cytokinins coordinate that of the root. Such cross-talk enables
the plant to continuously adapt to an ever-changing environment.
For instance, in nitrogen-starved maize roots resupplied
with nitrate, synthesis of tZ-type cytokinin level increases in the
root, which is then delivered to the xylem sap and results in an
increase in cytokinin-responsive gene expression in the shoot (10).
This sequence of events indicates the importance of long-distance
cytokinin transport as a signal that modulates shoot growth.
The translocation of cytokinins was demonstrated in grafting
experiments using a quadruple knockout mutant of cytokinin
synthesis genes, adenosine phosphate isopentenyl transferase
(IPT)1;3;5;7 (atipt1;3;5;7), and a double mutant of tZ-type cytokinin
synthesis genes, cytochrome P450 monooxygenases,
CYP735A1 and CYP735A2. These experiments revealed that
wild-type rootstocks can complement the growth of the mutant
shoot scions, indicating that tZ-type cytokinins synthesized in
the wild-type root are delivered to the mutant shoot and
stimulate its growth (8, 10). However, the molecular mechanism
underlying the translocation was hitherto obscure. Here,
we report that a member of the ABCG subfamily of ABC
proteins, AtABCG14, is required for the root-to-shoot translocation
of cytokinins.