Purification and characterization of a (1-->3)-beta-D-glucan endohydrolase from rice (Oryza sativa) bran

β-Glucan and its nanocomposites in sustainable agriculture and environment: an overview of mechanisms and applications

β-Glucan is an eco-friendly, biodegradable, and economical biopolymer with important roles for acquiring adaptations to mitigate climate change in crop plants. β-Glucan plays a crucial role in the activation of functional plant innate immune system by triggering the downward signaling cascade/s, resulting in the accumulation of different pathogenesis-related proteins (PR-proteins), reactive oxygen species (ROS), antioxidant defense enzymes, Ca²⁺-influx as well as activation of mitogen-activated protein kinase (MAPK) pathway. Recent experimental studies have shown that β-glucan recognition is mediated by co-receptor LysMPRR (lysin motif pattern recognition receptor)-CERK1 (chitin elicitor receptor kinase 1), LYK4, and LYK5 (LysM-containing receptor-like kinase), as well as different receptor systems in plants that could be plant species-specific and/or age and/or tissue-dependent. Transgenic overexpression of β-glucanase, chitinase, and/or in combination with other PR-proteins like cationic peroxidase, AP24,thaumatin-likeprotein 1 (TLP-1) has also been achieved for improving plant disease resistance in crop plants, but the transgenic methods have some ethical and environmental concerns. In this regard, elicitation of plant immunity using biopolymer like β-glucan and chitosan offers an economical, safe, and publicly acceptable method. The β-glucan and chitosan nanocomposites have proven to be useful for the activation of plant defense pathways and to enhance plant response/systemic acquired resistance (SAR) against broad types of plant pathogens and mitigating multiple stresses under the changing climate conditions.

Penicillium citrinum UFV1 β-glucosidases: purification, characterization, and application for biomass saccharification

BACKGROUND

β-Glucosidases are components of the cellulase system, a family of enzymes that hydrolyze the β-1,4 linkages of cellulose. These proteins have been extensively studied due to the possibility of their use in various biotechnological processes. They have different affinities for substrates (depending on their source) and their activities can be used for saccharification of different types of biomass. In this context, the properties and the synergistic capacity of β-glucosidases from different organisms, to supplement the available commercial cellulase cocktails, need a comprehensive evaluation.

RESULTS

Two β-glucosidases belonging to GH3 family were secreted by Penicillium citrinum UFV. PcβGlu1 (241 kDa) and PcβGlu2 (95 kDa) presented acidic and thermo-tolerant characteristics. PcβGlu1 showed Michaelis-Menten kinetics for all substrates tested with K_m values ranging from 0.09 ± 0.01 (laminarin) to 1.7 ± 0.1 mM (cellobiose, C2) and k_cat values ranging from 0.143 ± 0.005 (laminarin) to 8.0 ± 0.2 s^-1 (laminaribiose, Lb). PcβGlu2 showed substrate inhibition for 4-methylumbelliferyl-β-d-glucopyranoside (MUβGlu), p-nitrophenyl-β-d-glucopyranoside (pNPβGlu), cellodextrins (C3, C4, and C5), N-octil-β-d-glucopyranoside, and laminaribiose, with K_m values ranging from 0.014 ± 0.001 (MUβGlu) to 0.64 ± 0.06 mM (C2) and k_cat values ranging from 0.49 ± 0.01 (gentiobiose) to 1.5 ± 0.2 s^-1 (C4). Inhibition constants (K_i) for PcβGlu2 substrate inhibition ranged from 0.69 ± 0.07 (MUβGlu) to 10 ± 1 mM (Lb). Glucose and cellobiose are competitive inhibitors of PcβGlu1 and PcβGlu2 when pNPβGlu is used as a substrate. For PcβGlu1 inhibition, K_i = 1.89 ± 0.08 mM (glucose) and K_i = 3.8 ± 0.1 mM (cellobiose); for PcβGlu2, K_i = 0.83 ± 0.05 mM (glucose) and K_i = 0.95 ± 0.07 mM (cellobiose). The enzymes were tested for saccharification of different biomasses, individually or supplementing a Trichoderma reesei commercial cellulose preparation. PcβGlu2 was able to hydrolyze banana pseudostem and coconut fiber with the same efficiency as the T. reesei cocktail, showing significant synergistic properties with T. reesei enzymes in the hydrolysis of these alternative biomasses.

CONCLUSIONS

The β-glucosidases from P. citrinum UFV1 present different enzymatic properties from each other and might have potential application in several biotechnological processes, such as hydrolysis of different types of biomass.

Modulating the function of a β-1,3-glucanosyltransferase to that of an endo-β-1,3-glucanase by structure-based protein engineering

A glycoside hydrolase (GH) family 17 β-1,3-glucanosyltransferase (RmBgt17A) from Rhizomucor miehei CAU432 (CGMCC No. 4967) shared very low sequence homology (∼20 % identity) with that of other β-1,3-glucanases,despite their similar structural folds. Structural comparison and sequence alignment between RmBgt17A and GH family 17 β-1,3-glucanases suggested important roles for three residues (Tyr102, Trp157, and Glu158) located in the substrate-binding cleft of RmBgt17A in transglycosylation activity. A series of site-directed mutagenesis studies indicated that a single Glu-to-Ala mutation (E158A) modulates the function of RmBgt17A to that of a β-1,3-glucanase. Mutant E158A exhibited high hydrolytic activity (39.95 U/mg) toward reduced laminarin, 348.5-fold higher than the wild type. Optimal pH and temperature of the purified RmBgt17A-E158A were 4.5 and 55 °C, respectively. TLC analysis suggested that RmBgt17A-E158A is an endo-β-1,3-glucanase. Our study provides novel insight into protein engineering of the substrate-binding cleft of glycoside hydrolases to modulate the function of transglycosylation and hydrolysis.

Structures of an active-site mutant of a plant 1,3-β-glucanase in complex with oligosaccharide products of hydrolysis

Plant endo-1,3-β-glucanases are involved in important physiological processes such as defence mechanisms, cell division and flowering. They hydrolyze (1→3)-β-glucans, with very limited activity towards mixed (1→3,1→4)-β-glucans and branched (1→3,1→6)-β-glucans. Here, crystal structures of the potato (Solanum tuberosum) endo-1,3-β-glucanase GLUB20-2 with the nucleophilic Glu259 residue substituted by alanine (E259A) are reported. Despite this active-site mutation, the protein retained residual endoglucanase activity and when incubated in the crystallization buffer with a linear hexameric substrate derived from (1→3)-β-glucan (laminarahexose) cleaved it in two different ways, generating trisaccharides and tetrasaccharides, as confirmed by mass spectrometry. The trisaccharide (laminaratriose) shows higher binding affinity and was found to fully occupy the -1, -2 and -3 sites of the active-site cleft, even at a low molar excess of the substrate. At elevated substrate concentration the tetrasaccharide molecule (laminaratetrose) also occupies the active site, spanning the opposite sites +1, +2, +3 and +4 of the cleft. These are the first crystal structures of a plant glycoside hydrolase family 17 (GH17) member to reveal the protein-saccharide interactions and were determined at resolutions of 1.68 and 1.55 Å, respectively. The geometry of the active-site cleft clearly precludes any (1→4)-β-glucan topology at the subsites from -3 to +4 and could possibly accommodate β-1,6-branching only at subsites +1 and +2. The glucose units at subsites -1 and -2 interact with highly conserved protein residues. In contrast, subsites -3, +3 and +4 are variable, suggesting that the mode of glucose binding at these sites may vary between different plant endo-1,3-β-glucanases. Low substrate affinity is observed at subsites +1 and +2, as manifested by disorder of the glycosyl units there.

A cryoprotective and cold-adapted 1,3-β-endoglucanase from cherimoya (Annona cherimola) fruit

A 1,3-β-glucanase with potent cryoprotective activity was purified to homogeneity from the mesocarp of CO2-treated cherimoya fruit (Annona cherimola Mill.) stored at low temperature using anion exchange and chromatofocusing chromatography. This protein was characterized as a glycosylated endo-1,3-β-glucanase with a Mr of 22.07kDa and a pI of 5.25. The hydrolase was active and stable in a broad acidic pH range and it exhibited maximum activity at pH 5.0. It had a low optimum temperature of 35°C and it retained 40% maximum activity at 5°C. The purified 1,3-β-glucanase was relatively heat unstable and its activity declined progressively at temperatures above 50°C. Kinetic studies revealed low k(cat) (3.10±0.04 s(-1)) and Km (0.32±0.03 mg ml(-1)) values, reflecting the intermediate efficiency of the protein in hydrolyzing laminarin. Moreover, a thermodynamic characterization revealed that the purified enzyme displayed a high k(cat) at both 37 and 5°C, and a low Ea (6.99 kJ mol(-1)) within this range of temperatures. In vitro functional studies indicated that the purified 1,3-β-glucanase had no inhibitory effects on Botrytis cinerea hyphal growth and no antifreeze activity, as determined by thermal hysteresis analysis using differential scanning calorimetry. However, a strong cryoprotective activity was observed against freeze-thaw inactivation of lactate dehydrogenase. Indeed, the PD50 was 8.7 μg ml(-1) (394 nM), 9.2-fold higher (3.1 on a molar basis) than that of the cryoprotective protein BSA. Together with the observed accumulation of glycine-betaine in CO2-treated cherimoya tissues, these results suggest that 1,3-β-glucanase could be functionally implicated in low temperature-defense mechanism activated by CO2.

A rice β-1,3-glucanase gene Osg1 is required for callose degradation in pollen development

Plant β-1,3-glucanases are involved in plant defense and development. In rice (Oryza sativa), 14 genes encoding putative β-1,3-glucanases have been isolated and sequenced. However, only limited information is available on the function of these β-1,3-glucanase genes. In this study, we report a detailed functional characterization of one of these genes, Osg1. Osg1 encodes a glucanase carrying no C-terminal extension. Osg1 was found to be expressed throughout the plant and highly expressed in florets, leaf sheaths, and leaf blades. Investigations using real-time PCR, immunocytochemical analysis, and a GUS-reporter gene driven by the Osg1 promoter indicated that Osg1 was mainly expressed at the late meiosis, early microspore, and middle microspore stages in the florets. To elucidate the role of Osg1, we suppressed expression of the Osg1 gene by RNA interference in transgenic rice. The silencing of Osg1 resulted in male sterility. The pollen mother cells appeared to be normal in Osg1-RI plants, but callose degradation was disrupted around the microspores in the anther locules of the Osg1-RI plants at the early microspore stage. Consequently, the release of the young microspores into the anther locules was delayed, and the microspores began to degenerate later. These results provide evidence that Osg1 is essential for timely callose degradation in the process of tetrad dissolution.

High-level production of a thermoacidophilic beta-glucosidase from Penicillium citrinum YS40-5 by solid-state fermentation with rice bran

A high yield of beta-glucosidase (EC 3.2.1.21) of 159.1 U/g-solid activity on 4-nitrophenyl beta-d-glucopyranoside (pNPG) was achieved by rice bran-based solid-state fermentation (SSF) of the recently characterized fungus Penicillium citrinum YS40-5. The enzyme was both thermophilic and acidophilic at the optimized temperature and pH of 70 degrees C and 5.0, respectively. Over 95% of the original beta-glucosidase activity was maintained after a prolonged storage at ambient temperature for 4 weeks. The kinetic parameters V(max), K(m) and K(I) were 85.93 U/mg, 1.2 mM and 17.59 mM with pNPG, and 72.49 U/mg, 32.17 mM and 8.29 mM with cellobiose, respectively. The protein band with beta-glucosidase activity was characterized by native PAGE followed by MUG-zymogram analysis, and its identity confirmed by nanoLC-MS/MS. A 3.43-fold synergistic effect by combining this beta-glucosidase with Trichoderma reesei cellulases was observed, indicating this enzyme could potentially be used for improving the efficiency of cellulosic bioconversion.

Expression of an endo-(1,3;1,4)-beta-glucanase in response to wounding, methyl jasmonate, abscisic acid and ethephon in rice seedlings

We isolated two rice endo-(1,3;1,4)-beta-glucanase genes, denoted OsEGL1 and OsEGL2, which encoded proteins that shared 64% amino acid sequence identity. Both the OsEGL1 and OsEGL2 genes were successfully expressed in Escherichia coli to produce functional proteins. Purified OsEGL1 and OsEGL2 proteins hydrolyzed (1,3;1,4)-beta-glucans, but not (1,3;1,6)-beta-linked or (1,3)-beta-linked glucopolysaccharides nor carboxymethyl cellulose, similar to previously characterized grass endo-(1,3;1,4)-beta-glucanases. RNA blot analysis revealed that the OsEGL1 gene is expressed constitutively not only in young roots of rice seedlings, but also in mature roots of adult rice plants. Little or no expression of the OsEGL2 gene was observed in all tissues or treatments tested, but database and RT-PCR analysis indicated it is expressed in ripening panicle. In rice seedling leaves, OsEGL1 gene expression significantly increased in response to methyl jasmonate, abscisic acid, ethephon and mechanical wounding. Mechanical wounding also increased the leaf elongation rate in rice seedlings by 16% relative to that of control seedlings at day 4 after treatment. The increase in the leaf elongation rate of rice seedlings treated under mechanical wounding was concomitant with an increase in OsEGL1 expression levels in seedling leaves.

Herbivore-induced callose deposition on the sieve plates of rice: an important mechanism for host resistance

The brown planthopper (Nilaparvata lugens Stål; BPH) is a specialist herbivore on rice (Oryza sativa) that ingests phloem sap from the plant through its stylet mouthparts. Electronic penetration graphs revealed that BPH insects spent more time wandering over plants carrying the resistance genes Bph14 and Bph15, but less time ingesting phloem than they did on susceptible plants. They also showed that their feeding was frequently interrupted. Tests with [(14)C]sucrose showed that insects ingested much less phloem sap from the resistant than the susceptible plants. BPH feeding up-regulated callose synthase genes and induced callose deposition in the sieve tubes at the point where the stylet was inserted. The compact callose remained intact in the resistant plants, but genes encoding beta-1,3-glucanases were activated, causing unplugging of the sieve tube occlusions in susceptible plants. Continuing ingestion led to a remarkable reduction in the susceptible plants' sucrose content and activation of the RAmy3D gene, leading to starch hydrolysis and ultimately carbohydrate deprivation in the plants. Our results demonstrate that BPH feeding induces the deposition of callose on sieve plates in rice and that this is an important defense mechanism that prevents insects from ingesting phloem sap. In response, however, the BPH can unplug sieve tube occlusions by activating beta-1,3-glucanase genes in rice plants.

Cloning, characterization and expression of OsGLN2, a rice endo-1,3-beta-glucanase gene regulated developmentally in flowers and hormonally in germinating seeds

We report here the isolation and characterization of a new endo-1,3-beta-glucanase (1,3-beta-GLU) cDNA, OsGLN2, that is expressed both in flowers and in germinating seeds of rice (Oryza sativa L.). The isolated OsGLN2 gene encoded a protein which displayed 72%, 93% and 92% identity at the amino acid level with those encoded by barley GII, rice Gns4 and glu1 1,3-beta-GLU genes, respectively. A GST-OsGLN2 recombinant protein expressed in Escherichia coli preferentially hydrolyzed Laminaria digitata 1,3;1,6-beta-glucan and liberated only oligosaccharides, suggesting that the enzyme can be classified as a 1,3-beta-GLU. Northern analysis with a 3'-UTR gene-specific probe revealed that OsGLN2 is expressed exclusively in the paleae and lemmas during flowering, and no expression of OsGLN2 was detected in other tissues such as leaf blades, leaf sheaths, stems, nodes and roots in mature rice plants. The OsGLN2 gene is also expressed in germinating seeds, where its expression is predominant in endosperms rather than embryos. In de-embryonated rice half-seeds, addition of gibberellin A3 (GA) greatly enhanced expression of the OsGLN2 gene, while the GA-induced gene expression was suppressed strongly by abscisic acid (ABA). This is the first report, to our knowledge, that OsGLN2 encodes a 1,3-beta-GLU and is expressed specifically in paleae and lemmas during flowering and in germinating seeds, where its expression is enhanced by GA and suppressed by ABA.

Characterization of rice endo-beta-glucanase genes (Gns2-Gns14) defines a new subgroup within the gene family

Thirteen new beta-glucanase-encoding genes have been identified in the rice genome. These genes, together with other monocot beta-glucanases, have now been classified into four subfamilies based on the structure and function of the genes. Two tandem gene clusters, Gns2-Gns3-Gns4 and Gns5-Gns6, were classified in the defense-related Subfamily A. Growth-related 1,3;1,4-beta-glucanase Gns1 was classified in Subfamily B. Gns7 and Gns8, together with the barley genes GVI and Hv34, represent Subfamily C. Gns9 and a beta-glucanase gene from wheat were grouped in Subfamily D. Genes in Subfamilies C and D have structures that are distinct from those of the other subfamilies, but there are very little data available on the biochemical or physiological roles of these genes. Gene expression in growing tissues and lack of gene induction in response to disease-related treatments suggest that Subfamilies C and D may function in control of plant growth.