"We do not develop products. We test what is scientifically possible."

The five three-storey buildings of the science park in Golm look a bit out of place from the nearby railway station. The impression changes, however, as you approach: glass facades and spacious inner courtyards with modern sculptures give a feeling of transparency and openness. This impression is confirmed on a tour of the workspaces where Ralph Bock and his colleagues carry out high-tech research into genetic engineering methods on plants. What is special about their work is that they transform the genetic material in the plastids as well as in the cell nucleus. A plant cell contains two other genome s besides the nuclear genome: a genome in the mitochondrions that supply the cell with oxygen, and one in the chloroplasts (plastids) that are responsible for photosynthesis.

It all happened over a billion years ago

The precursors of mitochondrions and plastids were free-living bacteria. More than a billion years ago a proteobacterium was enclosed by the cell membrane of a ‘primordial’ cell, then completely absorbed inside the cell and finally converted to a different function. Its new job was to produce energy through respiration. When plant cells emerged, the process of taking up a bacterium was repeated. This time a blue-green alga that was able, even then, to convert light into chemical energy, lent itself to the task. More organelles had arrived: what are now known as chloroplasts .

In the course of evolution, huge quantities of DNA from the organelles migrated to the nuclear genome. Today chloroplast DNA contains only a few genes: just 130 in the tomato compared with over 30,000 in the nuclear genome.

Although the number of genes in the plastids (chloroplasts) is small, each plant cell contains up to one hundred plastids, which in turn contain up to one hundred identical DNA copies. If new genes are inserted into plastid DNA, activity is triggered in all the DNA copies at the same time. This means that plastid DNA can produce up to 40% more protein than nuclear DNA. This is what makes plastid transformation so interesting for biotechnology applications. Melanie Oey, a PhD student in the research team led by Ralph Bock, reports on preliminary results in producing a protein with antibiotic effect in tobacco plants using plastid transformation. "Up to 70% of the whole protein in the plant is an active antibiotic substance," she enthuses.

Biologically safe because of maternal inheritance?

Plastids have a special feature: in most plants their genes are inherited maternally, i.e. the (male) pollen of these plants does not contain any plastid DNA. This unusual inheritance model means that when plastids are genetically modified, the new gene sequences are not present in the pollen. The risk of transgenes spreading could be minimised using this strategy (biological confinement). Whether this actually works in practice, or whether plastid DNA is in fact occasionally found in pollen is something that Ralph Bock and his team have attempted to discover. To this end they have taken unmodified plants as female crossing partners and pollinated them using pollen from tobacco plants with genetically modified chloroplasts. They examined the progeny from around two million seeds and found that the spread of transgenes from chloroplasts via pollen is very small. The probability is less than one in fifty thousand.

There is another question that has occupied the scientists in Potsdam over recent years: Can genes from plastids enter the cell nucleus? Over very long evolutionary periods this can indeed occur, Ralph Bock explains. But here too he and his colleagues have been able to show that the risk is very small.

So much for the results in the greenhouse under standard conditions. Now the question is whether these findings are transferable to field conditions. Stress factors in the environment could have an impact on pollen inheritance or on the transfer of genes from plastids to the cell nucleus. In Ralph Bock’s current project he is therefore subjecting tobacco plants with a modified plastid genome to field-related stress conditions, such as heat, cold and drought.

Cereals – the great challenge

Ralph Bock and his colleagues have already developed a plastid transformation technology for the tomato, while another research group has succeeded in doing the same for cotton. But the real challenge, according to Ralph Bock, is still how to apply the technology in the most important agricultural crops - wheat, rice and maize.

Plastid transformation in cereals is difficult because these plants are hard to regenerate. "With tobacco we place a piece of leaf on a culture medium and get dozens, hundreds if we want, of new plants. If we use the same medium with maize nothing happens. No one knows why." In order to obtain only successfully transformed plants, multiple selection and regeneration stages are required on a culture medium containing an antibiotic. Moreover, cereals are naturally resistant to the antibiotic spectinomycin, which is generally used in plastid transformation, which means that it cannot be used to distinguish between transgenic and non-transgenic plants. "So there are a number of technical problems that occur during transformation of cereals. It should be possible to solve them all, but it will take time," predicts Ralph Bock.