Figure 2.

Spatial and temporal factors govern the formation of lateral root primordia. The regular spacing of lateral root primordia arises from pulses of auxin signaling in the basal meristem. The basal meristem encompasses the set of cells that are transitioning from the meristematic region into the elongation zone and thus includes cells that are undergoing division as well as elongation. In seedlings that are grown in constant light, pulses of auxin signaling, highlighted in purple, occur with a periodicity of 15 h. The response to auxin signaling in the xylem cells primes the adjacent pericycle cells (shown in red) so that they are competent to become lateral root founder cells (shown in green) upon a second, auxin-dependent signal in the differentiation zone (De Smet et al. 2007). As such, the pulses of auxin in the basal meristem coupled with the continuous growth of the root leads to the observed regular spacing of lateral root primordia. HAG, hours after germination.