Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer).
Tropism is the directional growth of an organism, typically a plant, in response an external stimulus, such as light or gravity. Growth towards the stimulus is referred to as positive tropism, and away from the stimulus as negative tropism.
The authors identify novel NEGATIVE GRAVITROPIC RESPONSE OF ROOTS genes in Medicago and Arabidopsis. Mutants display a spectacular phenotype: gravity is still perceived, but the direction of growth is reversed: roots always grow upwards.
Through a combination of experimental and computational approaches, the interplay between the plant hormone auxin and the auxin-induced PLETHORA transcription factors is shown to control zonation and gravity-prompted growth movements in plants.