Maize plants that “know” how to make friends could save on fertilizers: Researchers have found that the roots of particularly high-yield maize plants increasingly release substances into the soil that allow beneficial bacteria to thrive. These microbes stimulate the formation of side roots and thereby enable improved nitrogen uptake from the substrate. These findings could lead to the breeding of varieties that require less fertilizer and thus have less impact on the environment, say the scientists.
For a long time they were mainly considered villains – but it is now clear that special microbes fulfill important functions for many living things. In humans and animals, a healthy intestinal flora ensures, among other things, optimal absorption of nutrients and a strong immune system. In plants, the counterpart to the intestinal flora is the microbial society in the root area: Studies have already shown how important certain fungi and bacteria in the soil are for nutrient uptake and for the health of the plants.
It is also known that some plant species also specifically promote the development of favorable interrelationships: They release substances into the soil via their roots that have a beneficial effect on the environment for their microbial friends. In breeding, however, this aspect has hardly been used in a targeted manner so far. “The interactions of the root network with soil organisms have generally still not been researched,” says plant researcher Peng Yu from the University of Bonn. With the current study, Yu and his colleagues have now been able to make an important contribution to understanding the interrelationships in the soil.
Gifts to tiny friends
At the beginning there was the clarification of the question why different maize varieties differ so clearly in their yield. In their search for the cause, the researchers finally came across a peculiarity of the high-performance plants: they increasingly produce the enzyme flavone synthase 2 in their roots Yu. As he explains, the enzyme is a tool for producing the actually relevant active substances: “The plants use it to produce certain molecules from the group of flavonoids and release them into the soil,” says Yu.
As the researchers explain, flavonoids are primarily known as coloring substances in flowers and fruits. But apparently they also have an important effect in the soil: They ensure that certain soil bacteria accumulate around the roots. As the researchers found, these are representatives of the Oxalobacteraceae. The presence of these microbes in turn stimulates the formation of lateral branches in the roots, their studies have shown. “This enables the maize to absorb more nitrogen from the subsoil, among other things,” explains Frank co-author Hochholdinger. “For this reason, the plant then grows faster, especially when the nitrogen supply is scarce.”
Microbes make roots sprout
The researchers were able to impressively document the importance of microbe accumulation through their experiments. For this they used the maize breeding line LH93, which grows rather measly compared to high-performance varieties. That could be changed, however, if you planted these plants in soil in which the line 787 had previously grown, which releases many flavonoids into the soil: LH93 grew particularly well in this substrate. However, the effect did not materialize when the scientists sterilized the soil before repotting. This thus confirmed that the enriched bacteria are actually responsible for promoting growth, because the sterilization had killed them.
The scientists were able to demonstrate that the effect of the microbes promotes root growth using a maize mutant that normally hardly develops side roots. The experiments showed that the oxalobacteria acted like a kind of drug in these plants: When the researchers added them to the soil, the roots of these mutants also branched out and ensured an improved supply of nutrients. How the oxalobacteria lead to more intensive root penetration is still unclear. The researchers now want to pursue this question through further investigations.
Plant breeding potential
But the results are already showing potential for agriculture, say Yu and his colleagues: “If we can grow crops so that they increase their nutrient supply through the help of bacteria themselves, that could significantly reduce the environmental impact,” says Yu . The main focus is on nitrogen. This nutrient is of central importance for plant growth, which is why the content in the soil is often greatly increased by fertilization measures when cultivating useful plants.
However, this is problematic for various reasons: The nitrogen can get into rivers and lakes or into the groundwater and cause negative effects there. In addition, nitrogen compounds can turn into nitrogen oxides or ammonia gas and thus contribute to the greenhouse effect, among other things. In addition, the production of nitrogenous fertilizers requires a lot of energy. This means: It makes sense in many ways to optimally use the natural potential of plants for their own nitrogen supply.
Source: University of Bonn, specialist article: Nature Plants, doi: 10.1038 / s41477-021-00897-y