![]() Removal of this cis-regulatory element effectively uncouples PIN transcription from the CRF-dependent regulation and attenuates plant cytokinin sensitivity. Here we show that the cytokinin response factors (CRFs) 21 transcriptionally control PIN-FORMED ( PIN) genes encoding auxin transporters at a specific PIN CYTOKININ RESPONSE ELEMENT ( PCRE) domain. However, thus far, the components of the transcriptional complex that directly control PIN transcription in response to cytokinin are unknown. ![]() Through the cytokinin receptor ARABIDOPSIS HISTIDINE KINASE3 (AHK3) and the downstream signalling components ARABIDOPSIS RESPONSE REGULATOR (ARR1) and ARR12, cytokinin has been shown to activate SHY2/IAA3 (SHY2), a repressor of auxin signalling that negatively regulates the PIN auxin transporters 10. Cytokinin has been shown to influence cell-to-cell auxin transport by modification of the expression of several auxin transport components and thus to modulate auxin distribution important for root development 10, 11, 19, 20. Well-established are the mutual regulation of metabolic 18 and signalling pathways 2, 8, as well as the cytokinin-mediated modulation of auxin transport 10, 11, 12. ![]() Importantly, a complex network of interactions and feedback circuits interconnects both pathways and determines the final outcome of the individual hormone actions. The principal pathways that manage their metabolism, distribution, and perception and the backbone molecular components have been identified 14, 15, 16, 17. The hormones auxin and cytokinin are essential to control plant growth and development including early embryogenesis 1, 2 and postembryonic organogenic processes, such as root 3, 4, 5 and shoot 6 branching, phyllotaxis 7, shoot 8, 9 and root apical meristem activity 10, 11 and vascular development 12, 13.
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