Dynamics of expression and distribution of cytokinin oxidase/dehydrogenase in developing maize kernels

Maturing maize kernels are a rich source of cytokinins and cytokinin oxidase/ dehydrogenase activity, but the relationship between kernel development, cytokinin levels, the induction of cytokinin oxidase/ dehydrogenase and the control of cell division is not known. Using polyclonal antibodies raised against recombinant maize cytokinin oxidase/ dehydrogenase, we investigated the appearance of cytokinin oxidase/ dehydrogenase ( ZmCKX1) in both hybrid and inbred maize kernels as a function of time after pollination. Cytokinin oxidase/ dehydrogenase was detected by five days after pollination ( 5 DAP) in a hybrid line, but significantly later in inbred lines. The bulk of the cytokinin oxidase/ dehydrogenase detected was associated with the embryo and placental/ chalazal region of the kernels rather than with the endosperm. We identified additional maize sequences in the database that appear to encode cytokinin oxidase/ dehydrogenase gene family members and correspond closely with a subset of the ten cytokinin oxidase/ dehydrogenase genes identified in the rice genome. Gene expression of Zmckx1 was examined by RT- PCR in immature kernels and compared with that of three putative maize cytokinin oxidase/ dehydrogenase homologs. We conclude that the manipulation of kernel cytokinin levels to increase endosperm cell division will require a more detailed understanding of specific expression patterns and localization of multiple cytokinin oxidase/ dehydrogenases within kernels.