Morpho-molecular characterization of Ceratobasidium sp.: A mycorrhizal fungi isolated from a rare epiphytic orchid Gastrochilus calceolaris (J. E. Sm.) D. Don

A mycorrhizal fungus, Ceratobasidium sp. GC (NCBI Gene Bank Accession no GQ369961) associated with the roots of an epiphytic orchid Gastrochilus calceolaris was investigated by cultural morphology, microscopic features and molecular analysis of Internal Transcribed Spacer (ITS) regions sequences of nuclear ribosomal DNA. The colony appearance of the fungal endophyte was fluffy growth pattern and the colour of the young colony was milky white on both surfaces that turned in to brown at maturity on the upper surface and deep brown on reverse surface. The microscopic features of the fungus i.e. hyphal diameter, multi-nucleate vegetative cells, right angle branching pattern with slight constriction at branching point and a dolipore septum near the branching point were observed. All the characters corroborated the identity with anamorphic Rhizoctonia like fungi. The ITS regions sequences of nrDNA and phylogenetic analysis based on the neighbor-joining method showed clustered with Rhizoctonia like fungi, and the maximum identity (98.28%) with Ceratobasidium RR and Ceratobasidium FPUB isolated from Rhynchostylis retusa and Dactylorrhiza hetagera, respectively. Thus, the ITS of nrDNA sequences validated the morphological data. This is the first report of orchid mycorrhizal fungi from Bangladesh. Introduction Mycorrhizal fungi are fundamental in orchid growth and metabolism, and influence the distribution and rarity of these delicate plants. The existence of mycorrhizal associations has been established since long in orchids with certain heterogeneous groups of fungi (Hadley, 1982). It is a biotrophic mutualistic symbiosis prevalent in more then 90% of the higher plants in which both partners benefited in terms of nutrients exchange, evolution and fitness. It is one of the most significant events in the successful colonization of land by plants and the evolution of biotrophic root-inhabiting symbioses. Now-a-day, it is the most interesting area of research throughout the globe. Seed germination in orchids is totally dependent on mycobionts due to lack of sufficient food reserves (Smith and Read, 1997). Orchids are categorized as myco-heterotropic, holomycotropic and mixotropic based on their nutritional dependency on fungal partner (Dearnaley, 2007). The orchids which depend on mycorrhizal fungi during seed germination stages only are referred as mycoheterotropic. The achlorophyllous orchids entirely depend on mycorrhizal fungi for nourishment throughout their life cycle are categorized as holomycotropic. The third orchid nutritional mode exists mixotrophy. Such orchids are photosynthetic at the adult stage but augment their nutrition requirements specially carbon via mycorrhizal fungi (Dearnaley, 2007). Mixotrophic orchids may be an evolutionary step between autotrophic and mycoheterotropic orchids (Julou et al., 2005). Nevertheless, all orchids are mycoheterotrophic during their early stages of growth, development and survival in nature. Mycorrhizal fungi *Corresponding author: Email: musharof20bd@yahoo.com