Annals of Botany News in Brief

In vitro axenic germination and cultivation of mixotrophic Pyroloideae (Ericaceae)

Pyroloids, forest sub-shrubs of the Ericaceae family, are an important model for their mixotrophic nutrition, which mixes carbon from photosynthesis and from their mycorrhizal fungi. They have medical uses but are difficult to cultivate ex situ; in particular, their dust seeds contain undifferentiated, few-celled embryos, whose germination is normally fully supported by fungal partners. Their germination and early ontogenesis thus remain elusive.

An optimized in vitro cultivation system of five representatives from the subfamily Pyroloideae was developed to study the strength of seed dormancy and the effect of different media and conditions (including light, gibberellins and soluble saccharides) on germination. Figura et al. analysed the plants for morphological, anatomical and histochemical development.

Representatives of in vitro grown seedlings of species used in this study.
Representatives of in vitro grown seedlings of species used in this study. (A) Pyrola minor cultivated in the dark, (B) Pyrola minor cultivated in the light, and (C) Moneses uniflora, (D) Orthilia secunda, (E) Chimaphila umbellata and (F) Monotropa uniflora cultivated in the dark. Scale bars = 0.5 mm. Image: Figura et al. 2018.

Thanks to this novel cultivation method, which breaks dormancy and achieved up to 100 % germination, leafy shoots were obtained in vitro for representatives of all pyroloid genera (Moneses, Orthilia, Pyrola and Chimaphila). In all cases, the first post-germination stage is an undifferentiated structure, from which a root meristem later emerges, well before formation of an adventive shoot.

This cultivation method can be used for further research or for ex situ conservation of pyroloid species. After strong seed dormancy is broken, the tiny globular embryo of pyroloids germinates into an intermediary zone, which is functionally convergent with the protocorm of other plants with dust seeds such as orchids. Like the orchid protocorm, this intermediary zone produces a single meristem: however, unlike orchids, which produce a shoot meristem, pyroloids first generate a root meristem.