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Abstract

Plants continuously maintain pools of totipotent stem cells in their apical meristems from which elaborate root and shoot systems are produced. In Arabidopsis thaliana, stem cell fate in the shoot apical meristem is controlled by a regulatory network that includes the CLAVATA (CLV) ligand–receptor system and the homeodomain protein WUSCHEL (WUS). In addition, it has been known for decades that phytohormones such as auxin and cytokinin are important for meristem regulation. We are interested to understand how these diverse signals are integrated at the systems level and locally translated at the shoot apical meristem. We have found that WUS, a positive regulator of stem cells, directly represses the transcription of several two-component ARABIDOPSIS RESPONSE REGULATOR genes (ARR5, ARR6, ARR7 and ARR15), which act in the negative feedbackloop of cytokinin signalling. Our data show that ARR genes are active in the shoot meristem and negatively influence meristem function. Thus ARR repression by WUS might be necessary for proper stem cell regulation in Arabidopsis. Consistent with this hypothesis, a mutant ARR7 allele, which mimics the active, phosphorylated form, causes the arrest of shoot apical meristems and conversely, a loss-of-function mutation in a maize ARR homologue was recently shown to cause enlarged meristems.