Non Nutritional Environmental Variables

As well as carbon and nitrogen nutrition, discussed above, many more environmental variables affect fruit body initiation and development (reviewed by Jennings, 1995; Moore, 1998a; Scrase and Elliott, 1998; Kues and Liu, 2000). Such is the bulk of the literature that we can do little more here than list the major observations.

As the above discussions of metabolism imply, fruit body development requires oxidative metabolism (glycolysis and TCA cycle activity are often amplified) and good aeration is, not surprisingly, associated with successful fruiting. This means not only oxygen but also various volatile metabolites including carbon dioxide. Elevated carbon dioxide concentrations can suppress basidiome initiation in S. commune (Raudaskoski and Salonen, 1984). In Agaricus, increased elongation of the stem occurs with elevated CO2, accumulated naturally from respiration, whereas cap and gills expand and spores mature more rapidly when CO2 is removed (Turner, 1977). It has been argued that the morphogenetic effect on maturation of the fruit body may have ecological advantage: CO2-enhanced elongation of the stem would raise the gills away from the surface of the substratum where the concentration of CO2 might be expected to be higher than in the wider atmosphere because of the respiratory activity of microorganisms in the casing soil (Turner, 1977).

High CO2 levels promote formation of long hyphal compartments in S. commune. It has been argued (Raudaskoski and Salonen, 1984) that a wood decomposer like S. commune is likely to experience elevated CO2 within the wood as respiratory CO2 accumulates. Mycelium that reaches the surface of the wood, however, will be exposed to CO2 reduced to the atmospheric normal. Such mycelium will be able to form the shortened cells and more compact branching habit, and be predisposed to fruit body formation.

Light has diverse effects on formation of reproductive structures in different basidiomycetes, increasing or decreasing their number, affecting their development or determining whether or not they are produced (Carlile, 1970; Elliott, 1994). In general, the most effective parts of the spectrum are the near-ultraviolet and blue wavelengths, typical of the shaded and litter-covered forest floor. There are indications that the photoreceptor involved in fruit body morphogenesis may be membrane bound. In some fungi levels of intermediary metabolites and coenzymes, and activities of several enzymes respond very rapidly to changes in illumination. The vegetative mycelia of many Ascomycota require exposure to light before they will produce fruit bodies and/or asexual spores, and show specificity not only for particular wavelengths but also for a particular dosage of light radiation. In some Basidiomycota, sequential light exposures are responsible for initiating and programming fruit body morphogenesis, and periods of darkness between illumination events are important. Again, blue (400-520 nm) to near-ultraviolet (320-400 nm) light is the most effective and the work suggests that at least two photosensitive systems operate in fungi, one stimulated by near-ultraviolet and the other by blue light. Because their absorption spectra parallel the action spectra of the blue light photoresponses, carotenes and flavins appear to be the best candidates for photoreceptors.

Production of fruit bodies in vitro typically occurs over a more restricted range of temperature than that which will support mycelial growth. Optimum temperatures for fruit body production are generally lower than those most favourable for mycelial growth. In Basidiomycota most information relates to species adopted as laboratory models or for commercial cultivation. Cultivated species frequently need a temperature downshift (by 5-10 °C) and lower CO2 concentrations for fruiting, e.g. A. bisporus, C. cinereus, Flammulina velutipes, Kuehneromyces mutabilis, Lentinula edodes, Pholiota nameko, Pleurotus ostreatus, Stropharia rugosa-annulata and V volvacea (Chang and Hayes, 1978; Stamets, 1993). This list includes compost-grown fungi as well as some wood-chip/straw and log-grown wood decomposers, and is not unrepresentative of the wider community of saprotrophic fungi, so it may be that most Basidiomycota require a temperature downshift. A prolonged downshift is not always required; thus, fruit body initiation in F. velutipes, which fruits in nature during late autumn to spring, occurs at a continuous regime of 20 °C or following 12 h at 15 °C (Kinugawa and Furukawa, 1965). Interestingly, the optimum temperature for both mycelial growth and production of fruit body initials by A. bisporus is 24°C (Flegg, 1972, 1978a, 1978b). However, temperature downshift is required for further development of initials beyond a cap diameter of ^2 mm.The fruit bodies develop normally when the temperature is lowered to 16°C. So, as with the reaction to nitrogen sources mentioned above, the implication is that formation of fruit body initials/primordia is an aspect of mycelial growth, but their proper development requires a further morphogenetic switch. It is tempting to conclude that these in vitro responses reflect the organism's natural response to seasonal changes.

Relative humidity (RH) affects fruit body initiation. Relatively high humidity is usually conducive to initiation of fruiting (Stamets, 1993), though it prevents initiation in Polyporus ciliatus (Plunkett, 1956). The water content of the resource may be even more critical. There is a balance between too high a water content that reduces aeration and too low a water potential that provides insufficient water for development (Scrase and Elliott, 1998; Ohga, 1999a; Kashangura et al., 2006). There is variability between strains; Pleurotus sajor-caju was able to produce primordia at —2.5 MPa but none at —3.5 MPa even though they were able to grow under these xeric conditions (Kashangura et al., 2006). pH can affect fruit body development, being optimal for several species at 6-7 (Kiies and Liu, 2000), but pH 4 for L. edodes (Ohga, 1999b).

Physical constraints influence fruit body formation in vitro. Sexual reproduction is often initiated when the growing mycelium reaches an obstacle such as the edge of the dish or barriers placed onto the surface of the medium (the 'edge effect' or 'check to growth'). Reproductive structures often arise when mycelial growth had been arrested, by either physical or chemical means (Moore, 1998a). A physical barrier is not absolutely necessary for the 'edge effect', rather the important determining factor is the disturbance in metabolism which results from either encountering the edge of the dish or a major change in nutritional value of the substrate. Thus, different sorts of barrier and different sorts of medium transition are able to disturb the progress of metabolism sufficiently to initiate fruit body formation. The same applies to physical injury to the mycelium, which can stimulate fruit body formation (Leslie and Leonard, 1979a). Fruiting response to mechanical injury in S. commune is determined by at least four genes (Leslie and Leonard, 1979a, 1979b), showing that a number of different parallel routes lead to fruit body formation.

Inter- and intraspecific interactions can stimulate reproductive development. In interactions with other fungi this is at least partly a result of damage to vegetative hyphae (Rayner and Boddy, 1988). Many A. bisporus strains fruit only when associated with bacteria, e.g. pseudomonads, apparently not due to production of stimulatory compounds but to removal of inhibitory compounds (De Groot et al., 1998). When competing with C. cinereus in agar culture, C. congregatus fruited from a much smaller resource volume than when growing alone (Schmit, 1999). In contrast, interactions can result in a fungus being confined to territory, e.g. a decay column in wood, that is too small to support fruit body production by that species. Fruit bodies are assembled from contributions of a number of cooperating hyphal systems, usually of the same individual. Hyphal interactions are controlled by the somatic and mating incompatibility systems (Chiu and Moore, 1999) that maintain mycelial individuality. Fruit bodies of somatically compatible Basidiomycota can fuse when the fruit bodies develop in extremely close proximity, as is commonly seen when resupinate fruit bodies meet on wood, and also with stipitate basidiomata, e.g. a fused cap with three stems of Boletus (Xerocomus) chrysenteron in Kibby (2006). However, hyphal cooperation is so fundamental that it can even lead to the formation of chimeric fruit bodies. Mixed cultures of two genetically different heterokaryons can produce basidio-mata comprising both dikaryons, as seen with P. nameko (Babasaki et al., 2003). Even more extreme is the case of fruit bodies of Coprinus consisting of two different species, C. miser and C. pellucidus (Kemp, 1977). The hymenium comprised a mixed population of basidia bearing the distinctive spores of the two species but the chimera extended throughout the fruit body as both species could be recovered by outgrowth from stem segments. All of these features can be interpreted as aspects of the tolerance of imprecision in fungal morphogenesis which has been discussed elsewhere (Moore, 1998a, 1998b, 2005; Moore et al., 1998).

Once fruit bodies have been produced environment, particularly temperature and RH, can affect spore production. For example, in the field spore production by Hericium erinaceus is highest at about midday reflecting diurnal temperature and

RH (McCracken, 1970). In the laboratory, at 85-95% RH, spore production increased from a minimum at 0°C to a maximum at 24-27°C, and ceased at 31-33°C. At 20°C, sporulation was greater at 30% RH than at 90% RH (McCracken, 1970).

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