Membrane regulation of cytokinin-mediated cell division in Arabidopsis.

Controlled proteolytic activation of membrane-bound transcription factors (MTFs) is an efficient adaptation strategy that ensures prompt transcriptional responses to intrinsic and environmental changes in eukaryotes. The proteolytic processing liberates active transcription factors from the membranes, which subsequently enter the nucleus and turn on downstream target genes. In the December issue of Plant Cell, we have demonstrated that an Arabidopsis membrane-bound NAC transcription factor, designated NTM1, is activated by proteolytic cleavage through regulated intramembrane proteolysis (RIP). The transcriptionally active NTM1 form induces a subset of CDK inhibitor genes (KRPs), resulting in reduced cell division. We have also shown that cytokinins regulate NTM1 activity by modulating its protein stability via an ubiquitin (Ub)-mediated protein degradation pathway, defining a unique molecular scheme by which cytokinins regulate cell division. It is thus envisioned that both positive and negative signaling components would be required for tight control of cell cycling by cytokinins. In this addendum, we propose a working hypothesis in which environmental stresses affect cell division by regulating NTM1 expression or NTM1 processing step.