Monday, March 28, 2016

March Orchid Science: Orchids in Medicine, Conservation, Evolution, and Symbiosis

Hey guys, lately I've been interested in learning more about what goes on in the world of orchid research.  I'd like to try a column where I summarize science articles about orchids.  Some of these studies are more technical than others, and I will do my best to convey their findings in an understandable manner.

As the end of March is approaching, here is a wrap-up of this month's orchid science news.


    Gastrodia elata orchid
    Photo Credit
    Qwert1234, Wikimedia commons

    Gastrodia elata is an orchid species used in Sichuan cuisine and traditional Chinese medicine. It is a rootless, leafless plant, which grows from an underground tuber that produces a flowering stalk as shown in the image above. 

    Gastrodin is a chemical compound produced by Gastrodia elata, which is responsible for some of its medicinal properties. Studies have investigated gastrodin's medical potential in a variety of contexts, including everything from anxiety to diabetes and dementia. We know that Gastrodia elata produces high levels of gastrodin during growth of juvenile tubers, but we don't know how the plant synthesizes this chemical.

    In this study, the authors used a technique called de novo transcriptome sequencing to compare gene expression levels in germinated seeds versus juvenile plants. This is the first time that anyone has done a comprehensive genetic study of this orchid. 

    The resulting data gives us information about which genes are active in Gastrodia elata at different stages of the orchid's early growth. The authors then proceeded to identify two novel genes which encode for enzymes that may be involved in the biosynthesis of gastrodin.

    Authors: Tsai CC, Wu KM, Chiang TY, Huang CY, Li SJ, Chiang YC.
    Published in BMC Genomics (March 9, 2016)
    Comparative transcriptome analysis of Gastrodia elata (Orchidaceae) in response to fungus symbiosis to identify gastrodin biosynthesis-related genes


      Epicactis orchid species examined in Jacquemyn et al 2016
      Photo Credits:
      Epicactis helleborine, by Amadej Trnkoczy (Flickr gallery)
      Epicactis are a genus of terrestrial orchids; these species are hardy and can survive in a broad range of environments. Epicactis helleborine, originally a European species, has spread across Asia, northern Africa and North America.  It is increasingly considered invasive in parts of the US, earning it the moniker "weed orchid."

      This study looked at mycorrhizal fungus communities associated with three epicactis species (Epipactis helleborineEpipactis neerlandica, and Epipactis palustris).  Many orchid species require the presence of symbiotic root fungi (mycorrhiza) throughout life for optimal growth and development. Terrestrial orchids rely on specific fungi to trigger seed germination and/or initiate development of orchid seedlings. 

      Epicactis helleborine and Epicactis neerlandica have only recently evolved to be separate species.  However, they occupy extremely different environments; E. helleborine grows primarily in forests, and E. neerlandica is found almost exclusively in coastal dunes.  E. palustris is an evolutionarily more distant species that also grows in coastal dune habitats.

      This study found a total of 105 different kinds (taxa) of mycorrhizal fungi among the roots of these three orchid species. Each orchid species had a unique mycorrhizal community. Out of the 105 types of fungus identified in the study, only 8 were shared among all three orchid species. The differences were the most pronounced between the forest-growing orchid (E. helleborine) and the two dune species (E. neerlandica and E. palustris).

      Figure 3. A. from Jacquemyn et al 2016 
      This venn diagram depicts how many taxa of mycorrhizal fungi are unique to each orchid species, and how many are shared between them.

      Authors: Jacquemyn H, Waud M, Lievens B, Brys R
      Published in Annals of Botany (March 5, 2016)
      Differences in mycorrhizal communities between Epipactis palustris, E. helleborine and its presumed sister species E. neerlandica.

      Phalaenopsis aphrodite var formosana
      Photo Credit: Orchi (wikimedia commons image)

      Pollination and seed maturation works differently in orchids than it does in most flowers.  Like most other things, it is slower.  Much slower.  In most flowers, the ovule structure and the embryo sac have fully developed by the time the flower opens.  Once a flower is pollinated, the pollen reaches the ovule within hours, fertilizing the embryo, which then develops into a seed.

      Orchid flowers, on the other hand, will not even begin developing an ovule until after pollination has occurred.  This is an energy-saving strategy by the plant, which will only put in resources to develop the ovule if pollination has already occurred.

      One consequence of this delay is that the ovule must have enough time to mature after the orchid first senses pollination. As a result, orchids can have a delay of days, or even months between pollination and fertilization. In Vanda suavis, pollen can take up to 10 months to fertilize the flower.

      The authors in this study observed the timing of pollination and fertilization in Phalaenopsis aphrodite var. foromsana. They purchased seedlings from a commercial supplier.  The orchids produced flower stalks after about 2 months of cultivation, and the first flowers opened 1-2 months after that.

      The authors hand-pollinated these flowers and observed how the flower developed in response to pollination. They found that the pollen grains did not even begin to germinate until 3 days after pollination.  After that, it took 60-65 days for the pollen to reach the ovules deep in the body of the flower, and complete fertilization.  The timing of this 2-month journey for the pollen matched the timing of ovule maturation.

      Authors: Chen JC, Fang SC


      This study looked at seed germination and seedling development of Cynorkis purpurea in a lab setting.  Cynorkis purpurea is a terrestrial orchid that generally lives only in gallery forests in the Central Highlands of Madagascar.  

      The authors of this study wanted to understand how various symbiotic fungi influence seed germination and seedling development of Cynorkis purpurea.  They collected seeds from wild orchids in Madagascar, and attempted to grow them in the lab.  Some seeds were grown in the presence of orchid mycorrhizal fungi, while other seeds were grown in conditions without fungi.

      One of the things I was struck by in this study, was how slowly orchid seeds develop, even under optimal conditions.  Panel "d" in the image below shows what a 'successfully developed' seedling looked like after 3 months of growth.  At less than 1/2 a millimeter in diameter, the seedling is still nearly microscopic.

      Figure from Rafter et al, 2016, showing the germination stages of Cynorkis purpurea seeds

      a. What a Cynorkis purpurea seed looks like before germination 
      b. A successfully germinated seed (2-4 weeks)
      c. A fully germinated orchid seed shows fungal tendrils growing along its surface 
      d. After 12 weeks, seeds grown in favorable conditions will have developed the start of a shoot

      The study found that while Cynorkis purpurea seeds do not require symbiotic fungi for germination, seedlings developed dramatically better when they were paired with symbiotic fungi. The authors compared three different genera of fungus, and found that sebacina fungi were the most effective at promoting orchid seedling development. 

      One of the control conditions, which added sucrose to the growth media to simulate the effect of fungal presence, also significantly boosted seed germination and development. 

      Authors: Rafter M, Yokoya K, Schofield EJ, Zettler LW, Sarasan V.
      Published in Micorrhiza (March 17, 2016)
      Non-specific symbiotic germination of Cynorkis purpurea (Thouars) Kraezl., a habitat-specific terrestrial orchid from the Central Highlands of Madagascar


      Example of 4 orchid species that use CAM photosynthesis
      Photo Credits:
      Dendrobium terminale, (C) by G. Meyer 1990, Swiss Orchid Foundation at the Herbarium Jany Renz. Botanical Institute, University of Basel, Switzerland.
      Crassulacean acid metabolism, or CAM photosynthesis, is an adaptation that allows some plants to conserve water in arid conditions.  It is mostly found in succulents.  However, among approximately 25,000 known species of orchids, about 10,000 are estimated to use CAM photosynthesis.  

      Photosynthesis requires that plants take in carbon dioxide (CO2) from the air. However, when a plant opens its pores (stoma) to intake carbon dioxide, it also loses water through evaporation. In order to survive in water-scarce environments, some plants have evolved a way to take in CO2 at night, when temperatures are cooler and evaporation is less of a problem.  CAM photosynthesis is a method by which some plants can accumulate and store CO2 at night for use in photosynthesis during daytime.  Plants that use CAM photosynthesis can require as much as 80% less water than other kinds of plants.

      Scientists are interested in studying plants which use CAM photosynthesis, because it can be useful for developing drought-resistant crops.  This study did the first comparative genetics look at CAM in orchids. The authors examined CAM-related genes from 4 orchid species that had the CAM adaptation (Cymbidium atropurpureum,  Phalaenopsis mannii, Phalaenopsis equestris, and Dendrobium terminale), 9 orchid species that did not have CAM, and 12 other non-orchid species.

      The authors concluded that CAM may have evolved in orchids through changes in expression level of a few key genes involved in carbon fixation during photosynthesis.

      Authors: Zhang L, Chen F, Zhang GQ, Niu S, Xiong JS, Lin Z, Cheng ZM, Liu ZJ
      Published in The Plant Journal (March 9, 2016)
      Origin and mechanism of crassulacean acid metabolism in orchids as implied by comparative transcriptomics and genomics of the carbon fixation pathway.

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