Mullite: Crystal Structure and Related Properties Schneider, Hartmut; Fischer, Reinhard X.; Schreuer, Jürgen
Journal of the American Ceramic Society,
October 2015, Letnik:
98, Številka:
10
Journal Article
Recenzirano
Mullite is certainly one of the most important oxide materials for both conventional and advanced ceramics. Mullite belongs to the compositional series of orthorhombic aluminosilicates with the ...general composition Al2(Al2+2xSi2‐2x)O10‐x. Main members are sillimanite (x = 0), stoichiometric 3/2‐mullite (x = 0.25), 2/1‐mullite (x = 0.40), and the SiO2‐free phase ι‐alumina (x = 1, crystal structure not known). This study gives an overview on the present state of research regarding single crystal mullite. Following a short introduction, the second part of the review focuses on the crystal structure of mullite. In particular, the characteristic mullite‐type structural backbone of parallel chains consisting of edge‐sharing MO6 octahedra and their specific cross‐linkage by TO4 tetrahedra is explained in detail, the role of cation disorder and structural oxygen vacancies is addressed, and the possibility of cation substitution on different sites is discussed. The third part of the study deals with physical properties being relevant for technical applications of mullite and includes mechanical properties (e.g., elasticity, compressibility, strength, toughness, creep), thermal properties (e.g., thermal expansion, heat capacity, atomic diffusion, thermal conductivity), electrical conductivity, and optical properties. Special emphasis is put on structure–property relationships which allow for interpretation of corresponding experimental data and offer in turn the possibility to tailor new mullite materials with improved properties. Finally, the reported anomalies and discontinuities in the evolution of certain physical properties with temperature are summarized and critically discussed.
Nematode-trapping fungi (NTF) are a diverse and intriguing group of fungi that live saprotrophically but can switch to a predatory lifestyle when starving and in the presence of nematodes. NTF like ...Arthrobotrys oligospora or Duddingtonia flagrans produce adhesive trapping networks to catch and immobilize nematodes. After penetration of the cuticle, hyphae grow and develop inside the worm and secrete large amounts of hydrolytic enzymes for digestion. In many microbial pathogenic interactions small-secreted proteins (SSPs) are used to manipulate the host. The genome of D. flagrans encodes more than 100 of such putative SSPs one of which is the cysteine-rich protein CyrA. We have chosen this gene for further analysis because it is only found in NTF and appeared to be upregulated during the interaction. We show that the cyrA gene was transcriptionally induced in trap cells, and the protein accumulated at the inner rim of the hyphal ring before Caenorhabditis elegans capture. After worm penetration, the protein appeared at the fungal infection bulb, where it is likely to be secreted with the help of the exocyst complex. A cyrA-deletion strain was less virulent, and the time from worm capture to paralysis was extended. Heterologous expression of CyrA in C. elegans reduced its lifespan. CyrA accumulated in C. elegans in coelomocytes where the protein possibly is inactivated. This is the first example that SSPs may be important in predatory microbial interactions.
The filamentous fungus
is a common postharvest contaminant of food and feed, and some strains are plant pathogens. Many processes in
are triggered by light. Interestingly, blue light inhibits ...sporulation, and red light reverses the effect, suggesting interactions between light-sensing systems. The genome encodes a phytochrome (FphA), a white collar 1 (WC-1) orthologue (LreA), an opsin (NopA), and a cryptochrome (CryA) as putative photoreceptors. Here, we investigated the role of FphA and LreA and the interplay with the high-osmolarity glycerol (HOG) mitogen-activated protein (MAP) kinase pathway. We created loss-of function mutations for
,
, and
using CRISPR-Cas9 technology. Sporulation was reduced in all three mutant strains already in the dark, suggesting functions of the photoreceptors FphA and LreA independent of light perception. Germination of conidia was delayed in red, blue, green, and far-red light. We found that light induction of
(
lock-
ontrolled
ene in
and light-induced gene in
) and the catalase gene
depended on FphA, LreA, and HogA. Light induction of
(a putative ferrochelatase gene) and
(
light regulated, unknown function) required LreA and HogA but not FphA. Blue- and green-light stimulation of alternariol formation depended on LreA. A lack of FphA or LreA led to enhanced resistance toward oxidative stress due to the upregulation of catalases and superoxide dismutases. Light activation of FphA resulted in increased phosphorylation and nuclear accumulation of HogA. Our results show that germination, sporulation, and secondary metabolism are light regulated in
with distinct and overlapping roles of blue- and red-light photosensors.
Light controls many processes in filamentous fungi. The study of light regulation in a number of model organisms revealed an unexpected complexity. Although the molecular components for light sensing appear to be widely conserved in fungal genomes, the regulatory circuits and the sensitivity of certain species toward specific wavelengths seem different. In
, most light responses are triggered by blue light, whereas in
, red light plays a dominant role. In
, both blue and red light appear to be important. In
, photoreceptors control morphogenetic pathways, the homeostasis of reactive oxygen species, and the production of secondary metabolites. On the other hand, high-osmolarity sensing required FphA and LreA, indicating a sophisticated cross talk between light and stress signaling.
•First description of CRISPR/Cas9 application in A. alternata.•Establishment of a pyrG mutant.•Use of GFP.
The filamentous fungus Alternaria alternata is a potent producer of many secondary ...metabolites, some of which like alternariol or alternariol-methyl ether are toxic and/or cancerogenic. Many Alternaria species do not only cause post-harvest losses of food and feed, but are aggressive plant pathogens. Despite the great economic importance and the large number of research groups working with the fungus, the molecular toolbox is rather underdeveloped. Gene deletions often result in heterokaryotic strains and therefore, gene-function analyses are rather tedious. In addition, A. alternata lacks a sexual cycle and classical genetic approaches cannot be combined with molecular biological methods. Here, we show that CRISPR/Cas9 can be efficiently used for gene inactivation. Two genes of the melanin biosynthesis pathway, pksA and brm2, were chosen as targets. Several white mutants were obtained after several rounds of strain purification through protoplast regeneration or spore inoculation. Mutation of the genes was due to deletions from 1bp to 1.5kbp. The CRISPR/Cas9 system was also used to inactivate the orotidine-5-phosphate decarboxylase gene pyrG to create a uracil-auxotrophic strain. The strain was counter-selected with fluor-orotic acid and could be re-transformed with pyrG from Aspergillus fumigatus and pyr-4 from Neurospora crassa. In order to test the functioning of GFP, the fluorescent protein was fused to a nuclear localization signal derived from the StuA transcription factor of Aspergillus nidulans. After transformation bright nuclei were visible.
We have evaluated more than 7000 crystal structures of zeolites in terms of the values of their bond-lengths T–O (T = Si, Al, P, Zn, Be, Ge, B, As, Ga, Co) and their variability as well as the ...flexibility of their bond angles O–T–O. Out of these known crystal structure descriptions of zeolites, we have selected 1179 which have estimated standard deviations of their T–O bond lengths of 0.01 Å or less. For the most common bond lengths, we obtain 1.603(11) Å for 1323 mean tetrahedral Si–O, 1.736(8) Å for 416 Al–O, and 1.522(9) Å for 228 P–O. It is unsettling that the spread of each of these values is large: about 0.07 Å within the population studied by us. Furthermore, these values disagree by several hundredths of an Ångström from some of the mean values of Si–O, Al–O, and P–O compiled from nonzeolitic types of compounds. This is at variance with the widespread conviction that such T–O distance values should be relatively constant across different types of inorganic compounds. Ever since Cruickshank (Acta Crystallogr. 1956, 9, 757), it has been known that high atomic displacement factors shorten observed bond lengths. Corrections for this effect were applied in the past but have become lately rarer. We find that much of the variance observed by us in the bond lengths is due to the fact that topologically different zeolite framework types have different atomic displacement parameters of their oxygen atoms. Thus, it makes no sense to search for mean tetrahedral bond lengths in TO4 tetrahedra. Instead, a particular mean bond length, e.g., Si–O, can be only characteristic for a Si–O bond for one given framework type as we show for the topologically different framework types CAN, FER, MFI, NAT, and SOD. Even after bond length corrections for differing displacement parameters, the mean Si–O bonds range from 1.601 Å for FER to 1.629 Å for SOD. The observed angles O–T–O cover a range from 94.5 to 129.1° averaging around a tetrahedral angle of 109.5°. The largest deviations from the tetrahedral angle occur for tetrahedra with long mean T–O distances of the T-atoms. Our results, properly applied, can be useful as an input for distance least squares calculations on zeolites and for checking on the results of crystal structure refinements or of theoretical calculations.
Filamentous fungi constitute a large group of eukaryotic microorganisms that grow by forming simple tube-like hyphae that are capable of differentiating into more-complex morphological structures and ...distinct cell types. Hyphae form filamentous networks by extending at their tips while branching in subapical regions. Rapid tip elongation requires massive membrane insertion and extension of the rigid chitin-containing cell wall. This process is sustained by a continuous flow of secretory vesicles that depends on the coordinated action of the microtubule and actin cytoskeletons and the corresponding motors and associated proteins. Vesicles transport cell wall-synthesizing enzymes and accumulate in a special structure, the Spitzenkörper, before traveling further and fusing with the tip membrane. The place of vesicle fusion and growth direction are enabled and defined by the position of the Spitzenkörper, the so-called cell end markers, and other proteins involved in the exocytic process. Also important for tip extension is membrane recycling by endocytosis via early endosomes, which function as multipurpose transport vehicles for mRNA, septins, ribosomes, and peroxisomes. Cell integrity, hyphal branching, and morphogenesis are all processes that are largely dependent on vesicle and cytoskeleton dynamics. When hyphae differentiate structures for asexual or sexual reproduction or to mediate interspecies interactions, the hyphal basic cellular machinery may be reprogrammed through the synthesis of new proteins and/or the modification of protein activity. Although some transcriptional networks involved in such reprogramming of hyphae are well studied in several model filamentous fungi, clear connections between these networks and known determinants of hyphal morphogenesis are yet to be established.
In recent years, a large number of precisely determined crystal structure determinations of zeolites, that is compounds based on open frameworks of coordination tetrahedra, has accumulated. This ...gives us the opportunity to evaluate them in terms of the flexibility of their bond angles T–X–T which serve as hinges between the coordination tetrahedra in zeolites containing various SiO
4
, AlO
4
, PO
4
and similarly sized ions. Out of about 7,000 known crystal structure descriptions of zeolites we have selected 1187 which are based on single crystal diffraction studies and have estimated standard deviations of their T–O bond length of 0.01 Å or less. The mean Si–O–Si angle in zeolitic frameworks with T-sites fully occupied by one type of T atoms is 152.9° and these angles range from 127.0° to 180°, the mean for Al–O–Si is 141.9°, ranging from 124.8° to 179.3°. Straight angles Si–O–Si do occur in some zeolites.
One kind of the most extremely polarized cells in nature are the indefinitely growing hyphae of filamentous fungi. A continuous flow of secretion vesicles from the hyphal cell body to the growing ...hyphal tip is essential for cell wall and membrane extension. Because microtubules (MT) and actin, together with their corresponding motor proteins, are involved in the process, the arrangement of the cytoskeleton is a crucial step to establish and maintain polarity. In Saccharomyces cerevisiae and Schizosaccharomyces pombe, actin-mediated vesicle transportation is sufficient for polar cell extension, but in S. pombe, MTs are in addition required for the establishment of polarity. The MT cytoskeleton delivers the so-called cell-end marker proteins to the cell pole, which in turn polarize the actin cytoskeleton. Latest results suggest that this scenario may principally be conserved from S. pombe to filamentous fungi. In addition, in filamentous fungi, MTs could provide the tracks for long-distance vesicle movement. In this review, we will compare the interaction of the MT and the actin cytoskeleton and their relation to the cortex between yeasts and filamentous fungi. In addition, we will discuss the role of sterol-rich membrane domains in combination with cell-end marker proteins for polarity establishment.
Abstract
Salicylic acid is a phenolic phytohormone which controls plant growth and development. A methyl ester (MSA) derivative thereof is volatile and involved in plant-insect or plant-plant ...communication. Here we show that the nematode-trapping fungus
Duddingtonia flagrans
uses a methyl-salicylic acid isomer, 6-MSA as morphogen for spatiotemporal control of trap formation and as chemoattractant to lure
Caenorhabditis elegans
into fungal colonies. 6-MSA is the product of a polyketide synthase and an intermediate in the biosynthesis of arthrosporols. The polyketide synthase (ArtA), produces 6-MSA in hyphal tips, and is uncoupled from other enzymes required for the conversion of 6-MSA to arthrosporols, which are produced in older hyphae. 6-MSA and arthrosporols both block trap formation. The presence of nematodes inhibits 6-MSA and arthrosporol biosyntheses and thereby enables trap formation. 6-MSA and arthrosporols are thus morphogens with some functions similar to quorum-sensing molecules. We show that 6-MSA is important in interkingdom communication between fungi and nematodes.
Contamination of food and feed with toxin-producing fungi is a major threat in agriculture and for human health. The filamentous fungus Alternaria alternata is one of the most widespread postharvest ...contaminants and a weak plant pathogen. It produces a large variety of secondary metabolites with alternariol and its derivatives as characteristic mycotoxin. Other important phyto- and mycotoxins are perylene quinones (PQs), some of which have anticancer properties. Here, we discovered that the PQ altertoxin (ATX) biosynthesis shares most enzymes with the 1,8-dihydroxynaphthalene (1,8-DHN) melanin pathway. However, melanin was formed in aerial hyphae and spores, and ATXs were synthesized in substrate hyphae. This spatial separation is achieved through the promiscuity of a polyketide synthase, presumably producing a pentaketide (T4HN), a hexaketide (AT4HN), and a heptaketide (YWA1) as products. T4HN directly enters the altertoxin and DHN melanin pathway, whereas AT4HN and YWA1 can be converted only in aerial hyphae, which probably leads to a higher T4HN concentration, favoring 1,8-DHN melanin formation. Whereas the production of ATXs was strictly dependent on the CmrA transcription factor, melanin could still be produced in the absence of CmrA to some extent. This suggests that different cues regulate melanin and toxin formation. Since DHN melanin is produced by many fungi, PQs or related compounds may be produced in many more fungi than so far assumed.
Mycotoxins are a major threat for human health. Food safety control relies on the identification of the toxins or the detection of the expression of the respective genes. The latter method, however, relies on the knowledge of the biosynthetic pathway and the key genes. Alternaria alternata is a major food contaminant and produces many different mycotoxins with altertoxins and other perylene quinones as prominent examples. Here, we discovered that the biosynthetic pathway for altertoxins shares most of the enzymes with the dihydroxynaphthalene (DHN) melanin pathway. Because the DHN melanin pathway is widespread among fungi, the production of mycotoxins of the perylene quinone class could be more widespread than so far anticipated.
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