Plant-bacteria partnership reveals how roots access iron locked in soil

A new study led by researchers from the Max Planck Institute for Plant Breeding Research and the Leibniz Institute of Plant Genetics and Crop Plant Research has uncovered how Arabidopsis plants adapt their chemical communication with the root microbiota to optimize iron acquisition from acidic or calcareous soils. Iron is crucial for plant growth, but many soils contain insoluble forms of iron that roots cannot directly absorb.

Plants have developed strategies to unlock and acquire this essential micronutrient, and the role of the root microbiota in this process is key. Healthy plants colonized by diverse microbial communities release specialized molecules, such as coumarins, into the rhizosphere to mobilize bio-unavailable iron. Arabidopsis thaliana and many crop species activate an iron starvation response when iron levels are low.

This response involves releasing coumarins, such as sideretin and fraxetin, which can help mobilize iron from soil. The study found that these coumarins interact with a wide range of root-associated bacteria and help mobilize iron, making it more accessible to the plant. Sideretin is primarily released in acidic soils, while fraxetin is released in calcareous soils.

These coumarins can be utilized by bacteria through two mechanisms to mobilize iron. At acidic pH, sideretin undergoes reductive dissolution, reducing trivalent iron (Fe(III)) to the more soluble divalent form (Fe(II)), and bacteria help recycle used-up sideretin for further iron reduction. At near-neutral pH, fraxetin interacts with a bacterial siderophore, diverting some of the siderophore-mobilized iron for plant acquisition.

The presence of these bacterial traits in the root microbiota suggests that they evolved long before land plants appeared and play an important role in adapting to changing environmental conditions. When tested in the laboratory, most root-associated bacteria could rescue plants from iron deficiency, regardless of soil iron status.

This suggests that the capacity of bacteria to mobilize iron at different pH levels is widespread and can be activated on demand by the plant through the release of different coumarins in response to soil pH.