Mitochondria in plants can be inherited from the father more often than expected. The findings come from Wageningen University & Research (WUR), the Max Planck Institute of Molecular Plant Physiology (MPIMP) and The Chinese University of Hong Kong (CUHK). The study was recently published in Nature Plants. Plant breeders see this as a possible new tool for their toolkit.
It is explained so neatly in our schoolbooks. Half your DNA package comes from your father, the other half from your mother. That holds for humans, most animals and most plants. As with almost everything in biology, it is a bit more complicated. The standard story applies to the DNA in the nucleus. But cells also contain other organelles with their own DNA: cytoplasmic genomes. Think of chloroplasts in plant cells, or mitochondria: the cell's energy factories. For a long time, the idea was that these organelles and their DNA are passed on only through the mother.
Leaking fathers
Why those organelles are inherited only through the mother is still unclear. "People often think it is because egg cells are much larger than sperm cells," says plant biologist Kin Pan Chung of Wageningen University & Research, who carried out the research together with scientists in Germany and Hong Kong. "But the differences in gamete size cannot explain it all. For example, we see that the single-celled green alga Chlamydomonas reinhardtii also has two sexes, where the gametes are the same size. Yet there too, only one of the two sexes passes on the organelles during reproduction."
In recent years it has become clearer that the maternal inheritance mode is not always so strict. Researchers discovered that organelles sometimes do pass on via the paternal line in plants. For chloroplasts, that happened in 0.0016 % of cases; they call this 'paternal leakage'. For mitochondria, the idea remained that it did not occur in most flowering plants. Until Chung and colleagues devised a method to test this properly in tobacco plants, an important model species related to potato and tomato. "Paternal mitochondria turned out to occur in as many as 0.18 % of cases. That was a huge surprise."
Over seven percent
The researchers used a mutant tobacco plant with a mutation in the mitochondrial DNA. The result: the plant produces no pollen grains and is therefore male sterile. Chung and colleagues crossed these mutants with paternal plants containing functional mitochondria. Part of the offspring developed into healthy plants with normal pollen grains, indicating that these plants had received mitochondria from the father.
They then went a step further to find out which genes are involved in promoting the transmission of paternal mitochondria. "We created a tobacco mutant that does not produce a specific exonuclease-enzymes that cut up cytoplasmic DNA in pollen. Without this exonuclease, there is a higher abundance of mitochondrial DNA in pollen, therefore allowing a higher paternal transmission rate."
On top of that, they grew the paternal plants in the cold. It was already known that the cold treatment might lead to a higher share of organelles in sperm cells. And indeed, by combining the mutant and the cold, the proportion of offspring inheriting paternal mitochondria rose to over seven percent.
Less snipping for hybrid seeds
The research is fundamental, but a possible application is already emerging. Hybrid seeds are popular with breeders and users. By crossing a selected father and mother line, you get seeds that are often stronger and more consistent in quality than the individual parent lines. To make such hybrids, you want to prevent plants from fertilising themselves. Chung: "A breeder can prevent self-fertilisation by cutting all the stamens out of the plant. That is effective, but very labour-intensive. It can also be done by creating male-sterile lines." With the new knowledge about paternal mitochondria, an extra route may become possible. "To really use this, you still need to develop it much further," Chung agrees, "but we show here that it can work in principle."
Read the paper published in Nature Plants.
