Core Mycorrhizal Fungi Promote Seedling Growth in Dendrobium officinale: An Important Medicinal Orchid

Abstract

The critically endangered orchid Dendrobium officinale, valued for its medicinal properties, depends on specific seedling-associated mycorrhizal fungi (SAMF) for successful early-stage seedling development. However, conservation efforts are often hindered by difficulties in obtaining suitable SAMF, leading to poor seedling establishment in both natural and cultivated environments. In this study, we explored the growth-promoting effects of SAMF and evaluated the performances of synthetic fungal combinations. Our results demonstrated that mycorrhizal fungi, widely distributed across multiple habitats with high isolation frequencies, significantly promoted the growth of D. officinale, with specific fungi favoring different growth parameters. Tulasnella sp. TP-2 and TP-3 significantly improved stem diameter and plant height by 2.622 mm and 4.621 cm, while Tulasnella sp. TP-8 significantly increased tillering by a factor of 4.47. Additionally, Tulasnella sp. TP-11 and TP-13 markedly increased the number of new leaves (4.45) and new roots (2.688), respectively, identifying them as essential core OMFs for D. officinale seedlings. Contrary to expectations, synthetic fungal combinations composed of core orchid mycorrhizal fungi (core OMFs) did not exhibit synergistic growth-promoting effects. Instead, pronounced offset effects were observed, indicating that interactions between fungi may introduce competition or inhibition, limiting their collective ability to enhance plant growth. Our results confirmed that the core OMFs significantly promoted the growth of D. officinale seedlings. These core OMFs can serve as essential components in specialized microbial fertilizers for D. officinale, improving growth efficiency and yield, and supporting the sustainable development of the D. officinale industry.

Keywords: growth-promoting fungi; seedling growth; seedling-associated mycorrhizal fungi; synergistic/offset effects; synthetic fungal combinations.

Figure 1

Colonization of mycorrhizal fungi in the roots of D. officinale seedlings after symbiotic cultivation. (a) Seedlings inoculated with TP-11, TP-13, TP-8, TP-3, and TP-2, and cultivated for 90 days. (b,c) Ink staining results and resin sections of seedling roots inoculated with TP-8 cultivated for 60 days, revealing abundant formation of pelotons (red arrows), indicating successful fungal colonization. (d,e) Colonization status of seedlings inoculated with TP-13 and TP-21 after 30 days of symbiosis. (f) Illustration of entire root system ink-staining results of seedlings inoculated with Syncom9 after 90 days of symbiosis. The deep-blue areas (green arrows) denote locations of fungal colonization in the roots.

Figure 2

Isolation frequencies of 27 SAMF in six original habitats of D. officinale: GN (Guangnan County, Yunnan), DXLM (Langshan Mountain, Hunan), KSLM (Langshan Mountain, Hunan), LD (Luding County, Sichuan), SM (Shimian County, Sichuan), and LT (Luotian Town, Chongqing). Numbers in parentheses indicate the total isolation frequencies of each fungus across the six habitats. Red numerals denote strains with isolation frequencies > 10, reflecting their high prevalence in seedling stage. “*” indicates significant growth promotion, e.g., * leaf indicates significant enhancement of seedling leaf growth; p < 0.05.

Figure 3

Performance of D. officinale seedlings after 90 days of symbiotic cultivation with mycorrhizal fungi TP-2, TP-3, TP-8, TP-11, and TP-13 in terms of various growth parameters. (a) TP-2, (b) TP-3, (c) TP-8, (d) TP-11, (e) TP-13. Gray-shaded area represents the performance of non-inoculated control group (CK) seedlings. Light-gray-shaded area represents the mean values of growth for seedlings inoculated with all fungal treatments.

Figure 4

The impact of different synthetic fungal combinations on the growth of D. officinale seedlings. (a–f) Represent the effects of different synthetic fungal combinations on the new leaves, new roots, sprouting tillers, plant height, stem diameter, and dry weight in seedlings of D. officinale, respectively. Different uppercase/lowercase letters indicate significant differences among different treatments, p < 0.05, “ns” indicates no significant differences among different treatments. “***” indicates significant differences among different synthetic fungal combinations, p < 0.001.

Figure 5

Synergistic and offset effects of synthetic fungal combinations on the plant height, new root, and dry weight in seedlings of D. officinale. (a–c) Synergistic effects, (d–f) Offset effects. Box plots depict the deviation index between the effects of combination inoculations and the maximum or minimum effects of single fungi inoculations. Colored circles represent the mean effects of single fungi inoculations for each core OMF. Black circles represent the mean effects of combination inoculations.