Compatible and Incompatible Mycorrhizal Fungi With Seeds of Dendrobium Species: The Colonization Process and Effects of Coculture on Germination and Seedling Development.

Guang-Hui Ma,Jiang-Yun Gao,Xiang-Gui Chen,Marc-André Selosse

doi:10.3389/fpls.2022.823794

Abstract

Orchids highly rely on mycorrhizal fungi for seed germination, and compatible fungi could effectively promote germination up to seedlings, while incompatible fungi may stimulate germination but do not support subsequent seedling development. In this study, we compared the fungal colonization process among two compatible and two incompatible fungi during seed germination of Dendrobium officinale. The two compatible fungi, i.e., Tulasnella SSCDO-5 and Sebacinales LQ, originally from different habitats, could persistently colonize seeds and form a large number of pelotons continuously in the basal cells, and both fungi promoted seed germination up to seedling with relative effectiveness. In contrast, the two incompatible fungi, i.e., Tulasnella FDd1 and Tulasnella AgP-1, could not persistently colonize seeds. No pelotons in the FDd1 treatment and only a few pelotons in the AgP-1 treatment were observed; moreover, no seedlings were developed at 120 days after incubation in either incompatible fungal treatment. The pattern of fungal hyphae colonizing seeds was well-matched with the morphological differentiation of seed germination and seedling development. In the fungal cocultural experiments, for both orchids of D. officinale and Dendrobium devonianum, cocultures had slightly negative effects on seed germination, protocorm formation, and seedling formation compared with the monocultures with compatible fungus. These results provide us with a better understanding of orchid mycorrhizal interactions; therefore, for orchid conservation based on symbiotic seed germination, it is recommended that a single, compatible, and ecological/habitat-specific fungus can be utilized for seed germination.

Highlights

Orchids show a series of floral characteristics distinct from those of most other flowering plants
In the SSCDO-5 treatment, most seeds turned green with embryos swollen (Stage 1), fungal hyphae congregated at the basal end outside of seeds, and a few pelotons in the basal cells of enlarged seeds were observed (Figure 1b1)
The results showed that the pattern of fungal hyphae colonizing seeds was well-matched with the morphological differentiation of seed germination and seedling development, and compatible fungi could persistently colonize seeds and quickly promote the conversion of germinated seeds into seedlings

Summary

Introduction

Orchids show a series of floral characteristics distinct from those of most other flowering plants. Orchid mycorrhizal fungi (OMFs) belong to the so-called rhizoctonias, a polyphyletic group of fungi belonging to Tulasnellaceae and Ceratobasidiaceae (in the order Cantharellales), as well as Serendipitaceae (in the order Sebacinales; Smith and Read, 2008; Dearnaley et al, 2012; Weiß et al, 2016). These taxa are not mycorrhizal in other plants, with the exception of some of their subclades (Selosse et al, 2021). It is commonly known that orchid seeds are colonized by one or more mycorrhizal fungi to form protocorms (Leake, 1994; Smith and Read, 2008). Symbiotic fungi undergo a taxonomic and functional bottleneck during orchid seed germination, and the interaction between partners, rather than intrinsic fungal traits, may be involved in the bottleneck of fungal symbionts during orchid seed germination (Meng et al, 2019b)

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Conclusion