A mutually inhibitory interaction between auxin and cytokinin positions the auxin signalling maximum along the xylem axis.

(A) A schematic of the Arabidopsis root tip. (B) Localisation of AHP6 mRNA via in situ hybridization. (C) The cytokinin signalling domain reported by pARR5::GUS. (D) The auxin signalling domain reported by pIAA2::GUS. (E–G) Expression patterns of (E) PIN1; (F) PIN3; and (G) PIN7 fused with GFP. (H) Schematic of the parsimonious model showing the mutually inhibitory interactions between auxin and cytokinin which are proposed to generate complementary signalling domains. The network is active in all cells but shown in only two, representative of the procambium/pericycle and the xylem axis; the faint elements are proposed to be downregulated in the corresponding domain. (I) Simulation of a longitudinal section of a root shows uneven auxin accumulation throughout the stele, with outer cells (pericycle) accumulating higher levels of auxin (parameter settings and PIN localisation as described in [5]). Arrowheads mark the protoxylem position in cross sections; scale bars represent 10μm. In the model schematic, yellow arrows represent PIN transporters and pink arrows represent passive auxin influx.