Two-component circuitry in Arabidopsis cytokinin signal transduction

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Abstract
Cytokinins are essential plant hormones that are involved in shoot meristem and leaf formation, cell division, chloroplast biogenesis and senescence. Although hybrid histidine protein kinases have been implicated in cytokinin perception in Arabidopsis, the action of histidine protein kinase receptors and the downstream signalling pathway has not been elucidated to date. Here we identify a eukaryotic two-component signalling circuit that initiates cytokinin signalling through distinct hybrid histidine protein kinase activities at the plasma membrane. Histidine phosphotransmitters act as signalling shuttles between the cytoplasm and nucleus in a cytokinin-dependent manner. The short signalling circuit reaches the nuclear target genes by enabling nuclear response regulators ARR1, ARR2 and ARR10 as transcription activators. The cytokinin-inducible ARR4, ARR5, ARR6 and ARR7 genes encode transcription repressors that mediate a negative feedback loop in cytokinin signalling. Ectopic expression in transgenic Arabidopsis of ARR2, the rate-limiting factor in the response to cytokinin, is sufficient to mimic cytokinin in promoting shoot meristem proliferation and leaf differentiation, and in delaying leaf senescence.