When plants reproduce sexually, the genetic material from the cell nuclei of the father and mother are combined. The chloroplasts, which are important for photosynthesis, on the other hand, are only passed on from the mother plant to the offspring – this was previously thought. A new study now shows that under certain circumstances, including exposure to cold, paternal chloroplast genetic material can also be inherited. This could open up new possibilities for plant breeding.
When the precursors of today’s animal and plant cells developed more than a billion years ago, a symbiosis that is still successful today emerged: formerly independent protozoa with useful properties were taken up into other cells and integrated into their cell metabolism. The mitochondria, the power plants of animal and plant cells, and the chloroplasts, which enable the plant cells to carry out photosynthesis, developed from such unicellular organisms. According to previous assumptions, however, these cell organelles are only passed on from the mother to the offspring. In plants, it is believed, the chloroplasts either don’t get into the pollen grain at all, or they are broken down before the pollen fertilizes a female egg cell. A recombination of maternal and paternal chloroplast genome therefore seemed impossible.
Pollen under stress
A study by a team led by Kin Pan Chung from the Max Planck Institute for Molecular Plant Physiology in Potsdam contradicts this. Thus, although the inheritance of paternal chloroplasts is rare, it occurs more frequently under certain conditions. “Because inheritance from both parents is expected to strongly influence the stability and evolution of the organelle genome, we investigated which environmental and genetic factors determine how chloroplasts are inherited,” the researchers report.
Chung and his team examined more than four million tobacco plants under different environmental conditions. In order to be able to distinguish paternal chloroplasts from maternal ones, the researchers first grew father plants whose chloroplasts are resistant to an antibiotic. They then exposed these plants to various stressors such as heat, cold, drought and strong light during pollen ripening. They then pollinated untreated mother plants with the pollen treated in this way.
Green spots in seedling culture
Chung and his colleagues grew the seeds resulting from this cross on a nutrient medium that was mixed with the appropriate antibiotic. Because the maternal chloroplasts were killed by the antibiotic, seedlings that received only the chloroplasts of the mother plant remained colorless. If a seedling was green, however, it must have inherited chloroplasts from the father plant. “It’s not easy to stay motivated when you’re looking at thousands of little plants, always looking for that one green spot. We were correspondingly enthusiastic when an effect actually became apparent in the cold experiments,” says Chung’s colleague Stephanie Ruf.
In the next step, the team investigated which mechanism ensures that paternal chloroplasts are more likely to be inherited when it is cold. “We know that cold slows down the work of enzymes in plant metabolism. Therefore, we suspected that an enzyme might be involved in blocking paternal inheritance of chloroplasts,” explains Chung’s colleague Enrique Gonzalez-Duran. In fact, the researchers came across an enzyme called DPD1 that normally destroys the chloroplast genetic material during pollen ripening. If they grew plants in which this protein was defective, the pollen actually contained chloroplast genome, as the researchers demonstrated with the help of laser scanning microscopy and PCR tests.
New perspectives for plant breeding and evolutionary biology
Although the father plants manipulated in this way produced slightly less functional pollen than plants with intact DPD1, the researchers were able to pollinate female plants and grow seedlings with these pollen as well. And indeed: the rate of paternal chloroplast inheritance was significantly increased, similar to exposure to cold. When the researchers combined gene manipulation and cold stress, the inheritance rate rose to two to three percent. “That may not sound like much at first, but it is gigantic compared to a one in 100,000 chance that something like this will take place under normal conditions,” says Chung.
The findings open up new perspectives for plant breeding: “Because it was previously thought that mitochondria and chloroplasts were only inherited together and only from the mother, there was no way of passing on the properties encoded in their genome separately. The possibility of inheriting the chloroplasts from the father’s side with a simple cold treatment could open the door to completely new breeding programs,” explains Chung’s colleague Ralph Bock. This insight is also important for evolutionary biology. “Our results show how important it is to take environmental conditions into account in genetic research,” says Bock. “It will now be exciting to investigate whether maternal and paternal inherited chloroplasts actually exchange genetic material with each other,” adds Chung.
Source: Kin Pan Chung (Max Planck Institute for Molecular Plant Physiology, Potsdam) et al., Nature Plants, doi: 10.1038/s41477-022-01323-7