Cytokinin–auxin crosstalk in cell type specification

• Several conserved master regulators of cytokinin and auxin signalling exist in different specification contexts.
• Auxin and cytokinin interact transcriptionally and non-transcriptionally to activate specified founder cells.
• This interaction occurs via crosstalk and cross-regulation, especially for the cell cycle.
• The role of auxin and cytokinin in cell type specification is cell type- and context-specific.

Auxin and cytokinin affect cell fate specification transcriptionally and non-transcriptionally, and their roles have been characterised in several founder cell specification and activation contexts. Similarly to auxin, local cytokinin synthesis and response gradients are instructive, and the roles of ARABIDOPSIS RESPONSE REGULATOR 7/15 (ARR7/15) and the negative cytokinin response regulator ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6, as well as auxin signalling via MONOPTEROS/BODENLOS, are functionally conserved across different developmental processes. Auxin and cytokinin crosstalk is tissue- and context-specific, and may be synergistic in the shoot apical meristem (SAM) but antagonistic in the root. We review recent advances in understanding the interactions between auxin and cytokinin in pivotal developmental processes, and show that feedback complexity and the multistep nature of specification processes argue against a single morphogenetic signal.