Figure 1
Cytokinin and auxin signaling in Arabidopsis. Single lines indicate cell membranes; double lines represent the chromosome; bent arrows indicate positions of transcription initiation. (a) Cytokinin binding to a receptor histidine protein kinase (RHK) such as AHK3 triggers kinase autophosphorylation and initiates a phosphorelay cascade [5]. The phosphoryl group (blue sphere) transfers to a receiver domain in the receptor and subsequently to a histidine phosphotransfer protein (HPT), triggering HPT translocation to the nucleus. There, the phosphorylation is relayed to an Arabidopsis response regulator (ARR) such as ARR1. B-type RRs (B-RR) activate transcription of cytokinin-responsive genes, some of which contain a GAT(T/C) DNA sequence motif [7]. Cytokinin-responsive A-type RRs (A-RR) act to repress cytokinin signaling. (b) Auxin signaling is based on auxin-dependent, proteasome-mediated degradation of AUX/IAA repressors (see [9] and references therein). AUX/IAA proteins dimerize with and repress the activity of transcription factors in the AUXIN RESPONSE FACTOR (ARF) family, which bind TGTCTC-containing DNA sequence elements in promoters of auxin-responsive genes. Auxin-dependent gene expression is mediated by the release of ARF proteins from AUX/IAA repression as a result of proteasome-mediated degradation of AUX/IAA proteins. Auxin serves as the switch by binding to an F-box protein such as TRANSPORT INHIBITOR RESPONSE1 (TIR1) and enhancing its interaction with AUX/IAA proteins, increasing the rate of AUX/IAA ubiquitination (Ub) by the Skp1-Cul-F-box E3 ubiquitin ligase complex SCFTIR1.