Orchid Reintroduction Based on Seed Germination-Promoting Mycorrhizal Fungi Derived From Protocorms or Seedlings

Seed-Borne Endophytes and Their Host Effects

In the process of long-term co-evolution, endophytes and host plants benefit from and interact with each other, resulting in positive effects such as promoting plant growth, enhancing resistance, producing beneficial secondary metabolites, and negative effects such as carrying pathogens and producing toxins. In addition to the vegetative organs, plant seeds are also colonized by diverse endophytes and serve as vectors for the transmission of endophytes across plant generations. Seed endophytes, termed seed-borne endophytes (SBEs), have attracted much attention because these endophytes are involved in the assembly of the plant association microbiome and exert effects on progeny plants through vertical transfer. However, the importance of SBEs may still be underestimated. The present paper reviews the diversity, origin, and vertical transmission of seed endophytes, as well as their interaction and function with hosts, so as to provide a reference for future research and application of seed endophytes.

Efficiency of mycorrhizal fungi for seed germination and protocorms development of commercial Cattleya species (Orchidaceae)

Orchidaceae is one of the largest plant families and stands out for its wide variety of flowers with ornamental and environmental importance. Cattleya is one of the main commercial genera, presenting a great diversity of species and hybrids that attract the attention of collectors, orchid enthusiasts, and consumers. In their natural environment, orchids associate with mycorrhizal fungi, which are responsible for providing carbon and other nutrients during seed germination. This study investigated the potential of mycorrhizal fungi isolated from the genus Cattleya for in vitro symbiotic germination of seeds from three contrasting Cattleya species, comparing them with non-symbiotic germination in a commercially used culture medium for orchid propagation. The isolated fungi were molecularly identified through phylogenetic analyses of DNA sequences using the ITS (Internal Transcribed Spacer) region. Three isolates obtained were identified as Tulasnella amonilioides, and through microscopic evaluations, the formation of monilioid cells was observed, a morphological characteristic previously unknown for this species. The T. amonilioides isolates were efficient in promoting seed germination of Cattleya bicolor, Cattleya walkeriana and Cattleya jongheana and accelerated the germination process when compared with the non-symbiotic commercial medium, showing to be promised for commercial seed production of these orchids species.

Antioxidant and cytotoxic properties of protocorm-derived phenol-rich fractions of Dendrobium amoenum

BACKGROUND

Dendrobium amoenum is known for its aesthetic and medicinal values but it is threatened due to loss of wild resources. Plant tissue culture promotes wild resource protection and paves the way for secondary metabolite production. In this study, protocorms developed via in-vitro seed cultivation were used for bioactive secondary metabolite production. The objectives of this study were to evaluate total phenolic and flavonoid contents, to identify the bioactive secondary metabolites, to explore the antioxidants and cytotoxic properties of in-vitro-derived protocorms extracts of D. amoenum.

METHODS

Seeds of D. amoenum were cultivated on 10% coconut water, 0.25 and 0.5 mg/L BAP supplemented full-strength and half-strength MS medium to produce protocorms for the isolation of bioactive components. A distinct yellow fraction (DAYF), light-green fraction (DALGF), green fraction (DAGF), and dark-green fraction (DADGF) were obtained from methanol extract on a methanol-based Sephadex LH-20 column. The total phenol and flavonoid contents along with the antioxidant and cytotoxic properties of the fractions were evaluated. The compounds in active DAYF were identified using a GC-MS.

RESULTS

On a full-strength solid MS medium supplemented with 10% coconut water, approximately 95% of the seeds grew into protocorms, while 88.33% did so on a full-strength liquid MS medium. The DAYF had a total phenol content of 206.38 μg of GAE and a total flavonoid content of 101.88 μg of QE. Owing to these high contents, the DAYF inhibited 50% of the DPPH free radicals at a concentration of 63.73 μg/ml. Similarly, it also reduced the growth of HeLa cells by 50% at 67.03 μg/ml and U2OS cells by 50% at 207.40 μg/ml, while it was nontoxic to normal human epithelium cells. Bioactive phenolic compounds 2-methoxy-4-vinylphenol (1), 3,4-dimethoxy-phenol (2), 2-methoxy-4-(1-propenyl)-phenol (3), 2,6-dimethoxy-4-(2-propenyl)-phenol (4), 3-methoxy-1,2-benzenediol (5) were identified in the DAYF.

CONCLUSION

Protocorms of D. amoenum could serve as sources of bioactive secondary metabolites highlighting their potential in alternative medicine.

Shifts in bacterial community composition during symbiotic seed germination of a terrestrial orchid and effects on protocorm development

Fungi and bacteria often occupy very similar niches; they interact closely with each other, and bacteria can provide direct or indirect benefits to plants that form mutualistic interactions with fungi. In orchids, successful seed germination largely depends on compatible mycorrhizal fungi, but whether and how bacteria contribute to seed germination and protocorm development remains largely unknown. Here, we performed field and laboratory experiments to assess the potential role of bacteria in mediating seed germination and protocorm development in the terrestrial orchid Gymnadenia conopsea. Our results suggested that bacterial and fungal communities differ between developmental stages in the germination process. The diversity of bacterial and fungal communities and their interaction network in germinating seeds (Stage 1) differed significantly from those in later developmental stages (Stages 2-5). Pseudomonas gradually became the dominant bacterial group as the protocorms matured and showed a positive association with Ceratobasidiaceae fungi. Seed germination tests in vitro demonstrated that co-inoculation of Ceratobasidium sp. GS2 with Pseudomonas isolates significantly improved protocorm growth and development, suggesting that the observed increase in Pseudomonas abundance during protocorm development directly or indirectly improves the growth of germinating seeds. Overall, our findings indicate that bacteria may exert non-negligible effects on seed germination of orchids and, therefore, offer valuable perspectives for future strategies for conservation and cultivating orchid species.

IMPORTANCE

It is well known that orchid seeds depend on mycorrhizal fungi to supply the necessary nutrients that support germination in natural environments. Apart from fungi, bacteria may also be involved in the germination process of orchid seeds, but so far, their role has not been intensively studied. This research provides evidence that bacterial community composition changes during seed germination of the terrestrial orchid Gymnadenia conopsea. Interestingly, in vitro experiments showed that Pseudomonas spp., which were the most dominant bacteria in the later germination stages, improved protocorm growth. These results suggest that bacteria contribute to the germination of orchid seeds, which may open new perspectives to apply bacteria as a biofertilizer in the introduction and restoration of G. conopsea populations.

Insights into dynamic coenocytic endosperm development: Unraveling molecular, cellular, and growth complexity

The endosperm, a product of double fertilization, is one of the keys to the evolution and success of angiosperms in conquering the land. While there are differences in endosperm development among flowering plants, the most common form is coenocytic growth, where the endosperm initially undergoes nuclear division without cytokinesis and eventually becomes cellularized. This complex process requires interplay among networks of transcription factors such as MADS-box, auxin response factors (ARFs), and phytohormones. The role of cytoskeletal elements in shaping the coenocytic endosperm and influencing seed growth also becomes evident. This review offers a recent understanding of the molecular and cellular dynamics in coenocytic endosperm development and their contributions to the final seed size.

Enhancing Seed Germination of Cremastra appendiculata: Screening and Identification of Four New Symbiotic Fungi in the Psathyrellaceae Family

Several coprinoid fungi have been identified as promotors of Cremastra appendiculata seed germination, while others appear ineffective. This study aimed to discern which genera within the Psathyrellaceae family exhibit this capability and to identify the most effective coprinoid fungi for the cultivation of C. appendiculata. We collected 21 coprinoid fungi from diverse sources and symbiotically cultured them with C. appendiculata seeds. 9 fungi were found to induce seed germination and support seed development, specifically within the genera Coprinellus, Tulosesus, and Candolleomyces. In contrast, fungi that failed to promote germination predominantly belonged to the genera Coprinopsis and Parasola. Notably, four fungi-Coprinellus xanthothrix, Coprinellus pseudodisseminatus, Psathyrella singeri, and Psathyrella candolleana-were documented for the first time as capable of enhancing C. appendiculata seed germination. Strain 218LXJ-10, identified as Coprinellus radians, demonstrated the most significant effect and has been implemented in large-scale production, underscoring its considerable practical value. These findings contribute vital scientific insights for the conservation and sustainable use of C. appendiculata resources.

Orchids acquire fungal carbon for seed germination: pathways and players

To germinate in nature, orchid seeds strictly rely on seed germination-promoting orchid mycorrhizal fungi (sgOMFs) for provision of carbon nutrients. The underlying delivery pathway, however, remains elusive. We develop here a plausible model for sugar transport from sgOMFs to orchid embryonic cells to fuel germination. Orchids exploit sgOMFs to induce the formation of pelotons, elaborate intracellular hyphal coils in orchid embryos. The colonized orchid cells then obtain carbon nutrients by uptake from living hyphae and peloton lysis, primarily as glucose derived from fungal trehalose hydrolyzed by orchid-specific trehalases. The uptake of massive fungally derived glucose is likely to be mediated by two classes of membrane proteins, namely, sugars will eventually be exported transporters (SWEETs) and H+-hexose symporters. The proposed model serves as a launch pad for further research to better understand and improve orchid seed germination and conservation.

Discovery of seed germinating fungi (Mycetinis scorodonius) from Gastrodia elata Bl. f. glauca S. chow in Changbai Mountain and examination of their germination ability

Multi-generational asexual reproduction of Gastrodia elata Bl. will cause seedling species degeneration. Sexual reproduction of Gastrodia elata Bl. seed is an effective method to solve the problem of degeneration. The development of Gastrodia elata Bl. seeds cannot be separated from the germination fungus. However, there are few strains of germination fungus in production, and there is also the problem of species degradation in application for many years. It is very important for the sexual reproduction of Gastrodia elata Bl. to isolate more new strains of excellent germination fungus from the origin. This study used the Gastrodia elata Bl. f. glauca S. chow seeds germination vegetative propagation corms capture method to isolate its symbiotic germination fungus, and comprehensively identified the species of germination fungus by colony morphology, ITS, sporocarps regeneration and germination function, and compared the growth characteristics and germination ability with other germination fungus (Mycena purpureofusca, Mycena dendrobii and Mycena osmundicola). The germination fungus was isolated from the vegetative propagation corms of Gastrodia elata Bl. f. glauca S. chow seeds and named GYGL-1. After comprehensive identification, GYGL-1 was Mycetinis scorodonius. Compared with other germination fungus, GYGL-1 has fast germination speed, vigorous growth, and high germination ability for Gastrodia elata Bl. f. glauca S. chow seeds. Innovated the isolation method of Gastrodia elata Bl. seeds germination fungus, obtained the regenerated sporocarps of the germination fungus, and discovered that Mycetinis scorodonius has a new function of germinating Gastrodia elata Bl. f. glauca S. chow seeds, enriching the resource library of Gastrodia elata Bl. germination fungus.

Comparative transcriptomics and proteomics analysis of the symbiotic germination of Paphiopedilum barbigerum with Epulorhiza sp. FQXY019

Introduction

Paphiopedilum barbigerum is currently the rarest and most endangered species of orchids in China and has significant ornamental value. The mature seeds of P. barbigerum are difficult to germinate owing to the absence of an endosperm and are highly dependent on mycorrhizal fungi for germination and subsequent development. However, little is known about the regulation mechanisms of symbiosis and symbiotic germination of P. barbigerum seeds.

Methods

Herein, transcriptomics and proteomics were used to explore the changes in the P. barbigerum seeds after inoculation with (FQXY019 treatment group) or without (control group) Epulorhiza sp. FQXY019 at 90 days after germination.

Results

Transcriptome sequencing revealed that a total of 10,961 differentially expressed genes (DEGs; 2,599 upregulated and 8,402 downregulated) were identified in the control and FQXY019 treatment groups. These DEGs were mainly involved in carbohydrate, fatty acid, and amino acid metabolism. Furthermore, the expression levels of candidate DEGs related to nodulin, Ca2+ signaling, and plant lectins were significantly affected in P. barbigerum in the FQXY019 treatment groups. Subsequently, tandem mass tag-based quantitative proteomics was performed to recognize the differentially expressed proteins (DEPs), and a total of 537 DEPs (220 upregulated and 317 downregulated) were identified that were enriched in processes including photosynthesis, photosynthesis-antenna proteins, and fatty acid biosynthesis and metabolism.

Discussion

This study provides novel insight on the mechanisms underlying the in vitro seed germination and protocorm development of P. barbigerum by using a compatible fungal symbiont and will benefit the reintroduction and mycorrhizal symbiotic germination of endangered orchids.

Fungal communities associated with early immature tubers of wild Gastrodia elata

Full myco-heterotrophic orchid Gastrodia elata Bl. is widely distributed in Northeast Asia, and previous research has not fully investigated the symbiotic fungal community of its early immature tubers. This study utilized Illumina sequencing to compare symbiotic fungal communities in natural G. elata immature tubers and their habitats. LEfSe (Linear Discriminant Analysis Effect Size) was used to screen for Biomarkers that could explain variations among different fungal communities, and correlation analyses were performed among Biomarkers and other common orchid mycorrhizal fungi. Our results illustrate that the symbiotic fungal communities of immature G. elata tubers cannot be simply interpreted as subsets of the environmental fungal communities because some key members cannot be traced back to the environment. The early growth of G. elata was related to a small group of fungi, such as Sebacina, Thelephora, and Inocybe, which were also common mycorrhizal fungi from other orchids. In addition, Mycena, Auricularia, and Cryptococcus were unique fungal partners of G. elata, and many new species have yet to be discovered. Possible symbiotic Mycena should be M. plumipes and its sibling species in this case. Our results provide insight into the symbiotic partner switch and trophic pattern change during the development and maturation of G. elata.

Bioactive steroids from seed germination supporting fungus (Ceratobasidium GS2) of the terrestrial orchid Gymnadenia conopsea

Almost all orchids rely on mycorrhizal fungus to support their seed germination. To date, the effect of active components in mycorrhizal fungus on orchid seed germination largely remains unknown. In this study, we aimed to investigate the impact of active components found in mycorrhizal fungus on orchid seed germination. Specifically, we focused on a terrestrial orchid Gymnadenia conopsea and its host-specific seed germination supporting fungus Ceratobasidium GS2. In total, several steroids (1-7) were isolated from this fungus. Notably, compounds 1, 2, 4, and 5 exhibited significant enhancements in protocorm volume. Moreover, compounds 1-6 demonstrated strong promotion of protocorm differentiation. These findings suggest that steroids may play a crucial role in the symbiotic germination of G. conopsea seeds. Future studies should continue to explore the specific mechanisms through which these steroids exert their effects, contributing to our understanding of orchid biology and mycorrhizal interaction.

Symbiotic seed germination and seedling growth of mycorrhizal fungi in Paphiopedilum hirsutissimun (Lindl.Ex Hook.) Stein from China

Similar to other orchid species, Paphiopedilum hirsutissimum (Lindl.ex Hook.) Stein, relies on nutrients provided by mycorrhizal fungus for seed germination and seedling development in the wild owing to a lack of endosperm in its seeds. Therefore, obtaining suitable and specialized fungi to enhance seed germination, seedling formation, and further development is considered a powerful tool for orchid seedling propagation, reintroduction, and species conservation. In this study, we investigated the diversity, abundance, and frequency of endophytic fungal strains in the root organs of P. hirsutissimum. One family and five genera of the fungi were isolated and identified through rDNA-ITS sequencing. The ability of isolated fungi to germinate in vitro from the seeds of this species was evaluated, and the development of P. hirsutissimum protocorm has been described. The findings showed that the treatments inoculated with endophytic fungal DYXY033 may successfully support the advanced developmental stage of seedlings up to stage 5. In addition, scanning electron microscopy (SEM) revealed that the mycelium of this strain began to invade from either end of the seeds up to the embryo, extending rapidly from the inside to the outside. Its lengthening resulted in the bursting of the seed coat to form protocorms, which developed into seedlings. The results showed that DYXY033 has a high degree of mycobiont specificity under in vitro symbiotic seed germination conditions and is a representative mycorrhizal fungus with ecological value for the species. In summary, this strain may particularly be significant for the protection of P. hirsutissimum species that are endangered in China. In the long run, it may also contribute to global efforts in reintroducing orchid species and in realizing in situ restorations of threatened orchid populations.

Mycorrhizal specificity differences in epiphytic habitat: three epiphytic orchids harbor distinct ecological and physiological specificity

Orchidaceae has diversified in tree canopies and accounts for 68% of vascular epiphytes. Differences in mycorrhizal communities among epiphytic orchids can reduce species competition for mycorrhizal fungi and contribute to niche partitioning, which may be a crucial driver of the unusual species diversification among orchids. Mycorrhizal specificity-the range of fungi allowing mycorrhizal partnerships-was evaluated by assessment of mycorrhizal communities in the field (ecological specificity) and symbiotic cultures in the laboratory (physiological specificity) for three epiphytic orchids inhabiting Japan. Mycorrhizal communities were assessed with co-existing individuals growing within 10 cm of each other, revealing that ecological specificity varied widely among the three species, ranging from dominance by a single Ceratobasidiaceae fungus to diverse mycobionts across the Ceratobasidiaceae and Tulasnellaceae. In vitro seed germination tests revealed clear differences in physiological specificity among the three orchids, and that the primary mycorrhizal partners contributed to seed germination. In vitro compatibility ranges of three orchids strongly reflect the mycorrhizal community composition of wild populations. This suggests that differences in in situ mycorrhizal communities are not strongly driven by environmental factors, but are primarily due to physiological differences among orchid species. This study shows that the symbiotic strategy among the epiphytic orchid species varies from specialized to generalized association, which may contribute to biotic niche partitioning.

Effect of different substrates on in vitro symbiotic seed germination for soilless production of Anacamptis laxiflora orchid

In recent years, the orchid species have become endangered due to overuse and habitat destruction. As with most flowering plants, seed production is the primary strategy for reproduction in orchids. Orchids produce tiny seeds consisting of a seed coat and a rudimentary embryo. However, it lacks the endosperm, which is generally required as the primary energy source during germination. The only way to germinate orchid seeds is to get nutrients from an external source. In nature, this is achieved by mycorrhizal symbiosis. This study used Ceratobasidium sp. inoculation of Anacamptis laxiflora (Lam.) seeds combined with media with various organic substrates to determine their effectiveness on germination and seedling development by in vitro culture. The highest germination rate (35.78%) was obtained in the medium with addition of young hazelnut leaves. Then, soilless ex vitro symbiotic germination was performed on young hazelnut leaves, the most effective organic substrate. Seed germination was determined to be 19.01% in this medium while 14.87% seedlings with developed leaves and roots were formed. For the first time, success was achieved by producing A. laxiflora from seed in ex vitro conditions without soil and adapting it to nature.

Characterization of a Plant Growth-Promoting Endohyphal Bacillus subtilis in Fusarium acuminatum from Spiranthes sinensis

Successful seed germination and seedling growth in orchids require an association with mycorrhizal fungi. An endophytic Fusarium fungal strain YZU 172038 exhibiting plant growth-promoting (PGP) ability was isolated from the roots of Spiranthes sinensis (Orchidaceae). The harboring endohyphal bacteria were detected in the hypha by SYTO-9 fluorescent nucleic acid staining, fluorescence in situ hybridization (FISH), and PCR amplification of the 16S rDNA gene's region. Consequently, one endohyphal bacterium (EHB) - a strain YZSR384 was isolated and identified as Bacillus subtilis based on morphology, phylogenetic analysis, and genomic information. The results indicated that the strain YZSR384 could significantly promote the growth of rice roots and shoots similar to its host fungus. Its indole acetic acid (IAA) production reached a maximum of 23.361 μg/ml on the sixth day after inoculation. The genome annotation revealed several genes involved in PGP traits, including the clusters of genes encoding the IAA (trpABCDEFS), the siderophores (entABCE), and the dissolving phosphate (pstABCS and phoABDHPR). As an EHB, B. subtilis was first isolated from endophytic Fusarium acuminatum from S. sinensis.

Isolation and identification of beneficial orchid mycorrhizal fungi in Paphiopedilum barbigerum (Orchidaceae)

Seed germination and seedling development in nearly all orchid species rely on a symbiotic relationship with mycorrhizal fungi; however, this is not the case with all mycorrhizal fungi. This study aims to provide an understanding about the important role of mycorrhiza in seed germination and growth of Paphiopedilum barbigerum. Therefore, we isolated and identified endophytic fungi from the roots of wild P. barbigerum. The beneficial mycorrhizal fungi Epulorhiza sp. FQXY019 and Tulasnella calospora FQXY017 were screened by seed symbiotic germination tests and found to promote seed germination. However, only the seeds inoculated with FQXY019 progressed from the seed germination to rooting stage. This shows that mycorrhizal fungi and P. barbigerum have a specific relation at different growth phases. In addition, we selected FQXY019 and inoculated it into MS medium, B5 medium, OMA medium, and PDA medium. The results showed that FQXY019 co-cultured on PDA significantly promoted the increase in seedling fresh weight, leaf length, and root length (p < .01). Furthermore, it significantly promoted the root number and leaf number of seedlings compared with those co-cultured on MS, B5, and OMA media and control (p < .05). Thus, this study demonstrated the promoting effect of Epulorhiza sp. FQXY019 on seed germination and seedling development, making it an alternative method for the artificial propagation of P. barbigerum.

Large-Scale In Vitro Multiplication and Phytochemical Analysis of Himantoglossum affine (Boiss.) Schltr.: An Endangered Euro-Mediterranean Terrestrial Orchid

Himantoglossum affine is a threatened terrestrial orchid. We aimed to optimize asymbiotic seed germination and direct embryogenesis and to analyze the phytochemical profile and physico-biochemical analysis of leaf and tuber. The individual use of organic nitrogen compounds resulted in higher germination efficiencies, while the shortest times to germination were observed using coconut water plus casein hydrolysate. Plantlets grown on media supplemented with pineapple juice and peptone had the highest plantlet length and weight. For embryogenesis, the highest regeneration rate (44%) and embryo number/explant (10.12 ± 2.08) were observed in young protocorm-like body (PLB) explants with 0.5 mg/L naphthalene acetic acid (NAA) and 1 mg/L thidiazuron (TDZ). During the acclimatization process, the scattered vascular tubes converted to fully developed vascular tissues, ensuring maximum sap flux. Gas chromatography–mass spectrometry analysis identified 1,2,3-propanetriol, monoacetate, 4H-pyran-4-one, 2,3-dihydro-3,5-dihydroxy-6-methyl, and 2-butenedioic acid, 2-methyl-, (E)- as the most prevalent compounds. We reported higher contents of total phenolics and flavonoids and antioxidant activity compared to other terrestrial orchids. The glucomannan content (36.96%) was also higher than starch content (31.31%), comparable to those reported in other tuberous orchids. Based on the fragmentation of H. affine populations in the Middle East and Euro-Mediterranean countries due to over-harvesting, climate change, and/or human impact, our procedure offers a tool for the re-introduction of in vitro-raised plants to threatened areas.

The challenges of growing orchids from seeds for conservation: An assessment of asymbiotic techniques

Lewis Knudson first successfully germinated orchid seeds asymbiotically on artificial medium in 1922. While many orchid species have since been grown asymbiotically, the tremendous variation in how species respond to artificial medium and growth conditions ex situ has also become apparent in the past century. In this study, we reviewed published journal articles on asymbiotic orchid seed germination to provide a summary of techniques used and to evaluate if these differ between terrestrial and epiphytic species, to identify areas where additional research is needed, and to evaluate whether asymbiotic germination could be used more often in ex situ conservation. We found articles reporting successful asymbiotic germination of 270 species and 20 cultivars across Orchidaceae. Researchers often used different techniques with epiphytic versus terrestrial species, but species-specific responses to growth media and conditions were common, indicating that individualized protocols will be necessary for most species. The widespread success in generating seedlings on artificial media suggests that asymbiotic techniques should be another tool for the conservation of rare orchid species. Further advances are needed in understanding how to introduce mycorrhizae to axenically grown orchids and to maximize the viability of seedlings reintroduced into natural habitats to fully utilize these methods for conservation.