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Hans-Peter Braun, Leibniz Universität Hannover, Germany

Mistletoe is missing the machinery to make energy

It’s a biological assembly line as classic as the Model T’s. To produce energy, mitochondrial power plants in a cell use electron transport chains to convert electrons to adenosine triphosphate (ATP), the cell’s energy currency. Now, two independent groups reveal how evolution has severely messed with the production line in European mistletoe, the holiday adornment best known as an excuse for surprise smooches.

Researchers first noticed something was wrong when genomic sequencing turned up a dearth of mitochondrial genes for coding the protein subunits that make up the electron transport chain’s first station, dubbed complex I. Missing such a critical piece was unheard of in multicellular organisms. Perhaps the genes had simply skipped from the mitochondrial to the nuclear genome, researchers surmised. But this seemed highly unlikely, given the number of genes that had disappeared.

To find out which parts of the assembly line machinery had vanished, researchers extracted proteins from mitochondria in the mistletoe’s leaves and compared them to those in a small flowering weed of the mustard family called Arabidopsis thaliana. Although they found evidence for the machinery of other stations in the mistletoe transport chain, neither team detected any sign of complex I, they report today in Current Biology. Furthermore, the other transport chain stations—complexes II to V—were found to have 14% to 44% less protein than Arabidopsis did, suggesting that mistletoe is unable to produce all the energy it needs using this system.

So how does mistletoe adjust? Scientists say the flora has made some extreme adjustments to its metabolism. But it also helps that the plants are parasites. Hanging from trees rather than doorways, the mistletoe can make ATP by breaking down sugar pilfered from their hosts. That stolen sugar just might be enough to make up for their faulty electron transport chain assembly line.