Antagonists and inhibitors of calcium accumulation do not impair gravity perception though they adversely affect the gravitropic responses of Coprinus cinereus stipes

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.

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.

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.

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.