Factors regulating the expression of cell cycle genes in individual buds of Populus

The early and differential responses of the individual buds along a shoot have remained largely unknown due to the difficulties of analyzing early indicators that allow the monitoring of the effects of subtle changes in the environment on the growth activity of the individual bud. To overcome this problem, we transformed poplar [Populus tremula (L.) × P. alba (L.)] with two chimeric genes, Pcdc2a-gus and Pcyc1At-gus, the expression of which is closely linked to cell division in Arabidopsis thaliana (L.) Heynh. We analyzed the expression levels of both chimeric genes in individual buds of the same tree, and under different conditions known to promote or retard growth in the buds. The expression levels of both chimeric genes were found to reflect closely the growth activity of the buds. After decapitation of the shoot, the expression of Pcdc2a-gus and Pcyc1At-gus revealed rapid and selective changes in the cell cycle, even when no morphological changes were observed. Furthermore, on the basis of the expression of the chimeric cell cycle genes, different degrees of growth activity and dormancy could be discriminated in the axillary buds. In addition, the expression of Pcyc1At-gus was found to be closely associated with the day length, which is critical for dormancy induction in poplar.