SCI1, the first member of the tissue-specific inhibitors of CDK (TIC) class, is probably connected to the auxin signaling pathway.

The recent finding of a tissue-specific cell cycle regulator (SCI1) that inhibits cell proliferation/differentiation in the upper pistil points to an unanticipated way of controlling plant morphogenesis. The similarity between the SCI1 RNAi-silenced plants and some auxin-related phenotypes suggested that SCI1 could be involved in the auxin signaling pathway. To address this hypothesis, we analyzed the expression of three auxin-related genes in transgenic plants in which SCI1 was silenced and overexpressed. The results showed that the expression levels of the auxin-related genes largely correlated with the SCI1 expression level. Additionally, we analyzed the Arabidopsis SCI1 upstream regulatory region and found putative cis-acting elements also present in the AtCYCB1;1 AtYUC1, AtYUC2 and AtYUC4 URRs, suggesting a cell cycle- and auxin-related transcriptional regulation. Based on our previous and the current studies, we propose SCI1 as a signal transducer engaging auxin signaling and cell division/differentiation.