Severely reduced cytokinin movement is required to establish a cytokinin gradient across the Arabidopsis root via diffusion from a local biosynthesis source in the xylem axis.

To establish a cytokinin gradient without altering the diffusion coefficient of cytokinin, we introduced bidirectional membrane permeability. Decreasing the membrane permeability (moving right within rows) concentrates cytokinin in the source cells (here, the xylem axis), but no morphogen gradient is established, because the diffusion within the apoplast evens out the distribution within the rest of the root. Decreasing the diffusion coefficient (downwards in columns) in the apoplast transforms the step-wise cytokinin distribution pattern into a true gradient. Parameter values are given in Table 6.