Volume 10 Part 1 Article 34: Biochemical Changes during Growth and Development of Agaricus bisporus

Volume 10 Part 1 Article 34
Year 1979
Title: Biochemical Changes during Growth and Development of Agaricus bisporus
Author: D.A. Wood

Abstract:

The culture medium used for the production of mycelium and for the subsequent formation of fruit bodies of Agaricus bisporus is normally based on non sterile composted plant residues. Some information is available regarding the nutritional requirements for mycelial growth of A. bisporus (Treschow, 1944). Mycelium can be produced in reasonable yield on a medium containing glucose, a nitrogen source such as glutamate or ammonia, thiamin to meet a vitamin requirement together with salts and various trace elements. To date no fruiting has been achieved on such defined media or even on more complex undefined media which allow fruit body production of other laboratory grown basidiomycete fungi The only medium supporting fruit body production m high yield is compost.

Little is known of the nutrition of fungus in compost except for the work of Gerrits (1969). He made quantitative analyses of the major polymer fractions of compost during mycelial growth and fruiting and concluded that hgno-protem polymers formed the largest fraction degraded during mycelial growth (spawn run) and he also suggested that there was a change in fungal metabolism during fruiting which led to a higher rate of cellulose and hemicellulose degradation. Since the composts he examined were unsterile, no evidence was presented to suggest that it was the mycelium of A. bisporus alone which was responsible for the degradations which were measured nor for the changes which were observed in their rate.

It is likely, however, that the major classes of polymer such as Ligno protein, cellulose and hemicellulose, which are found in composted straw serve as the major source of carbon and nitrogen for the nutrition of the mushroom The strategy of the first part of work presented here has been to examine enzymes which might either be involved m their depolymensation or in the release from such compounds of nutrients which may m turn be assimilated by the fungal mycelium.

It has been shown that laccase activity accumulates in compost during its colonisation by mycelium of A. bisporus and that this activity rapidly declines at the time of fruiting (Turner, 1974). Further work showed that cellulase behaved in the opposite fashion, a large increase m activity taking place at the time of fruiting (Turner et al., 1975). This work did not exclude the possibility that the enzymes assayed and/or their change m activity could be due to their origin from other microorganisms present in the compost The correlation of enzyme changes and fruiting observed could thus have been coincidental In the experiments reported here, further examination was carried out on the enzymes described by Turner et al. (1975) and also on other depolymerases such as xylanase, lammarmase, and protease to see if any of these enzymes also show marked activity changes during fruiting. In addition, this work was intended to provide further evidence about the source and time of change of activity of these enzymes during fruiting to see if the enzymes are produced by A. bisporus and to determine whether the changes are associated with fruiting initiation or with later events in fruiting. To do this, enzyme levels have been examined m strains genetically blocked in fruiting, and in axenic cultures of A. bisporus which arrested their development at various stages of fruiting.

Turner et al. (1975) also showed that there was a positive correlation between fruit body development and the production of ethylene by nonaxenic cultures of A. bisporus. The ethylene they found could have originated from the mycelium of A. bisporus, from the microorganisms in the compost or casing layer, from a combination of the metabolism of A. bisporus with that of other microorganisms or from the wheat straw compost itself. To determine the source of this ethylene, cultures were fruited axenically by the method of Long and Jacobs (1974) and examined for ethylene production during mycelial growth and fruiting.

During differentiation of both basidiomycetes and other microorganisms activities of intracellular proteases increase, (Iten and Matile, 1970; Betz and Weiser, 1976; Mandelstam, 1976). It is thought that these proteases are involved in increased rates of protein turnover associated with development. This protein turnover supplies ammo acids to allow synthesis of proteins not previously present during development. Chang and Chan (1973) have shown that new proteins are formed both m different tissues and at different times of development of fruit bodies of Volvariella volvaceae. To see if similar processes are involved in fruiting of A. bisporus intracellular protease activity was assayed at various stages of development.

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