Search for Candidate Genes Important for Balanced Growth of Shoots
and Roots. Good candidates for transporters involved in the
communication between the root and shoot are transporters that
are colocalized with cytokinin biosynthesis genes in the root. To
identify such candidates, we searched for genes with high transcript
levels in root phloem companion cells (11), as this cell type
expresses IPTs and is likely to be involved in cytokinin export. In
addition, we searched for genes that are highly coexpressed with
the most important cytokinin synthesis gene, IPT3 (Table S1),
and genes induced by cytokinin treatment, both according to the
literature (12) and the electronic fluorescent pictograph browser
(Fig. S1A). AtABCG14 was the only gene that satisfied the three
selection criteria. Induction of AtABCG14 by cytokinin treatment
was confirmed by RT-PCR using RNA extracted from
cytokinin-treated Arabidopsis seedlings (Fig. S1B). Thus, we hypothesized
that AtABCG14 has a role in cytokinin transport. To
test this hypothesis, we obtained the corresponding knockout
mutant (SK_15918; Fig. S2E) and analyzed its phenotypes.