Topic 21.11

Cytokinins Promote Cell Expansion and Greening in Cotyledons

The promotion of cell enlargement by cytokinins is most clearly demonstrated in the cotyledons of dicots with leafy cotyledons, such as mustard, cucumber, and sunflower. The cotyledons of these species expand as a result of cell enlargement during seedling growth. Cytokinin treatment promotes additional cell expansion, with no increase in the dry weight of the treated cotyledons.

Leafy cotyledons expand to a much greater extent when the seedlings are grown in the light than in the dark, and cytokinins promote cotyledon growth in both light- and dark-grown seedlings. As with auxin-induced growth, cytokinin-stimulated expansion of radish cotyledons is associated with an increase in the mechanical extensibility of the cell walls. However, cytokinin-induced wall loosening is not accompanied by proton extrusion, as it is in auxin-enhanced cell elongation. Neither auxin nor gibberellin promotes cell expansion in cotyledons.

Web Figure 21.11.A The effect of cytokinin on the expansion of radish cotyledons. The experiment described here shows that the effects of light and cytokinin are additive. T₀ represents germinating radish seedlings before the experiment began. The detached cotyledons were incubated for 3 days (T₃) in either darkness or light with or without 2.5 mM zeatin. In both the light and the dark, zeatin-treated cotyledons expanded more than in the control. (From Huff and Ross 1975.) (Click image to enlarge.)