphodium lolii and its host perennial ryegrass (Lolium perenne) in three different tissues (immature leaf, blade, and sheath). Unbiased direct-infusion MS using a linear ion trap mass spectrometer allowed metabolic effects to be determined free of any preconceptions and in a high-throughput fashion. Not only the full MS(1) mass spectra (range 150-1,000 mass-to-charge ratio) were obtained but also MS(2) and MS(3) product ion spectra were collected on the most intense MS(1) ions as described previously (Koulman et al., 2007b). We developed a novel computational methodology to take advantage of the MS(2) product ion spectra collected. Several heterogeneous MS(1) bins (different MS(2) spectra from the same nominal MS(1)) were identified with this method. Exploratory data analysis approaches were also developed to investigate how the metabolome differs in perennial ryegrass infected with N. lolii in comparison to uninfected perennial ryegrass. As well as some known fungal metabolites like peramine and mannitol, several novel metabolites involved in the symbiosis, including putative cyclic oligopeptides, were identified. Correlation network analysis revealed a group of structurally related oligosaccharides, which differed significantly in concentration in perennial ryegrass sheaths due to endophyte infection. This study demonstrates the potential of the combination of unbiased metabolite profiling using ion trap MS and advanced data-mining strategies for discovering unexpected perturbations of the metabolome, and generating new scientific questions for more detailed investigations in the future.

A surrogate method for comparison analysis of salivary concentrations of Xylitol-containing products

Background

Xylitol chewing gum has been shown to reduce Streptococcus mutans levels and decay. Two studies examined the presence and time course of salivary xylitol concentrations delivered via xylitol-containing pellet gum and compared them to other xylitol-containing products.

Methods

A within-subjects design was used for both studies. Study 1, adults (N = 15) received three xylitol-containing products (pellet gum (2.6 g), gummy bears (2.6 g), and commercially available stick gum (Koolerz, 3.0 g)); Study 2, a second group of adults (N = 15) received three xylitol-containing products (pellet gum, gummy bears, and a 33% xylitol syrup (2.67 g). For both studies subjects consumed one xylitol product per visit with a 7-day washout between each product. A standardized protocol was followed for each product visit. Product order was randomly determined at the initial visit. Saliva samples (0.5 mL to 1.0 mL) were collected at baseline and up to 10 time points (~16 min in length) after product consumption initiated. Concentration of xylitol in saliva samples was analyzed using high-performance liquid chromatography. Area under the curve (AUC) for determining the average xylitol concentration in saliva over the total sampling period was calculated for each product.

Results

In both studies all three xylitol products (Study 1: pellet gum, gummy bears, and stick gum; Study 2: pellet gum, gummy bears, and syrup) had similar time curves with two xylitol concentration peaks during the sampling period. Study 1 had its highest mean peaks at the 4 min sampling point while Study 2 had its highest mean peaks between 13 to 16 minutes. Salivary xylitol levels returned to baseline at about 18 minutes for all forms tested. Additionally, for both studies the total AUC for the xylitol products were similar compared to the pellet gum (Study 1: pellet gum – 51.3 μg.min/mL, gummy bears – 59.6 μg.min/mL, and stick gum – 46.4 μg.min/mL; Study 2: pellet gum – 63.0 μg.min/mL, gummy bears – 55.9 μg.min/mL, and syrup – 59.0 μg.min/mL).

Conclusion

The comparison method demonstrated high reliability and validity. In both studies other xylitol-containing products had time curves and mean xylitol concentration peaks similar to xylitol pellet gum suggesting this test may be a surrogate for longer studies comparing various products.

The major pathways of carbohydrate metabolism in the ectomycorrhizal basidiomycete Laccaria bicolor S238N

The primary carbohydrate metabolism of an ectomycorrhizal fungus and its transcriptional regulation has never been characterized at the genome scale although it plays a fundamental role in the functioning of the symbiosis. In this study, the genome sequence of the ectomycorrhizal basidiomycete Laccaria bicolor S238N-H82 was explored to construct a comprehensive genome-wide inventory of pathways involved in primary carbohydrate metabolism. Several genes and gene families were annotated, including those of the glycolysis, pentose phosphate pathway, tricarboxylic acid cycle, and trehalose and mannitol metabolism. The transcriptional regulation of these pathways was studied using whole-genome expression oligoarrays and quantitative polymerase chain reaction in free-living mycelium, ectomycorrhizas and fruiting bodies. Pathways of carbohydrate biosynthesis and catabolism are identical in L. bicolor compared with other sequenced saprotrophic basidiomycetes. Ectomycorrhiza and fruiting body development induced the regulation of a restricted set of transcripts of the glycolytic, mannitol and trehalose metabolisms.

Secreted proteins of Uromyces fabae: similarities and stage specificity

Uromyces fabae on Vicia faba is a model system for obligate biotrophic interactions. Searching for potential effector proteins we investigated the haustorial secretome of U. fabae (biotrophic stage) and compared it with the secretome of in vitro grown infection structures, which represent the pre-biotrophic stage. Using the yeast signal sequence trap method we identified 62 genes encoding proteins secreted from haustoria and 42 genes encoding proteins secreted from in vitro grown infection structures. Four of these genes were identical in both libraries, giving a total of 100 genes coding for secreted proteins. This finding indicates a strong stage-specific regulation of protein secretion. Similarity with previously identified proteins was found for 39 of the sequences analysed, 28 of which showed similarity to proteins identified among members of the order Uredinales only. This might be taken as an indication for possible roles in virulence and host specificity unique to the Uredinales.

Saccharomyces cerevisiae SSD1 orthologues are essential for host infection by the ascomycete plant pathogens Colletotrichum lagenarium and Magnaporthe grisea

Fungal plant pathogens have evolved diverse strategies to overcome the multilayered plant defence responses that confront them upon host invasion. Here we show that pathogenicity of the cucumber anthracnose fungus, Colletotrichum lagenarium, and the rice blast fungus, Magnaporthe grisea, requires a gene orthologous to Saccharomyces cerevisiae SSD1, a regulator of cell wall assembly. Screening for C. lagenarium insertional mutants deficient in pathogenicity led to the identification of ClaSSD1. Following targeted gene replacement, appressoria of classd1 mutants retained the potential for penetration but were unable to penetrate into host epidermal cells. Transmission electron microscopy suggested that appressorial penetration by classd1 mutants was restricted by plant cell wall-associated defence responses, which were observed less frequently with the wild-type strain. Interestingly, on non-host onion epidermis classd1 mutants induced papilla formation faster and more abundantly than the wild type. Similarly, colonization of rice leaves by M. grisea was severely reduced after deletion of the orthologous MgSSD1 gene and attempted infection by the mutants was accompanied by the accumulation of reactive oxygen species within the host cell. These results suggest that appropriate assembly of the fungal cell wall as regulated by SSD1 allows these pathogens to establish infection by avoiding the induction of host defence responses.

Generation of a wheat leaf rust, Puccinia triticina, EST database from stage-specific cDNA libraries

Thirteen cDNA libraries constructed from small amounts of leaf rust mRNA using optimized methods served as the source for the generation of 25 558 high-quality DNA sequence reads. Five life-cycle stages were sampled: resting urediniospores, urediniospores germinated over water or plant extract, compatible, interactive stages during appressorium or haustorium formation just before sporulation, and an incompatible interaction. mRNA populations were subjected to treatments such as full-length cDNA production, subtractive and normalizing hybridizations, and size selection methods combined with PCR amplification. Pathogen and host sequences from interactive libraries were differentiated in silico using cereal and fungal sequences, codon usage analyses, and by means of a partial prototype cDNA microarray hybridized with genomic DNAs. This yielded a non-redundant unigene set of 9760 putative fungal sequences consisting of 6616 singlets and 3144 contigs, representing 4.7 Mbp. At an E-value 10(-5), 3670 unigenes (38%) matched sequences in various databases and collections but only 694 unigenes (7%) were similar to genes with known functions. In total, 296 unigenes were identified as most probably wheat and ten as rRNA sequences. Annotation rates were low for germinated urediniospores (4%) and appressoria (2%). Gene sets obtained from the various life-cycle stages appear to be remarkably different, suggesting drastic reprogramming of the transcriptome during these major differentiation processes. Redundancy within contigs yielded information about possible expression levels of certain genes among stages. Many sequences were similar to genes from other rusts such as Uromyces and Melampsora species; some of these genes have been implicated in pathogenicity and virulence.

Metabolic processes and carbon nutrient exchanges between host and pathogen sustain the disease development during sunflower infection by Sclerotinia sclerotiorum

Interactions between the necrotrophic fungus Sclerotinia sclerotiorum and one of its hosts, Helianthus annuus L., were analyzed during fungal colonization of plant tissues. Metabolomic analysis, based on (13)C- and (31)P-NMR spectroscopy, was used to draw up the profiles of soluble metabolites of the two partners before interaction, and to trace the fate of metabolites specific of each partner during colonization. In sunflower cotyledons, the main soluble carbohydrates were glucose, fructose, sucrose and glutamate. In S. sclerotiorum extracts, glucose, trehalose and mannitol were the predominant soluble carbon stores. During infection, a decline in sugars and amino acids was observed in the plant and fungus total content. Sucrose and fructose, initially present almost exclusively in plant, were reduced by 85%. We used a biochemical approach to correlate the disappearance of sucrose with the expression and the activity of fungal invertase. The expression of two hexose transporters, Sshxt1 and Sshxt2, was enhanced during infection. A database search for hexose transporters homologues in the S. sclerotiorum genome revealed a multigenic sugar transport system. Furthermore, the composition of the pool of reserve sugars and polyols during infection was investigated. Whereas mannitol was produced in vitro and accumulated in planta, glycerol was exclusively produced in infected tissues and increased during colonization. The hypothesis that the induction of glycerol synthesis in S. sclerotiorum exerts a positive effect on osmotic protection of fungal cells and favors fungal growth in plant tissues is discussed. Taken together, our data revealed the importance of carbon-nutrient exchanges during the necrotrophic pathogenesis of S. sclerotiorum.

Aircraft measurement of organic aerosols over China

Lower to middle (0.5-3.0 km altitude) tropospheric aerosols (PM2.5) collected by aircraft over inland and east coastal China were, for the first time, characterized for organic molecular compositions to understand anthropogenic, natural, and photochemical contribution to the air quality. n-Alkanes, fatty acids, sugars, polyacids are detected as major compound classes, whereas lignin and resin products, sterols, polycyclic aromatic hydrocarbons, and phthalic acids are minor species. Average concentrations of all the identified compounds excluding malic acid correspond to 40-50% of those reported on the ground sites. Relative abundances of secondary organic aerosol (SOA) components such as malic acid are much higher in the aircraft samples, suggesting an enhanced photochemical production over China. Organic carbon (OC) concentrations in summer (average, 24.3 microg m(-3)) were equivalent to those reported on the ground sites. Higher OC/EC (elemental carbon) ratios in the summer aircraft samples also support a significant production of SOA over China. High loadings of organic aerosols in the Chinese troposphere may be responsible to an intercontinental transport of the pollutants and potential impact on the regional and global climate changes.

Decoding the mannitol enigma in filamentous fungi

Mannitol is a 6-carbon polyol that is among the most abundant biochemical compounds in the biosphere. Mannitol has been ascribed a multitude of roles in filamentous fungi including carbohydrate storage, reservoir of reducing power, stress tolerance and spore dislodgement and/or dispersal. The advancement of genetic manipulation techniques in filamentous fungi has rapidly accelerated our understanding of the roles and metabolism of mannitol. The targeted deletion of genes encoding proteins of mannitol metabolism in several fungi, including phytopathogens, has proven that the metabolism of mannitol does not exist as a cycle and that many of the postulated roles are unsupported. These recent studies have provided a much needed focus on this mysterious metabolite and make this a fitting time to review the roles and metabolism of mannitol in filamentous fungi.

Safeners coordinately induce the expression of multiple proteins and MRP transcripts involved in herbicide metabolism and detoxification inTriticum tauschii seedling tissues

Chemicals called safeners protect cereal crops from herbicide toxicity. Proteomic methods (2-D PAGE and LC-MS/MS) were utilized to identify safener- and/or herbicide-regulated proteins in three tissues (root, leaf, and coleoptile) of Triticum tauschii seedlings to better understand a safener's mechanism of action. Growth experiments showed that the safener cloquintocet-mexyl protected seedlings from injury by the herbicide dimethenamid. In total, 29 safener-induced and 10 herbicide-regulated proteins were identified by LC-MS/MS. These proteins were classified into two major categories based on their expression patterns, and were further classified into several functional groups. Surprisingly, mutually exclusive sets of proteins were identified following herbicide or safener treatment, suggesting that different signaling pathways may be recruited. Safener-responsive proteins, mostly involved in xenobiotic detoxification, also included several new proteins that had not been previously identified as safener-responsive, whereas herbicide-regulated proteins belonged to several classes involved in general stress responses. Quantitative RT-PCR revealed that multidrug resistance-associated protein (MRP) transcripts were highly induced by safeners and two MRP genes were differentially expressed. Our results indicate that safeners protect T. tauschii seedlings from herbicide toxicity by coordinately inducing proteins involved in an entire herbicide detoxification pathway mainly in the coleoptile and root, thereby protecting new leaves from herbicide injury.

Mannitol metabolism in the phytopathogenic fungus Alternaria alternata

Mannitol metabolism in fungi is thought to occur through a mannitol cycle first described in 1978. In this cycle, mannitol 1-phosphate 5-dehydrogenase (EC 1.1.1.17) was proposed to reduce fructose 6-phosphate into mannitol 1-phosphate, followed by dephosphorylation by a mannitol 1-phosphatase (EC 3.1.3.22) resulting in inorganic phosphate and mannitol. Mannitol would be converted back to fructose by the enzyme mannitol dehydrogenase (EC 1.1.1.138). Although mannitol 1-phosphate 5-dehydrogenase was proposed as the major biosynthetic enzyme and mannitol dehydrogenase as a degradative enzyme, both enzymes catalyze their respective reverse reactions. To date the cycle has not been confirmed through genetic analysis. We conducted enzyme assays that confirmed the presence of these enzymes in a tobacco isolate of Alternaria alternata. Using a degenerate primer strategy, we isolated the genes encoding the enzymes and used targeted gene disruption to create mutants deficient in mannitol 1-phosphate 5-dehydrogenase, mannitol dehydrogenase, or both. PCR analysis confirmed gene disruption in the mutants, and enzyme assays demonstrated a lack of enzymatic activity for each enzyme. GC-MS experiments showed that a mutant deficient in both enzymes did not produce mannitol. Mutants deficient in mannitol 1-phosphate 5-dehydrogenase or mannitol dehydrogenase alone produced 11.5 and 65.7 %, respectively, of wild type levels. All mutants grew on mannitol as a sole carbon source, however, the double mutant and mutant deficient in mannitol 1-phosphate 5-dehydrogenase grew poorly. Our data demonstrate that mannitol 1-phosphate 5-dehydrogenase and mannitol dehydrogenase are essential enzymes in mannitol metabolism in A. alternata, but do not support mannitol metabolism operating as a cycle.

Single-cell transcript profiling of barley attacked by the powdery mildew fungus

In many plant-pathogen interactions, there are several possible outcomes for simultaneous attacks on the same leaf. For instance, an attack by the powdery mildew fungus on one barley leaf epidermal cell may succeed in infection and formation of a functional haustorium, whereas a neighboring cell attacked at the same time may resist fungal penetration. To date, the mixed cellular responses seen even in susceptible host leaves have made it difficult to relate induced changes in gene expression to resistance or susceptibility in bulk leaf samples. By microextraction of cell-specific mRNA and subsequent cDNA array analysis, we have successfully obtained separate gene expression profiles for specific mildew-resistant and -infected barley cells. Thus, for the first time, it is possible to identify genes that are specifically regulated in infected cells and, presumably, involved in fungal establishment. Further, although much is understood about the genetic basis of effective papilla resistance associated with mutant mlo barley, we provide here the first evidence for gene regulation associated with effective papilla-based nonspecific resistance expressed in nominally "susceptible" wild-type barley.

[Physiological function of arginine and its metabolites in plants]

L-arginine is an important and unique amino acid in plants. It serves not only as an important nitrogen reserve and recycling, but also as a precursor of the biosynthesis of polyamines, nitric oxide and so on. Polyamines and nitric oxide are important messengers involved in almost all physiological and biochemical processes, growth & development, and adaptation of plants to stress. Arginine decarboxylase, arginase and nitric oxide synthase are the key enzymes in L-arginine catabolism, in which polyamines are formed through ADC or arginase-ODC pathway while nitric oxide is formed through the NOS pathway. The relative activity of these three enzymes can control the direction of arginine metabolism. Arginine content keeps higher level in roots during overwinter period. The arginine metabolism plays important role in perception and adaptation of plant to environmental disturbances.