B-type cyclin modulation in response to carbon balance in callus of Populus alba

In plants, sucrose is the principal transported carbon compound. Sucrose and/or glucose and fructose are relevant in the regulation of meristematic cell proliferation. The aim was to define the sugar balance and the relation- ship between the osmoregulation processes and the cell division patterns in callus of Populus alba in response to different sucrose medium concentrations. Callus prolifer- ation and metabolism were assessed through biometric measures, non-structural soluble carbohydrates quantifica- tion, osmotic potential determination, as well as by quan- tification of mRNA accumulation of B-type cyclin genes. Calli were cultured on proliferation medium without sucrose or supplemented with 10, 20 or 30 g l-1 of sucrose. A large amount of non-structural soluble carbohydrates was used to restore the osmotic balance between callus and medium; upon reaching the equilibrium, carbon was then used for cell division. The initial investment of carbon for osmoregulation processes can explain the differences in the lag phase duration in response to the decrease of medium sucrose concentration. However, calli cultured on medium added with 30 g l-1 of sucrose used carbon both to adjust their osmotic potential and to restore the growth whilst the calli on sucrose free medium entered in a quiescent state. The growth rates compared to the transcript accumulation trends suggested that a threshold effect, rather than a quantitative regulation model, governed the relation between CycB gene transcription and cell division. The findings showed that poplar calli used the carbon following a ''hierarchical'' model based on their physiological state and the sugar concentration available in the medium.