Rockcress (Arabidopsis) are a genus of the Brassicaceae family. They are small flowering plants related to cabbage and mustard.

One species, Thale Cress (Arabidopsis thaliana), are one of the model organisms for studying plant sciences, including genetics and plant development. Similarly as mouse and fruit fly (Drosophila) are used as model organisms to understand human biology, Arabidopsis thaliana is used to study agronomic crops.

The genome of Arabidopsis, consisting of five chromosomes, has been sequenced. There are 30644 known genes in the Arabidopsis genome.

Arabidopsis has been used extensively in the study of the genetic basis of phototropism, chloroplast alignment, and other light influenced processes. Study of this organism has led to discoveries such as positive phototropic responses of root cells to red, far red and infrared light.

While gravitropic response of Arabidopsis root organs is the predominant tropic effect in these organs, specimens treated with mutagens and then selected for the absence of gravitropic action have shown both negative phototropic response to blue or white light, and positive phototropic response to red light. The photosensitive pigments phytochrome A and phytochrome B mediate this red light based phototropic response.

Common names include Thale Cress, Mouse-ear Cress, and Mustard Weed.