Although many horizontal gene transfer events in plants involve mitochondrial sequences, evidence for cell-to-cell movement of mitochondria in plants is lacking.

Researchers investigated the cell-to-cell movement of mitochondria using two tobacco plants Nicotiana tabacum and Nicotiana sylvestris. The authors used an experimental system in which the cytoplasm of N. tabacum was replaced with the cytoplasm of Nicotiana undulata, which carries a sterility-causing mitochondrial genome that makes N. tabacum male flowers sterile. 

N. sylvestris flowers are fertile, and the authors could detect the rare transfer of mitochondrial DNA (mtDNA) from N. sylvestris to N. tabacum by monitoring flowering events in N. tabacum plants regenerated from graft junctions, as the transfer of mtDNA restored fertile flower anatomy.

The authors identified branches with fertile flowers in regenerated plants, indicating cell-to-cell movement of mitochondria through the graft junction. When the authors analyzed the mitochondrial genomes, they found extensive recombination, and identified orf293 as the gene likely responsible for male sterility.

The cell-to-cell movement of mitochondria during grafting may recapitulate the horizontal mtDNA transfer that occurred during evolution. According to the authors, such intercellular movement may be used to transfer mtDNA to other species. 

http://www.pnas.org/content/early/2016/03/02/1518644113