The tasty and versatile Xianggu mushroom is a favourite on Chinese dinner tables, in haute cuisine as in more homely fare. It can cost from under a hundred dollars to a few hundred dollars per catty, depending on the quality. Different grades of Xianggu are the result of differences in the strains or in the cultivation processes.
To Develop a Breeding Programme for Xianggu
There have not been, however, any breeding programmes for Xianggu similar to plant breeding programmes. Traditionally the better strains have been selected only after time-consuming and labour-intensive screening, which adds to the overall production cost. But since 1993, Prof. H.S. Kwan of the Xianggu mushroom molecular biology research programme at The Chinese University of Hong Kong has striven to make Xianggu more palatable to both the taste buds and the pocket. The programme tries to develop and apply state-of-the-art molecular biological technology for the improvement of Xianggu mushroom strains, and three of its projects have received funding from the Research Grants Council three years in a row since 1993. They are :
- Generation of Molecular Markers for Construction of a Genetic Map and Characterization of Genetic Determinants of Some Phenotypic Traits in Shiitake Mushroom (Lentinula edodes) 1993 (HK$756,000)
- Identification and Molecular Cloning of Differentially Expressed Genes Involved in Lignocellulose Utilization in Shiitake Mushroom (Lentinula edodes) 1994 (HK$583,000)
- Project Molecular Cloning and Time Course Studies of Genes Specially Expressed during Fruit Body Development in Shiitake Mushroom (Lentinula edodes) 1995 (HK$713,000)
The projects have different focuses but share in common two objectives: (1) to plot a molecular genetic map of the Xianggu mushroom, and (2) to isolate and manipulate genes that are important for the cultivation of quality Xianggu mushroom.
To Plot a Genetic Map
To facilitate the construction of strains with desirable qualities, researchers need the help of a molecular genetic map to identify and locate the agronomically important genes in the Xianggu mushroom genome. Prof. Kwan and his team have so far generated over 70 molecular markers and located two mating-factor genes. A molecular genetic map has been plotted based on this information (see figure below). It is hoped that eventually genes that are conducive to tastiness, succulence, and rapid growth can be identified, isolated, and reshuffled into a single strain. Plentiful yields of quality Xianggu mushroom may thus result.
Gene Manipulation
Researchers set their eyes on two types of genes in particular: those involved in fruiting body development, and those involved in the degradation of lignocellulosic materials in the growth substrate. Once such genes have been identified, they will
be cloned for further analysis. Gene cloning is the process by which a vector inserted with an isolated gene is placed inside a suitable organism, often some bacterium, to produce multiple copies of that gene.
Fruiting Body Development
Fruiting body development refers to the actual process of fungal growing from mycelium into mushrooms. It is a very complex process which has been poorly studied in the past. Advances in the polymerase-chain-reaction-based RNA fingerprinting method
have now enabled researchers to identify and clone genes that are expressed specifically during Xianggu's fruiting body development. More than 10 putative 'fruiting genes' have been identified, several have been cloned and analysed, and more are expected
to undergo these processes. The functions of these genes can be inferred by comparing their DNA sequence with the known genes in the GeneBank of the National Center for Biotechnological Information in the United States. The information thus acquired will
be used to identify the key steps during fruiting body development and to construct a model of the process. 'Fruiting genes' may also be manipulated to shorten the fruiting process and to increase yields.
Degradation of Lignocellulosic Materials
Mushrooms do not carry out photosynthesis but rely on extracellular enzymes to degrade complex organic matters into soluble substances as nutrition. As materials that are most widely adopted as growth substrates for mushrooms are lignocellulosic, researchers in Prof. Kwan's laboratory want also to identify genes that are expressed specifically during growth on lignocellulosic substrates. They do it by RNA fingerprinting and other approaches, and a relevant gene has been identified and cloned. This
gene can be shuttled back into the mushroom to improve growth and fruiting efficiency.
Wide Applicability of Research Results
Many researchers are now beginning to delve into the myriads of possibilities of the Xianggu mushroom, such as using it to make health foods and for seasoning, analysing its anti-tumour/anti-viral properties, and tapping its other medicinal potentials. Growing better mushrooms tomorrow will certainly affect more than just the palate.
Prof. Kwan Hoi-shan received his Ph.D. from the University of California at Davis in 1982 and is currently associate professor in the Department of Biology at The Chinese University of Hong Kong. His research interests include molecular genetic analysis of edible mushrooms (Lentinula edodes) for strain typing, genetic maps, and breeding programmes; molecular cloning, analysis and utilization of cellulose and ligninase genes from edible mushrooms; and molecular genetic studies of fruit-body development of shiitake mushrooms (Lentinula edodes). Since 1986, Prof. Kwan has been the recipient of five RGC/UGC Earmarked Research Grants and three Croucher Foundation Research Grants.
