Nitrogen, crucial for plant growth, drives legumes like beans and chickpeas to adapt to nitrogen-poor soils. They form root nodules housing bacteria capable of extracting nitrogen from the air. These rhizobia receive sugars from the plant in exchange for the fixed nitrogen.
A study from the University of Cologne, the University of Copenhagen, and the Max Planck Institute for Plant Breeding Research in Cologne has shed light on this symbiosis. Published in Science, their research details the vital role of root barriers in regulating the delicate metabolism between plants and bacteria.
The Caspary strip, a waterproof barrier in plant roots, acts as a gatekeeper, controlling the flow of water and nutrients into the plant's vascular system. This barrier develops alongside the external nodules. The formation of these nodules is also finely regulated: when nitrogen is scarce, roots send an alarm signal, the CEP1 peptide, to the leaves, which then enhance nodule formation.
The research team discovered that the Caspary strip controls the associated signaling pathways. They studied the legume Lotus japonicus. When the scientists removed the Caspary strip, the plants formed nodules very slowly on nitrogen-poor soil. This was not due to the leaky barrier, but because no CEP1 signal was produced. The plants could not detect and respond to the nitrogen deficiency.
Inside the nodules, the researchers also found a compact version of the Caspary strip, regulating the exchange between plant and bacteria. Without this barrier, sugar flows freely from the plant into the nodules. This causes bacteria to multiply further, but they no longer produce nitrogen compounds as nutrients for the plant.
"The study provides new insights into how plants and microbes interact 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 ensure the partnership remains fair.