The dual inoculation of endophytic fungi and bacteria promotes seedlings growth in Dendrobium catenatum (Orchidaceae) under in vitro culture conditions

Orchid endophytes harbored in roots may have important interactions with the orchids themselves. However, few studies have demonstrated whether the coexistence of endophytic fungi and bacteria could affect the growth and development of orchid species. Four epiphytic fungi and four epiphytic bacteria were isolated from Dendrobium catenatum, and five "fungi + bacteria" combinations were obtained to dual inoculate with in vitro asymbiotic germinated D. catenatum seedlings. Significant variation was found for the five "fungi-bacteria" combinations on seedlings growth compared with non-inoculated treatment and corresponding single inoculated treatment. Compared with corresponding single inoculation, PY (Pyrenochaeta sp.) + HE2 (Herbaspirillum sp.) showed synergistic effects on increase in fresh weight and lateral root number, additive effects on tiller number and root tip number; EP2 (Epulorhiza sp.) + HE1 (Herbaspirillum sp.) showed synergistic effects on root tip number, additive effects on increase in fresh weight and lateral root number. Three of the combinations, namely PA (Paraphoma sp.) + HE2 (Herbaspirillum sp.), EP2 (Epulorhiza sp.) + EN1 (Enterobacter sp.) and EP2 (Epulorhiza sp.) + EN2 (Enterobacter sp.), gave additive effects but no synergistic effects on seedling growth which suggested the presence of competition for nutrition. PY (Pyrenochaeta sp.) + HE2 (Herbaspirillum sp.) expressed the best promotion than its corresponding single fungi or bacteria inoculation. All dual and single endophytes inoculations showed significant increase in root branching, with only one exception of PA. The study shows that co-inoculation with endophytic fungi and bacteria could be feasible to promote orchid growth. We suggest that not only an understanding of the mycorrhizal biology but also the balance of other various endophytes should be a necessary consideration for orchid conservation in future.