A nondescript plant has overturned one of the bedrock assumptions of biology. Unlike every other animal and plant ever studied, its cells do not have the equipment to make energy. It is not clear how it survives.
All animals, plants and fungi are “eukaryotes”: they are made up of cells that are much more complex than those of bacteria. All eukaryotic cells contain tiny sausage-shaped objects called mitochondria, which are their energy source.
Mitochondria are almost entirely essential for eukaryotes. Some single-celled eukaryotes, such as certain yeasts, have few or no mitochondria. But multicellular eukaryotes – all plants and animals, including us – rely on their mitochondria absolutely.
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So researchers were shocked to discover that the mitochondria of mistletoe () are virtually disabled. Two teams independently found the same thing.
Weird plant
The key thing mitochondria do is make large quantities of a molecule called adenosine triphosphate (ATP). This carries lots of energy, which other parts of the cell can use to keep working.
Mitochondria make ATP using a chain of five large enzymes. But mistletoe has completely lost the first one – called Complex I – as well as the genes that make it.
“It’s not been believed that any multicellular organism can live without Complex I,” says of Leibniz University in Hannover, Germany.
The other four enzymes were still there. But they were in short supply compared with a control plant called .
“All the other complexes were reduced in abundance by 50 to 80 per cent,” says of the Max Planck Institute of Molecular Plant Physiology in Potsdam-Golm, Germany. “This means ATP production would drop by a factor of four.”
Mistletoe mystery
The challenge now is to find out how mistletoe makes its energy instead. Its lifestyle offers some possible clues: mistletoe is semi-parasitic, and gets a lot of its energy by stealing from the trees it grows on.
Complex I is a big molecule, so making it takes a lot of energy. Perhaps instead, mistletoe saves this energy and makes ATP another way.
It might use a chemical process called glycolysis, in which sugars and other carbohydrates are “burned” without oxygen. Making ATP by glycolysis is around 12 times less efficient than doing it in mitochondria, but it may be that it saves energy in the long run, because the mistletoe can steal the sugars from the host tree.
However, this might not be the whole story. Mistletoe’s host trees are dormant in winter, so would not supply much water and sugar to the mistletoe – yet the mistletoe remains green and healthy throughout winter.
Green blobs
There is another possibility. Mistletoe might instead make some ATP in its chloroplasts. These green nodules are where plant cells perform photosynthesis harness energy from light to make sugars from carbon dioxide and water.
All chloroplasts make ATP using ATP synthase, one of the enzymes also found in mitochondria. But they usually use up all the ATP they make to drive photosynthesis.
Braun has preliminary evidence that mistletoe chloroplasts contain abnormally high amounts of ATP synthase could make enough ATP to make up for the disabled mitochondria.
If mistletoe uses its chloroplasts in this way, Braun says, it would explain how it gets through winter.
Current Biology
Current Biology
