Hyphal ultrastructure in fruit-body primordia of the basidiomycetesSchizophyllum commune andCoprinus cinereus

Summary Light and electron microscopic investigations of fruit-body primordia of the basidiomycetesSchizophyllum commune andCoprinus cinereus showed that hyphal fusions are common in specific areas of the fruit-body primordia of both species. In these areas conspicuous amounts of a mucilaginous substance occur between the closely packed hyphae. A possible function of this mucilage is to aid the adhesion of hyphae prior to fusion. In the fruit-body primordium ofCoprinus cinereus the dolipore/parenthesome septum is surrounded by an additional hemispherical membranous cap (outer cap). Such a cap was never found inSchizophyllum commune fruit-body primordia.     

GROWTH PATTERNS AND MOTILITY OF SPIROGYRA SP. AND CLOSTERIUM ACEROSUM1,2

Spirogyra and Closterium exhibit active motility. This motility is associated with the secretion of pectic mucilage from the cells. The gliding of these cells is not directed toward light but photosynthesis is the energy source for it. The secretion of mucilage causes older Closterium cultures to become thick gelatinous clusters. Spirogyra filaments when undisturbed grow to form thick multistranded rings. This growth pattern might result from the tendency of the filaments to rotate on their long axis.     

Immunogold-cytochemical labelling of β-glucosidase in the white-rot fungus Coriolus versicolor

Summary Immunogold cytochemical labelling of hyphal sections of Coriolus versicolor showed that -glucosidase was localised in the extracellular mucilage, cell wall layers and cell interior in hyphae grown on glucose-rich malt extract medium whereas in hyphae grown with carboxymethylcellulose (CMC) as sole carbon source, most labelling was in the cell wall layers and cell interior. Little mucilage was visible around hyphae from these cultures. Hyphae from beechwood cultures showed gold labelling of -glucosidase in mucilage and fungal cell walls with some intracellular labelling. Biochemical studies of enzyme activity showed that similar amounts of enzyme were detected in the growth medium when cultures were grown on CMC medium, in agitated liquid cultures or in stationary cultures. In agitated cultures grown on glucose-rich malt extract, the activity of -glucosidase in the medium was 100 times less than that detected in stationary cultures on the same medium. However activity in the hyphae of stationary CMC-grown cultures was similar to that in hyphae from stationary glucose-rich cultures. These data confirm the patterns of gold labelling observed in hyphae from stationary cultures on glucose-rich malt extract when -glucosidase was immobilised in the extracellular mucilage layer around the hyphae. In this paper we propose that a primary function of the extracellular mucilage produced by hyphae of C. versicolor in vivo is to serve as a matrix for immobilisation of -glucosidase. Its substrate, cellobiose, which is released as a result of endo-and exoglucanase hydrolysis of cellulose, is absorbed and retained by the gel filtration properties of the mucilage, so encountering the immobilised -glucosidase. Glucose produced by this reaction is retained within the mucilage matrix around the hyphae before intracellular absorption.Offprint requests to: C. S. Evans     

Enzymatic studies of the extracellular mucilage of two aquatic hyphomycetes,Lemonniera aquatica andMycocentrospora filiformis

The effect of three carbohydrate-digesting enzymes, β-glucuronidase, lyticase and α-mannosidase and three proteolytic enzymes, α-chymotrypsin, papain and pronase E, on the strength of conidial attachment ofLemonniera aquatica andMycoentrospora filiformis was determined using the LH_Fowler cell Adhesion Measurement Module. Carbohydrate-digesting enzyme treatments showed significant differences in number of attached and detached, conidia versus control samples; little or no effect was observed for the proteolytic enzymes. Scanning and transmission electron microscopy showed different degrees of mucilage digestion by the carbohydrate-digesting enzymes on the germ hyphae, hyphae subtending appressoria, and appressoria of the two species. The loss of mucilage integrity and decrease in mucilage thickness were more pronounced on the hyphal sheaths than on the appressorial sheaths. Lyticase caused the most severe damage to the mucilage and cytoplasm of both fungi, particularlyL. aquatica. β-Glucuronidase and α-mannosidase exhibited more effective mucilage digestion onM. filiformis than onL. aquatica. Results indicate that the mucilage of the two species is mainly polysaccharide, containing more β-1,3-glucans than β-glucuronide and α-mannosyl residues. Variability of mucilage composition exists between these species and also between different structures of the same fungus.     

ANALYSIS OF MUCILAGE SECRETION AND EXCRETION IN MICRASTERIAS (CHLOROPHYTA) BY MEANS OF IMMUNOELECTRON MICROSCOPY AND DIGITAL TIME LAPSE VIDEO MICROSCOPY1

Two different, independent, and alternative modes of mucilage excretion were found in the unicellular green alga Micrasterias denticulata Bréb. under constant culture conditions. The cells were capable of either excreting mucilage over all their cell surface or they extruded mucilage from one of their polar ends, which enabled directed movement such as photoorientation or escape from unfavorable environmental conditions. By means of a polyclonal antibody raised against Micrasterias mucilage, the secretory pathway of Golgi derived mucilage vesicles from their origin to their discharge was analyzed by means of conventional and energy filtering TEM. Depending on the stage of the cell cycle, mucilage vesicles were subjected to maturation processes. This may occur either after they have been pinched off from the dictyosomes (e.g. during cell growth) or when still connected to trans‐Golgi cisternae, as in the case of interphase cells. Only fully grown mature vesicles contained mucilage in its final composition as indicated by antibody labeling. After fusion of mucilage vesicles with vacuoles, no immunolabeling was found in vacuoles, indicating that the vesicle content was digested. Mucilage vesicles fused with the plasma membrane in areas of cell wall pores but were also able to excrete mucilage at any site directly through the respective cell wall layer. This result disproves earlier assumptions that the pore apparatus in desmids are the only mucilage excreting areas at the cell surface. Both mechanisms, excretion through the pores and through the cell wall, lead to formation of mucilage envelopes covering the entire cell surface.     

THE MORPHOGENESIS AND POSSIBLE EVOLUTIONARY ORIGINS OF FUNGAL SCLEROTIA

1. Fungal sclerotia are able to survive adverse conditions for long periods and they are formed by many important plant pathogens. An understanding of the factors involved in their initiation and development may lead to a method of repressing their formation in nature, thereby reducing the chances of survival of fungi that depend on them as persistent resting stages in their life-cycles. Also, data on sclerotial morphogenesis may be applicable to other multihyphal fungal structures. 2. There are three types of sclerotial development. The most primitive and least common is the loose type, which is illustrated by Rhizoctonia solani. The sclerotium forms by irregular branching of the mycelium followed by intercalary septation and hyphal swelling. When mature, it consists of loosely interwoven hyphae that are rich in food reserves and darkly pigmented. The main types of development are terminal and lateral. The former develops from the coalescence of initials that are produced by a well-defined pattern of branching at the tip of a hypha or tips of closely associated hyphae, e.g. Botrytis cinerea. Lateral sclerotia are formed by the interweaving of side branches of one or several main hyphae. When only one main hypha is involved the sclerotium is of the lateral, simple type, e.g. Sclerotinia gladioli. If several main hyphae give rise to a sclerotium, the term strand type has been used. Sclerotium rolfsii is the classical example. 3. There is a considerable literature on the effects of environmental conditions on the initiation, development and maturation of sclerotia but few attempts have been made to interpret the data. Phenolics and/or polyphenol oxidases have been found to be connected with morphogenesis of the protoperithecium of Neurospora crassa, the perithecium of Podospora anserina and of Hypomyces sp. and the basidiocarp of Schixophyllum commune. A close correlation has been shown between melanin synthesis and microsclerotial development by Verticillium but there appears to be no literature on the role of phenolics and polyphenol oxidases in the morphogenesis of sclerotia. Possibly these substances may inhibit growth of the apices of main hyphae by changing the permeability of the membrane, by inducing a thickening of the cell wall at the tip or by reducing the plasticity of the wall. Such a check in growth could trigger-off the formation of initials close to the margin of the colony or elsewhere in the culture. Sulphydryl groups and disulphide bonds are of great significance in morphogenesis of organisms and are probably involved in sclerotial initiation. The formation of a large number of hyphal branches is a prerequisite for sclerotial initiation and mycelial branching is possible only if there is plasticity of hyphal walls. The ability of the wall to be moulded is possibly related to changes in the sulphur linkages of the protein of the protein-carbohydrate complexes of the cell wall and could be influenced by sulphur availability or the activity of specific enzymes. 4. After a sclerotial primordium has been initiated, further increase in size will depend on the continued, active translocation of nutrients to the site of development. Movement of nutrients to sclerotia is through a few translocatory hyphae. Presumably, nutrients will continue to move into the young sclerotium as long as a concentration or pressure gradient is maintained. Energy and substances for the formation of new branches are supplied in this way and as the requirements for hyphal branches are reduced, excess nutrients become available for conversion to inactive or insoluble reserves and for exudation. The exudates are often complex, consisting of proteins, including enzymes, lipids and carbohydrates. Many sclerotia have a mucilaginous matrix in which the medullary hyphae are embedded. Sclerotium-forming, fungal species that are not regarded as having such a matrix appear to secrete a layer of mucilage over the surface of sclerotial hyphae. This mucilage could have a morphogenetic function and serve as an adhesive which loosely binds hyphae together. More permanent unions are by hyphal fusions or anastomoses. 5. The sclerotium matures within a few days of attaining its maximum size. The rind effectively seals off the medullary hyphae from the surroundings and the translocatory hyphae cease to function. Thus the sclerotium is isolated both physiologically and nutritionally. The endogenous reserves enable the structure to exist in the absence of exogenous nutrients and then, when conditions become suitable, to germinate. 6. The sclerotium appears to provide an example of convergent evolution whereby analogous structures, which have become adapted to resist adverse conditions, have evolved. Data are available mainly for Typhula spp. and ScZerotinia spp. Sclerotia may be degenerate sexual reproductive structures, hyphal aggregates that have developed from closely interwoven conidiophores and undifferentiated conidia or they may be modified vegetative structures.     

ACTIVE GLIDING MOTILITY IN AN ARAPHID MARINE DIATOM,ARDISSONEA (FORMERLY SYNEDRA) CRYSTALLINA1

Active gliding movement over long distances was observed and filmed in the marine pennate diatom Ardissonea (Synedra) crystallina (Agardh) Kütz. Typical speeds measured ca. 1–2 μm-s^?1. Motion wax often smooth and steady; however, discontinuous jerky motions and rolling movements were common. Motion, was associated with secretion of twin or, less commonly, single straight trails of mucilage from one end of the cell. In a few instances, reversal in direction was related to cessation of mucilage secretion at one end and commencement at the other. Temporary cessation of movement due to an obstruction was accompanied by a build-up of mucilage at one end of the cell. Mucilage was apparently secreted at two specific sites at each end of the cell and was stained by alcian blue. Persistent trails were visible under scanning electron microscopy (SEM). SEM confirmed that cells had no raphes or labiate processes. The apparent site of secretion was a deep groove formed at the junction of the valve and valvocopula (first girdle band) at each end of the cell. Transmission electron microscopy confirmed the presence of mucilage vesicles in the cytoplasm, but these were not in any manner obviously related to secretion nor was any morphological structure associated with secretion. Cells often become epiphytic through secretion of a terminal stipe. Both stipe secretion and movement may involve the same structural differentiation of the frustule. These results demonstrate a previously unrecorded type of diatom motility. The mechanism, involves mucilage secretion and appears similar to that seen, for example, in some other algae such as the desmids (green algae).     

Mucilage in Cacti: Its Apoplastic Capacitance, Associated Solutes, and Influence on Tissue 5

Mucilage content in the stems of four sympatric cactus speciesvaried from none for Ferocactus acanthodes, 19% by dry weightfor Opuntia basilaris, 26% for Opuntia acanthocarpa, and 35%for Echinocereus engelmannii. Although the mucilage differedchemically among the species (the arabinose content ranged from17% to 51% of the sugar monomers), its relative capacitance(change in relative water content per unit change in water potential)remained about 15 Mpa^–1. The relative capacitance of thewater-storage parenchyma averaged 1·04 Mpa^–1 andwas consistent with the mucilage content, being lowest for F.acanthodes and highest for E. engelmannii. Mucilage isolatedfrom hydrated tissue was accompanied by solutes with an osmoticpressure of about 0·2 MPa. Such associated solutes influencethe water-release characteristics of mucilage and hence itsrole as an apoplastic capacitor. In particular, extracellularsolutes can facilitate the release of appreciable mucilage-boundwater to the cells at tissue water potentials occurring duringthe initial phases of drought. Key words: Echinocereus engelmannii, Ferocactus acanthodes, Opuntia acanthocarpa, Opuntia basilaris, water potential isotherms     

ASSOCIATION OF MUCILAGE WITH THE LIGULE OF SEVERAL SPECIES OF SELAGINELLA

Mucilage bodies of protein and some polysaccharide were closely associated with ligules of Selaginella kraussiana (Kunze) A. Braun grown in the greenhouse, and Selaginella wallacei Hieron. and Selaginella oregana D.C. Eat. from natural habitats. With S. wallacei and S. oregana, several mucilage bodies were associated with some ligules. No mucilaginous material was associated with the ligules of Selaginella douglasii Hook, et Grev., Selaginella densa Rydb. or Selaginella apoda (L.) Fernald from natural habitats. When stem tips of S. kraussiana and S. douglasii were placed in sterile culture on a medium with sucrose, massive amounts of mucilage appeared around the ligules in the tight interstices of the stem tip leaves and extended out on the surface of the leaves. The mucilage from a cultured stem tip of S. kraussiana contained 0.034 μgm of protein, 14.6 μgm of apparent glucose equivalents as polysaccharide and no significant amounts of phenolics. In stem tips with tightly appressed leaves, either ligules or young leaves could have produced the mucilage; however, in more mature regions of the stem where internodal elongation had separated the leaves, mucilage coated only the apical portions of the ligules and was not associated with leaves. No other possible source of mucilage (bacteria or fungi) was detected among the leaves. When stem tips of S. wallacei, S. densa and S. apoda were placed in culture, mucilage was not produced. Since mucilage is closely associated with the ligule of some species of Selaginella growing in nature, the greenhouse, or tissue culture, the ligule may be glandular.     

CELLULAR MOVEMENT IN THE CENTRIC DIATOM ODONTELLA SINENIS1

Cultured, actively growing cells of Odentella sinensis secrete mucilage, forming gelatinous masses; the mucilage can be visualised with Alcian Blue. When examined live with the light microscope, many cells exhibited continuous small shuffing and rocking movements that could last for long periods (30-40 min); the cells, however, were not translocated and remained relatively fixed in position with respect to their neighbours. Ultrastructural examination of these cells showed prominent aggregations of mucilage vesicles, derived from the Golgi bodies, at the base of the labiate processes, each of which is close to an elevation bearing an ocellus. In Ditylum brightwellii, similar aggreations of these vesides were also located at teh labiate processes; this diatom, too, secretes mucilage but does not have ocelli. We conclude that the movements observed in O. sinenisis are an indirect result of active muilage secretion through the labiate process. It has been suggested that the raphe may have evolved from the labiate process; our conclusion, therefore, has phylogenetic implications, suggesting a functional as well as a morphological relationship between the two valve structures.     

THE DEVELOPMENT OF MUCILAGINOUS RAPHIDE CRYSTAL IDIOBLASTS IN YOUNG LEAVES OF TYPHA ANGUSTIFOLIA L. (TYPHACEAE)

Raphide crystal idioblast initiation occurs in the uppermost region of intercalary meristems in young leaves of Typha angustifolia L., and development proceeds acropetally. Idioblast differentiation commences with a loss of stored lipids, depletion of starch from amyloplasts, enlargement of the nucleus and nucleolus, cell elongation, and the formation of a central vacuole. Crystalloplastids are formed via dedifferentiation of amyloplasts, followed by an increase in plastid number as cell volume increases with cell elongation. Crystalloplastid membranes stain intensely with periodic acid-thiocarbohydrazide-silver proteinate (PA-TCH-SP). Following crystal production within the central vacuole, crystalloplastids differentiate lobed regions, dense with plastid ribosomes, thylakoids, lamellae, and plastoglobuli. Mucilage, which stains with PA-TCH-SP, appears to be formed at the tonoplast in the central vacuole and follows differentiation of crystalloplastid lobes. Crystal chambers are surrounded by lamellae during mucilage accumulation and the crystals undergo a change in shape. Lobed crystalloplastids may be involved in vacuolar mucilage formation in these types of raphide crystal idioblasts.     

Leaflet secretory structures of five taxa of the genus Zornia J.F. Gmel. (Leguminosae, Papilionoideae, Dalbergieae) and their systematic significance

A survey of the secretory structures of leaflets has been carried out for five taxa of the genus Zornia J.F. Gmel. to assess possible taxonomic value of the glands, taking into account that these taxa, Zornia curvata Mohlenbr., Z. gemella Willd. ex Vogel, Z. glabra Desv., Z. latifolia Sm. and Z. reticulata Sm., have overlapping diagnostic characters. The leaflet secretory structures of the five studied taxa of Zornia were mucilage epidermis, mucilage cavities, and idioblasts secreting phenolic compounds. Mucilage epidermis is found in all five aforementioned taxa. Mucilage cavities are observed on both epidermal surfaces of the leaflets in four taxa, the exception being Z. glabra, whose cavities occur only on the abaxial surface. Idioblasts secreting phenolic compounds were detected only in the mesophyll of Z. latifolia and Z. reticulata. The joint occurrence of mucilage epidermis with mucilage cavities seems to be of unifying value for the five taxa analyzed. The position of mucilage cavities and the occurrence of secretory idioblasts in the mesophyll of the leaflets are of diagnostic value, providing a key to enable identification of the taxa studied. This study reveals that the secretory structures provide important information in support of systematic studies of the Leguminosae. In relation to the five taxa of Zornia studied, current results suggest recognition of four species: Z. curvata, Z. glabra, Z. latifolia, and Z. reticulata.     

Toward understanding the ecological role of mucilage in seed germination of a desert shrub Henophyton deserti: interactive effects of temperature, salinity and osmotic stress

Aims

Seeds of Henophyton deserti (Brassicaceae), an endemic saharan shrub in south Tunisia, produce a pectinaceous mucilage layer that can imbibe a large amount of water when wetted. The aim of this study was to explore the role of mucilage in seed germination of this shrub under heterogeneous stressful environments.

Methods

Germination of both intact and demucilaged seeds was tested over wide ranges of temperature, and in iso-osmotic solutions of NaCl and PEG. Recovery of germination after NaCl and Polyethylene Glycol (PEG)-6000 treatment was also tested. The effect of mucilage on water uptake was measured and the structure of the seed investigated.

Results

A considerable proportion of seed mass (30 %) is made up of mucilage, which is extremely hydrophilic and able to increase seed mass by 550 % over dry seeds. Mucilage water uptake appears to be unaffected by salt concentration, while higher concentrations of PEG inhibit mucilage hydration. Mucilage decreases germination specifically at 10 °C and this effect can be interpreted in relation to oxygen uptake. High concentrations of NaCl and PEG decrease both germination percentage and rate, with some greater tolerance at 15 °C and 20 °C versus 25 °C. Recovery was higher from higher concentrations of NaCl and PEG and lower temperatures, with a clear inhibitory effect of mucilage.

Conclusions

The study has shown that the mucilage of H. deserti may act as a physical barrier for regulating diffusion of water and oxygen to the inner tissue of the seed and thereby prevent germination under unsuitable conditions.     

A highly differentiated glomeromycotean association with the mucilage-secreting, primitive antipodean liverwort Treubia (Treubiaceae): clues to the origins of mycorrhizas

Thallus anatomy in three species of the primitive liverwort genus Treubia (Metzgeriidae, Treubiales) was studied by light and electron microscopy. The thallus exudes copious mucilage, a feature shared elsewhere in liverworts only with the mycotrophic subterranean axes of the allied genus Haplomitrium. The central strand in the thallus midrib has a unique histological organization and harbors an intra- and intercellular infection by a glomeromycotean fungus that is far more highly differentiated than most of the glomeromycotean associations described to date. The fungus enters the thallus via clefts in the ventral epidermis along the midrib and colonizes the parenchyma above, forming intracellular coils and prominent, relatively short-lived, hyphal swellings. Above the zone with intracellular colonization is a tissue area containing mucilage-filled intercellular spaces; here the fungus is entirely intercellular and forms abundant pseudoparenchymatous structures and, in more mature parts of the thalli, large hyphae with thick multistratose walls. Mucilage in Treubia differs in histochemistry and origin from that produced by apical papillae, via hypertrophied Golgi, in all other bryophytes. Remarkable parallels between fungal associations in Treubia, Haplomitrium, and Lycopodium, all members of very ancient lineages, suggest that these associations epitomize very early stages in the evolution of glomeromycotean symbioses.     

Degradation of seed mucilage by soil microflora promotes early seedling growth of a desert sand dune plant

In contrast to the extensive understanding of seed mucilage biosynthesis, much less is known about how mucilage is biodegraded and what role it plays in the soil where seeds germinate. We studied seed mucilage biodegradation by a natural microbial community. High‐performance anion‐exchange chromatography (HPAEC) was used to determine monosaccharide composition in achene mucilage of Artemisia sphaerocephala. Mucilage degradation by the soil microbial community from natural habitats was examined by monosaccharide utilization tests using Biolog plates, chemical assays and phospholipid fatty acid (PLFA) analysis. Glucose (29.4%), mannose (20.3%) and arabinose (19.5%) were found to be the main components of achene mucilage. The mucilage was biodegraded to CO₂ and soluble sugars, and an increase in soil microbial biomass was observed during biodegradation. Fluorescence microscopy showed the presence of mucilage (or its derivatives) in seedling tissues after growth with fluorescein isothiocyanate (FITC)‐labelled mucilage. The biodegradation also promoted early seedling growth in barren sand dunes, which was associated with a large soil microbial community that supplies substances promoting seedling establishment. We conclude that biodegradation of seed mucilage can play an ecologically important role in the life cycles of plants especially in harsh desert environments to which A. sphaerocephala is well‐adapted.     

MUCILAGE PROCESSING AND SECRETION IN THE GREEN ALGA CLOSTERIUM. I. CYTOLOGY AND BIOCHEMISTRY1

Placoderm desmids (Conjugates, Chlorophyta) such as Closterium exhibit a gliding locomotory behavior. This results from the forceful extrusion of an acidic polysaccharide from one pole of the cell causing the cell to glide in the opposite direction. A biochemical and cytological analysis of gliding behavior was performed. The mucilage is a high molecular weight polysaccharide rich in glucuronic acid and fucose. Under normal growth conditions, 3 μg of mucilage is produced per cell in 30 days. Mucilage production increased 3–4 fold in cells challenged with low phosphate or nitrate conditions. A polyclonal antibody was raised against the mucilage and used in immunofluorescence studies. These results show that upon contact with another object Closterium aligns itself parallel to that object by a “jack-knife” motion. Subsequently, large amounts of mucilage are released to form elongate tubes enmeshing the cell with that object. In post-cytokinetic phases of the cell cycle, mucilage is extruded only through the pole of the developing semi-cell. Chlorotetracyclene-labeling of mucilage-secreting cells shows a correlation between calcium-rich loci on the cell surface and sites of mucilage release.     

Labellar anatomy and secretion in Bulbophyllum Thouars (Orchidaceae: Bulbophyllinae) sect. Racemosae Benth. & Hook. f.

Background and Aims

Floral secretions are common in Bulbophyllum Thouars, and the labella of a number of Asian species are said to produce secretions rich in lipids that act as food rewards for insect pollinators. Although some of these reports are based on simple histochemical tests, a much greater number are anecdotal and, hitherto, neither the ultrastructure of the labellum nor the secretory process has been investigated in detail. Furthermore, sophisticated histochemical approaches have generally not been applied. Here, both the labellar structure and the secretory process are investigated for four species of Asian Bulbophyllum sect. Racemosae Benth. & Hook. f., namely Bulbophyllum careyanum (Hook.) Spreng., B. morphologorum Kraenzl., B. orientale Seidenf. and B. wangkaense Seidenf., and compared with those of unequivocal lipid-secreting orchids.

Methods

Labellar, secretory tissue was investigated using light microscopy, scanning electron microscopy, transmission electron microscopy and histochemistry.

Key Results

The adaxial median longitudinal groove of the labellum contained secretory tissue comprising palisade-like epidermal cells, similar to those of certain lipid-secreting Oncidiinae Benth. However, these cells and their secretions gave positive results mainly for protein and mucilage, and their organelle complement was consistent with that of cells involved in protein and mucilage synthesis. Sub-cuticular accumulation of secretion resulted in cuticular distension and blistering. The sub-epidermal layer of isodiametric parenchyma contained starch and, like the epidermal cells, ultrastructure consistent with mucilage synthesis. Lipids were mainly confined to the cuticle, and hardly any intracellular lipid droplets were observed.

Conclusions

It is proposed that mucilage is produced by dictyosomes present in the palisade-like epidermal cells. Mucilage precursors may also be produced by these same organelles in sub-epidermal cells and are thought to pass along the symplast via plasmodesmata into the adjoining palisade-like secretory cells, which contain abundant arrays of rough endoplasmic reticulum. Here, they become chemically modified and form a protein-rich, mucilaginous secretion that, following vesicle-mediated transport across the cytoplasm, traverses the cell wall and accumulates in blisters formed from the distended cuticle. Rupture of these blisters releases the secretion onto the labellar surface. However, in certain species, there is some evidence that the secretion may traverse the cuticle via cuticular pores, and micro-channels may permit the passage of fragrance. Hydrolysis of sub-epidermal starch probably generates the carbohydrate and, together with mitochondria, much of the energy required for the secretory process. This anatomical organization resembles that found in certain lipid-secreting, Neotropical species of Bulbophyllum and Oncidiinae, but since the chemical composition of their secretions is different, and these taxa occur on a separate continent and have different insect pollinators, parallelism of floral anatomy is likely.     

Arbuscular mycorrhizal structure and fungi associated with mosses

We investigated the colonization and diversity of arbuscular mycorrhizal (AM) fungi associated with 24 moss species belonging to 16 families in China. AM fungal structures, i.e. spores, vesicles, hyphal coils (including intracellular hyphae), or intercellular nonseptate hyphae, were found in 21 moss species. AM fungal structures (vesicles, hyphal coils, and intercellular nonseptate hyphae) were present in tissues of 14 moss species, and spores and nonseptate hyphae on the surface of gametophytes occurred in 15 species. AM fungal structures were present in 11 of the 12 saxicolous moss species and in six of the ten terricolous moss species, but absent in two epixylous moss species. AM fungal structures were only observed in moss stem and leaf tissues, but not in rhizoids. A total of 15 AM fungal taxa were isolated based on trap culture with clover, using 13 moss species as inocula. Of these AM fungi, 11 belonged to Glomus, two to Acaulospora, one to Gigaspora, and one to Paraglomus. Our results suggest that AM fungal structures commonly occur in most mosses and that diverse AM fungi, particularly Glomus species, are associated with mosses.     

Oldest fossil basidiomycete clamp connections

A rachis of the fossil filicalean fern Botryopteris antiqua containing abundant septate hyphae with clamp connections is preserved in a late Visean (Mississippian; ~330 Ma) chert from Esnost (Autun Basin) in central France. Largely unbranched tubular hyphae pass from cell to cell, but may sometimes produce a branch from a clamp connection. Other clamp-bearing hyphae occur clustered in individual cells or small groups of adjacent host cells. These hyphae may be tubular, catenulate with numerous hyphal swellings, or they may display a combination of both. The Visean hyphae with clamp connections predate Palaeancistrus martinii, the heretofore oldest direct fossil evidence of Basidiomycota, by some 25 Ma.     

Anastomosis Formation and Nuclear and Protoplasmic Exchange in Arbuscular Mycorrhizal Fungi

We observed anastomosis between hyphae originating from the same spore and from different spores of the same isolate of the arbuscular mycorrhizal fungi Glomus mosseae, Glomus caledonium, and Glomus intraradices. The percentage of contacts leading to anastomosis ranged from 35 to 69% in hyphae from the same germling and from 34 to 90% in hyphae from different germlings. The number of anastomoses ranged from 0.6 to 1.3 per cm (length) of hyphae in mycelia originating from the same spore. No anastomoses were observed between hyphae from the same or different germlings of Gigaspora rosea and Scutellospora castanea; no interspecific or intergeneric hyphal fusions were observed. We monitored anastomosis formation with time-lapse and video-enhanced light microscopy. We observed complete fusion of hyphal walls and the migration of a mass of particles in both directions within the hyphal bridges. In hyphal bridges of G. caledonium, light-opaque particles moved at the speed of 1.8 ± 0.06 μm/s. We observed nuclear migration between hyphae of the same germling and between hyphae belonging to different germlings of the same isolate of three Glomus species. Our work suggests that genetic exchange may occur through intermingling of nuclei during anastomosis formation and opens the way to studies of vegetative compatibility in natural populations of arbuscular mycorrhizal fungi.