Agricultural Applications

Transcription

1 The Mycota 11 Agricultural Applications Bearbeitet von F Kempken 1. Auflage Buch. XVIII, 388 S. Hardcover ISBN Format (B x L): 19,3 x 27 cm Gewicht: 2200 g Weitere Fachgebiete > Chemie, Biowissenschaften, Agrarwissenschaften > Agrarwissenschaften > Ackerbaukunde, Pflanzenbau Zu Inhaltsverzeichnis schnell und portofrei erhältlich bei Die Online-Fachbuchhandlung beck-shop.de ist spezialisiert auf Fachbücher, insbesondere Recht, Steuern und Wirtschaft. Im Sortiment finden Sie alle Medien (Bücher, Zeitschriften, CDs, ebooks, etc.) aller Verlage. Ergänzt wird das Programm durch Services wie Neuerscheinungsdienst oder Zusammenstellungen von Büchern zu Sonderpreisen. Der Shop führt mehr als 8 Millionen Produkte.

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4 1 Application and Potential of Molecular Approaches to Mushrooms Paul A. Horgen 1 and Alan Castle 2 CONTENTS I. Edible Mushrooms on a Global Scale A. Specialty Mushrooms B. White Button Mushroom, Agaricus bisporus II. The State of Breeding and Strain Improvement at the Turn of the Century A. Breaking Down Life Cycle Barriers B. Genomics in Mushrooms C. The State of A. bisporus Nuclear DNA Heterothallic Forms of A. bisporus in Nature Somatic Contact and the Potential for Breeding D. The Mitochondrial Genome and Its Potential Role in Strain Performance E. Extrachromosomal Elements in Mitochondria of Agaricus Species F. Transformation in Agaricus III. Genetic Improvement of A. bisporus: Perspectives for the Future IV. Conclusions References I. Edible Mushrooms on a Global Scale The information that will be discussed in this chapter deals mainly with the white button mushroom, Agaricus bisporus, but we will also present some background information on other mushrooms. There are more than 38,000 kinds of mushrooms in the world and these vary considerably in color, size and shape. Only a fraction of these 38,000+ types of mushrooms are grown or harvested for commercial purposes. The most significant of these include the specialty mushrooms and the white button mushroom. 1 Department of Botany, Erindale Campus, University of Toronto, Mississauga, Ontario L5L 1C6, Canada 2 Department of Biological Sciences, Brock University, St. Catharines, Ontario L2S 3A1, Canada A. Specialty Mushrooms Specialty or exotic varieties of mushrooms are commercially grown in the United States, Western Europe and the Orient, as well as in other parts of the world. These exotic mushrooms include Shiitake (Lentinula edodes), Maitake (Grifola frondosa), Nameko (Pholiota nameko), Enoki (Flammulina velutipes), Pom Pom (Hericium erinaceus), Oyster (Pleurotus spp.), Portabella, Crimini (Agaricus bisporus), and others. Worldwide production has steadily increased over the last years as these varieties have gained in popularity. Shiitake, Portabella, and Oyster are the most popular, followed by the Enoki, Maitake, Nameko and Pom Pom (Molin 1995). USDA (United States Department of Agriculture) figures have indicated that the volume of sales for commercially grown specialty mushrooms in the US market has tripled over the last decade. The value of the sales rose in 1999 for Shiitake mushrooms to 8.24 million pounds, Oyster mushrooms to 3.53 million pounds and all other specialty mushroom sales totaled 1.20 million pounds (USDA Mushroom Industry Report 1999). Growers and breeders of specialty mushrooms and of A. bisporus are faced with very similar challenges. These include identifying novel substrates for growth and production, optimizing bioconversion of these substrates, controlling diseases and pests, developing characteristics of agronomic importance, including improved rate of production and post-harvest quality, and offering mushroom consumers increased choice. In addition, interest in the medicinal properties of specialty mushrooms is growing. Molecular genetic approaches to the challenges and opportunities confronting the specialty mushroom industry parallel efforts on A. bisporus. Molecular characteristics are being developed and mapped relative to desired traits; novel strains and crosses are verified with these markers and transformation procedures are optimized and applied to specific The Mycota XI Agricultural Applications Kempken (Ed.) Springer-Verlag Berlin Heidelberg 2002

5 4 P.A. Horgen and A. Castle situations. The reader is directed to the following references for details and specific goals of research programs on specialty mushrooms (Royse 1997; Chen et al. 2000; Honda et al. 2000; Lee et al. 2000; Ramirez et al. 2000). B. White Button Mushroom, Agaricus bisporus The cultivation of the white button mushroom is by far the most successful component of the mushroom industry worldwide. Production in 1999 was over 2 million metric tons with a retail value in excess of US $10 billion (Faostat Database 2000). The history of mushroom cultivation is extensive, dating back to the seventeenth century, when the industry first started in France (van Griensven 1988; Khush et al. 1995). During the last 300 years, three major events have dramatically affected the mushroom industry to position it where it is as we enter the new millennium. The first was the development of mushroom spawn in the late 1800s. This allowed for a new companion industry to develop (the spawn industry which is the seed component of mushroom production), which ensured the delivery of pure, disease-free strains (van Griensven 1988; Khush et al. 1995). The second was the study and development of effective composting technology, which really was advanced in the 1940s and 1950s. This is an area where the industry still expends considerable resources as we enter the new century. Because of the semi-sterile nature of mushroom cultivation, there is a prevailing feeling within the white button mushroom industry that virtually all problems that develop can be managed, controlled and solved by manipulation of the compost and the growth environment. The third most significant development in the button mushroom industry was the selection of the Horst U1 (and also U3) spawn strains in 1978 by Gerda Fritsche (see review, Fritsche 1991). Because of the nature of the A. bisporus life cycle, strain improvement and breeding have been problematic over the last three centuries. The most successful and dramatic improvement in A. bisporus resulted from the least complex of breeding strategies, selection and maintenance of phenotypic variants. This important advancement dramatically illustrated the potential that a sustained breeding effort (Fritsche 1983) could bring to this enormously profitable industry. A complete review of these three developments is described by Khush et al. (1995). Although the sustained breeding effort that Fritsche described has not occurred, this article will deal mainly with the development and application of modern genetic approaches to mushroom strain improvement which could greatly add to this effort.we will mainly focus on Agaricus bisporus, but have also provided some background information of specialty mushrooms. For general reviews of mushroom breeding, see Raper (1985); Fritsche (1991); Horgen and Anderson (1993); Khush et al. (1995); Stoop and Mooibroek (1999); and Honda et al. (2000). Recent references on breeding Agaricus bisporus include Kerrigan (2000), Loftus et al. (2000) and Sonnenberg (2000). All are included in the Proceedings of the 15th International Congress on the Science and Cultivation of Edible Fungi (Van Griensven 2000). Numerous other papers on specific aspects of the cultivation and breeding of several mushroom species are included. II. The State of Breeding and Strain Improvement at the Turn of the Century A. Breaking Down Life Cycle Barriers Early advances in the understanding of A. bisporus breeding and genetics were made by Charles Miller, Carlene Raper, and Tim Elliott (see review by Khush et al. 1995).The most serious issue relating to the difficulties in breeding A. bisporus during the last century has been the secondarily homothallic life cycle of the fungus. Most basidiospores receive two post-meiotic nuclei (reviewed by Khush et al. 1995). Furthermore, all of the prevailing data suggests that post-meiotic events are non-random (Royse and May 1982) such that non-sister nuclei, i.e., those that contain non-sister chromatids, are preferentially packaged into the binucleate basidiospores (Khush et al. 1995). In addition, all data collected to date suggest that crossing-over events in meiosis are extremely rare in A. bisporus. The combination of non-random post-meiotic nuclear packaging and a greatly reduced frequency of crossing-over results

6 Application and Potential of Molecular Approaches to Mushrooms 5 in over 90% of loci that are heteroallelic in both parents and offspring (see review by Khush et al. 1995). The mating type locus is also maintained in a heteroallelic state and most basidiospores produce a fertile mycelium upon germination. At no point in the life cycle, with the exception of relatively rare basidiospores, does this species produce a haploid, monokaryotic cell which can be readily crossed with an appropriate breeding partner. These data would all suggest that traditional breeding strategies with the button mushroom are extremely slow and laborious, but nevertheless could result in improved strains (Fritsche 1983; Raper 1985). After the pioneering work of Miller, Raper and Elliott, the first major advancement in the development of a modern breeding program came in identifying stable genetic markers that could be followed in crosses. The classic isozyme work of May and Royse (1981) and Royse and May (1982) were the first studies in which multiple markers were presented. This work resulted in the identification of approximately 10 useful isozyme loci (Khush et al. 1995). The development and adoption of DNA-based markers greatly increased the number of characters that could be exploited (Castle et al. 1987, 1988; Loftus et al. 1988; Horgen and Anderson 1993). Restriction fragment length polymorphisms (RFLPs), and later randomly amplified polymorphic DNA (RAPDs) and amplified fragment length polymorphisms (AFLPs), both genetic markers produced by the polymerase chain reaction (PCR), potentially provided an unlimited number of unique markers (Khush et al. 1992). These genetically undefined markers were soon followed by gene markers, which continue to be added each year (Sonnenberg et al. 1996, 1999; Ospina-Giraldo et al. 2000). There are now several hundred genetic markers deposited in GenBank (for a list of accession numbers, access the website <url> and search the protein or nucleotide database with the keyword Agaricus ). An initial genetic map was published of the A. bisporus genome (Kerrigan et al. 1993) with RFLP, RAPD, and isozyme markers. The map continues to evolve and expand with the addition of other defined genetic loci (Horgen et al. 1996; Imbernon et al. 1996; Callac et al. 1997). A second important advancement in moving the button mushroom towards a rational breeding strategy was the increase in our collective understanding of the meiotic process. The secondarily homothallic life cycle was described by early studies prior to the establishment of DNA-based markers (Evans 1959; Miller 1971) which were used to elucidate the unusual events occurring surrounding meiosis. Studies using RFLP markers (Summerbell et al. 1989; Allan et al. 1992) and PCR-based markers (Khush et al. 1992) reconfirmed that the isolates of A. bisporus studied favored a life style termed intramixus by Kerrigan (1990). Because meiosis was so unusual in this species, Allen et al. (1992) hypothesized that perhaps it rarely occurred during the production of basidiospores. Their results, however, instead established that meiosis does predominate (Allen et al. 1992). It was suggested that low levels of recombination are common and that nonrandom chromosome segregation occurs. In the most detailed study of meiosis to date, Kerrigan et al. (1993) reported that chromosome segregation in meiosis I is random. We have established that independent assortment occurs by the nonparental genotypes observed in homokaryotic spores, or in protoplast regenerates of germinated heterothallic spores (Summerbell et al. 1989; Allen et al. 1992; Khush et al. 1992; Kerrigan et al. 1993). Non-sister pairings of post-meiotic nuclei appear to be preferentially packaged during basidiospore production in commercial, wild-collected and hybrid isolates (Khush et al. 1995). The net result of the A. bisporus life cycle is the maintenance of the parental genotype through what Kerrigan has called pseudo-clonal lineages (Kerrigan 1990). Heteroallelism predominates at the mating-type locus, and the binucleate spores are self-fertile (Khush et al. 1995). A third important development that occurred was the development of pulse field gel electrophoresis (PFGE) which resulted in resolution of A. bisporus chromosomes (Royer et al. 1991; Sonnenberg et al. 1991; Horgen et al. 1996) and the publication and expansion of the genetic map (Kerrigan et al. 1993). A. bisporus heterokaryons (dikaryotic state) possess 13 pairs of chromosomes (Fig. 1.1) ranging in size from 1.4 Mb to 3.65 Mb (Horgen et al. 1996; Sonnenberg et al. 1996). The genetic map, proposed by Kerrigan et al. (1993), suggested a number of linkage groups that could be associated with the chromosome-sized DNAs. Although the physical map for A. bisporus is developing slowly, several loci have been placed on specific chromosomes, and a partial representation is shown in Table 1.1. The mating-type locus