Abstract:[Objective] To investigate the community structure, diversity, and plant-growth- promoting functions of endophytic bacteria in the protocorms of Epidendrum sp. and mine the core bacterial groups associated with the seed germination. [Methods] We collected five samples including three types of protocorms (germinated on MS1 medium, pine bark, and rotten oak leaves) and two types of substrates (pine bark and rotten oak leaves). High-throughput sequencing of the 16S rRNA gene was employed to compare the diversity and community composition of bacterial communities under different germination conditions. The conventional method was used to isolate the endophytic bacteria from the protocorms germinated on pine bark and rotten oak leaves (i.e., from symbiotic seed germination), and the growth-promoting potential of the isolates was evaluated. [Results] A total of 2 735 operational taxonomic units (OTUs) were obtained from the five samples, belonging to 876 genera, 453 families of 41 phyla, among which Proteobacteria and Actinobacteria were the dominant phyla. The results of principal coordinates analysis showed that there were differences in the bacterial community structure between the protocorms of Epidendrum sp. and the substrates, and the community structure of endophytic bacteria in the protocorms germinated on the MS1 medium was closest to that on pine bark. Functional prediction indicated that the endophytic bacteria in the protocorms germinated on rotten oak leaves had higher nitrogen-fixing ability than that in other types of protocorms. Nineteen isolates were obtained from protocorms geminated on pine bark and rotten oak leaves, belonging to 16 species of 12 genera. Tumebacillus flagellatus, Bradyrhizobium cenepequi, and Weizmannia ginsengihumi were the common species in the protocorms germinated on pine bark and rotten oak leaves. Pseudomonas koreensis and W. ginsengihumi had the potential of solubilizing phosphorus and producing indole-3-acetic acid (IAA) and siderophores. [Conclusion] The Epidendrum sp. protocorms germinated in different environments harbor rich endophytic bacteria. The endophytic bacteria isolated from the protocorms from symbiotic seed germination had plant growth-promoting functions, such as fixing nitrogen, solubilizing phosphorus, and producing IAA and siderophores. This study provides a scientific basis for mining the microbial resources related to seed germination of orchids and studying the interactions between orchids and microorganisms.
