Abstract
BackgroundMycorrhizal associations play a key role in the life cycle and evolutionary history of orchids. All orchids grow from extremely small seeds that are lacking in reserves, and germination and growth into an underground heterotrophic, achlorophyllous stage depend upon symbiotic fungi to provide nutrient. However, the nutritional physiology between this symbiosis and green-leaved orchids is still unclear. To understand further how these associations affect growth and carbon utilization of green orchids, the green orchids were inoculated with two symbiotic fungi isolated from the roots of a wild orchid (Dendrobium officinale) in vitro and 13C stable isotope signature experiments were designed to analyze carbon nutrition acquisition.ResultsAfter two months, both fungi had formed mycorrhizal associations with the host roots. Moreover, the growth rate was more rapid for the mycorrhizal seedlings than for the non-mycorrhizal seedlings. The mycorrhizal seedlings not only absorbed more 13C from the substrate, but also the S3-mycorrhizal seedlings assimilated more atmospheric 13CO2 due to significantly higher effective quantum yield of photosystem II, compared with the non-mycorrhizal seedlings. These results suggested that the green orchids could receive more C nutrition from the substrate due to symbiotic fungi, and photosynthesis capacity of the green D. officinale could be enhanced by the S3 fungus, therefore carbon nutrition acquisition also increased. As a result, the S1- and S3- mycorrhizal seedlings showed markedly higher biomass and polysaccharides contents than the non-mycorrhizal seedlings.ConclusionsThese results improve our understanding of the mycorrhizal functioning in the green Dendrobium and show some potential application in the cultivation of D. officinale.Electronic supplementary materialThe online version of this article (doi:10.1186/1999-3110-54-23) contains supplementary material, which is available to authorized users.
Highlights
Mycorrhizal associations play a key role in the life cycle and evolutionary history of orchids
We sequenced the internal transcribed region (ITS)-5.8S rDNA sequences of the re-isolated fungus from the roots inoculated with S1 as well as the Large subunit gene of mitochondrial rDNA (mtLSU) sequences of the re-isolated fungus from the S3-inoculated roots and verified that these fungi were the same as those used for the first inoculations
In this study, two endophytic fungi (S1 and S3) sampled from wild D. officinale formed in vitro mycorrhizal associations with host roots, thereby markedly enhancing seedling growth
Summary
Mycorrhizal associations play a key role in the life cycle and evolutionary history of orchids. A division of orchids into three physiological types (fully autotrophic, fully mycoheterotrophic (MH), partial MH or mixotrophs) is based on carbon nutrition (Dearnaley et al 2012) Regardless of their carbon at adult stage, all orchids grow from extremely small seeds that are lacking in reserves, and germination and growth into an underground heterotrophic, achlorophyllous stage (protocorm stage) depend upon symbiotic fungi to provide carbon nutrient Stable isotope signatures have proven that some fully autotrophic terrestrial orchids can acquire carbon, nitrogen, phosphate via the “up-flow” pathway through their symbiotic fungi (Cameron et al 2006, 2007, 2008; Hynson et al 2012) Standard experiments with those signatures have been conducted with media containing labeled substrates. Whether the ability of CO2 assimilation for green orchids is influenced by its fungal partner still needs experimental confirmation
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