The odd life of an underground orchid

Beautiful and bizarre, Rhizanthella gardneri is a critically endangered species of orchid in the state of Western Australia that spends its entire life cycle underground. It’s a parasite, extracting sustenance from a fungus species that lives symbiotically with the roots of the broom brush in the Western Australia outback. Despite having lost the ability to photosynthesize its own food, this subterranean orchid still retains its chloroplasts – cell sub-units with their own genes which in most plants carry out photosynthesis. Rhizanthella gardneri has the fewest chloroplast genes found in any plant, and they are genes that are not involved in photosynthesis. These remaining genes and their functions could provide new insights into critical processes in the lives of plants.

This unusual orchid is critically endangered, with only fifty known plants in the wild, found in five locations in Western Australia. Because of its rarity, the locations of the orchids are a secret. They are also very difficult to find. Professor Mark Brundrett of the Wheatbelt Orchid Rescue Project said in a press release,

We needed all the help we could get since it often took hours of searching under shrubs on hands and knees to find just one underground orchid!

Rhizanthella gardneri leads a very peculiar life. The plant spends its entire growth cycle underground; even when it flowers, the blooms are several centimeters below the soil surface. Unlike most other plants, this orchid does not photosynthesize its own food but has instead evolved a parasitic relationship with a fungus associated with the roots of the broom brush shrub. (Certain types of fungi live symbiotically with some kinds of plants – the fungi provide the plants with mineral nutrients and water, and in turn, the host plants provide the fungi with photosynthesized carbohydrates.) Dr. Etienne Delannoy, the lead author of a scientific paper about Rhizanthella gardneri recently published in Molecular Biology and Evolution, told EarthSky,

Yes, that’s really an amazing plant! For example, there’s a very tight relationship between the orchid, the fungus, and the broom bush, to such an extent that the seeds of this orchid can germinate only when infected by this particular fungus, provided that the fungus is actually mycorrhizing [living in symbiosis with] the broom bush. The seeds are fleshy which is unique to orchids. They can be eaten by rats and will still germinate.

While the unusual life of this orchid certainly captures the imagination, it holds another secret, deep in its cells.

Photosynthesis is the process by which plants use sunlight to convert water and carbon dioxide to oxygen and sugars. This is done in chloroplasts – organelles in plant cells that give leaves their green color. Organelles are sub-units in cells with a specific function, and contain their own DNA. Scientists theorize that chloroplasts originated from free-living photosynthetic microbes called cyanobacteria that were incorporated into cells that would eventually evolve to become plants. Over the course of evolution, some of the cyanobacteria genes in chloroplasts were either lost or exported to the nucleus of the plant cells.

Most plants and algae have about 110 genes in their chloroplasts, but not all of those genes are encoded for photosynthesis. Sorting out the functions of those other genes has been difficult to do in photosynthesizing plants. But the cells in the non-photosynthesizing underground orchid still retain their chloroplasts, and those chloroplasts should only contain genes that encode for functions other than photosynthesis. Dr. Delannoy and his team sequenced the chloroplast genome of Rhizanthella gardneri and found that it only has 37 genes, the smallest number known in any plants. Those 37 genes contain the instructions for synthesizing four important plant proteins. This discover has provided a significant step toward understanding the full purpose of chloroplasts in plant cells, and could help scientists understand the evolution and functions of other cell organelles.

Rhizanthella gardneri, an orchid that lives its entire life underground, has no need for photosynthesis having become a parasite to a fungus living a symbiotic relationship with a type of woody shrub in the Western Australia outback. Compared to other plants, this orchid has the fewest number of genes in its chloroplast (a sub-unit of the plant cell that has its own genome). A primary function of chloroplasts in plants is photosynthesis, but since this orchid no longer photosynthesizes, those genes left in its chloroplasts that are also found in other plants serve a different purpose. Understanding the functions in the chloroplasts of Rhizanthella gardneri will provide scientists with valuable insights into this underground orchid of Western Australia as well as processes that are essential for plant life.

How did plants come to have seeds?

George Whitesides says nanotech will teach us plants’ secrets

February 16, 2011

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