Gibberellic Acid Induces Asymbiotic Germination of the Obligate Mycoheterotroph Pterospora andromedea (Ericaceae)

Studies of the Monotropoideae (monotropes; Ericaceae), a monophyletic group of nonphotosynthetic, mycoheterotrophic, and often rare or endangered plants, have been limited by the inability to propagate them. Monotropes associate with specific fungal hosts, and the only previously known method of seed germination was induction by host fungi or closely related fungi. In order to overcome very low monotrope seed germination rates and to facilitate further study and conservation efforts, we developed a method using gibberellic acid (GA) to induce asymbiotic germination. Pterospora andromedea Nutt. (Monotropoideae, Ericaceae) and Sarcodes sanguinea Torr. (Monotropoideae, Ericaceae) seeds from California were exposed to their fungal symbiont or to agarose infused with 0–1 mM GA and then scored for germination. Continuous exposure to 0.5 mM GA for two months induced 75% P. andromedea germination, compared to only 21% with its host fungus, Rhizopogon salebrosus A.H. Sm. (Basidiomycota). Even short GA exposure (one or three days) significantly enhanced germination (69% and 90%, respectively). The highest germination rate was observed with exposure to 0.5 mM GA for three or 14 d. The closely related S. sanguinea required a three-month exposure to GA and even then produced far lower germination rates (,1%). Nevertheless, this is the only known method of inducing monotrope germination without the presence of a specific fungal symbiont. In the case of P. andromedea, exogenous GA stimulates germination at rates far higher than that achieved with its fungal symbiont. Application of GA to induce monotrope germination may be used to examine the early stages of mycoheterotroph development, to improve assays for seed viability, and potentially to aid conservation efforts.