Just as shoots are negatively gravitropic and positively phototropic, roots are positively gravitropic and negatively phototropic. Blue light has been shown to stimulate negative phototropism in roots. In the shoot, we saw that PIN3 changed from symmetric to asymmetric plasma membrane localization in the starch sheath following unilateral blue light treatment (see textbook Chapter 18). In dark-treated roots, PIN3 shows symmetric localization on the plasma membranes of columella cells of the root cap. Following unilateral blue light exposures, PIN3 shows asymmetric localization on the illuminated side (closest to the light source). The auxin stream is redirected to the lit side of the root, and roots bend away from the light source.