Ultrastructural changes during the symbiotic seed germination of Gastrodia elata with fungi, with emphasis on the fungal colonization region

Background Gastrodia elata is a fully mycoheterotrophic orchid and has long been used in traditional Chinese medicine. The life cycle of G. elata requires an association with two different fungi-Mycena for seed germination and Armillaria for tuber growth. The association with Armillaria is representative of the phytophagous type of orchid mycorrhiza: the intracellular hyphae are lysed without forming condensed pelotons. However, whether the association with Mycena during seed germination belongs to the same type of orchid mycorrhiza is unknown. Results Histological and ultrastructural studies revealed several notable features in different developmental stages. First, a thickened cell wall with papillae-like structures appeared during fungal penetration in the suspensor end cell, epidermal cells and cortical cells of germinating embryos. In addition, the formation of two distinctive cell types in the colonized region of a protocorm (i.e., the passage canal cell filled with actively growing fungal hyphae) can be observed in the epidermal cell, and the distinctive digestion cell with a dense cytoplasm appears in the cortex. Finally, within the digestion cell, numerous electron-dense tubules form a radial system and attach to degrading fungal hyphae. The fungal hyphae appear to be digested through endocytosis. Conclusions The present study provides important structural evidence for the phytophagous type of orchid mycorrhiza in the symbiotic germination of G. elata with Mycena. This case demonstrates a particular nutrient transfer network between G. elata and its litter-decaying fungal partner.