Plants communicate with their shoots through chemical signals produced in their roots when soil microorganisms are present. Researchers discovered that a molecule called N-hydroxypipecolic acid acts like an on-off switch controlled by two proteins, FMO1 and UGT76B1. Beneficial fungi suppress the ‘off’ switch, allowing this signal to travel to leaves where it boosts immunity against disease. Different amounts of this signal have different effects: small amounts help the plant grow, while large amounts strengthen defenses but slow growth.