LMU biologists decipher the ionome of chloroplasts and create the foundations for new biotech strategies.
Plants fix 258 billion tons of CO₂ in their chloroplasts through photosynthesis every year. For these cell organelles to work properly, they require certain minerals – particularly ions of the metals iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn). Disruptions of ion homeostasis impair photosynthesis and thus growth and yields. A team with members from Munich, Bochum, Columbia (MO), and Saarbrucken, led by LMU biologist Professor Hans-Henning Kunz has now deciphered the chloroplast ionome – the totality of metal ions in the chloroplast – of various plant species. Their findings form an important basis for elucidating the regulation of mineral homeostasis in chloroplasts and developing new biotech strategies.

Many molecular mechanisms for maintaining ion homeostasis in chloroplasts remain unknown. A major key to further advances is to be found in the characterization of the chloroplast ionome, which has not been adequately described before now. To fill this knowledge gap, the researchers investigated the elemental composition of chloroplasts and leaves from the model plant Arabidopsis thaliana and three further species – the metal hyperaccumulator Arabidopsis halleri, Pisum sativum, and Nicotiana benthamiana – and analyzed similarities and differences.

“We found that metal concentrations in the chloroplasts were quite similar across all species. Even for the hyperaccumulator A. halleri, which had about 23 times more zinc in its leaf tissue than the other species, chloroplast zinc contents were at around the same level as that of the other plants,” says Lorenz Holzner, lead author of the study. “This suggests that plants keep chloroplast metal contents within a certain range in order to protect photosynthesis from excess metal.”

An exception were the iron ions, whose levels in the chloroplasts of A. halleri were twice as high as in A. thaliana. Accordingly, the researchers used iron to investigate whether the chloroplast ionome can be influenced by means of genetic modifications. “And indeed, we found that the mutation of a specific iron transport protein occasioned a 14-fold enrichment of iron in the chloroplasts,” explains Holzner.

The authors hypothesize that the excess iron in the chloroplasts is fixed in specific iron storage proteins (ferritins), as the iron content decreased again considerably in mutants without ferritin. “Our study shows that chloroplasts can be converted into large iron stores with the help of ferritins,” says Kunz. “Overall, our findings furnish important starting points for the development of new biotech concepts to allow us to strengthen the resilience of plants, improve yields, and adapt the nutrient content of our food according to our needs.”