The enzymatic and biochemical properties of the proteins encoded by five potato cytokinin oxidase/
dehydrogenase (CKX)-like genes functionally expressed in yeast and the effects of tuber dormancy
progression on StCKX expression and cytokinin metabolism were examined in lateral buds isolated from
field-grown tubers. All five putative StCKX genes encoded proteins with in vitro CKX activity. All five
enzymes were maximally active at neutral to slightly alkaline pH with 2,6-dichloro-indophenol as the
electron acceptor. In silico analyses indicated that four proteins were likely secreted. Substrate dependence
of two of the most active enzymes varied; one exhibiting greater activity with isopentenyl-type
cytokinins while the other was maximally active with cis-zeatin as a substrate. [3H]-isopentenyladenosine
was readily metabolized by excised tuber buds to adenine/adenosine demonstrating that CKX
was active in planta. There was no change in apparent in planta CKX activity during either natural or
chemically forced dormancy progression. Similarly although expression of individual StCKX genes varied
modestly during tuber dormancy, there was no clear correlation between StCKX gene expression
and tuber dormancy status. Thus although CKX gene expression and enzyme activity are present in
potato tuber buds throughout dormancy, they do not appear to play a significant role in the regulation
of cytokinin content during tuber dormancy progression.