Kinetics and mechanics of stem gravitropism in Coprinus cinereus

Gene expression profiling reveals large regulatory switches between succeeding stipe stages in Volvariella volvacea

The edible mushroom Volvariella volvacea is an important crop in Southeast Asia and is predominantly harvested in the egg stage. One of the main factors that negatively affect its yield and value is the rapid transition from the egg to the elongation stage, which has a decreased commodity value and shelf life. To improve our understanding of the changes during stipe development and the transition from egg to elongation stage in particular, we analyzed gene transcription in stipe tissue of V. volvacea using 3′-tag based digital expression profiling. Stipe development turned out to be fairly complex with high numbers of expressed genes, and regulation of stage differences is mediated mainly by changes in expression levels of genes, rather than on/off modulation. Most explicit is the strong up-regulation of cell division from button to egg, and the very strong down-regulation hereof from egg to elongation, that continues in the maturation stage. Button and egg share cell division as means of growth, followed by a major developmental shift towards rapid stipe elongation based on cell extension as demonstrated by inactivation of cell division throughout elongation and maturation. Examination of regulatory genes up-regulated from egg to elongation identified three potential high upstream regulators for this switch. The new insights in stipe dynamics, together with a series of new target genes, will provide a sound base for further studies on the developmental mechanisms of mushroom stipes and the switch from egg to elongation in V. volvacea in particular.

Microscopic observations of the early development of Pleurotus pulmonarius fruit bodies

From observations made by light microscopy, transmission electron microscopy, environmental-scanning and cryoscanning electron microscopy we conclude that the expansion of the young fruit body of Pleurotus pulmonarius involves considerable vacuolation of hyphae but no marked inflation of cell dimensions. There is evidence for an extensive extracellular matrix (ECM), the components of which must be under the control of the hyphae which the ECM surrounds. However the ECM in these fruit bodies is a dilute material. It is easily lost during specimen preparation and is evident only when certain techniques are used to preserve the fluid surface of the hyphae. Observations of the hyphal and fruit body structures with a range of conventional microscopic techniques are crucial to complement the information obtained through physiological and molecular studies for understanding the cellular changes that occur during mushroom development.

Life history and developmental processes in the basidiomycete Coprinus cinereus

Coprinus cinereus has two main types of mycelia, the asexual monokaryon and the sexual dikaryon, formed by fusion of compatible monokaryons. Syngamy (plasmogamy) and karyogamy are spatially and temporally separated, which is typical for basidiomycetous fungi. This property of the dikaryon enables an easy exchange of nuclear partners in further dikaryotic-monokaryotic and dikaryotic-dikaryotic mycelial fusions. Fruiting bodies normally develop on the dikaryon, and the cytological process of fruiting-body development has been described in its principles. Within the specialized basidia, present within the gills of the fruiting bodies, karyogamy occurs in a synchronized manner. It is directly followed by meiosis and by the production of the meiotic basidiospores. The synchrony of karyogamy and meiosis has made the fungus a classical object to study meiotic cytology and recombination. Several genes involved in these processes have been identified. Both monokaryons and dikaryons can form multicellular resting bodies (sclerotia) and different types of mitotic spores, the small uninucleate aerial oidia, and, within submerged mycelium, the large thick-walled chlamydospores. The decision about whether a structure will be formed is made on the basis of environmental signals (light, temperature, humidity, and nutrients). Of the intrinsic factors that control development, the products of the two mating type loci are most important. Mutant complementation and PCR approaches identified further genes which possibly link the two mating-type pathways with each other and with nutritional regulation, for example with the cAMP signaling pathway. Among genes specifically expressed within the fruiting body are those for two galectins, beta-galactoside binding lectins that probably act in hyphal aggregation. These genes serve as molecular markers to study development in wild-type and mutant strains. The isolation of genes for potential non-DNA methyltransferases, needed for tissue formation within the fruiting body, promises the discovery of new signaling pathways, possibly involving secondary fungal metabolites.

Gravitropism of basidiomycetous fungi--on Earth and in microgravity

In order to achieve perfect positioning of their lamellae for spore dispersal, fruiting bodies of higher fungi rely on the omnipresent force gravity. Only accurate negatively gravitropic orientation of the fruiting body cap will guarantee successful reproduction. A spaceflight experiment during the STS-55 Spacelab mission in 1993 confirmed that the factor gravity is employed for spatial orientation. Most likely every hypha in the transition zone between the stipe and the cap region is capable of sensing gravity. Sensing presumably involves slight sedimentation of nuclei which subsequently causes deformation of the net-like arrangement of F-actin filament strands. Hyphal elongation is probably driven by hormone-controlled activation and redistribution of vesicle traffic and vesicle incorporation into the vacuoles and cell walls to subsequently cause increased water uptake and turgor pressure. Stipe bending is achieved by way of differential growth of the flanks of the upper-most stipe region. After reorientation to a horizontal position, elongation of the upper flank hyphae decreases 40% while elongation of the lower flank slightly increases. On the cellular level gravity-stimulated vesicle accumulation was observed in hyphae of the lower flank.

Comparing plant and fungal gravitropism using imitational models based on reiterative computation

Mathematical models which imitate plant gravitropic responses were used to compare plant and fungal gravitropism with kinetic data from the agarics Coprinus cinereus and Flammulina velutipes. Similarities were: bending depends on differential growth; growth of the organ is most intensive just behind the apex; gravitropisms exhibit a substantial time delay. Differences were: the agaric stem apex always returns to the vertical (some plant organs show stable plagiogravitropic growth); curvature compensation occurred in C. cinereus; C. cinereus stems rarely overshot or oscillated around the vertical although data for F. velutipes showed a single overshoot and oscillation. The work focused attention on the need for data on detection-level thresholds, angle-response and acceleration-response relationships in fungi, and the need for detailed observations of gravitropism kinetics in a larger number and wider range of fungi.

Gravitropic bending of fruiting bodies--a model based on hyphal gravisensing and cooperativity

Gravitropic bending of the winter mushroom Flammulina velutipes is achieved by differential growth of the apical part of the stem, the transition zone. Ultrastructural analysis revealed that bending is due to the relaxation of tissue tensions at the lower flank of the stem where hyphal extension growth is promoted in contrast to the upper flank. Extension of lower flank hyphae is preceded by a conspicuous accumulation of microvesicles in the cytosol and their subsequent fusion with the vacuolar compartment, leading to a large volume increase. The hypothesis is put forward that all hyphae in the transition zone are capable of gravisensing. It is derived from experiments with transition zone segments, which exhibit negative gravitropic response independent from their origin within the stem. A model is presented which connects individual gravisensing of the hyphae with a cooperative response within the stem or small segments of the stem. An essential step is the transmission of positional information, by each hypha with respect to the gravitational vector, to the surroundings. The existence of a soluble growth regulator, which is enriched at the lower flank of the stem, is discussed. A gradient could be formed which precedes the gradient of microvesicle formation, and thereby determines the change of growth direction.

Coordinated cell elongation alone drives tropic bending in stems of the mushroom fruit body of Coprinus cinereus

During tropic bending in the stem of the mushroom fruit body of Coprinus cinereus the majority of extension occurred in the upper 20-30% of the stem. By attaching inert markers to the stem, it was shown that the outer flank of the bend initially has a faster rate of extension, although the inner flank matches this growth rate later in the response. Thus bending results from differential enhancement of growth rate rather than sustained differences. Large voids, up to 85 micrometers in diameter, observed in tropically bent stems showed no significant difference in number between inner and outer flanks but are implicated in bending because of their absence from unbent stems. Such voids may prevent the propagation of cracks through the stem tissue during bending. Creases at the external and lumen surfaces were also peculiar to bent stems and could represent constrictions caused by localized accumulation of stresses. Cell morphometric analysis of transverse sections of both flanks of the bend revealed no significant differences in hyphal diameter, distribution, or populations of cell types, but cells of the outer flank were four to five times longer than those of the inner. Thus, tropic bending requires only an increase in length of pre-existing inflated hyphae in the outer flank tissue.

Gravimorphogenesis in agarics

The shape changes which occur in agaric fruit bodies in response to change in the direction of gravity, usually referred to as gravitropism are morphogenetic changes. Our interest in what we prefer to call gravimorphogenesis is to use it to examine morphogenesis experimentally. We are examining two agarics, Coprinus cinereus and Flammulina velutipes, and applying the best available technologies, including video analysis, all forms of electron microscopy, computer-aided image analysis and experiments in orbit in Spacelab. Responses to gravity of the two organisms differ in ways which can be related to their ecological and structural adaptations. C. cinereus reacts extremely rapidly; its fruit body can regain the vertical within 3 h of being placed horizontal, whereas F. velutipes requires 12 h to bend through 90 degrees. The fungi also differ in the bulk of tissue involved in the response. In Coprinus, a zone extending several cm down from the apex is normally involved in bending. In Flammulina, gravisensing is limited to a region just a few mm immediately below the cap, although curvature is performed in a zone of up to 2 cm below. Flammulina cultures were flown on the Spacelab D-2 mission in 1993, and fruit body disorientation in orbit provides the first definitive proof that 'gravitropism' really is a response to the unidirectional gravity vector. Experiments with different clinostat rotation rates in Flammulina indicate that the perception threshold is about 10(-4) x g. Analysis of different times of exposure to an altered gravity vector prior to clinorotation in Coprinus reveals that the perception time is 7 minutes and that continued response requires continued exposure. Cell size determinations in Coprinus demonstrate that cells of the stem increase in length, not diameter, to produce the growth differential. In Flammulina a unique population of highly electron-transparent microvacuoles changes in distribution; decreasing in upper cells and increasing in the lower cells in a horizontal fruit body within a few minutes of disorientation. These are thought to contribute to vacuolar expansion which accompanies/drives cell elongation. Application of a variety of metabolic inhibitors indicates that the secondary messenger calcium is also involved in regulating the growth differentials of gravimorphogenesis but that gravity perception is unaffected by inhibitors of calcium signalling. In both Flammulina and Coprinus, gravity perception seems to be dependent on the actin cytoskeleton since cytochalasin treatment suppresses gravitropic curvature in Flammulina and, in Coprinus, significantly delays curvature without affecting stem extension. This, together with altered nuclear motility observed in living hyphae during reorientation suggests that gravity perception involves statoliths (possibly nuclei) acting on the actin cytoskeleton and triggering specific vesicle/microvacuole release from the endomembrane system.

Morphometric analysis of cell size patterning involved in gravitropic curvature of the stipe of Coprinus cinereus

During gravitropic bending of the stipe of Coprinus cinereus the majority of elongation occurred in the apical region of the lower surface of the stipe, although some elongation was seen throughout the stipe. The final rate of elongation was similar at both the upper and lower stipe surfaces but the lower surface achieved this rate first (close to the reaction time 25 min), whilst the upper surface of the stipe only attained its final elongation rate after a period of acceleration of 150 min. Detailed morphometric analysis of cell size patterning in transverse sections revealed no significant differences in cross sectional area, spatial or proportional distribution of different cell types between the upper and lower regions of the gravitropic bend. Measurements of longitudinal cell size revealed significant differences in compartment size between the lower and upper region. Hyphal compartments of lower regions of the bend were on average four to five times longer than those of the upper region.

Graviresponses in fungi

Although the orientation of mycelial hyphal growth is usually independent of the gravity vector, individual specialised hyphae can show response to gravity. This is exemplified by the sporangiophore of Phycomyces, but the most striking gravitropic reactions occur in mushroom fruit bodies. During the course of development of a mushroom different tropisms predominate at different times; the young fruit body primordium is positively phototropic, but negative gravitropism later predominates. The switch between tropisms has been associated with meiosis. The spore-bearing tissue is positively gravitropic and responds independently of the stem. Bracket polypores do not show tropisms but exhibit gravimorphogenetic responses: disturbance leads to renewal of growth producing an entirely new fruiting structure. Indications from both clinostat and space flown experiments are that the basic form of the mushroom (overall tissue arrangement of stem, cap, gills, hymenium, veil) is established independently of the gravity vector although maturation, and especially commitment to the meiosis-sporulation pathway, requires the normal gravity vector. The gravity perception mechanism is difficult to identify. The latest results suggest that disturbance of cytoskeletal microfilaments is involved in perception (with nuclei possibly being used as statoliths), and Ca2(+)-mediated signal transduction may be involved in directing growth differentials.

The role of calcium accumulation and the cytoskeleton in the perception and response of Coprinus cinereus to gravity

The role of Ca2+ in the gravitropic perception and/or response mechanism of Coprinus cinereus was examined by treating stipes with inhibitors of Ca2+ transport and calmodulin. Inhibitors had no effect on gravity perception but significantly diminished gravitropism. It is concluded that, under the conditions tested, Ca2+ is not involved in gravity perception by Coprinus stipes, but does contribute to transduction of the gravitropic impulse. The results would be consistent with regulation of the gravitropic bending process requiring accumulation of Ca2+ within a membrane-bound compartment. Treatment of stipes with an actin inhibitor caused a significantly delayed response, a result not observed with the Ca2+ inhibitors. This suggests that cytoskeletal elements may be involved directly in perception of gravity by Coprinus stipes while Ca(2+)-mediated signal transduction may be involved in directing growth differentials.

Distribution of mechanical stress is not involved in regulating stipe gravitropism in Coprinus cinereus

Removal of large segments of the apical part of the stipe of Coprinus cinereus (extending to about half its length) affected neither the ability of the stipe to show gravitropic bending nor its ability to compensate the curvature so induced and adjust to the vertical. However, gravitropic reaction time was directly proportional to the amount of stipe removed. Application of lateral loads of up to 20 g had no adverse effects on adjustment of the stipe to the vertical and continued vertical growth. It is concluded that sensing the distribution of extracellular mass and/or mechanical stress is unlikely to be a component of the control of gravitropic bending in C. cinereus stipes.