Scientists have believed for a long time that the role of plant immune system was only to distinguish between friend and foe and to fend off pathogens, but it is also involved in accommodating beneficial microorganisms in the plant when required. Researchers from the Max Planck Institute for Plant Breeding Research in Cologne, Germany in collaboration with an international consortium of other laboratories discovered this relationship between the model plant Arabidopsis thaliana and the fungus Colletotrichum tofieldiae. The plant tolerates the fungus when it needs help in obtaining soluble phosphate from the soil and rejects the microbe if it can accomplish this task on its own.

Plant growth is possible when plants have access to soluble phosphate in the soil. Most plants maintain a mycorrhiza, the fungal mesh around their roots that supplies them with vital soil-derived nutrients. Arabidopsis is one of the few plants without a mycorrhiza. Instead, it engages in a beneficial relationship with the soil fungus C. tofieldiae which converts insoluble phosphate in the soil into soluble phosphate and releases the nutrient via the fungal mesh to its plant host. The research team discovered that an intact innate immune system is needed for the symbiosis and allows the fungus to take up residence in the plant's roots only if the plant is not able to obtain enough soil phosphate on its own. However, if phosphate is plentiful, the plant launches a massive immune response.

For more details, read the news release at the Max Planck Institute for Plant Breeding Research website.

Figure: Fluorescent microscopy image of a root of Arabidopsis thaliana (violet) surrounded by a fungal mesh of Colletotrichum tofieldiae (green). The mesh also grows within the root cells.