Dendrobium seeds also need to be germinated with the help of “partners”

Researchers from Xishuangbanna Tropical Botanical Garden of the Chinese Academy of Sciences (hereinafter referred to as Banna Botanical Garden) and Pu’er College constructed a symbiotic germination system with Dendrobium dendrobium and Glucosis GC-15 (Tulasnella sp.) with important ornamental and medicinal value, and studied in detail the development mode of symbiotic germination of orchid seeds by morphological anatomical means. Recently, the relevant research results were published in the international journal “Horticulture”.

Orchid seeds are generally considered to be undifferentiated embryonic structures, without endosperm, and germination of seeds under natural conditions must be dependent on fungi. Orchid seeds germinate slowly, first going through the development stage of the original bulb before the seedlings are formed. Protobulb is a unique development stage of orchids, which is the key period for seedlings to be built and to build mycorrhizal symbiosis, which determines whether seeds can develop smoothly to form seedlings. Studying the developmental characteristics and functions of protobulbs is of great significance to understand the relationship between orchids and fungi. Due to the small size of the original bulb, the internal morphological structure is rarely studied in depth, and there has been great controversy about the properties and structure of the original bulb.

In this study, the germination process of Dendrobium dendrobium seeds can be divided into 6 stages according to morphological and anatomical characteristics. Protobulb formation occurs in stage 2; The protobulb undergoes obvious tissue differentiation at stage 3, with embryonic cells at the top developing into cotyledons and the mid-upper center differentiating into the apical meristem (SAM).

They also found that the glutinous mycelium invades the embryo through the seed hole at the first stage when the seed begins to absorb water and expand, and later enters through pseudoroots and proliferates in specific parenchymal cells in the middle and lower parts of the protobulb, but the hyphae are eventually dissolved by the protobulb and supplied to the protobulb as polysaccharide nutrients.

The researchers said that the morphological studies of the germination process of Dendrobium dendrobium seeds showed that the mature embryos of the seeds had planned a development “blueprint”, and the polar differentiation of the embryos formed two functional areas, which developed into cotyledons and stem apex meristem and symbiotic fungal colonization areas, respectively, exercising the functions of photosynthesis, seedling vegetative body completion, and nutrient supply.

The seemingly simple embryo completes the complex development process with the help of the “partner” of the symbiotic fungus, and obtains tissue organs such as cotyledons and stem apical meristems, pseudoroots and mycorrhizae to form the functional body of “protobulbs”, thus opening the life journey of orchid plant seedlings.

Gao Xinzhen, a master’s student of Banna Botanical Garden, is the first author of the paper, and Luo Yan, researcher of the Plant Diversity and Conservation Research Group, and Shao Shi, associate researcher of the Rapid Evolution and Conservation Group, become the co-corresponding authors of the paper. (Source: Hu Minqi, China Science News)

Scanning electron microscopy of Dendrobium dendrobium seeds germination at different developmental stages and mycelial masses in the original bulb. Photo courtesy of the research group

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