Arabidopsis plants have been engineered with a "sensor " (the Adh gene promoter) that monitors cellular physiology and stress and a "reporter gene" (encoding beta-glucuronidase - GUS) that can generate a signal visible to researchers.

When stress, such as hypoxia or microgravity, is detected by the plants, the promoter induces the expression of the reporter gene which in turn makes a product that turns blue when incubated with the proper substrate (x-glucuronic acid).

In other words, plants that are not stressed do not show a color change when incubated with the substrate (below left), but plants that are responding to stress turn blue (right).