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Some plants can survive months without water, only to turn green again after a brief rain. A recent study by the Universities of Boone and Michigan showed that this is not due to the “miracle gene”. Rather, this ability is the result of a whole network of genes, nearly all of which are also present in more vulnerable varieties. The results have already appeared online previously in “The Plant Journal.” The print version will be published soon.
In their study, the researchers took a closer look at a species long studied at the University of Bonn – the resurrection plant Craterostigma plantagineum. It gets its name quite right: in times of drought, one might think that it is dead. But even after months of drought, a little water is enough to revive it. “At our institute, we have been studying how a plant does this work for many years,” explains Prof. Dr. Dorothea Bartels from the Institute for Molecular Physiology and Biotechnology of Plants (IMBIO) at the University of Bonn.
Her interests include genes responsible for drought tolerance. It is becoming increasingly clear that this ability is not the result of a single “miracle gene”. Instead, there are a large number of genes involved, most of which are also present in species that are not well adapted to drought.
The plant contains eight copies of each chromosome
In the current study, Bartel’s team, jointly with researchers from the University of Michigan (USA), analyzed the complete genome of Craterostigma plantagineum. And this is very intricately constructed: While most animals have two copies of each chromosome—one from the mother, one from the father—Craterostigma has eight. This “eightfold” genome is also called an octopus. In contrast, we humans are diploid.
“Such proliferation of genetic information can be observed in many plants that have evolved under extreme conditions,” says Bartels. But why is that? Possible cause: If the gene was present in eight copies instead of two, it could in principle be read four times faster. Thus the octopus genome can enable large amounts of the required protein to be produced very quickly. This ability also appears to be important for the development of drought tolerance.
In Craterostigma, some genes associated with greater drought tolerance are duplicated. These include the so-called ELIPs – an acronym that stands for Early Photoinducible Proteins – they are rapidly triggered by light and protect against oxidative stress. It occurs at high copy numbers in all drought tolerant species. “Craterostigma has approximately 200 nearly identical ELIP genes located in large clusters of ten or twenty copies on different chromosomes,” Bartels explains. So drought-tolerant plants can rely on an extensive network of genes that they can rapidly regulate if a drought occurs.
Drought-sensitive species usually have the same genes – albeit with lower numbers of copies. This is also not surprising: the seeds and pollen of most plants often remain able to germinate after long periods without water. So they also have a genetic program to protect against drought. “However, this program is usually turned off upon germination and cannot be reactivated afterwards,” the botanist explains. “In the epiphytes, in contrast, they remain active.”
Most species “can” tolerate drought
Drought tolerance, then, is something the vast majority of plants “can do.” The genes that confer this ability may have appeared very early in the course of evolution. However, these webs are more efficient in drought tolerant species and, moreover, are not only active at certain stages of the life cycle.
However, not every cell in Craterostigma plantagineum has the same “desiccation program” either. This was shown by researchers from the University of Dusseldorf who also participated in the study. For example, different drought network genes are activated in roots during drought than in leaves. This result is not unexpected: leaves, for example, need to protect themselves from the harmful effects of the sun. They are helped in this by ELIPs, for example. With sufficient moisture, the plant forms photosynthetic pigments that at least partially absorb radiation. This natural protection largely fails during a drought. Roots, in contrast, don’t have to worry about sunburn.
The study improves understanding of why some species suffer from dehydration. In the long run, it could contribute to the reproduction of crops such as wheat or maize that are better adapted to drought. In times of climate change, demand for them is likely to increase more than ever in the future.
Participating institutions and financing:
In addition to the University of Bonn, Michigan State University (USA) and Heinrich Heine University Düsseldorf participated in the study. The work was funded by the US National Science Foundation (NSF) and the German Research Foundation (DFG).
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