Tiny orchid seeds need fungi to help them grow

The Orchid family (Orchidaceae) is one of the largest families of flowering plants (alongside the Asteraceae, the daisy family). There are over 26,000 species of orchids with 100-200 new ones discovered every year (1). This is a HUGE number (to put it into perspective there are only around 10,000 bird and 5000 mammal species worldwide).

The early purple orchid (Orchis mascula) is found in UK woodlands and flowers in May. Image by Glyn Baker (CC BY-SA 2.0) via Wikimedia Commons.

Orchids are found on every continent except Antarctica in a wide variety of habitats from desert fringes to rainforests. Tropical regions have the richest variety in orchids, most of which grow perched on trees or rocks. A few members of the family such as vanilla are lianas (woody vines) that grow up trees.

Orchids are well-known for their beautiful flowers, but for me, the most fascinating feature of orchids is that they need to form interactions with fungi to be able to grow. Orchid seeds are tiny (less than 1mm long) and contain little or no endosperm (seed energy store) (2). Therefore, to germinate and grow into a mature plant, the seeds need be colonized by friendly fungi, known as mycorrhizal fungi. The fungi provide the main source of carbohydrates (e.g. sugars) for the seedling until it is able to produce its own through photosynthesis. As the chances of an individual orchid seed meeting a suitable mycorrhizal fungus are small, individual orchids can produce millions of seeds (2).

The mycorrhizal fungi can also provide other nutrients such as nitrogen (3). Although most orchids do eventually produce their own carbohydrates by photosynthesis, many continue to receive “top up” carbohydrates from their fungal partner and can maintain interactions with mycorrhizal fungi throughout their lifetime (3).

The interactions between orchids and mycorrhizal fungi resemble the symbioses (mutually-beneficial relationships) formed between other land plants and arbuscular-mycorrhizal fungi (3). However, the orchid-mycorrhizal fungi interactions appear to be a one-sided transfer of nutrients to the orchid instead of mutual exchange. There does not appear to be a benefit to the fungi, which can grow and reproduce without the orchid (3). Orchids can therefore be considered to be parasites of their fungal partners. How the orchids manage to persuade or “trick” the fungi into forming an interaction remains a bit of a mystery, but it is clear that orchids are more than just a pretty face…

Phalaenopsis bellina grows on branches of trees in the shady lowland rainforests of Malaysia and Borneo. Image by Jan Witowski (CC BY-SA 3.0) via Wikimedia Commons.

1. Kew: The Orchid Family (Orchidaceae). (Retrieved 13/03/14) http://www.kew.org/plants-fungi/for-gardeners/orchids/

2. Kew Millenium Seed Bank Blog: Orchid seeds – Nature’s tiny treasures by Wolfgang Stuppy. (Retrieved 13/03/14).  http://www.kew.org/news/kew-blogs/millennium-seed-bank/orchid-seeds-natures-tiny-treasures.htm

3. Rasmussen and Rasmussen (2009) Orchid mycorrhiza: implications of a mycophagous life style. OIKOS.

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