RESUMO
Plants continuously maintain pluripotent stem cells embedded in specialized tissues called meristems, which drive long-term growth and organogenesis. Stem cell fate in the shoot apical meristem (SAM) is controlled by the homeodomain transcription factor WUSCHEL (WUS) expressed in the niche adjacent to the stem cells. Here, we demonstrate that the bHLH transcription factor HECATE1 (HEC1) is a target of WUS and that it contributes to SAM function by promoting stem cell proliferation, while antagonizing niche cell activity. HEC1 represses the stem cell regulators WUS and CLAVATA3 (CLV3) and, like WUS, controls genes with functions in metabolism and hormone signaling. Among the targets shared by HEC1 and WUS are phytohormone response regulators, which we show to act as mobile signals in a universal feedback system. Thus, our work sheds light on the mechanisms guiding meristem function and suggests that the underlying regulatory system is far more complex than previously anticipated.
Assuntos
Proteínas de Arabidopsis/metabolismo , Arabidopsis/metabolismo , Proteínas de Homeodomínio/metabolismo , Reguladores de Crescimento de Plantas/metabolismo , Caules de Planta/genética , Transdução de Sinais/genética , Fatores de Transcrição/metabolismo , Transcrição Gênica , Proteínas de Arabidopsis/genética , Diferenciação Celular/genética , Diferenciação Celular/fisiologia , Citocininas/genética , Citocininas/metabolismo , Regulação da Expressão Gênica de Plantas/genética , Proteínas de Homeodomínio/genética , Meristema/genética , Meristema/metabolismo , Reguladores de Crescimento de Plantas/genética , Brotos de Planta/genética , Brotos de Planta/metabolismo , Caules de Planta/metabolismo , Células-Tronco/citologia , Células-Tronco/metabolismo , Fatores de Transcrição/genéticaRESUMO
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). Phytohormones such as auxin and cytokinin are also important for meristem regulation. Here we show a mechanistic link between the CLV/WUS network and hormonal control. 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-feedback loop of cytokinin signalling. These data indicate that ARR genes might negatively influence meristem size and that their repression by WUS might be necessary for proper meristem function. Consistent with this hypothesis is our observation that a mutant ARR7 allele, which mimics the active, phosphorylated form, causes the formation of aberrant shoot apical meristems. Conversely, a loss-of-function mutation in a maize ARR homologue was recently shown to cause enlarged meristems.
