Legumes such as peas and beans form intimate and mutually beneficial partnerships (symbioses) with nitrogen-fixing bacteria, rhizobia. The plant benefits from an enhanced supply of nitrogen, ‘fixed’ from the air by the rhizobia, while the bacteria benefit from protective accommodation inside special structures, called root nodules, that supply nutrients from the host plant. A different type of symbiosis is formed between the roots of many plant species and soil fungi, called mycorrhizal fungi. Both types of complex plant-microbe interactions are crucial for supplying plants with nutrients, but many details of how these symbioses develop remain unclear.