Trichoderma reesei Dehydrogenase, a Pyrroloquinoline Quinone-Dependent Member of Auxiliary Activity Family 12 of the Carbohydrate-Active Enzymes Database: Functional and Structural Characterization

Tilletia horrida glycoside hydrolase family 128 protein, designated ThGhd_7, modulates plant immunity by blocking reactive oxygen species production

Tilletia horrida is an important soilborne fungal pathogen that causes rice kernel smut worldwide. We found a glycoside hydrolase family 128 protein, designated ThGhd_7, caused cell death in Nicotiana benthamiana leaves. The predicted signal peptide (SP) of ThGhd_7 targets it for secretion. However, loss of the SP did not affect its ability to induce cell death. The 23-201 amino acid sequence of ThGhd_7 was sufficient to trigger cell death in N. benthamiana. ThGhd_7 expression was induced and upregulated during T. horrida infection. ThGhd_7 localised to both the cytoplasm and nucleus of plant cells, and nuclear localisation was required to induce cell death. The ability of ThGhd_7 to trigger cell death in N. benthamiana depends on RAR1 (required for Mla12 resistance), SGT1 (suppressor of G2 allele of Skp1), and BAK1/SERK3 (somatic embryogenesis receptor-like kinase 3). Heterologous overexpression of ThGhd_7 in rice reduced reactive oxygen species (ROS) production and enhanced susceptibility to T. horrida. Further research revealed that ThGhd_7 interacted with and destabilised OsSGT1, which is required for ROS production and is a positive regulator of rice resistance to T. horrida. Taken together, these findings suggest that T. horrida employs ThGhd_7 to disrupt ROS production and thereby promote infection.

Genomic and Transcriptomic Comparisons of the Twig Blight Pathogen, Passalora sequoiae, with Mycosphaerellaceae Foliar and Conifer Pathogens

Passalora sequoiae is a foliar pathogen to conifer tree species. In this study, we conducted whole-genome and transcriptome analyses on isolates of P. sequoiae collected from symptomatic Leyland cypress leaves from a Christmas tree farm in Mississippi. The objectives for this research were to elucidate the pathogenicity mechanisms of P. sequoiae by characterizing the genome and transcriptome and possibly identify unique and shared predicted genes in comparison with non-conifer/canker and foliar pathogens in the family Mycosphaerellaceae. P. sequoiae was found to be similar to other foliar Mycosphaerellaceae pathogens and likely represents a hemibiotrophic lifestyle based on comparisons across pathogens. The genome and in planta transcriptome highlighted some unique features of P. sequoiae: the significant presence of chitin synthases and fructose-degrading carbohydrate-degrading enzymes, trans-AT PKS genes, and antibiotic gene clusters that were unique to P. sequoiae compared with the other Mycosphaerellaceae species genomes. Several transcripts that were highly expressed in planta were identified as effectors, yet the functions were not characterized. These targets provide ample resources to continue to characterize pathogen-conifer host interactions in conifer foliar pathogens. Furthermore, this research helps build genomic resources for an important plant pathogen on Leyland cypress that will further our ability to develop novel management practices that could begin with breeding for resistance.

Fucose as a nutrient ligand for Dikarya and a building block of early diverging lineages

Fucose is a deoxyhexose sugar present and studied in mammals. The process of fucosylation has been the primary focus in studies relating to fucose in animals due to the presence of fucose in Lewis antigens. Very few studies have reported its presence in Fungi, mostly in Mucoromycotina. The constitution of 25% and 12% of this sugar in the carbohydrates of cell wall in the respective Umbelopsis and Mucorales strains boosts the need to bridge the gap of knowledge on fucose metabolism across the fungal tree of life. In the absence of a network map involving fucose proteins, we carried out an in-silico approach to construct the fucose metabolic map in Fungi. We analyzed the taxonomic distribution of 85 protein families in Fungi including diverse early diverging fungal lineages. The expression of fucose-related protein-coding genes proteins was validated with the help of transcriptomic data originating from representatives of early diverging fungi. We found proteins involved in several metabolic activities apart from fucosylation such as synthesis, transport and binding. Most of the identified protein families are shared with Metazoa suggesting an ancestral origin in Opisthokonta. However, the overall complexity of fucose metabolism is greater in Metazoa than in Fungi. Massive gene loss has shaped the evolutionary history of these metabolic pathways, leading to a repeated reduction of these pathways in most yeast-forming lineages. Our results point to a distinctive mode of utilization of fucose among fungi belonging to Dikarya and the early diverging lineages. We speculate that, while Dikarya used fucose as a source of nutrients for metabolism, the early diverging group of fungi depended on fucose as a building block and signaling compound.

Tracking the gastrointestinal digestive and metabolic behaviour of Dendrobium officinale polysaccharides by fluorescent labelling

Recently, Dendrobium officinale polysaccharide (DOP), a typical acetylated glucomannan, has been widely applied in functional foods owing to its excellent bioactivity. However, the insufficiency of studies on in vivo process severely limits the further utilization of DOP. The aim of this study was to systematically investigate the gastrointestinal digestive behaviour of DOP after oral administration by labelling it with two fluorescein aminopyrene-1,3,6-trisulfonic acids, trisodium salt (APTS) and cyanine 7.5 (Cy7.5). Combining the results of NIR imaging and HPGPC, we found that DOP was poorly absorbed directly in the prototype form; instead, DOP moved with the intestinal contents to the distal part of the intestine, where Bacteroides aggregated for a prolonged time and was metabolized to oligosaccharide-like substances. In contrast, the digestive degradation of DOP in pseudo-sterile mice with a targeted clearance of Bacteroides significantly weakened, which provided the basis and direction for the subsequent search for more specific metabolic pathways of DOP in vivo.

The Role of Glycoside Hydrolases in Phytopathogenic Fungi and Oomycetes Virulence

Phytopathogenic fungi need to secrete different hydrolytic enzymes to break down complex polysaccharides in the plant cell wall in order to enter the host and develop the disease. Fungi produce various types of cell wall degrading enzymes (CWDEs) during infection. Most of the characterized CWDEs belong to glycoside hydrolases (GHs). These enzymes hydrolyze glycosidic bonds and have been identified in many fungal species sequenced to date. Many studies have shown that CWDEs belong to several GH families and play significant roles in the invasion and pathogenicity of fungi and oomycetes during infection on the plant host, but their mode of function in virulence is not yet fully understood. Moreover, some of the CWDEs that belong to different GH families act as pathogen-associated molecular patterns (PAMPs), which trigger plant immune responses. In this review, we summarize the most important GHs that have been described in eukaryotic phytopathogens and are involved in the establishment of a successful infection.

Sugar oxidoreductases and LPMOs - two sides of the same polysaccharide degradation story?

Lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of glycosidic bonds in recalcitrant polysaccharides such as chitin and cellulose and their discovery has revolutionized our understanding of enzymatic biomass conversion. The discovery of LPMOs raises interesting new questions regarding the roles of other oxidoreductases and abiotic redox processes in biomass conversion. LPMOs need reducing power and an oxygen co-substrate and biomass degrading ecosystems contain a multitude of redox enzymes that affect the availability of both. For example, biomass degrading fungi produce multiple sugar oxidoreductases whose biological functions so far have remained somewhat enigmatic. It is now conceivable that these redox enzymes, in particular H2O2-producing sugar oxidases, could play a role in fueling and controlling LPMO reactions. Here, we shortly review contemporary issues in the LPMO field, paying particular attention to the possible roles of sugar oxidoreductases.

Characterization of the CAZy Repertoire from the Marine-Derived Fungus Stemphylium lucomagnoense in Relation to Saline Conditions

Even if the ocean represents a large part of Earth's surface, only a few studies describe marine-derived fungi compared to their terrestrial homologues. In this ecosystem, marine-derived fungi have had to adapt to the salinity and to the plant biomass composition. This articles studies the growth of five marine isolates and the tuning of lignocellulolytic activities under different conditions, including the salinity. A de novo transcriptome sequencing and assembly were used in combination with a proteomic approach to characterize the Carbohydrate Active Enzymes (CAZy) repertoire of one of these strains. Following these approaches, Stemphylium lucomagnoense was selected for its adapted growth on xylan in saline conditions, its high xylanase activity, and its improved laccase activities in seagrass-containing cultures with salt. De novo transcriptome sequencing and assembly indicated the presence of 51 putative lignocellulolytic enzymes. Its secretome composition was studied in detail when the fungus was grown on either a terrestrial or a marine substrate, under saline and non-saline conditions. Proteomic analysis of the four S. lucomagnoense secretomes revealed a minimal suite of extracellular enzymes for plant biomass degradation and highlighted potential enzyme targets to be further studied for their adaptation to salts and for potential biotechnological applications.

A Conserved Glycoside Hydrolase Family 7 Cellobiohydrolase PsGH7a of Phytophthora sojae Is Required for Full Virulence on Soybean

Phytopathogens deploy glycoside hydrolases (GHs) to disintegrate plant cell walls for nutrition and invasion. However, the pathogenic mechanisms of the majority of GHs in virulence remain unknown, especially in oomycetes. In this study, a Phytophthora sojae gene encodes a GH7 family cellobiohydrolase, named PsGH7a, was identified. PsGH7a was highly induced during the cyst germination and infection stages. PsGH7a is conserved in oomycetes, and shares a high amino acid sequence identity (>85%) within Phytophthora genus. The recombinant PsGH7a catalyzes the hydrolysis of β-1,4-glucan and avicel, which represent the major components of cellulose in plant cell wall. The mutation of catalytic residue Glu236 to alanine resulted in a lower catalytic activity. In addition, the PsGH7a promotes Phytophthora invasion, while the mutant can not. Notably, PsGH7a protein triggers hypersensitive cell death in diverse plants. PsGH7a knockout mutants were generated via CRISPR/Cas9 system, to investigate its biological function. Compared to wild-type strain P6497, the mutants showed reduced virulence on susceptible soybean, indicates PsGH7a is indispensable to P. sojae virulence.

PQQ-GDH - Structure, function and application in bioelectrochemistry

This review summarizes the basic features of the PQQ-GDH enzyme as one of the sugar converting biocatalysts. Focus is on the membrane -bound and the soluble form. Furthermore, the main principles of enzymatic catalysis as well as studies on the physiological importance are reviewed. A short overview is given on developments in protein engineering. The major part, however, deals with the different fields of application in bioelectrochemistry. This includes approaches for enzyme-electrode communication such as direct electron transfer, mediator-based systems, redox polymers or conducting polymers and holoenzyme reconstitution, and covers applied areas such as biosensing, biofuel cells, recycling schemes, enzyme competition, light-directed sensing, switchable detection schemes, logical operations by enzyme electrodes and immune sensing.

Improved Production of Pyrroloquinoline Quinone by Simultaneous Augmentation of Its Synthesis Gene Expression and Glucose Metabolism in Klebsiella pneumoniae

Klebsiella pneumoniae can naturally synthesize pyrroloquinoline quinone (PQQ), but current low yield restricts its commercialization. Here, we reported that PQQ production can be improved by simultaneously intensifying PQQ gene expression and glucose metabolism. Firstly, tandem repetitive tac promoters were constructed to overexpress PQQ synthesis genes. Results showed that when three repeats of tac promoter were recruited to overexpress PQQ synthesis genes, the recombinant strain generated 1.5-fold PQQ relative to the strain recruiting only one tac promoter. Quantitative real-time PCR (qRT-PCR) revealed the increased transcription levels of PQQ synthesis genes. Next, fermentation parameters were optimized to augment the glucose direct oxidation pathway (GDOP) mediated by PQQ-dependent glucose dehydrogenase (PQQ-GDH). Results demonstrated that the cultivation conditions of sufficient glucose (≥ 32 g/L), low pH (5.8), and limited potassium (0.7 nmol/L) significantly promoted the biosynthesis of gluconic acid, 2-ketogluconic acid, and PQQ. In optimum shake flask fermentation conditions, the K. pneumoniae strain overexpressing PQQ synthesis genes under three repeats of tac promoter generated 363.3 nmol/L of PQQ, which was 2.6-fold of that in original culture conditions. In bioreactor cultivation, this strain produced 2371.7 nmol/L of PQQ. To our knowledge, this is the highest PQQ titer reported so far using K. pneumoniae as a host strain. Overall, simultaneous intensification of pqq gene expression and glucose metabolism is effective to improve PQQ production.

The membrane-bound sorbosone dehydrogenase of Gluconacetobacter liquefaciens is a pyrroloquinoline quinone-dependent enzyme

Membrane-bound sorbosone dehydrogenase (SNDH) of Gluconacetobacter liquefaciens oxidizes l-sorbosone to 2-keto-l-gulonic acid (2KGLA), a key intermediate in vitamin C production. We constructed recombinant Escherichia coli and Gluconobacter strains harboring plasmids carrying the sndh gene from Ga. liquefaciens strain RCTMR10 to identify the prosthetic group of SNDH. The membranes of the recombinant E. coli showed l-sorbosone oxidation activity, only after the holo-enzyme formation with pyrroloquinoline quinone (PQQ), indicating that SNDH is a PQQ-dependent enzyme. The sorbosone-oxidizing respiratory chain was thus heterologously reconstituted in the E. coli membranes. The membranes that contained SNDH showed the activity of sorbosone:ubiquinone analogue oxidoreductase. These results suggest that the natural electron acceptor for SNDH is membranous ubiquinone, and it functions as the primary dehydrogenase in the sorbosone oxidation respiratory chain in Ga. liquefaciens. A biotransformation experiment showed l-sorbosone oxidation to 2KGLA in a nearly quantitative manner. Phylogenetic analysis for prokaryotic SNDH homologues revealed that they are found only in the Proteobacteria phylum and those of the Acetobacteraceae family are clustered in a group where all members possess a transmembrane segment. A three-dimensional structure model of the SNDH constructed with an in silico fold recognition method was similar to the crystal structure of the PQQ-dependent pyranose dehydrogenase from Coprinopsis cinerea. The structural similarity suggests a reaction mechanism under which PQQ participates in l-sorbosone oxidation.