Figure 10.

Model Summarizing Key Conclusions about Polar Auxin Transport Streams and Lateral Root Growth.

Basipetal auxin transport mediated by MDR4 (1) does not play a detectable role in lateral root growth and development. In the mature region of the root where lateral roots are produced, MDR1 expression is restricted to the stele (2) where it mediates acropetal auxin transport (3). MDR1 expression in the early primordium (4) is responsible for concentrating auxin there. However, even in the absence of the MDR1-mediated auxin accumulation, lateral root primordia progress through the early stages of development apparently normally. MDR1-mediated acropetal auxin transport within the lateral root (5) is necessary for normal elongation of the lateral root and also for producing the local auxin maximum at the apex of the lateral root.