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The SCF complex containing the TIR1 F-box protein provokes AUX/IAA degradation in response to auxins ( Gray et al., 2001). Auxins direct the degradation of auxin/indole-3-acetic acid (AUX/IAA) proteins that act as auxin signaling repressors. Selective protein degradation is a critical component of both growth hormone signaling and cell cycling ( Deshaies, 1999 Hellman and Estelle, 2002 Risseeuw et al., 2003 Vierstra, 2003). They induce CYCD3-1 expression, and transgenic plants overexpressing CYCD3-1 are cytokinin-autotrophic in leaf callus culture ( Riou-Khamlichi et al., 1999). Cytokinins regulate the G1/S and G2/M transitions as well as progression through the S-phase ( De Veylder et al., 2003). Among these, the best characterized are cytokinins, which are most directly related to cell division. The roles of cytokinins, auxins, brassinosteroids, abscisic acid, jasmonic acid, and ethylene have all been demonstrated ( Dewitte and Murray, 2003).

Plant growth hormones affect cell cycle progression. The phase transitions are coordinately regulated by a battery of cell cycle regulators, including cyclins, cyclin-dependent kinases (CDKs), and CDK inhibitors that have been termed inhibitors of Cdc2 kinases or KIP-related protein (KRPs) in plants ( De Veylder et al., 2003). The cell cycle consists of alternating phase transitions of DNA replication (S-phase) and chromosome partitioning (M-phase) separated by gap phases (G1- and G2-phases). These results indicate that the membrane-mediated activation of NTM1 defines a molecular mechanism by which cytokinin signaling is tightly regulated during cell cycling.Д¬ell division plays a crucial role in virtually all aspects of growth and developmental processes in plants ( Meijer and Murray, 2001 De Veylder et al., 2003 Dewitte and Murray, 2003). Interestingly, cytokinins were found to regulate NTM1 activity by controlling its stability. Consistent with a role for NTM1 in cell cycling, a Ds element insertional mutant was morphologically normal but displayed enhanced hypocotyl growth with accelerated cell division.

Accordingly, a subset of cyclin-dependent kinase (CDK) inhibitor genes (the KIP-related proteins) was induced in this mutant with a significant reduction in histone H4 gene expression and in CDK activity. Cell proliferation was greatly reduced in an Arabidopsis mutant with retarded growth and serrated leaves in which a transcriptionally active NTM1 form was constitutively expressed. Here, we show that a membrane-bound NAC (for NAM, ATAF1/2, CUC2) transcription factor, designated NTM1 (for NAC with transmembrane motif1), is activated by proteolytic cleavage through regulated intramembrane proteolysis and mediates cytokinin signaling during cell division in Arabidopsis thaliana. The proteolytic cleavage liberates active transcription factors from the membranes that can enter the nucleus and evokes rapid transcriptional responses to incoming stimuli. An exquisite example is the proteolytic activation of membrane-bound transcription factors. Controlled release of membrane-tethered, dormant precursors is an intriguing activation mechanism that regulates diverse cellular functions in eukaryotes.

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