Why the mitochondrial genome of lettuce is like a minced salad

The mitochondria, "the power station of the cell". Somewhere in the distant past, one bacterium had entered another cell and never left it, retaining some of its own DNA. For billions of years, mitochondria have gone from mother to offspring of most eukaryotic organisms, generating energy for the cell and playing a role in metabolism and programmed cell death.

All eukaryotes have them, but that does not mean that all mitochondria are identical. Plant mitochondria are actually quite different from those of animals. A new article published on August 30 in PLOS Genetics by Alex Kozik, Beth Rowan and Richard Michelmore at UC Davis Genome Center and Alan Christensen (who was on sabbatical at UC Davis of the University of Nebraska-Lincoln) shows how different he is.

The mitochondrial genomes of plants are larger and more complex than those of animal cells. Their size, sequence and arrangement may vary considerably, although the protein coding sequences are conserved.

It is almost universally thought that the genomes of mitochondrial plants are unique circular chromosomes (master circles) and that scientific papers and textbooks show them as such, write Kozik and his colleagues. But in recent years, it has become apparent that they are much more complex.

DNA of minced salad

Researchers used sequencing and long-read DNA microscopy to determine the structure of mitochondrial genomes in domestic lettuce (Lactua sativa) and two related wild relatives, L. saligna and L. serriola.

They convincingly show that plant mitochondrial DNA is a dynamic mixture of branching structures, ribbons and rings. In fact, mitochondrial DNA has been found more often in the form of branched or linear structures than in the form of circles and has never been found in the form of a large main circle.

While the genes themselves have been preserved, gene blocks have been blended. It looks more like a chopped salad: the basic ingredients can be mixed in different ways.

"Our data suggest that plant mitochondrial genomes should be presented as multiple-sequence units showing their variable and dynamic connections, rather than as circles," the authors wrote. This correction of the widespread concept of mitochondrial genome structure is the basis for future work on the dynamics and evolution of the mitochondrial genome in various plants.