Development of Mycorrhizal Associations in Caladenia tentaculata

Abstract

A greater understanding of the orchid-mycorrhizal relationship will assist in the development of improved ex-situ propagation and cultivation methods and appropriate in-situ management techniques for the many threatened Australian terrestrial orchids. The development of mycorrhizal infection in the Australian terrestrial orchid, Caladenia tentaculata, was studied by comparing the in-vitro colonization of embryos infected by one of two fungal isolates prepared from collars of the same orchid growing in-situ. One fungal isolate stimulated germination of the orchid seed and supported subsequent growth (the compatible isolate), but the other did not (the incompatible isolate). Inoculated and un-inoculated orchid embryos and protocorms were sampled frequently after propagation, fresh mounted and visualized with ultraviolet (UV) light, fixed and either cleared or resin-embedded for light microscopy and hand sectioned for scanning electron microscopy (SEM). The compatible isolate characteristically infected the seed through the suspensor cells resulting in healthy seedlings (protocorms). This infection was restricted to the basal cells. The meristematic, starch storage and vascular tissues remained uninfected. Two layers of cells containing morphologically distinct fungal coils (pelotons) formed under the epidermal cells during compatible colonization. The pelotons within the inner-most layer were digested and the cells harboring these pelotons were re-infected with new hyphae. Multicellular rhizoids developed from the basal epidermal cells. The incompatible isolate either failed to infect the seed or penetrated the epidermal cells or developing rhizoids. Chance suspensor infection stimulated distorted protocorm growth, although infection was not restricted to basal cells, and the starch storage and meristematic tissues also were colonized. The distribution of the developing rhizoids was irregular unlike that during compatible infection.