Cytokinin deficiency causes distinct changes of sink and source parameters in tobacco shoots and roots.

Cytokinin deficiency causes pleiotropic developmental changes such as reduced shoot and increased root growth. It was investigated whether cytokinin-deficient tobacco plants, which overproduce different cytokinin oxidase/dehydrogenase enzymes, show changes in different sink and source parameters, which could be causally related to the establishment of the cytokinin deficiency syndrome. Ultrastructural analysis revealed distinct changes in differentiating shoot tissues, including an increased vacuolation and an earlier differentiation of plastids, which showed partially disorganized thylakoid structures later in development. A comparison of the ploidy levels revealed an increased population of cells with a 4C DNA content during early stages of leaf development, indicating an inhibited progression from G2 to mitosis. To compare physiological characteristics of sink leaves, source leaves and roots of wild-type and cytokinin-deficient plants, several photosynthetic parameters, content of soluble sugars, starch and adenylates, as well as activities of enzymes of carbon assimilation and dissimilation were determined. Leaves of cytokinin-deficient plants contained less chlorophyll and non-photochemical quenching of young leaves was increased. However, absorption rate, photosynthetic capacity (F(v)/F(m) and J(CO2 max)) and efficiency (Phi CO(2 app)), as well as the content of soluble sugars, were not strongly altered in source leaves, indicating that chlorophyll is not limiting for photoassimilation and suggesting that source strength did not restrict shoot growth. By contrast, shoot sink tissues showed drastically reduced contents of soluble sugars, decreased activities of vacuolar invertases, and a reduced ATP content. These results strongly support a function of cytokinin in regulating shoot sink strength and its reduction may be a cause of the altered shoot phenotype. Roots of cytokinin-deficient plants contained less sugar compared with wild-type. However, this did not negatively affect glycolysis, ATP content, or root development. It is suggested that cytokinin-mediated regulation of the sink strength differs between roots and shoots.

Enhanced cytokinin degradation in leaf primordia of transgenic Arabidopsis plants reduces leaf size and shoot organ primordia formation.

The plant hormone cytokinin is a key morphogenic factor controlling cell division and differentiation, and thus the formation and growth rate of organs during a plant's life cycle. In order to explore the relevance of cytokinin during the initial phase of leaf primordia formation and its impact on subsequent leaf development, we increased cytokinin degradation in young shoot organ primordia of Arabidopsis thaliana by expressing a cytokinin oxidase/dehydrogenase (CKX) gene under control of the AINTEGUMENTA (ANT) promoter. The final leaf size in ANT:CKX3 plants was reduced to ∼27% of the wild-type size and the number of epidermal cells was reduced to ∼12% of the wild type. Kinematic analysis revealed that cell proliferation ceased earlier and cell expansion was accelerated in ANT:CKX3 leaves, demonstrating that cytokinin controls the duration of the proliferation phase by delaying the onset of cell differentiation. The reduction of the cell number was partially compensated by an increased cell expansion. Interestingly, ANT:CKX3 leaf cells became about 60% larger than those of 35S:CKX3 leaves, indicating that cytokinin has an important function during cell expansion as well. Furthermore, ANT:CKX3 expression significantly reduced the capacity of both the vegetative as well as the generative shoot apical meristem to initiate the formation of new leaves and flowers, respectively. We therefore hypothesize that the cytokinin content in organ primordia is important for regulating the activity of the shoot meristem in a non-autonomous fashion.