Genome analysis of a drought-tolerant plant

Some plants can survive months without water, only to turn green again after a brief downpour. A new study shows that this is not due to a ‘miracle gene’; rather, this ability is a consequence of a whole network of genes, almost all of which are also present in more vulnerable varieties.

The researchers took a close look at a species that has long been studied at the University of Bonn — the ‘resurrection plant’, Craterostigma plantagineum. In times of drought, one might think 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 the plant does this for many years,” said Prof Dr Dorothea Bartels, from the Institute of Molecular Physiology and Biotechnology of Plants (IMBIO) at the University of Bonn. She and her team have now collaborated with the University of Michigan to analyse the complete genome of Craterostigma plantagineum, with their results published in The Plant Journal.

The resurrection plant Craterostigma plantagineum — in irrigated condition (left), desiccated (centre) and then ‘resurrected’ (right). Image ©AG Bartels/University of Bonn.

While most animals have two copies of each chromosome — one from the mother, one from the father — Craterostigma has eight. An ‘eightfold’ genome such as this is also called octoploid.

“Such a multiplication of genetic information can be observed in many plants that have evolved under extreme conditions,” Bartels said. A probable reason for this is that if a gene is present in eight copies instead of two, it can, in principle, be read four times as fast. An octoploid genome can therefore enable large quantities of a 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 tolerance to drought are even further replicated. These include the so-called ELIPs (early light inducible proteins), as they are rapidly switched on by light and protect against oxidative stress. They occur in high copy numbers in all drought-tolerant species.

“Craterostigma has close to 200 ELIP genes that are nearly identical and are located in large clusters of 10 or 20 copies on different chromosomes,” Bartels said. Drought-tolerant plants can therefore presumably draw on an extensive network of genes that they can rapidly upregulate in the event of drought.

Drought-sensitive species usually have the same genes, albeit in lower copy numbers. This is not surprising: the seeds and pollen of most plants are often still able to germinate after long periods without water, so they also have a genetic program to protect against drought.

“However, this program is normally switched off at germination and cannot be reactivated afterwards,” Bartels said. “In resurrection plants, in contrast, it remains active.”

Drought tolerance, then, is theoretically possible in the vast majority of plants, as the genes that confer this ability probably emerged very early in the course of evolution. However, these networks are more efficient in drought-tolerant species and are not active only at certain stages of the life cycle.

That said, not every cell in Craterostigma plantagineum has the same ‘drought program’, as shown by researchers from the Heinrich Heine University Düsseldorf who were also involved in the study. For instance, different drought network genes are active in roots during desiccation than in leaves. This finding is not unexpected: leaves, for instance, need to protect themselves against the damaging effects of the sun, and are helped in this by ELIPs. With sufficient moisture, the plant forms photosynthetic pigments that at least partially absorb radiation. This natural protection largely fails during drought. Roots, in contrast, do not have to worry about sunburn.

The study thus improves understanding of why some species suffer so little from drought. In the long term, it could therefore contribute to the breeding of crops such as wheat or corn that cope better with drought — and in times of climate change, these are likely to be in great demand.

Top image ©AG Bartels/University of Bonn.