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Abstract

It has been known for decades that some phytohormones, such as cytokinin and auxin, play an essential role in meristem regulation, however, the underlying molecular mechanisms remained largely elusive. In Arabidopsis thaliana, stem cell fate in the shoot apical meristem is controlled genetically by a negative feed back loop of CLAVATA3 (CLV3) and the homeodomain transcription factor WUSCHEL (WUS). We have shown previously that WUSCHEL, which positively regulates stem cell fate, represses the transcription of several A-type ARABIDOPSIS RESPONSE REGULATOR genes (ARR5, ARR6, ARR7 and ARR15), in the meristem. Since A-type ARR genes act in the negative feed back loop of cytokinin signaling, this finding provided the first mechanistic link between hormone signaling and plant stem cell regulation. However, we did not find any visible phenotype in ARR5, ARR6 and ARR7 single mutants, while strong alleles of ARR15 caused female gametophytic lethality. To circumvent the lethality problem, we constructed conditional alleles by inducible artificial microRNAs against ARR15 to further explore its functions in the shoot apical meristem. Our results show that down-regulation of ARR15 or expression of constitutively active forms of ARR15 cause meristematic defects, including disturbed phyllotaxis. Consistent with the observed phenotypes, RNA in situ hybridization showed that ARR15 is expressed in the shoot apical meristem. Our data indicate that ARR15 function might be one of the central elements in the regulation of meristem activity.