Phytochromes can also affect root growth. It has been well documented that gravitropism is the main tropism in roots. However, a recent study has shown that phototropism also plays a role. A red light induced positive phototropism has been recently recorded in an experiment that used Arabidopsis to test where in the plant had the most effect on a positive phototropic response. The experimenters utilized an apparatus that allowed for root apex to be zero degrees so that gravitropism could not be a competing factor. When placed in red light, Arabidopsis roots displayed a curvature of 30 to 40 degrees. This showed a positive phototropic response in the red light. They then wanted to pinpoint exactly where in the plant light is received. When roots were covered there was little to no curvature of the roots when exposed to red light. In contrast, when shoots were covered, there was a positive phototropic response to the red light. This proves that lateral roots is where light sensing takes place. In order to further gather information regarding the phytochromes involved in this activity, phtochrome A, B, D and E mutants, and WT roots were exposed to red light. Phytochrome A and B mutants were severely impaired. There was no significant difference in the response of "phyD" and "phyE" compared with the wildtype, proving that "phyA" and "phyB" are responsible for positive phototropism in roots.   