Arctic bacteria can help plants to survive drought

Drought is becoming an increasing problem for farmers. Changing rainfall patterns and increasing evaporation reduce soil moisture and reduce crop growth and yield, which threatens global food security and increases hunger in much of the world. Plant scientists and breeders are striving to engineer new varieties of crops that tolerate drought better, and microbiologists are investigating whether bacteria can help plants to adapt.

“We have investigated bacteria that originate from arctic grass species in Canada. They produce enzymes that can help plants to tolerate drought better. This is already a huge problem in the global South and is already becoming one in parts of the global North. If we succeed in understanding how these bacteria can enable plants to tolerate drought, we can develop new plant-associated bacteria that may mitigate some of the consequences,” explains a lead author, Peter Stougaard, Professor, Department of Environmental Science, Aarhus University, Denmark.

Less water in the soil

Unstressed plant roots normally balance indole-3-acetic acid (IAA), a growth hormone, and ethylene, a stress hormone. IAA promotes growth, and ethylene regulates ripening and ageing. Plants that are stressed, such as by drought or heat, produce more ethylene, which can inhibit IAA’s growth-promoting effects.

“If stress is transitory, getting plants to produce ethylene is very useful, because this slows their metabolism until the stressful circumstances pass and they can continue growing. But if this situation lasts too long, growth is inhibited and the yields decline significantly,” says Peter Stougaard.

Previous research has shown that the growth-inhibiting effects can be synthetically reduced: for example, by increasing the amount of the plant hormone auxin, which promotes root elongation so that plants can reach the dwindling supplies of groundwater. In addition, cytokinins can delay leaf ageing and promote further shoot growth, even under stressful conditions such as heat and drought.

“It turns out that 1-amino-cyclopropane-1-carboxylate (ACC) deaminase can protect plant growth against drought because this enzyme breaks down ACC, a precursor to ethylene. In addition to perhaps reducing the plant’s inherent stress, it also produces elongated roots, reducing the stress of lacking water and increasing water absorption. In addition, ACC deaminase can also briefly strengthen the plant’s resistance to oxygen-limited conditions,” notes Peter Stougaard.

Grass species in Ellesmere Island

Since plants do not naturally produce ACC deaminase, they have to get help from bacteria, such as those from arctic grass bacteria.

“We knew that this bacterium could make cytokinin. However, we used some bioinformatic tools to compare gene sequences from the bacterium with others and discovered that some were similar to those known from other ACC deaminases. We therefore assumed that it could presumably be the same as reported elsewhere,” says Peter Stougaard.

Twelve years ago, researchers in Canada had already characterised these bacteria, including the cytokinin- and ACC deaminase–producing bacteria, in the soil around the roots of some grass species on Ellesmere Island, where they potentially play a part in nutrient exchange – and thus in the interaction between plants and soil. The researchers named the interesting cytokinin- and ACC deaminase–producing bacterium Pseudomonas fluorescens G20–18^T but later renamed it.

“We discovered that it had been characterised as the wrong species. In our world, naming things correctly is important because otherwise this creates problems in publications and taxonomic descriptions. We therefore named the bacterium as a new species, Pseudomonas hormoni G20-18^T,” explains Peter Stougaard.

A sea of hormones

It turns out that P. hormoni G20-18^T can communicate with the environment, including with plant roots. The roots that have P. hormoni G20-18^T on or in them are protected, but if the roots are exposed to stress, the bacteria break down ACC, the precursor to ethylene, and this avoids producing too much ethylene, maintaining the balance between ethylene and IAA. 

“In addition, the bacteria will cause the root cells to divide more and elongate the roots to better reach groundwater. In addition to the plants’ IAA influencing the formation of ACC deaminase, we found that other metabolites such as amino acids affected ACC deaminase production. So the bacteria and the plants cross-talked using chemical molecules,” says Peter Stougaard.

The bacterium was named P. hormoni G20-18^T because, in addition to ACC deaminase, it produces several hormones that plants also naturally produce, including the cytokinins isopentenyl adenosine, trans-zeatin ribose, dihydrozeatin riboside and IAA. P. hormoni G20-18^T is thus an example of a bacterium that appears to be able to help plants to adapt to the stressful situations that future climate change will create.

“In the future, drought, flooding, oxygen-limited conditions in the soil and higher temperatures will increasingly stress crops. We hope that studying P. hormoni G20-18^T and other such bacteria will enable us to contribute new knowledge that can be used to develop new environmentally friendly and natural bioinoculants for agriculture. In other words, natural and beneficial microorganisms that can improve plant growth and health,” adds Peter Stougaard.

Under what circumstances?

According to the researchers, the results are extremely promising. However, several problems need to be solved before drought-affected soil can be fertilised with the climate-friendly bacteria.

“One challenge is that P. hormoni G20-18^T is gram-negative and does not form spores. It would have been somewhat easier if it formed spores like Bacillus. Then it could be mixed more efficiently with the seeds, and the spores themselves would also be more drought-resistant. However, P. hormoni G20-18^T is endophytic and can live inside the plants without harming them. But this means that once P. hormoni G20-18^T gets into the plant, it is protected – so that is the good thing about it,” says Peter Stougaard.

It is equally important to determine whether the bacteria’s magical enzymes work as well for warm wheat plants at our latitudes as they do in arctic grasses.

“But how can P. hormoni G20-18^T produce hormones and under what circumstances? Because using P. hormoni G20-18^T to mediate drought tolerance is not useful if it does not actually synthesise the necessary enzymes. Knowing the background to all of this is therefore important. So we are working intensively to investigate these hormone-producing bacteria and wheat grown under various drought conditions,” concludes Peter Stougaard.