Plants produce a countless number of metabolites with quite diverse functions. Many of these metabolites are not only useful to the plant itself, but also have positive effects in humans and animals. The extraction of these compounds in sufficient quantities from the naturally producing resources is often laborious and costly. Some metabolites can be produced in bacteria, but how about producing them in their natural hosts – the plants themselves? The research team led by Ralph Bock at the Max Planck Institute of Molecular Plant Physiology in Potsdam-Golm strives to accomplish exactly that. In their latest study, published in the journal Current Biology, the researchers demonstrated how new metabolic pathways can be introduced into plants to allow the production of high-value compounds. Their approaches include the exploitation of a natural phenomenon that allows the exchange of genes between different species.
For their studies, the researchers chose the colorant astaxanthin, an orange pigment belonging to a big group of compounds called carotenoids. Astaxanthin occurs naturally in some marine bacteria and microalgae, and it protects the cells against damage by UV radiation and other stressful environmental conditions. In animals who eat these microorganisms, it protects the cells against stress and diseases by slowing down or completely preventing the harmful oxidation of biomolecules. Shrimps and lobsters are orange colored, because astaxanthin is taken up with their food and accumulates in the animals.
Some fishes, such as salmon and trout, also use astaxanthin to protect their tissues from oxidative stress. This may be because the fish are exposed to high stress levels during their long migration from the ocean to the upper reaches of rivers where they spawn. The incorporation of astaxanthin, which is obtained with their diet, colors the flesh of the fish red. In salmon and trout farms, where fish are bred for the market to protect the natural fish stocks, the fish cannot feed on astaxanthin-containing plankton or small animals. Although, when farmed, the fish may not necessarily need the colorant, without astaxanthin-containing food, the flesh of the fish remains white to grey and does not show the typical orange color appealing to the consumer. For this reason, astaxanthin is in high demand as feed additive in fish farming. Unfortunately, extraction of the pigment from microorganisms or animals is extremely expensive. This is because astaxanthin is only present in fairly small concentrations in these marine organisms, thus requiring complicated and expensive isolation processes. Moreover, large amounts of non-usable waste products accumulate causing ecological concerns and economic problems.
The production of this high-value colorant in plants could deliver significant advantages and reduce production expenses dramatically. This is the reason why researchers are looking into possibilities to integrate the natural metabolic pathway of astaxanthin synthesis into plants by genetic engineering. Putting it into the DNA of the chloroplast is particularly attractive, as each plant cell contains large numbers of these organelles, but have only one nucleus. Thus, introduction of the genetic information into the chloroplast DNA allows high production rates of the pigment. An additional advantage lies in the high level of biological safety, as the chloroplasts are excluded from the pollen of the plant. This means that genetically engineered chloroplasts cannot be distributed in the environment via pollen.
Ralph Bock’s research group has taken the astaxanthin metabolic pathway from a marine bacterium and introduced it into the model plant tobacco. After all genes of the astaxanthin pathway had been implanted into the chloroplast genome, the tobacco plants shows a characteristic orange coloring due to the massive production of the colorant in the chloroplasts. Salmon-pink tobacco had been created.
The possibility to produce this valuable pigment in plants represents a considerable achievement. However, tobacco is a very poisonous plant, because it contains nicotine which is one of the strongest natural toxins. That is why it was desirable to use a nicotine-free plant for astaxanthin production. The researchers chose a close relative of the cigarette tobacco, the tree tobacco. This species does not contain any nicotine and, moreover, it is a perennial tree. Thus, once planted, the leafy biomass can be harvested from the tree over and over again. However, for technical reasons, it has not been possible to engineer the chloroplast DNA of this plant. That is why the research team has been searching for novel ways to introduce new genes into other plants.
“In previous grafting experiments, we had already shown that it is possible for plants to exchange chloroplast DNA between species”, explains Ralph Bock. This phenomenon, which commonly occurs in nature without any human intervention, is called horizontal gene transfer. It provides researchers with the opportunity to transfer foreign genes into so far non-transformable plants. After successful grafting of tobacco shoots onto plants of the tree tobacco, cells from the graft site were used to regenerate new plantlets by tissue culture. In this way, single tree tobacco cells that had received the chloroplast DNA from the astaxanthin-producing tobacco plant were regenerated into trees that now made the pigment.
This experiment nicely demonstrated the possibility of introducing foreign genes into non-transformable species by simply using horizontal gene transfer. In the future, this very simple transformation technology could also be applied to other important, but currently non-transformable plant species.