Surprising finds in a rare plant’s mitochondrial genome

Rare and ancient plant gobbles up entire mitochondria from other plants and holds onto them for eons.

The Science:

One of the oldest flowering plants, Amborella trichopoda, split off from the lineage of other flower plants about 200 million years ago. Analysis reveals that it has a record-setting amount of foreign DNA in its mitochondria, the powerhouse organelles that essentially make it possible for all living things to breathe. Amborella mitochondria have absorbed whole genomes of mitochondria from six other plant species, including one moss and three algae and two other flowering plants. The researchers propose a “mitochondrial fusion” model to explain this near-whole genome retention.

The Impact:

Sequencing the entire mitochondrial genome of Amborella, initiated under the DOE JGI’s Community Sequencing Program, was an effort that took eight years. But the effort has yielded several surprising insights, from the size of the genome (3.9 megabases) to the mechanism by which mitochondrial DNA transfer occurs.

Summary

Amborella only grows on Grande Terre, the main island of New Caledonia, a small nation 750 miles off the eastern coast of Australia. It’s the sole species in its family and genus. In a paper published December 20 in the journal Science, researchers described several unusual features of its mitochondrial DNA, which contain the equivalent of six foreign mitochondrial genomes. Many of these were swallowed whole, but instead of digesting them, incorporating the useful bits and casting off the rest, Amborella has held onto the genomes — nearly intact — for millions of years.

The findings provide strong evidence for the hypothesis that plant mitochondria can take up new traits by fusing with the mitochondria of other species. Many plant species live on Amborella shrubs. When wounded, Amborella often generates rapid growth at those locations, a virtual petri dish for mitochondria of different species to come into direct contact with each other. This, coupled with a low rate of losing mitochondrial genes over time, has created Amborella’s huge mitochondrial DNA bank.

The project involved many researchers at four different institutions (the DOE Joint Genome Institute, Indiana University, Pennsylvania State University and the Research Institute for Development in New Caledonia) over eight years. “In short, getting a complete assembly of the Amborella mitochondrial genome took years and the effort of several people,” said senior author, Jeffrey Palmer. “But it was well worth it, because the genome is so utterly unusual, amazing and full of surprises and mechanistic insights.”

Palmer cited several motivating factors for plumbing the genomic entrails of Amborella’s mitochondria. “Although they contain a relatively small number of genes, plant mitochondrial genomes are disproportionately important in terms of basic metabolism. This is because they encode most of the key proteins involved in respiration.” He also pointed to defects in plant mitochondrial genes that can cause cytoplasmic male sterility — when plants fail to produce functional pollen — a critically important agronomic trait for hybrid seed production. He said that findings from plant mitochondrial genome studies can also be extrapolated to other mitochondrial genomes, to shed light, for instance, on the mechanisms of cellular aging.

Contact

Jeffrey Palmer
University of Indiana
[email protected]

Publication

Danny Rice, et al. Horizontal Transfer of Entire Genomes via Mitochondrial Fusion in the Angiosperm Amborella. Science 20 December 2013, Vol. 342 no. 6165
DOI: 10.1126/science.1241089

Funding

Department of Energy, Office of Science
National Institutes of Health
National Science Foundation