What role does the seed coat play during symbiotic seed germination in orchids: an experimental approach with Dendrobium officinale

Abstract

Background Orchids require specific mycorrhizal associations for seed germination. During symbiotic germination, the seed coat is the first point of fungal attachment, and whether the seed coat plays a role in the identification of compatible and incompatible fungi is unclear. Here, we compared the effects of compatible and incompatible fungi on seed germination, protocorm formation, seedling development, and colonization patterns in Dendrobium officinale; additionally, two experimental approaches, seeds pretreated with NaClO to change the permeability of the seed coat and fungi incubated with in vitro-produced protocorms, were used to assess the role of seed coat played during symbiotic seed germination. Results The two compatible fungi, Tulasnella sp. TPYD-2 and Serendipita indica PI could quickly promote D. officinale seed germination to the seedling stage. Sixty-two days after incubation, 67.8 ± 5.23% of seeds developed into seedlings with two leaves in the PI treatment, which was significantly higher than that in the TPYD-2 treatment (37.1 ± 3.55%), and massive pelotons formed inside the basal cells of the protocorm or seedlings in both compatible fungi treatments. In contrast, the incompatible fungus Tulasnella sp. FDd1 did not promote seed germination up to seedlings at 62 days after incubation, and only a few pelotons were occasionally observed inside the protocorms. NaClO seed pretreatment improved seed germination under all three fungal treatments but did not improve seed colonization or promote seedling formation by incompatible fungi. Without the seed coat barrier, the colonization of in vitro-produced protocorms by TPYD-2 and PI was slowed, postponing protocorm development and seedling formation compared to those in intact seeds incubated with the same fungi. Moreover, the incompatible fungus FDd1 was still unable to colonize in vitro-produced protocorms and promote seedling formation. Conclusions Compatible fungi could quickly promote seed germination up to the seedling stage accompanied by hyphal colonization of seeds and formation of many pelotons inside cells, while incompatible fungi could not continuously colonize seeds and form enough protocorms to support D. officinale seedling development. The improvement of seed germination by seed pretreatment may result from improving the seed coat hydrophilicity and permeability, but seed pretreatment cannot change the compatibility of a fungus with an orchid. Without a seed coat, the incompatible fungus FDd1 still cannot colonize in vitro-produced protocorms or support seedling development. These results suggest that seed coats are not involved in symbiotic germination in D. officinale.

A novel method to produce massive seedlings via symbiotic seed germination in orchids

“…Ecologically, the natural occurrences of OMFs can affect key processes such as seed germination and plant growth and survival ( Jacquemyn et al., 2012 ) and therefore result in the compatibility of mycorrhizal associations in orchids ( Põlme et al., 2018 ). The four OMFs used to create seed-fungus complexes in this study were originally from different geographical locations/habitats ( Table 1 ) and have been shown to be compatible with D. officinale and quickly promote seed germination up to the seedling stage with relative effectiveness ( Shao et al., 2019 ; Wang et al., 2021 ; Chen et al., 2022 ). Although seedlings were produced in the three fungal treatments, the seedling productivity varied greatly among treatments ( Figure 2 ).…”

The seed microbiomes of staple food crops

“…and Paenibacillus spp., with the ability to use host machinery in the synthesis of biocontrol metabolites, such as antibiotics, organic acids and lipopeptides, are already used as alternatives to chemical pesticides (Thomloudi et al, 2021 ). The close interactions of endophytes with plants during seed embryonic development make them unique compared with the rhizosphere colonizers (Chen et al, 2022 ). In fact, by soil inoculation and foliar spray of beneficial microbes at different growth stages, seed endophytes are being explored for their ability to enhance agricultural crop productivity (Mukherjee et al, 2022 ; Taheri et al, 2022 ).…”

“…The changes that are usually observed at the seeds' maturity stage with an increase in the seeds' osmotic pressure may cause a greater shift in the seed microbiomes (Abdelfattah et al, 2023 ). For example, it has been found that barely bacterial community compositions are associated with the total and reducing sugar contents and the viscosity of the seeds, thus possibly connected to the osmotic pressure tolerance of the endophytes (Chen et al, 2022 ). Therefore, it seems most microbes find seeds as their destinations based on their nature, compartments, maturity and cultivars.…”

The orchid seed coat: a developmental and functional perspective

“…This safeguards the entry of fungal hyphae into the embryo's future shoot apical zone, allowing proper shoot development. Moreover, the compatibility between the fungi and the orchids is a critical factor in determining the ultimate success of symbiotic seed germination (Chen et al 2022 ). Although the function of the seed coat is 'passive', the structural design enables it to play a role in the early stages of symbiotic seed germination.…”