Cytokinin and the cell cycle

• Cytokinins regulate cell proliferation, affecting mitosis and endoreplication.
• Cytokinins control the cell cycle both at the G1/S and G2/M transitions.
• Cytokinins have opposing effects on cell division in shoot and root meristems.
• The circuitry underlying cytokinin input into SAM and RAM function is described.
• A model for the opposing effect of cytokinin in the SAM and RAM is proposed.

The phytohormone cytokinin influences many aspects of plant growth and development, including a prominent role in the regulation of cell proliferation. How the cytokinin response pathway integrates into the machinery regulating progression through the cell cycle is only beginning to be appreciated. Cytokinin is generally considered to promote mitotic cell division in the shoot, but differentiation and transition to the endocycle in the root. Here we consider recent data on the inputs by which cytokinins positively and negatively regulate transitions through the cell cycle. Cytokinin positively regulates cell division and also serves a key role in establishing organization within shoot stem cell centers. Both auxin-dependent and auxin-independent mechanisms have been uncovered by which cytokinin stimulates the endocycle in roots. We conclude with a model that reconciles the opposing effects of cytokinin on shoot and root cell division.

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