Plants engineered for on-demand drought tolerance

Drought tolerance can be induced using a chemical already approved for agriculture.

Source: Ars Technica

Even under normal circumstances, drought is a regular occurrence in agricultural regions. Several human-driven trends, from groundwater depletion to climate change, are expected to aggravate natural water shortages. While crops can’t be expected to be very productive during times of drought, it might be possible to at least get them to better tolerate short periods of water scarcity without dying.

Efforts to that end have largely focused on traditional breeding between commercial crops and drought-tolerant relatives. But researchers  at the US University of California, Riverside are now reporting progress with an alternative approach: genetic engineering. They have taken a signalling network that plants normally use to respond to stresses such as lack of water and have rewired it so that it responds to a molecule that’s normally used to kill fungus.

The signalling network that was used normally responds to a chemical made by plants called abscisic acid. Its response triggers long-term changes by regulating the activity of genes. But it also has a short-term effect: it helps plants hold on to water. It does this by affecting what are called “guard cells,” which form part of the openings (called stomata) that plants use to regulate the flow of gases into and out of their leaves.

When stomata are fully open, critical gases like the carbon dioxide required for photosynthesis can enter the leaves while the oxygen produced by it can exit. Unfortunately for plants, water vapour that forms in the open interior spaces of a leaf can also escape through these same stomata. Abscisic acid causes guard cells to close off the stomata. While this process will slow down photosynthesis, it also lets the plants hold on to much more water, allowing them to better tolerate a period of going without.

Plants sense abscisic acid through a variety of receptors that latch on to this molecule, so the researchers picked one of these and tried to get it to latch on to something different. First they disabled a key piece of the protein that interacts with abscisic acid. Next, they targeted lots of mutations to the parts of the receptor that form a binding pocket for this molecule. Once the binding pocket was altered, they tested whether the protein stuck to any one of a panel of 15 different chemicals, all of which are already used in agriculture.

While they had several promising chemical-receptor combinations, they focused on those that latched on to a chemical (Syngenta’s mandipropamid), which is used in agriculture to kill fungi that attack plants. An abscisic acid receptor with three mutations bound weakly to the fungicide, and the team of scientists subjected this receptor to further mutations, selecting for enhanced binding; five additional mutations (one each in five different tests) were identified this way. So the authors engineered 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 small relative of mustard called Arabidopsis to do their initial tests.

In seeds, abscisic acid manages stress by keeping the seeds from germinating until conditions are favourable. With the genetically modified seeds, applying the fungicide delayed their germination compared to untreated seeds. When plants carrying the receptor grew out of these seeds, the authors tested the fungicide on them.

Normally, the loss of water vapour through stomata helps 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 glow red with a thermal camera. To show that this wasn’t something odd with Arabidopsis, the authors added the receptor to tomato plants and showed that they also heated up when the fungicide was applied.

But the key question is how the plants responded to low-water conditions. The answer, as shown above, is “very well.” The authors subjected plants to an 11-day water-free period and then returned them to regular watering. Twenty-four hours later, the genetically modified plants had bounced back. The regular ones, well …

Although we know a fair bit about the abscisic acid network, its activity normally changes over time as stresses come and go and plants adjust to their environment. The authors note that it will be important to determine if extended periods of activity will have some unforeseen consequences for the plants. Still, even some downsides may end up being much better than the consequences we can foresee from extended droughts.

Article reference Nature, 2014. DOI: 10.1038/nature14123  (About DOIs).

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