Symbiotic Culture of Three Closely Related Dendrobium Species Reveals a Growth Bottleneck and Differences in Mycorrhizal Specificity at Early Developmental Stages

Abstract

Mycorrhizal specificity, i.e., the range of fungi allowing mycorrhizal partnerships, differs among orchid species, but that at early developmental stages is unclear. We investigated whether mycorrhizal specificity during seed germination and seedling development differs among three Dendrobium species, D. officinale, D. okinawense and D. moniliforme, in vitro. Nine mycorrhizal fungal strains were obtained from the roots of these species and cultured with a seed of each Dendrobium species. Five to eight fungal strains stimulated seed germination, whereas one to four fungal isolates significantly promoted protocorm development in the three species. To evaluate effects on leafy seedling growth, seedlings obtained from asymbiotic culture were cultured with nine fungal isolates. D. officinale and D. okinawense showed specificity for a single Serendipitaceae or Tulasnellaceae isolate, whereas D. moniliforme exhibited specificity for three isolates of Serendipitaceae and Tulasnellaceae. Therefore, the three Dendrobium species had a growth bottleneck from seed germination to the protocorm stage, and mycorrhizal specificity of protocorm growth and seedling development in vitro varied among the species. Our findings imply divergent mycorrhizal specificity in Dendrobium species at early developmental stages. This study provides insights into the diversity of orchid mycorrhizal specificity, as well as valuable information for conservation of endangered orchids.

The epiphytic orchid Vanda falcata is predominantly associated with a single Tulasnellaceae fungus in adulthood, and Ceratobasidiaceae fungi strongly induce its seed germination in vitro

“…Several terrestrial orchid taxa, such as Cephalanthera species (Bidartondo & Read, 2008), Bletilla striata (Fuji et al, 2020), and Cyrtopodium glutiniferum (Pereira et al, 2015), associate with various mycobionts at the early seed germination stage, but only a subset of them promote subsequent orchid development. Such fungal reduction during plant development has been reported in the epiphytic orchid genus Dendrobium (Meng et al, 2019a; Zhang et al, 2022); thus, changes in mycorrhizal associations during orchid ontogeny may also be expected among epiphytic orchid species.…”

“…Adult plants of the epiphytic orchid D. okinawense are predominantly associated with a single Tulasnellaceae OTU across its distribution range (Rammitsu et al, 2021a), and this OTU shares 99.8% sequence similarity with TU11 of V. falcata. The main fungal partner of adult D. okinawense did not induce seed germination in vitro, but significantly enhanced the growth of leafy seedlings (Zhang et al, 2022). These studies suggest that TU11 may contribute to plant growth after leaf development rather than to seed germination in V. falcata.…”

“…The main fungal partner of adult D . okinawense did not induce seed germination in vitro, but significantly enhanced the growth of leafy seedlings (Zhang et al, 2022). These studies suggest that TU11 may contribute to plant growth after leaf development rather than to seed germination in V .…”

Mycorrhizal Fungi of Phalaenopsis japonica (Orchidaceae) and Their Role in Seed Germination and Seedling Development

“…A 6 mm plug of pre-cultured fungal isolates was inoculated onto the OMA medium, and the plates were cultured under a 12 h/12 h light/dark photoperiod at 25 ± 1 • C. As a control, plates with no fungal inoculum were also prepared. After 4 months of culturing, the seeds were counted under a stereomicroscope, and seed germination and protocorm development were scored on a scale of 0-6 as follows: stage 0, no germination; stage 1, seed with swollen embryo; stage 2, enlarged embryo with ruptured seed coat; stage 3, appearance of the rhizoid; stage 4, appearance of the proto-meristem; stage 5, emergence of the first leaf; and stage 6, emergence of the first root and further development (Supplementary Figure S1) [11]. Seed germination (%) per stage was calculated using the following formula: percentage seed germination = (number of seeds per germination stage/total number of viable seeds) × 100.…”

“…Diversity 2024, 16, 218 2 of 16 While orchids can establish associations with various fungal partners, not all associations have equal impacts on orchid growth and development [10,11]. Since mycorrhizal fungi vary significantly among orchid species [12,13], thorough elucidation of the diversity and function of the OMF associated with each orchid species is vital.…”