Molecular mechanism of cytokinin-activated cell division in Arabidopsis

Abstract

The hormone cytokinin regulates various aspects of plant development and physiology, largely by managing cell proliferation. Yang et al. show that cytokinin promotes nuclear localization of the transcription factor MYB3R4, which activates the expression of two importins and genes that tip the cell into the next phase of the cell cycle. The importins facilitate further MYB3R4 accumulation within the nucleus, accelerating the progression into mitosis. MYB3R4 and the importins dissipate when the nuclear membrane dissolves at prometaphase, so there is only one round of mitotic activation per cell cycle. Science , this issue p. 1350 The plant hormone cytokinin triggers a self-amplifying loop that drives cell proliferation and meristem growth. Mitogens trigger cell division in animals. In plants, cytokinins, a group of phytohormones derived from adenine, stimulate cell proliferation. Cytokinin signaling is initiated by membrane-associated histidine kinase receptors and transduced through a phosphorelay system. We show that in the Arabidopsis shoot apical meristem (SAM), cytokinin regulates cell division by promoting nuclear shuttling of Myb-domain protein 3R4 (MYB3R4), a transcription factor that activates mitotic gene expression. Newly synthesized MYB3R4 protein resides predominantly in the cytoplasm. At the G2-to-M transition, rapid nuclear accumulation of MYB3R4—consistent with an associated transient peak in cytokinin concentration—feeds a positive feedback loop involving importins and initiates a transcriptional cascade that drives mitosis and cytokinesis. An engineered nuclear-restricted MYB3R4 mimics the cytokinin effects of enhanced cell proliferation and meristem growth.