The green of a plant’s leaves reminds us that the energy that fuels life on Earth ultimately comes from the sun through the process of photosynthesis. But new research shows that some orchids have lost the ability to photosynthesise entirely. The sun, it seems, is not the only way to go.
Craig Barret and Brandon Sinn of West Virginia University, US, and Aaron Kennedy of America’s Department of Agriculture, have delved into the genomes of a particular group of “heterotrophic” plants: the leafless orchid genus Hexalectris.
Plants are termed heterotrophic if they do not derive their energy directly from the sun. They include insectivorous species, ones that feed on dead matter, parasites and symbiotes, and epiphytes that grow on other plants but do not take nutrients from them.
The nine species of Hexalectris seem mostly, and to various degrees, to have transitioned to a myco-heterotrophic lifestyle, meaning that they parasitise fungi and feed on the nutrients they produce. In orchids more generally, it is estimated that some 30 species have made this transition.
This non-photosynthetic strategy removes evolutionary selection pressure on the genes that produce photosynthetic apparatus.
Barret and his colleagues wondered to what degree this lack of selection pressure had led to the loss of photosynthetic genes, because it was unclear how many of the nine species had transitioned to full myco-heterotrophy, and how many remained partly dependent on the sun.
To measure this, they investigated the plastomes – the genome of a small organelle found in plant cells, called a plastid – of all nine Hexalectris species. Plastids are often filled with chlorophyll (making them, thus, chloroplasts) and are the site for the photosynthetic process.
The researchers’ work, published in the journal Molecular Biology and Evolution, revealed that at least four, perhaps five, of the orchids have lost genes necessary for photosynthesis over the past 10 to 30 million years.
Intriguingly, this appears to have happened independently each time. The researchers suggest that the leap to myco-heterotrophy is facilitated by the fact that all young orchid plants feed this way before they develop photosynthetic abilities.
The fact that so many species in just one small genus have moved away from dependence on photosynthesis has wider implications for orchids and, indeed, all plants.
“Our unprecedented finding of multiple, independent transitions to a fully myco-heterotrophic lifestyle in a single genus,” the authors write, “reveals that the number of such transitions among land plants is likely underestimated.”
It seems that more plants than we ever suspected have come to shun the sun.
Stephen Fleischfresser is a lecturer at the University of Melbourne's Trinity College and holds a PhD in the History and Philosophy of Science.
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