FUNGAL SYMBIOSES WITH ORCHID PROTOCORMS

Orchid species have minute seeds that in nature and in vitro, if compl ex polysaccharides are used as a carbon source, must be invaded by sym biotic fungi in order for germination to occur. Subsequent to germinat ion, a protocorm, develops from the simple embryo contained in the mat ure seed. A shoot apical meristem and adventitious roots are initiated at the chalazal end of the protocorm, events that are necessary for s eedling establishment. Most of the information pertaining to the inter action between symbiotic fungi and orchid protocorms has been obtained from laboratory experiments using terrestrial orchid species germinat ed in vitro in combination with fungal species that have been isolated tither from roots of orchid species or from diseased non-host plant s pecies. Fungal isolates were previously grouped in the form-genus Rhiz octonia although anamorphs and teleomorphs have now been placed in sev eral genera. Symbiotic fungal hyphae enter the embryo of imbibed seeds either through the suspensor or epidermal hairs and form coiled, bran ched structures called pelotons in parenchyma cells of the embryo. The peloton is separated from the parenchyma cell cytoplasm by a perifung al membrane and interfacial matrix material. The perifungal membrane s hares some characteristics with the peripheral plasma membrane but dif fers in that it does not show adenylate cyclase activity. Colonization of embryo cells induces nuclear hypertrophy, partly due to an increas e in DNA synthesis, and changes in both microtubules and actin filamen ts. Pelotons undergo degradation and during this phase pectins, beta-1 ,3 glucans and cellulose are deposited in the interfacial matrix surro unding senescing hyphae and hyphal clumps. The orchid protocorm-symbio tic fungal system shows promise for studies of plant cell-fungal inter actions at the molecular level.