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Horticulture Research International, Wellesbourne, Warwick, CV35 9EF, United Kingdom
R. Li
Department of Agriculture, Yunnan Agriculture University, Kunming 650201, China
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The mushroom (Agaricus bisporus) has a requirement for a "casing layer" that has specific physical, chemical and microbiological properties which stimulate and promote the initiation of primordia. Some of these primordia then may develop further into sporophores, involving differentiation of tissue. Wild and commercial strains of A. bisporus were cultured in axenic and nonaxenic microcosms, using a rye grain substrate covered by a range of organic and inorganic casing materials. In axenic culture, A. bisporus (commercial strain A15) was capable of producing primordia and mature sporophores on charcoal (wood and activated), anthracite coal, lignite and zeolite, but not on bark, coir, peat, rockwool, silica or vermiculite. Of six strains tested, only the developmental variant mutant, B430, produced rudimentary primordia on axenic peat-based casing material. However, none of these rudimentary primordia developed differentiated tissues or beyond 4 mm diameter, either on axenic casing material in the microcosms or in larger-scale culture. In larger-scale, nonaxenic culture, strain B430 produced severely malformed but mature sporophores in similar numbers to those of other strains. Typically, 3–6% of primordia developed into mature sporophores, but significant differences in this proportion, as well as in the numbers of primordia produced, were recorded between 12 A. bisporus strains.
Key words: aggregate, hypha, mycelium, primordium, Pseudomonas putida, sporophore
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). This initiation is followed by the formation of primordia, defined by Wood (1976) as structures greater than 1 mm diameter composed of a dense hyphal mesh with a smooth surface and visibly distinct from strands or knots of hyphae. Flegg (1979) referred to the latter as "mycelial aggregates". In addition to a nutritional substrate, e.g., compost (Noble et al 1998), most A. bisporus strains have a requirement for a separate "casing layer" that has specific physical, chemical and microbiological properties which stimulate and promote the initiation of primordia (Eger 1961, Flegg and Wood 1985). Primordia are formed after mycelial colonization of the nutritional substrate and casing layer. Development of primordia is stimulated by a reduction in temperature and CO2 (Flegg and Wood 1985) and by the presence of bacteria in the casing layer, of which Pseudomonas putida (Trevisan) Migula is regarded as most significant (Hayes et al 1969, Park and Agnihotri 1969). Long and Jacobs (1974) showed that activated charcoal could replace the function of stimulatory bacteria in the casing layer, possibly by adsorbing compounds that were inhibitory to initiation (Wood and Blight 1983). After initiation, some primordia may continue to develop in size and by differentiation of tissues into sporophores (Wood 1979).
Strains of A. bisporus have been found to vary markedly in their ability to form primordia axenically on malt-extract agar (MEA). Using 12 strains of commercial origin, Wood (1976, 1979) found that such primordia were formed asynchronously. Wannet et al (1999) found two wild strains from the Agaricus Recovery Programme (ARP) collection (Sylvan Spawn Laboratory Inc., West Hills Industrial Park, Kittaning, Pennsylvania), 116 and 155.8, which produced primordia after mycelial colonization of MEA, usually at the edge of the plate. Except in rare cases, none of the above strains produced sporophores in the absence of a casing layer, and none produced primordia on defined-agar media. Elliott and Wood (1978) isolated two developmental variant, mutant A. bisporus strains (B430 and B431) that produced primordia synchronously throughout mycelial growth on defined-minimal media but were not shown to produce mature sporophores.
The aims of the current work were to determine if: (i) axenic casing materials other than activated charcoal could stimulate initiation; (ii) the mutant strain B430 and other wild A. bisporus isolates could initiate primordia and develop sporophores on an axenic casing layer; (iii) primordia and sporophores of the above strains develop normally under nonaxenic conditions, compared with commercial A. bisporus strains.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, was used to examine growth in axenic casing materials (Fig. 1). The microcosms were filled with a 10 mm layer (30 g) of sterile rye grain "spawn" (Elliott 1985), which served as a nutritional substrate, colonized with the appropriate A. bisporus strain. This substrate was covered with 60 g (about 17 mm layer, surface area 64 cm2) of casing material, consisting of a mixture of black peat and CaCO3 (4:1 v/v) or other materials. Gamma irradiation (Technical Service Consultants Ltd, Heywood, Lancashire, United Kingdom) was used to produce axenic casing material, to avoid physical and chemical changes caused by autoclaving. The microcosms were incubated at 25 C until mycelium had started to colonize the surface of the casing layer, normally 3–5 d after the jars were filled. To promote conditions in the microcosms for initiation to occur, the CO2 concentration was reduced from about 0.4% v/v to 0.08–0.12% v/v by placing two sterile plastic caps, each containing 5 g self-indicating, 1–2.5 mm mesh soda lime granules, in each microcosm. The microcosms then were transferred to a 16 C cabinet; the soda lime was replaced when necessary, and the casing layer irrigated with up to 15 mL sterile water after about 14 d to maintain a matric potential of -1 to -2 kPa (Noble et al 1999). The matric potential, pH and electrical conductivity of casing materials were determined according to Noble et al (1999).
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1 mm diameter were recorded 21 d after transfer to 16 C; axenic or nonaxenic conditions were maintained in the microcosms for a further 21 d to determine if primordia developed into mature sporophores, i.e., Stage 5 (Hammond and Nichols 1976) pileus with broken veil and gills exposed. Initiation on axenic and nonaxenic casing materials. The types and sources of the 12 organic and inorganic casing materials used are shown in Table I. Peat and lignite were too acidic for mycelial growth; CaCO3, therefore, was added at 9% v/v to achieve a pH of 7.4 (Table I). The commercial white hybrid A. bisporus strain Sylvan A15 (Sylvan Spawn Ltd, Peterborough, United Kingdom) was used. Each of six replicates consisted of four blocks, each containing three pairs of jars. Each pair of jars contained the same casing material, one of each pair randomly selected to contain axenic material and other nonaxenic material. Casing materials were allocated to blocks within replicates following an alpha design, such that pairs of casing materials appeared within a block no more than twice. This allocation ensured that the precision of comparisons between pairs of casing materials was as equal as possible across all pairs.
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), to allow for proper adjustment for differences between blocks within replicates. This approach also enabled comparisons between treatments that allowed for the variation in the precision of different paired comparisons.
Axenic and nonaxenic initiation of A. bisporus strains.
These A. bisporus strains were used: commercial strain A15, UK wild strains 96-4, 97-3 (HRI Culture Collection, Wellesbourne, Warwick, United Kingdom), American wild strains JB10, JB157 (deposited as ARP116, ARP155, Sylvan Spawn Laboratory Inc.) and the developmental variant B430 (HRI Culture Collection). Strains ARP116 and ARP155 are var. burnettii with mainly tetrasporic basidia (Callac et al 1993); all other strains are var. bisporus with mainly bisporic basidia. This experiment was arranged as a split-plot design with six replicate blocks that each contained six pairs of jars. Each pair of jars contained the same strain, one with axenic and the other with nonaxenic casing material. Data from this experiment were analyzed using analysis of variance.
Flask culture –
Composted substrate (Noble et al 1998) (400 g) was filled to a depth of 80 mm in 2 L "Quickfit" multiadapter flasks (Fisher Scientific, Loughborough, United Kingdom) and covered with 180 g of the above peat and CaCO3 casing material to a depth of 28 mm (surface area 113 cm2). After autoclaving (axenic flasks only), the substrate was inoculated with two 5 mm plugs of A. bisporus mycelium grown on MEA, through holes made in the casing layer. The flasks were sealed with bacterial air vents and kept at 25 C until mycelium became visible at the surface of the casing material, which then was watered with 50 mL sterile water. The flasks then were transferred to a room at 18 C and connected to a filtered air flow of 15 L h-1 to maintain a CO2 concentration of 0.06–0.12% v/v. Primordia and sporophores were recorded in the same way as for culture in microcosms.
Initiation of A. bisporus strains in axenic and nonaxenic flask culture – These A. bisporus strains were used: commercial strain A15, UK wild strain 97-3, and the strain B430. Two replicate axenic and nonaxenic flasks were prepared with each strain, arranged in a randomized block design.
Larger-scale nonaxenic culture –
A. bisporus strains were cultured with polypropylene trays (60 x 40 x 18 [deep] cm) in a controlled-environment room, with cultural conditions as described in Noble et al (1999). Each tray contained 9 kg of the above composted substrate colonized with A. bisporus strains after inoculation with 1% w/w rye grain spawn as mentioned above. The substrate was covered with a 28 mm deep casing layer, which consisted of a 4:1 v/v mixture of black peat and CaCO3 (Noble et al 1999).
Initiation of A. bisporus strains in larger-scale nonaxenic culture –
These A. bisporus strains were used: commercial white hybrid strains 2100 (Amycel Ltd, Burton, Staffordshire, United Kingdom) and A15, UK wild strains 96-4, 97-2, 97-3, JFB II, JFB III (HRI Culture Collection), AMA4 (deposited as ARP174, Sylvan Spawn Laboratory Inc.) and American wild strains ARP116, ARP130, ARP155 (Sylvan Spawn Laboratory Inc.) and the strain B430. Strains ARP130 and ARP174 are var. burnettii and var. bisporus respectively. Four replicate trays of each strain were prepared; two of the trays were used for recording the population of primordia and sporophores, while the casing layer in the other two trays was not disturbed and only the number of sporophores was recorded. Each tray was divided into a 3 x 5 grid, each square measuring 10 x 10 cm. At each recording date, the casing layer in a randomly selected square was removed carefully so as not to disturb adjacent squares. The full depth of casing layer then was examined to record the number of primordia. After recording, all Stage 5 sporophores (Hammond and Nichols 1976) were removed and the disturbed casing layer sample was replaced over the compost in the square on the tray. Individual squares in the grid were examined once. Primordia and sporophores were recorded at 2 d intervals for 26 d after the first primordia were visible. The numbers of primordia and sporophores were recorded by examination of destructively sampled casing layer and recording the number of: (i) live primordia 1–3 mm diameter (small primordia) (Wood 1976, Flegg 1979); (ii) live primordia >3 mm diameter (large primordia); (iii) dead primordia 1 mm diameter; (iv) sporophores that had reached Stage 5 (Hammond and Nichols 1976). A small number of primordia that were attached to the base of sporophores were recorded in the above categories. The numbers of primordia in the recording areas were plotted against time. Estimates of primordia numbers on days between the recording dates were obtained by cubic interpolation between the observed points. Where this cubic interpolation gave negative estimates, values were reset to zero. A measure of the total production and survival of primordia was obtained by summing the daily counts (both observed and interpolated), giving an approximate area under the interpolated curve for each strain, referred to as the integral number of primordia. These parameters of growth curves were analyzed: (a) the area under the curve, i.e., the integral numbers of primordia in categories (i) to (iii) with time; (b) the peak value of primordial numbers in categories (i) to (iii).
Core (20 mm diameter) profiles of the casing layer also were examined with either a binocular microscope (x10) or scanning electron microscope (SEM) (x35–x100) after gold coating of frozen hydrated samples of casing material, with mycelium and primordia attached.
Two replicates of the 12 strains were arranged following a latinized row-column alpha design. Each replicate was arranged in a single layer of trays (three trays wide by eight trays deep), each strain being applied to a pair of adjacent trays. Within each pair of trays, one was left undisturbed, while the casing of the other was sampled on occasions during crop development (these treatments applied at a subplot level). The design ensured that each strain only appeared at most once in each of four blocks of trays along the length of the experiment and that the precision of comparisons between pairs of strains was as equal as possible across all pairs.
Data from this experiment first were analyzed using the REML approach to allow proper adjustment for differences between rows and columns within replicates, but with little evidence of any positional variation, a simpler analysis of variance was used.
Measurement of bacterial numbers in casing materials –
In all the experiments, bacteria were isolated from casing materials on nutrient and Pseudomonas isolation agars (PIA) (Difco Laboratories, Detroit, Michigan) to determine the total bacterial populations as colony forming units per g dry weight casing material (cfu g-1) and to estimate the proportion that were Pseudomonas spp. An estimate of the proportion of Pseudomonas spp. that were P. putida isolates was obtained with these tests (Stanier et al 1966, Lelliott and Stead 1987) on 30 cfus from the PIA isolation: (i) fluorescence under UV light on Pseudomonas Agar F (PAF) (Merck Ltd., Poole, Dorset, United Kingdom); (ii) inability to hydrolyze gelatin; (iii) arginine dihydrolaze test positive. Results from these tests on Pseudomonas spp. isolates previously were found to correspond to characterization by ribotyping (Fermor et al 2000).
Bacterial and primordial populations showed evidence of a mean-variance relationship. These populations respectively were subjected to a logarithmic or square root transformation before analysis. All differences in the results section were significant at P < 0.05, or if stated, P < 0.01 or 0.001.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Visual observation of primordia 0.5 mm or greater in diameter, in a larger casing material sample, therefore was a more reliable method for measuring the occurrence and amount of primordia. The development of primordia and sporophores was slower in the microcosm tests than in the larger-scale culture due to the lower temperature (16 C compared with 18 C) (Flegg 1979).
Primordia initiation on axenic casing materials –
The casing material in microcosms in which initiation did not occur became overgrown with mycelium i.e., stroma (Fletcher et al 1986), except in animal charcoal, where mycelial growth was poor. This might have been due to the high pH value of this casing material (Table I). After initiation on the casing materials, mycelial growth declined or stopped.
No primordia formed on axenic bark, coir, peat, rockwool, silica gel or vermiculite, or on animal charcoal (Fig. 2). Primordia formed on axenic and nonaxenic charcoal (wood and activated), coal, lignite and zeolite. Of the axenic materials, most primordia formed on activated charcoal, and of the nonaxenic materials, more primordia formed on coir, activated charcoal and peat than on the other materials (P < 0.01). In axenic and nonaxenic microcosm treatments in which primordia were recorded, 1–2 primordia developed into mature sporophores. Two primordia formed in one of the six replicate microcosms of the nonaxenic vermiculite treatment, and one of these developed into a mature sporophore.
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. In zeolite, bark, charcoal, coal and lignite casing materials, 18–62% of the bacteria were estimated to be Pseudomonas spp. but none of the isolated bacteria produced classical P. putida reactions. In coir and vermiculite casing materials, none of the isolated bacteria grew on PIA and therefore were unlikely to be Pseudomonas spp. No bacteria were detected in the axenic microcosms at the end of the experiment. The electrical conductivities of activated charcoal, animal charcoal, coir and lignite (range 922 to 1486 µS) were higher than those of the other casing materials (range 58 to 452 µS). Coir and silica gel had lower pH values and the charcoal sources had higher pH values than the other casing materials (Table I). However, there were no relationships between casing electrical conductivity or pH and the initiation of primordia.
Axenic and nonaxenic initiation of A. bisporus strains –
B430 was the only strain to produce primordia on axenic peat-based casing material, but here the number produced was less than on nonaxenic material (P < 0.001, Figs. 1 and 3). The other strains produced only vegetative mycelium in axenic peat-based casing material (Figs. 1 and 3). The rudimentary primordia of B430 were similar to those shown by Elliott and Wood (1978). Between 3–5 of these primordia in each axenic or nonaxenic microcosm reached a diameter of 4 mm, but none of the primordia formed differentiated tissues associated with developing sporophores (Wood 1979). For the other strains in nonaxenic microcosms, 1–2 primordia developed into mature sporophores. Strains A15 and B430 produced more primordia than the other strains in nonaxenic casing, whereas strain ARP116 produced the fewest (P < 0.01, Fig. 3).
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Initiation of A. bisporus strains in axenic and nonaxenic flask culture – As in the previous experiment, B430 was the only strain to produce primordia on axenic peat-based casing material, and again none of these primordia grew beyond a diameter of 4 mm and the number produced was less than on nonaxenic material (Table II). The other two A. bisporus strains produced only vegetative mycelium in axenic peat-based casing material, but all three strains produced mature sporophores in the nonaxenic flasks.
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Larger-scale nonaxenic culture of A. bisporus strains –
The first primordia were recorded 8–9 d (16 d for strain ARP155) after the casing material was applied, and the first sporophores attained Stage 5 development (Hammond and Nichols 1976) after 13–18 d (22 d for ARP155). A. bisporus (var. bisporus) strains had 80–98% bisporic basidia (50 observed basidia) and A. bisporus (var. burnettii) strains had 73–100% tetrasporic basidia. This result agrees with Callac et al (1993), who observed 17–100% bisporic basidia and 0–24% tetrasporic basidia for var. bisporus and 0–8% bisporic and 54–100% tetrasporic basidia for var. burnettii. Sporophores of all the wild strains had brown caps, with ARP116 and ARP130 being darker than JFB II, JFB III; the caps of 96-4, 97-2 and 97-3 were pale brown. Strain B430 produced severely distorted sporophores with gill and spore tissue on the pileus surface, similar to "rosecomb" described by Fletcher et al (1986), and split stipes (Fig. 4). However, gill tissue was normal, producing viable spores.
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Bacterial populations in the casing material on the trays were similar to those recorded on peat-based materials in the microcosms and flasks. They were not affected by the A. bisporus strains (105–107 cfu g-1), and 49% of the bacterial populations were estimated to be Pseudomonas spp. and 20% were estimated to be P. putida, according to the above tests.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, who found no fruiting of A. bisporus on sterile peat, rockwool or organic wastes (or on sand, brown coal, glasswool, glass beads and chopped bricks), but initiation occurred on these materials if they were treated with an aqueous-filtered extract of normal casing material. As here, Long and Jacobs (1974) and Wood and Blight (1983) found that initiation occurred on axenic activated charcoal, although our experiments also showed that other carbonized materials (wood charcoal, coal and lignite), as well as zeolite, could stimulate initiation in axenic culture. The observation made here—of mycelial growth declining or stopping after initiation on different casing materials—also was made by Eger (1962) after the introduction of stimulatory bacteria into the casing layer of sterile cultures of A. bisporus.
The results here indicate that different casing materials support different populations of bacteria, although these were not related to the numbers of initials formed. The proportions of fluorescent Pseudomonas spp. found in peat casing (16–56%) correspond to those of Miller et al (1995), who found that they were 14–41% of the total bacteria present. Hayes and Nair (1974) found that the proportion of Pseudomonas spp. increased from 34% of the total bacteria present at the time peat casing was applied to 95% of the total bacteria at the end of a mushroom crop. Although coir and vermiculite did not appear to support the growth of Pseudomonas spp., other genera of bacteria, Bacillus and Alcaligenes, have been shown to stimulate initiation of A. bisporus (Park and Agnihotri 1969, Fermor et al 2000).
Previous attempts to isolate the putative inhibitors to initiation from activated charcoal have been confounded by the large number of compounds adsorbed and identified (Grove 1981, Wood and Blight 1983). Of these, 1-octen-3-ol was found to inhibit primordial formation in plate culture. Here, more primordia were formed on axenic activated charcoal than on other axenic materials; this might be due to the relatively greater adsorption action of the material (Long and Jacobs 1974). Chemical analysis by gas chromatography-mass spectrometry of adsorbent, stimulatory and nonstimulatory casing materials might identify differences in compounds present. Initiation on zeolite might provide the opportunity to selectively adsorb putative fruiting inhibitors, using a range of synthetic zeolites with different adsorption characteristics (Eitel 1966).
Primordia initiation of different A. bisporus strains –
Tschierpe (1983) and Fritsche and Sonnenberg (1988) recorded differences in the number of primordia produced by different A. bisporus strains under similar cultural conditions, and Flegg (1979) found that this proportion could be altered by temperature. Here, an estimate of the proportion of primordia that developed into sporophores could be obtained by dividing the peak number of total primordia (including primordia that developed into sporophores before the peak) by the numbers of sporophores that developed (Table II, Fig. 5). These proportions are overestimates because primordia that developed between the peak value and Day 35 are not included and the number of dead initials might be underestimates as previously mentioned. In most strains, 2.7–6.1% of primordia developed into sporophores. In strains ARP116 and JFBIII, these figures were higher (10.4 and 15.6%), and in strains ARP130 and ARP155 they were lower (0.2 and 1.2%). The proportions found here correspond to those of Flegg (1979), who found that 3.2% of primordia of the strain D649 developed into sporophores at a cropping temperature of 16 C. He also suggested that the sporophores, which develop during a period of several weeks, arise from mycelial aggregates formed during the period immediately after casing is applied to the start of the first flush. This observation agrees with the results in this work, although in some strains (97-3 and ARP174) significant numbers of new primordia developed after the first flush of sporophores. Wannet et al (1999) found that all hyphal knots were formed before the first period of aggregate development on MEA. At each period of aggregate release or flush, about 2–5% of these knots developed synchronously into aggregates.
Strain ARP116 did not produce primordia on an axenic casing layer, although Wannet et al (1999) found that it produced primordia on axenic MEA. In nonaxenic culture, the production of primordia and sporophores of ARP116 followed a similar pattern to that in other A. bisporus strains. The developmental variant strain B430 was capable of initiating rudimentary primordia on axenic peat-based casing material but, similar to the observations of Elliott and Wood (1978) and Wannet et al (1999), the primordia were unable to grow beyond Stage 2 growth. Flegg and Wood (1985) found that light and electron microscope sections of primordia produced by B430 on axenic media showed them to be indistinguishable from normal primordia that were capable of further differentiation. As here, the rudimentary primordia consisted mainly of a disorientated mass of hyphae. The formation of normal primordia and the subsequent numbers of sporophores of B430 in nonaxenic culture were similar to those of other A. bisporus strains. This result contrasts with that of Elliott and Wood (1978), who were unable to demonstrate conclusively that B430 was capable of producing mature sporophores in vivo.
In flask culture, B430 primordia developed into mature sporophores using nonaxenic casing material but not with axenic casing material. This response indicates that the mechanism for producing rudimentary primordia in axenic casing material differs from that in nonaxenic casing material, where primordia capable of further differentiation are produced. The lack of sporophores of B430 in nonaxenic microcosms might have been due to the limited substrate weight (30 g compared with 400 g in flasks). Wood (1979) found that A. bitorquis strains produced hyphal aggregates on defined axenic media but, similar to B430, they were incapable of further differentiation. San Antonio and Peerally (1979) found no relationship between primordia formation of 22 strains of A. bitorquis on axenic media and sporophore production in culture on peat casing and compost. This report agrees with the results found here for primordia formation and sporophore production of A. bisporus strains on casing material and compost in nonaxenic culture. Rainey et al (1990) developed an in vitro Petri dish bioassay using A. bitorquis (strain W19) to determine the involvement of Pseudomonas spp. in stimulating initiation. However, subsequent tests (Fermor et al 2000) showed that the stimulatory response of W19 in the bioassay did not correspond with in vivo results obtained with A. bisporus in the microcosms used here.
This work supports the hypothesis (Flegg and Wood 1985) that inhibitors prevent initiation of primordia and that the putative inhibitors either can be metabolized by the casing microbiota (P. putida) or be adsorbed by certain casing materials. These putative inhibitors do not prevent the formation of rudimentary primordia in strain B430 but prevent the formation of primordia capable of differentiation. Further work is needed to determine whether the numbers of primordia and the proportion that develop into sporophores are related to the production of putative inhibitors by the mycelium of different A. bisporus strains, or their metabolism or adsorbtion by the casing layer.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Accepted for publication December 23, 2002.
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LITERATURE CITED |
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