Figure 9.

Diagram of MDR-Dependent Auxin Streams in the Apex of the Primary Root.

MDR1 is expressed in the central cylinder and the cortex of the root apex, where auxin is transported acropetally and centripetally. These processes help balance expansion rates of cells on sides of the growing zone to guide vertical root growth. MDR4 is an important contributor to basipetal auxin transport. It plays a role in controlling differential growth during gravitropism, perhaps by affecting the auxin asymmetry that drives the process.