Nitrogen is essential for plant growth. This is why legumes such as beans and chickpeas have adapted to life on nitrogen-poor soils. They form root nodules in which special bacteria can absorb nitrogen from the air. These rhizobia receive sugar from the plant in exchange for the fixed nitrogen.
A study by researchers from the University of Cologne, the University of Copenhagen and the Max Planck Institute for Plant Breeding Research in Cologne has now shed more light on this symbiosis. In the journal Science, they describe the crucial role of root barriers in regulating the sensitive metabolism between plants and bacteria.
Root barrier as a control center
The Caspary strip, a watertight barrier in plant roots, acts as a “bouncer”, deciding how much water and which nutrients enter the plant's vascular system. This root barrier develops at the same time as the external nodules. The formation of these nodules is also a finely regulated process: if too little nitrogen is available, the roots send an alarm signal in the form of the peptide CEP1 to the leaves, which then increase nodule formation.
It has now been shown that it is the Caspary stripe that controls the signaling pathways involved. The research team investigated the legume Lotus japonicus, a form of the common hornwort. When the scientists removed the Caspary strip, the plants were very slow to form nodules on nitrogen-poor soil. However, this was not due to the leaky barrier, but to the fact that CEP1 was no longer being produced. The plants were therefore unable to recognize the nitrogen deficiency and react to it.
Basis for a fair partnership
The researchers also found a compact version of the Caspary strip in the nodules themselves, which regulates the exchange between the plant and bacteria. Without this barrier, sugar passes unhindered from the plant into the nodules. As a result, bacteria continue to multiply, but no longer produce nitrogen compounds as nutrients for the plant.
“The study provides new insights into how plants and microbes interact with each other and establishes a new model system to investigate how a beneficial partnership can take place in a confined space,” summarizes lead author Tonni Grube Andersen. According to the researchers, plants have developed this finely tuned control system to keep the partnership fair.
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