Can the stress tolerance of moss be transferred to maize and other plants?
The moss Physcomitrella patens has long since been one of several popular model organisms used in research. The Physcomitrella patens genome was sequenced in 2007. Comparative analyses with other plant species show why the relatives of the moss were able to colonise land approximately 500 million years ago: the moss relatives developed a large number of mechanisms to protect them against drought and other stress factors, which made them real survivalists. Prof. Dr. Ralf Reski, head of the Department of Plant Biotechnology at the University of Freiburg, has contributed greatly to shaping the career of the moss Physcomitrella patens. In an interview with BIOPRO, Reski explained what agricultural crops can learn from their ancestral relatives.
Prof. Reski, what causes plants stress and how does this affect agriculture, especially in the face of the changing climate?
Reski: Plants are exposed to both biotic and abiotic stress. Biotic stress refers to stress from living organisms such as potato beetles, fungi and bacteria. Abiotic stress relates to unfavourable growth conditions. The most important stressors are extreme temperature, high salt concentrations, UV radiation and high heavy metal concentrations. It is generally believed that temperature, salt concentration and UV radiation will increase in the future as a consequence of the changing climate. These factors therefore represent the most limiting factors for agricultural productivity. It is thought that abiotic stress reduces crop yield by more than 20 per cent as plants are unable to adapt effectively to extreme environmental conditions. If the plants’ tolerance to these stress factors could be increased, it would be possible to boost the crop yield. This would not only be of major assistance in the current disaster in the Horn of Africa, where millions of Africans are at risk of starvation after two failed rainy seasons, but also with regard to the growing world population in general. Urban sprawl and desertification reduces available arable land. In addition, incorrect irrigation and the use of large amounts of fertilisers cause a build up of salt in the soil. These are the major reasons why efforts are being undertaken to increase the crop yield per hectare of arable land.
How do plants react to high salt concentrations in the soil?
Reski: Roots, which interact with and take up nutrients from the soil, are a plant’s first line of defence against high salt concentrations. High salt concentrations lead to disturbances in the physiological balance, the plants take up smaller amounts of nutrients, grow more slowly and wither. In addition, salt has an osmotic effect on the cells, which means that the cells dehydrate. The lack of intracellular water leads to the destruction of protein structure, vital processes get disrupted and the plants eventually die.
What can plants do to counteract abiotic stress?
Reski: Mosses are excellent models in the quest for answers to questions like this. In contrast to flowering plants, mosses are able to actively transport salt out of the cell. Physcomitrella has a protein that functions like a pump. At present, Australian researchers are trying to transfer the pump-encoding gene to crop plants. In addition to this salt pump, Physcomitrella has other stress response mechanisms to protect it against environmental stress, for example mechanisms that enable it to counteract stress caused by drought. Dryness is an osmotic stress factor, as are high salt concentrations in the soil. In both flowering plants and mosses, chaperones, i.e. proteins that maintain the proper conformation of other proteins, or molecules such as the amino acid proline are able to counteract the lack of intracellular water. By binding to proteins, proline maintains the proper functional conformation of proteins. In extreme cases of abiotic stress, plants attempt to switch from growth to reproduction. The early production of flowers of fruit trees (also known as bolting) can be an indicator of plant stress. In biological terms, bolting makes sense because plants that are exposed to high stress with a potential fatal outcome try to produce seeds that can survive adverse conditions. The recombination of genes in seeds might also generate offspring that are better adapted to a particular stress factor.
So, individual cells react to stress, and cells also communicate with each other in situations of stress. Do they communicate by way of hormones?
Reski: Yes, they do. Abscisic acid, or ABA for short, is a growth-inhibiting hormone that enables perennial plants to tolerate stressful conditions. ABA-mediated signalling nearly always mediates the same genetic programme that enables plants to switch from vegetative growth to reproduction. Physcomitrella patens is also an excellent model for studying plant response to stress factors: when plants experience high levels of stress they attempt to produce sexual spores that can survive unfavourable environmental conditions for long periods. In young moss plants, which are unable to go through the development programme as quickly as adult plants, ABA mediates the reprogramming of individual vegetative cells, i.e. plant growth slows down in favour of protective measures. These cells, which have an elongated shape and normally carry out photosynthesis, develop a circular shape under the influence of ABA and grow a thick cell wall. Surrounding cells die as a result of apoptosis and resting vegetative spores that can survive adverse environmental conditions develop. It is worth noting that ABA-mediated signalling is the fastest type of signalling known in mosses. It induces a cellular response within just a few minutes, and this response involves the production of transcription factors that regulate the genes of other proteins, thereby switching on signalling cascades that lead to the reprogramming of the cells.
You’ve mentioned current attempts to transfer stress tolerance genes to agricultural plants. Why is the moss Physcomitrella patens so important for plant biotechnology?
Reski: We use the moss to gain insights into genes, mechanisms and methods which can in principle also be used to genetically modify crops. Monsanto and BASF are currently working on the development of a maize variety that is able to produce 20 per cent greater yields under dry conditions than unmodified plants. This maize is currently being tested in the USA and is set to come to Europe. In a cooperative project with BASF several years ago, we succeeded in identifying moss genes that other plants do not possess, including genes that mediate the plants’ tolerance to stress or genes that are involved in the biosynthesis of polyunsaturated fatty acids. BASF has already transferred some of these genes into cultivated plants. Another possibility is to use mosses for the production of proteins that are of direct importance for humans, for example therapeutic antibodies. In contrast to flowering plants, mosses can be grown in bioreactors under the controlled and sterile conditions that are essential for drug production. However, the greatest difference between Physcomitrella patens and other plants is the ability to apply gene targeting, a genetic technique that uses homologous recombination to change an endogenous gene, delete or add genes and introduce point mutations. In the medical sector, gene targeting is used to produce knock-out mice in a process that takes around 18 months. On the other hand, knock-out mosses can be produced within the relatively short period of 8 weeks.
Germany is a world leader when it comes to its opposition to agricultural biotech. What is your view on this? Do you believe that this needs to be changed in view of the need to feed a growing world population?
Reski: Abundance is the key word in Europe. We have the luxury of deciding whether we want genetically modified crops or not. However, genetically engineered plants are essential to feed starving populations in other countries. This is why China, for example, is investing hundreds of millions of dollars in plant biotechnology. As I see it, as Europeans, we have two choices: we can either accept genetically engineered plants and continue our research, or we can decide against such plants, in which case we will be forced to import ever increasing quantities of food. Around 80 per cent of globally produced soya is already genetically modified. In Germany, the application of genetic engineering involving the use of plants is as controversial as the application of recombinant techniques for the production of therapeutic proteins a few years ago. Let me give you an example: In the 1980s, Germany quashed genetically engineered medicines. Protests at the time were directed at a company called Hoechst, which had developed a method to produce artificial insulin for diabetics, replacing a process that derived insulin from the pancreases of slaughtered pigs. The company battled against red tape before finally deciding to sell the patent. The then environment minister of the German state of Hesse (where Hoechst was located, ed. note), Joschka Fischer, refused permission for Hoechst (now part of Sanofi, ed. note) to open an insulin production plant and it was more than 10 years before the plant could be opened. This action led to many German companies deciding not to file patents related to genetically engineered products. Nowadays, all diabetics – including Germans - use insulin that is made with genetically engineered bacteria/yeast, which is, however, not produced by German companies. The German pharmaceutical industry, which was once regarded as the world's pharmacy, suffered a considerable setback from which it is recovering rather slowly. If I had to give a reason for this setback, I would say it was the protests and political decisions that blocked the production of artificial insulin in the 1980s.
I am sure German agriculture will suffer to the same extent as the German pharmaceutical industry if political acceptance of plant biotechnology does not increase. As I see it, green biotechnology and ecology go very well together. Let me give you an example to show what I mean. The Kaiserstuhl mountains are contaminated with large amounts of copper. The vines in the region are frequently attacked by fungi which can only be abated with fungicides or copper-sulphate solutions, thus leading to the accumulation of copper in the soil. If we were able to increase the vines’ resistance to fungi by using genetic engineering, I am sure we would not have this problem. Another example where genetic engineering could be an alternative to traditional methods in combating pests and increasing plant productivity is the Western corn rootworm that is currently spreading across cropland throughout Germany. In this case, genetically engineered maize that is resistant to the corn rootworm might be a good solution for combating the growing problem, but this has once again sparked disagreement between authorities, environmental groups and the producer. And yet another example: maybe some of us still recall the many thousands of bee colonies that were destroyed in the Rhine Valley in 2008. This was believed to have come about because of a chemical used to impregnate the maize seeds. These are all examples of the price we pay for the restrictions we put on green biotechnology. Another price we pay is the huge quantities of subsidies to the tune of several billion euros per year given to European farmers, in compensation, amongst other things, for losses sustained at harvest.
Professor Dr. Ralf Reski
Department of Plant Biotechnology
Faculty of Biology
University of Freiburg
Tel.: +49 (0)761/203-6969
Fax: +49 (0)761/203-6967