The top layer of leaf litter was removed around two Rhizanthella gardneri blooms. The bracts - a leaflike structure under the flower - of this rare, earth-dwelling plant spring open when the dirt is removed.
Credit: Mark Brundrett, University of Western Australia
SYDNEY: A rare orchid in Western Australia that spends its entire life underground is providing crucial insights into exactly which genes are essential for plant survival.
The critically endangered Rhizanthella gardneri, a native Australian orchid that lives its entire lifecycle underground, was found to be missing 70% of its chloroplast genes - genes typically associated with the conversion of sunlight into energy - which are essential to regular, above-ground plants.
The findings shed light on the vital non-photosynthetic function of chloroplasts, and could explain gene loss in other parasites, such as the Plasmodium parasite that causes malaria. “It’s the smallest chloroplast genome known in land plants and the genes that remain aren’t random," said lead author Etienne Delannoy from the University of Western Australia.
A lifetime underground
Although chloroplasts are most well-known for their role in photosynthesis, they also have poorly understood functions critical to plant life. Scientists suggest that chloroplasts are also linked to the synthesis of hormones, vitamins and fatty acids - but this has been difficult to study since photosynthesis itself masks the process in normal plants.
However, recent genetic sequencing has revealed that the Rhizanthella retains modified chloroplasts, and possesses a significantly reduced gene set compared to normal plants.
While a typical land plant contains around 110 genes in its chloroplast genome, Rhizanthella was found to contain only 37, a fraction of the normal amount. The researchers believe these genes are responsible for manufacturing two complex protein structures that can not be made anywhere else in the plant cell, but which are vital for plant survival.
Western Australia's little "gold nugget”
Not only is the Rhizanthella rare, (there's an estimated 50 individuals left in the wild), but it’s also notoriously difficult to find. The orchid depends on parasitising nutrients from a fungus associated with roots of broom bush (Melaleuca uncinata) - a woody shrub found in outback Western Australia. Since the flower remains concealed below the litter layer, it is located by the presence of the bush.
“We have to crawl around on our hands and knees and scrape the litter back for what could be hours, but it’s like finding a gold nugget,” said conservation biologist Mark Brundrett from the University of Western Australia, co-author of the study in the journal Molecular Biology and Evolution.
The total DNA from Rhizanthella gardneri was extracted using a single flower head, and allowed the complete chloroplast genome to be sequenced.
“Even though the chloroplasts aren’t photosynthesising anymore, whatever they're doing is important enough that the plant is keeping many copies per cell,” said co-author and plant molecular biologist Ian Small from the University of Western Australia.
