Results
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.
atabcg14 Exhibits Retarded Shoot Growth, Which Can Be Recovered
by tZ Application. The shoot-to-root ratio of atabcg14 seedlings
differed from that of the wild type (Fig. 1A and Fig. S2A). The
leaves of atabcg14 plants grown on synthetic plant growth agar
(MGRL-agar) medium (13) were smaller than those of the wild
type, whereas the roots were longer. Mature atabcg14 mutant
plants had considerably smaller rosette leaves (Fig. 1B) and
shorter (Fig. 1C) and thinner (Fig. S2B) stems than the wild type.
The number and size of xylem and phloem cells in the mutant
was strikingly reduced (Fig. 1D and Fig. S2 C and D), and lignin
content was decreased (Fig. S3). Furthermore, the mutant produced
fewer siliques and seeds per plant than the wild type, but
their seeds were larger (Fig. S2 F and G). The introduction of
pAtABCG14::sGFP::AtABCG14 genomic DNA (gDNA) into
atabcg14 restored the leaf size (Fig. 1B), shoot-to-root fresh
weight ratio (Fig. S2A), and stem diameter (Fig. S2B) to those of
the wild type.
The phenotypes of atabcg14 plants were similar to those described
for cytokinin biosynthesis (14) and receptor Arabidopsis
histidine kinase (AHK) mutants (15). To test our hypothesis that
the dwarf leaf phenotype of the atabcg14 mutant is due to impaired
cytokinin allocation to the shoot, we exogenously applied
cytokinin to the leaves of atabcg14. Indeed, daily spraying with
1 μM tZ rescued the small leaf (Fig. 1E and Fig. S4A) and short
stem (Fig. S4A) phenotypes. In contrast, a similar treatment
with iP did not rescue the dwarf phenotypes of the mutant
(Fig. S4).