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Through photosynthesis, plants use sunlight to convert water and carbon dioxide into energy-rich carbon compounds such as glucose. These compounds not only sustain the plant's own growth and development but are also channeled to the roots. A substantial share of this carbon is released into the soil in the form of organic substances, providing an important food source for microorganisms.

Researchers at the University of Bonn, together with Forschungszentrum Jülich and other partners, have now investigated how this process unfolds. Their have been published in Nature Communications.

Microbes in the rhizosphere

Microorganisms in the soil are usually limited in growth because energy-rich carbon sources are scarce. Around the root, however—in the so-called rhizosphere—conditions are different: plants exude organic compounds here that fuel microbial life.

Many of these microorganisms in turn support the plant's performance, for instance by supplying nutrients, protecting against pathogens, or helping it cope with drought stress.

The central question the researchers addressed was: to what extent do plants steer the composition of microbial communities at their roots? The results suggest that they exert at least some influence. In maize, the data indicate that different groups of microbes thrive in distinct root zones. Plants do not release their carbon evenly—instead, different parts of the root favor different microbial communities, leading to changes in the microbiome along the root system.

A view inside the root

The maize plants were cultivated at Forschungszentrum Jülich. To capture the underlying processes, scientists at the Institute of Plant Sciences combined two imaging techniques more commonly associated with medicine: magnetic resonance imaging (MRI) and positron emission tomography (PET).

MRI revealed the architecture of the maize roots in soil and enabled precise measurements.

PET, using the short-lived radioactive tracer ¹¹COâ‚‚ (with a half-life of about 20 minutes), tracked how freshly fixed carbon moved through the root system in the form of sugars.

By combining both methods, the Jülich researchers were able, for the first time, to non-destructively and at high resolution demonstrate that sugar accumulates very differently across root regions. This allowed them to target specific root types and sections for further analysis of their microbial communities.

In addition, a stable ¹³COâ‚‚ label was applied to determine which microorganisms actually consumed the root exudates—and how they responded to the uneven distribution of carbon in the rhizosphere. The microbial analyses themselves were carried out by teams at the Universities of Bonn and Cologne.

Implications for science and agriculture

In the longer term, insights into these processes could help to make better use of beneficial microorganisms in crop protection and growth promotion—for instance, to strengthen plants against drought or disease.

This is highly relevant for agriculture, where so-called biologicals and biostimulants have so far produced mixed results. Detailed knowledge of sugar flows and microbial communities may help such approaches become more consistent and effective.