From 13C-lignin to 13C-mycelium: Agaricus bisporus uses polymeric lignin as a carbon source

Plant biomass conversion by saprotrophic fungi plays a pivotal role in terrestrial carbon (C) cycling. The general consensus is that fungi metabolize carbohydrates, while lignin is only degraded and mineralized to CO2. Recent research, however, demonstrated fungal conversion of 13C-monoaromatic compounds into proteinogenic amino acids. To unambiguously prove that polymeric lignin is not merely degraded, but also metabolized, carefully isolated 13C-labeled lignin served as substrate for Agaricus bisporus, the world's most consumed mushroom. The fungus formed a dense mycelial network, secreted lignin-active enzymes, depolymerized, and removed lignin. With a lignin carbon use efficiency of 0.14 (g/g) and fungal biomass enrichment in 13C, we demonstrate that A. bisporus assimilated and further metabolized lignin when offered as C-source. Amino acids were high in 13C-enrichment, while fungal-derived carbohydrates, fatty acids, and ergosterol showed traces of 13C. These results hint at lignin conversion via aromatic ring-cleaved intermediates to central metabolites, underlining lignin's metabolic value for fungi

Developments in breeding of Agaricus bisporus var. bisporus : progress made and technical and legal hurdles to take

True breeding of button mushrooms has hardly been done in the last decades, despite this species being one of the most cultivated mushrooms worldwide. Research done in the last 20 years has identified and characterised new germplasm and improved our understanding of the genetic base for some traits. A substantial collection of wild-collected strains is now available and partly characterised for a number of important traits such as disease resistance and yield. Most of the variations found in a number of important agronomic traits have a considerable heritability and are thus useful for breeding. Genetic marker technology has also developed considerably for this mushrooms in the last decade and used to identify quantitative trait loci (QTL) for important agronomic traits. This progress has, except for one example, not resulted so far into new commercially varieties. One of the reasons lies in the typical life cycle of the button mushroom Agaricus bisporus var. bisporus which hampers breeding. Joint investment is needed to solve technical problems in breeding. Special attention is needed for the protection of new varieties. Due to its typical life cycle, it is very easy to generate so called “look-a-likes” from protected cultivars by screening fertile single spore cultures. A consensus has been reached within the mushroom (breeding) industry to consider this method as the generation of essentially derived varieties as defined in plant breeding

Critical factors involved in primordia building in agaricus bisporus : A review

The button mushroom Agaricus bisporus is an economically important crop worldwide. Many aspects of its cultivation are well known, except for the precise biological triggers for its fructification. By and large, for most basidiomycete species, nutrient availability, light and a drop in temperature are critical factors for fructification. A. bisporus deviates from this pattern in the sense that it does not require light for fructification. Furthermore its fructification seems to be inhibited by a self-generated factor which needs to be removed by microorganisms in order to initiate fruiting. This review explores what is known about the morphogenesis of fruiting initiation in A. bisporus, the microflora, the self-inhibitors for fruiting initiation and transcription factors involved. This information is subsequently contrasted with an overall model of the regulatory system involved in the initiation of the formation of primordia in basidiomycetes. The comparison reveals a number of the blank spots in our understanding of the fruiting process in A. bisporus.</p

Multi-trait QTL analysis for agronomic and quality characters of Agaricus bisporus (button mushrooms)

The demand for button mushrooms of high quality is increasing. Superior button mushroom varieties require the combination of multiple traits to maximize productivity and quality. Very often these traits are correlated and should, therefore, be evaluated together rather than as single traits. In order to unravel the genetic architecture of multiple traits of Agaricus bisporus and the genetic correlations among traits, we have investigated a total of six agronomic and quality traits through multi-trait QTL analyses in a mixed-model. Traits were evaluated in three heterokaryon sets. Significant phenotypic correlations were observed among traits. For instance, earliness (ER) correlated to firmness (FM), cap color, and compost colonization, and FM correlated to scales (SC). QTLs of different traits located on the same chromosomes genetically explains the phenotypic correlations. QTL detected on chromosome 10 mainly affects three traits, i.e., ER, FM and SC. It explained 31.4 % phenotypic variation of SC on mushroom cap (heterokaryon Set 1), 14.9 % that of the FM (heterokaryon Set 3), and 14.2 % that of ER (heterokaryon Set 3). High value alleles from the wild parental line showed beneficial effects for several traits, suggesting that the wild germplasm is a valuable donor in terms of those traits. Due to the limitations of recombination pattern, we only made a start at understanding the genetic base for several agronomic and quality traits in button mushrooms.</p

Variability in growth and biofilm formation of Listeria monocytogenes in Agaricus bisporus mushroom products

Foods and food production environments can be contaminated with Listeria monocytogenes and may support growth of this foodborne pathogen. This study aims to characterize the growth and biofilm formation of sixteen L. monocytogenes strains, isolated from mushroom production and processing environments, in filter-sterilized mushroom medium. Strain performance was compared to twelve L. monocytogenes strains isolated from other sources including food and human isolates. All twenty-eight L. monocytogenes strains showed rather similar growth performance at 20 °C in mushroom medium, and also significant biofilm formation was observed for all strains. HPLC analysis revealed the presence of mannitol, trehalose, glucose, fructose and glycerol, that were all metabolized by L. monocytogenes, except mannitol, in line with the inability of L. monocytogenes to metabolize this carbohydrate. Additionally, the growing behavior of L. monocytogenes was tested on whole, sliced and smashed mushroom products to quantify performance in the presence of product-associated microbiota. A significant increase of L. monocytogenes was observed with higher increase of counts when the mushroom products were more damaged, even with the presence of high background microbiota counts. This study demonstrated that L. monocytogenes grows well in mushroom products, even when the background microbiota is high, highlighting the importance to control (re)contamination of mushrooms

Growth performance of Listeria monocytogenes and background microbiota from mushroom processing environments

Interaction between Listeria monocytogenes and resident background microbiota may occur in food processing environments and may influence the survival of this pathogen in a factory environment. Therefore the aim of this study was to characterize the growth performance of microbiota isolated from the processing environments of frozen sliced mushrooms, and to investigate the competitive performance of L. monocytogenes when co-cultured with accompanying environmental microbiota. Acinetobacter, Enterobacteriaceae, Lactococcus and Pseudomonas were the most prominent background microbiota isolated from the processing environment of frozen sliced mushrooms. All individual microbiota strains were able to grow and form biofilm in filter-sterilized mushroom medium, with the mannitol-consumers Raoultella and Ewingella as top performers, reaching up to 9.6 and 9.8 log CFU/mL after 48 h incubation at room temperature. When L. monocytogenes mushroom isolates were co-cultured with the microbiota strains, L. monocytogenes counts ranged from 7.6 to 8.9 log CFU/mL after 24 h of incubation, while counts of the microbiota strains ranged from 5.5 to 9.0 log CFU/mL. Prolonged incubation up to 48 h resulted in further increase of L. monocytogenes counts when co-cultured with non-acidifying species Pseudomonas and Acinetobacter reaching 9.1 to 9.2 log CFU/mL, while a decrease of L. monocytogenes counts reaching 5.8 to 7.7 log CFU/mL was observed in co-culture with Enterobacteriaceae and acidifying Lactococcus representatives. In addition, L. monocytogenes grew also in spent mushroom media of the microbiota strains, except in acidified spent media of Lactococcus strains. These results highlight the competitive ability of L. monocytogenes during co-incubation with microbiota in fresh and in spent mushroom medium, indicative of its invasion and persistence capacity in food processing factory environments

Quantitative trait locus mapping for bruising sensitivity and cap color of Agaricus bisporus (button mushrooms)

White button mushrooms discolor after mechanical damage of the cap skin. This hampers the development of a mechanical harvest system for the fresh market. To unravel the genetic basis for bruising sensitivity, two haploid populations (single spore cultures) were generated derived from crosses between parental lines differing in discoloration after mechanical damage (bruising sensitivity). The haploids were crossed with different homokaryotic tester lines to generate mushrooms and allow assessment of the bruising sensitivity in different genetic backgrounds. Bruising sensitivity appears to be a polygenic highly heritable trait (H2: 0.88-0.96) and a significant interaction between genotypes and tester lines and genotypes and flushes was found. Using SNP markers evenly spread over all chromosomes, a very low recombination was found between markers allowing only assignment of QTL for bruising sensitivity to chromosomes and not to sub-regions of chromosomes. The cap color of the two parental lines of population 1 is white and brown respectively. A major QTL for bruising sensitivity was assigned to chromosome 8 in population 1 that also harbors the main determinant for cap color (brown versus white). Splitting offspring in white and non-white mushrooms made minor QTL for bruising sensitivity on other chromosomes (e.g. 3 and 10) more prominent. The one on chromosome 10 explained 31% phenotypic variation of bruising sensitivity in flush 2 in the subpopulations of population 1. The two parental lines of population 2 are both white. Major QTL of bruising sensitivity were detected on chromosome 1 and 2, contributing totally more than 44% variation of the bruising sensitivity in flush 1 and 54% variation of that in flush 2. A considerable consistency was found in QTL for bruising sensitivity in the different populations studied across tester lines and flushes indicating that this study will provide a base for breeding cultivars that are less sensitive for bruising allowing the use of mechanical harvest and automatic postharvest handling for produce for the fresh market. The low recombination between homologous chromosomes, however, underlines the need to introduce a normal recombination pattern found in a subspecies of the button mushroom

Effects of the mushroom-volatile 1-octen-3-ol on dry bubble disease

Dry bubble disease caused by Lecanicillium fungicola is a persistent problem in the cultivation of the white button mushroom (Agaricus bisporus). Because control is hampered by chemicals becoming less effective, new ways to control dry bubble disease are urgently required. 1-Octen-3-ol is a volatile that is produced by A. bisporus and many other fungi. In A. bisporus, it has been implicated in self-inhibition of fruiting body formation while it was shown to inhibit spore germination in ascomycetes. Here, we show that 1-octen-3-ol inhibits germination of L. fungicola and that enhanced levels of 1-octen-3-ol can effectively control the malady. In addition, application of 1-octen-3-ol stimulates growth of bacterial populations in the casing and of Pseudomonas spp. specifically. Pseudomonas spp. and other bacteria have been demonstrated to play part in both the onset of mushroom formation in A. bisporus, as well as the inhibition of L. fungicola spore germination. A potential role of 1-octen-3-ol in the ecology of L. fungicola is discussed.</p

Feeding growing button mushrooms : The role of substrate mycelium to feed the first two flushes

A number of experiments were done to further our understanding of the substrate utilization in button mushroom crops (Agaricus bisporus). An analysis of the degradation of dry matter of the substrate during a crop cycle revealed that for pin formation the upper 1/3rd layer is used, for the production of flush one all layers are involved and for flush two mainly the lower 1/3 layer is used. A reduction in substrate depth leads to a decrease in yield/m2 but an apparent increase in yield per tonne of substrate with a lower mushroom quality. A short daily interruption of the connection between the casing soil with the substrate results in a delay of the first flush. Interruptions with only part of the substrate did not lead to delay in production. Daily interruption of the connection with all or only part of the substrate leads to a shift in yield from flush one to flush two but the total yield remains unchanged. The mycelial biomass in the substrate increases from filling up to pinning, has a steeper increase during flush one, and is levelling off during flush two, indicating that in the period of venting and up to/including flush one, enzymes are secreted by growing hyphae generating nutrients to feed a fixed amount of mushroom biomass for two flushes. A sidewise extension of the substrate (without casing soil, thus not producing mushrooms) showed that the substrate at a distance more than somewhere between 20–50 cm away from the casing soil does not contribute to feeding mushrooms in the first two flushes. The observations are discussed with respect to relevant previous research