Gene Editing Reaches Plant Mitochondria

Modified gene editing machinery enables targeted disruptions of mitochondrial genes in rice and rapeseed plants.

 

Gene editing technologies have revolutionized the field of genetics, allowing researchers to make targeted changes to the DNA of various animal and plant nuclei, animal mitochondria, plant chloroplasts, and more. Missing from this list until recently, however, was plant mitochondrial DNA. The tools for delivering the necessary editing enzymes to plant mitochondria simply hadn’t been built.

Now, plant molecular biologist Shin-ichi Arimura of the University of Tokyo and colleagues have filled this gap, creating plant-friendly mitoTALENs—mitochondria-targeting gene editing tools based on transcription activator-like editing nucleases (TALENs).“This is an important paper—it’s the first demonstration that we can make targeted and heritable changes to mitochondrial DNA” in plants, Ian Small, a plant scientist at the University of Western Australia who was not involved in the research, writes in an email to The Scientist

 

HITTING DELETE: Agrobacteria carrying a plasmid encoding a gene-specific, mitochondria-homing mitoTALEN infect cultured plant cells and deliver the genetic payload to the cells’ nuclei (1), where the DNA integrates into the genome. The encoded mitoTALEN protein is then produced and enters the mitochondria, deleting a portion of the target gene—in this case, a gene causing male sterility (2). The resulting transgenic plant can self-fertilize, producing abundant seeds (3). Illustration by GEORGE RETSECK

 

Regular TALENs are composed of a DNA binding domain that can be readily engineered to recognize practically any DNA sequence, and a nuclease domain that chops up the DNA at that site, causing deletions. To target a mitochondrial gene, the team modified a plant-adapted TALEN such that it also included a mitochondrial homing signal, and engineered DNA binding domains to recognize particular genes of interest. The researchers then transferred a plasmid encoding a mitoTALEN into plants via agrobacteria—a common strategy used by plant geneticists.

In proof-of-principle experiments, the researchers designed two mitoTALENs, each targeting a particular mitochondrial gene: orf79 in rice and orf125 in rapeseed (canola). The resulting deletions enabled the researchers to confirm the genes’ hitherto suspected roles in male sterility—a natural phenomenon that prevents self-fertilization in certain hermaphroditic plants, thus promoting hybrid seed development. Indeed, disabling the genes reinstated self-fertilization in the two types of plants, the team showed.

Such male sterility genes, which are encoded in the maternally inherited mitochondria of certain plants, are desirable for agriculturalists wishing to produce hybrid crops that “grow faster, produce more, and are more resistant to disease,” explains Small. Thus, generally speaking, the goal is to activate such genes or introduce them into crop plants that lack them, rather than delete them, as Arimura’s team did.

While the plant mitoTALENs can’t yet deliver the “holy grail” of plant mitochondrial gene editing, this study is “an important first step,” says plant physiologist Ralph Bock of the Max Planck Institute of Molecular Plant Physiology who did not participate in the research. In the meantime, he adds, “one could use [the technology] to ask questions about the functions of mitochondrial genes.” (Nat Plants, 5:722–30, 2019)

Story by: Ruth Williams, for The Scientist, March 2020