Utrecht University: Plants and parasites can rapidly evolve symbiotic relationship

Within just a few generations, bacteria on plants can adapt from harmful to beneficial. Biologists report how parasitic bacteria can rapidly adapt to a new variant, helping both the plant and themselves. This discovery paves the way for developing special bacterial fertilizer for crops, the researchers say. The results were published in the journals Nature Communications and mBio.

Just like our gut is populated by countless microorganisms, plant roots are also a welcoming home for bacteria. At the roots, bacteria perform a variety of tasks that are beneficial to the plant. They protect the roots, help absorb water from the soil, and assist the plant by extracting nutrients from the soil.

Harmful bacteria also exist. They extract substances from the root, but do not provide any benefit or even lead to plant diseases. However, these kinds of parasitic bacteria can change within a few generations into useful organisms that actually help the plant. Biologists from Utrecht University and the University of York discovered this rapid evolution, and now report their results in the journals Nature Communications external linkand mBio external link.

Symbiosis after six transplants
The biologists discovered the transformation in Arabidopsis plants whose roots were covered with harmful Pseudomonas bacteria. When the plants became sick from the bacteria, after about four weeks, the biologists transplanted some of the bacteria to healthy plants. Then those plants also became sick, after which the biologists again transferred the bacteria from the sick plants to new, healthy ones.

After each transplant of bacteria, the new plants became less sick, the biologists observed. After six consecutive transplants, the plants didn’t even get sick from the bacteria at all. In fact, the plants grew better than normal.

The rate at which the bacteria evolved from parasitic to beneficial is very special. This has not been observed before.

Dr. Ronnie de Jonge
Dr. Ronnie de Jonge
Environmental Biology
“The rate at which the bacteria evolved from parasitic to beneficial is very special,” said Utrecht University biologist Ronnie de Jonge, one of the leaders of the study. “This has not been observed before.”

The enhanced plant growth was also beneficial to the Pseudomonas bacteria. De Jonge: “Bacteria living on plant roots depend on substances secreted by the root. More plant growth means more substances being excreted by the root. The bacteria benefit from that. We ended up seeing dozens of times more bacteria than in the early stages of the experiment.”

Especially when circumstances are less favourable, bacterial fertilizers could prove to be beneficial.
Bacterial fertilizer for crops
According to De Jonge, the rapid evolution of the bacteria opens up possibilities for making a kind of bacterial fertilizer for crops. By actively combing bacteria and plants, they can live in symbiosis with each other, leading to higher crop yields. Especially when conditions are less favourable for the plant, the bacterial fertilizer could prove to be beneficial.

Precision agriculture
It should even be possible to make special compositions of bacteria for specific conditions, said De Jonge. “In our research, we saw the bacteria responding very specifically to the composition of substances excreted by the root, and adapt to it quickly. By applying the same tactic, you could evolve bacteria specifically for plants growing, for example, in a field that often dries out. You then select the microorganisms that thrive under those specific dry conditions. They can help the plant cope with the drought.”

Beneficial bacteria could be evolved specifically for plants growing in unfavourable conditions, for example in a field that often dries out.

Dr. Ronnie de Jonge
Dr. Ronnie de Jonge
Environmental Biology
Adding helpful bacteria to plants definitely isn’t a pipe dream. Already, seed production companies can supply seeds with special coatings incorporating beneficial bacteria. After germination, bacteria from the coating help the plants deal with diseases or iron and phosphate deficiencies, for example.

Reseachers envision custom made bacterial coatings, aiding specific plants in specific conditions
Currently, breeders are working in a rather generic manner in this respect. They want to provide the coatings with different types of bacteria that are beneficial for all kinds of conditions. De Jonge and colleagues, on the other hand, envision custom made bacterial coatings. For specific plants and specific conditions, you could develop separate bacteria and incorporate them into the coatings. “Those bacteria can do their work in a targeted way and be very effective,” says De Jonge.

Proof of concept
More research is needed before such ‘precision coatings’ are feasible though, according to De Jonge. His team will further investigate how the interactions between plants and parasites work, and what circumstances play a role in this. “For this proof of concept, we used a fairly simple setup. We studied one plant, with one bacterium that is completely dependent on the plant for its energy. Now we also want to study more complex conditions, and further investigate how the composition of secreted substances around the root ultimately affects the micro-organisms.”

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