Even under normal conditions, droughts are common in agricultural areas. Several human-induced trends, from groundwater depletion to climate change, are expected to exacerbate natural water shortages. While crops cannot be expected to be very productive in times of drought, it may be possible to at least make them better tolerate short periods of water scarcity without dying.
Efforts to do so have largely focused on traditional breeding between commercial crops and drought-tolerant relatives. But researchers are now reporting progress with an alternative approach: genetic engineering. They’ve taken a signaling network that plants normally use to respond to stresses like lack of water and rewired it so that it responds to a molecule normally used to kill fungi.
The signaling network used normally responds to a chemical made by plants called abscisic acid. The response causes long-term changes by regulating the activity of genes. But it also has a short-term effect: it helps plants retain water. It does this by affecting what are called “guard cells,” which are part of the openings (called stomata) that plants use to regulate the flow of gases in and out of their leaves.
When the stomata are fully open, critical gases, such as the carbon dioxide needed for photosynthesis, can enter the leaves while allowing the oxygen produced by them to escape. Unfortunately for plants, water vapor that forms in the open interior spaces of a leaf can also escape through the same stomata. Abscisic acid ensures that guard cells close the stomata. While this process slows down photosynthesis, it also causes the plants to retain much more water, making them better able to tolerate a period without.
Plants sense abscisic acid through different receptors that attach to this molecule, so the researchers picked one and tried to get it to attach to something else. First, they knocked out an important part of the protein that interacts with abscisic acid. They then targeted many mutations to the parts of the receptor that form a binding pocket for this molecule. After changing the binding pocket, they tested whether the protein stuck to a panel of 15 different chemicals, all of which are already used in agriculture.
While they had several promising combinations of chemical receptors, they focused on those that latched on to a chemical (called mandipropamid), which is used in agriculture to kill fungi that attack plants. A three-mutation abscisic acid receptor bound weakly to the fungicide, and the team of scientists subjected this receptor to further mutations, selecting for improved binding; five additional mutations (one in five different tests) were identified in this way. So the authors developed a receptor with both the original three mutations and all five of the new ones.
The resulting receptor had a very strong affinity for the fungicide, so they put it back into plants, using a little relative of mustard called Arabidopsis to do their first tests.
In seeds, abscisic acid manages stress by preventing the seeds from germinating until conditions are right. In the genetically modified seeds, the application of the fungicide delayed their germination compared to untreated seeds. When plants carrying the receptor grew from these seeds, the authors tested the fungicide on them.
Normally, the loss of water vapor through the stomata helps to cool the leaves of the plant. The authors found that when the fungicide was applied, the genetically modified plants retained more heat — you could see them glowing red with a thermal camera. To show that this was not something strange Arabidopsisthe authors added the receptor to tomato plants and showed that they also warmed up when the fungicide was applied.
But the key question is how the plants reacted to low tide. The answer, as shown above, is “very good.” The authors subjected the plants to an 11-day water-free period and then watered them regularly again. Twenty-four hours later, the genetically modified plants had recovered. The usual, well…
Although we know quite a bit about the abscisic acid network, its activity normally changes over time as stress comes and goes and plants adapt to their environment. The authors note that it will be important to determine whether extended periods of activity will have some unforeseen consequences for the plants. Still, even some of the drawbacks may end up being much better than the consequences we can foresee from prolonged droughts.
Nature2014. DOI: 10.1038/nature14123 (About DOIs).