The cytokinin dehydrogenases (CKX; EC 1.5.99.12) are a protein family that maintains the endogenous
levels of cytokinins in plants by catalyzing their oxidative degradation. The CKX family in maize (Zea
mays L.) has thirteen members, only two of which - ZmCKX1 and ZmCKX10 - have previously been
characterized in detail. In this study, nine further maize CKX isoforms were heterologously expressed in
Escherichia coli, purified by affinity and ion-exchange chromatography and biochemically characterized.
ZmCKX6 and ZmCKX9 could only be expressed successfully after the removal of putative sequencespecific
vacuolar sorting signals (LLPT and LPTS, respectively), suggesting that these proteins are localized
to the vacuole. Substrate specificity analyses revealed that the CKX isoforms can be grouped into two
subfamilies: members of the first strongly prefer cytokinin free bases while members of the second
degrade a broad range of substrates. The most active isoform was found to be ZmCKX1. One of the
studied isoforms, ZmCKX6, seemed to encode a nonfunctional enzyme due to a mutation in a conserved
HFG protein domain at the C-terminus. Site-directed mutagenesis experiments revealed that this domain
is essential for CKX activity. The roles of the maize CKX enzymes in the development of maize seedlings
during the two weeks immediately after radicle emergence were also investigated. It appears that
ZmCKX1 is a key regulator of active cytokinin levels in developing maize roots. However, the expression
of individual CKX isoforms in the shoots varied and none of them seemed to have strong effects on the
cytokinin pool.