Abstract
Seeds of almost all orchids depend on mycorrhizal fungi to induce their germination in the wild. The regulation of this symbiotic germination of orchid seeds involves complex crosstalk interactions between mycorrhizal establishment and the germination process. The aim of this study was to investigate the effect of gibberellins (GAs) on the symbiotic germination of Dendrobium officinale seeds and its functioning in the mutualistic interaction between orchid species and their mycobionts. To do this, we used liquid chromatograph-mass spectrometer to quantify endogenous hormones across different development stages between symbiotic and asymbiotic germination of D. officinale, as well as real-time quantitative PCR to investigate gene expression levels during seed germination under the different treatment concentrations of exogenous gibberellic acids (GA3). Our results showed that the level of endogenous GA3 was not significantly different between the asymbiotic and symbiotic germination groups, but the ratio of GA3 and abscisic acids (ABA) was significantly higher during symbiotic germination than asymbiotic germination. Exogenous GA3 treatment showed that a high concentration of GA3 could inhibit fungal colonization in the embryo cell and decrease the seed germination rate, but did not significantly affect asymbiotic germination or the growth of the free-living fungal mycelium. The expression of genes involved in the common symbiotic pathway (e.g., calcium-binding protein and calcium-dependent protein kinase) responded to the changed concentrations of exogenous GA3. Taken together, our results demonstrate that GA3 is probably a key signal molecule for crosstalk between the seed germination pathway and mycorrhiza symbiosis during the orchid seed symbiotic germination.
Highlights
Orchidaceae is among the largest families of flowering plants, one that is fascinating and rich in species diversity, with diverse pollination mechanisms and a unique mycorrhizal symbiotic relationship [1]
We experimentally demonstrated that the seed germination of D. officinale on the oatmeal agar (OMA) medium with fungi is faster than seed germination on 1/2 Murashige & Skoog (1/2 MS) medium without fungi [7]
Many studies have revealed that plant hormones, especially gibberellins, are important factors affecting seed germination [10], and they are critical for the establishment of mycorrhizal symbiosis [18,19]
Summary
Orchidaceae is among the largest families of flowering plants, one that is fascinating and rich in species diversity, with diverse pollination mechanisms and a unique mycorrhizal symbiotic relationship [1]. Orchid mycorrhizae differ from other major types of mycorrhizae in that, besides mineral nutrients (e.g., P and N), this type of fungus supplies carbohydrates to the plant, especially in the early stages of seed germination and seedling development [2]. Seed germination and the subsequent development of the protocorm of almost all orchids is dependent either on mutualistic symbiosis with a compatible fungus, such as the member of Tulasnelloid, Sebacinaoid, and Ceratobasidiaceae under natural field conditions (symbiotic germination, SG) [5] or the replacement of the fungus by an exogenous nutrient substance in medium under controlled conditions (asymbiotic germination, AG) [6]. Symbiotic germination is acknowledged as being a unique and important topic of orchid seed biology
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