SCI1 Is a Direct Target of AGAMOUS and WUSCHEL and Is Specifically Expressed in the Floral Meristematic Cells

The specified floral meristem will develop a pre-established number of floral organs and, thus, terminate the floral meristematic cells. The floral meristematic pool of cells is controlled, among some others, by WUSCHEL (WUS) and AGAMOUS (AG) transcription factors (TFs). Here, we demonstrate that theSCI1(Stigma/stylecell-cycleinhibitor 1) gene, a cell proliferation regulator, starts to be expressed since the floral meristem specification ofNicotiana tabacumand is expressed in all floral meristematic cells. Its expression is higher in the floral meristem and the organs being specified, and then it decreases from outside to inside whorls when the organs are differentiating.SCI1is co-expressed withN. tabacum WUSCHEL(NtWUS) in the floral meristem and the whorl primordia at very early developmental stages. Later in development,SCI1is co-expressed withNAG1(N. tabacum AG) in the floral meristem and specialized tissues of the pistil.In silicoanalyses identifiedcis-regulatory elements for these TFs in theSCI1genomic sequence. Yeast one-hybrid and electrophoresis mobility shift assay demonstrated that both TFs interact with theSCI1promoter sequence. Additionally, the luciferase activity assay showed that NAG1 clearly activatesSCI1expression, while NtWUS could not do so. Taken together, our results suggest that during floral development, the spatiotemporal regulation ofSCI1by NtWUS and NAG1 may result in the maintenance or termination of proliferative cells in the floral meristem, respectively.