Agriculture Reference
In-Depth Information
AtNPF4.6 is identical to the previously characterized nitrate transporter NRT1.2
(Huang et al.
1999
). The
K
m
values of AtNPF4.6 for nitrate and ABA were about
5.9 mM and 5
ʼ
M, respectively (Kanno et al.
2012
; Huang et al.
1999
). A compe-
tition assay using a modified yeast two-hybrid system with the receptor complex
showed that nitrate did not compete with ABA as a substrate for AtNPF4.6 (Kanno
et al.
2013
), suggesting that ABA is the better substrate of this protein. In addi-
tion, AtNPF4.6 preferred the naturally occurring (
+
) enantiomer of ABA to the
synthetic (
−
) enantiomer and did not transport GA, IAA, or jasmonic acid. These
results indicate that AtNPF4.6 is a specific ABA transporter. AtNPF4.1, a closely
related homologue of AtNPF4.6 that was identified also as an ABA importer can-
didate, transported GA in addition to ABA (Kanno et al.
2012
). The physiological
functions of AtNPF4.1 remain to be determined.
Several other NPF members are reported to transport IAA or glucosinolates
(Nour-Eldin et al.
2012
; Krouk et al.
2010
). These findings indicate that NPF pro-
teins transport a variety of compounds. Thus, although several NPF proteins have
been characterized as nitrate or peptide transporters, careful investigation includ-
ing detailed analysis of their substrate specificities will be required to understand
their in vivo functions. The
nip
/
lated
mutants that are defective in an NPF pro-
tein in
Medicago truncatula
(MtNPF1.7) have defects in lateral root development
and root nodule formation (Veereshlingam et al.
2004
; Yendrek et al.
2010
). The
defect in lateral root development of
nip
/
lated
is partially rescued by exogenously
applied ABA (Liang et al.
2007
). Also, the mutants are less sensitive to exog-
enously applied ABA in terms of stomatal closure and seed germination (Liang
et al.
2007
). These observations indicate that MtNPF1.7/NIP/LATED is somehow
involved in the physiological responses mediated by ABA; however, ABA trans-
port activities of MtNPF1.7/NIP/LATED have not been reported and this protein
may function as a high-affinity nitrate transporter (Bagchi et al.
2012
).
3.4 Further Prospects
Endogenous ABA levels are regulated by several steps during biosynthesis and
catabolism. Thus, transmembrane transport of ABA precursors and metabolites
as well as bioactive ABA will all contribute to define the ABA concentration at
a specific site. ABA is derived from carotenoids, and early stages of ABA bio-
synthesis take place in chloroplasts (plastids). After the cleavage of epoxycarot-
enoids by 9-
cis
epoxycarotenoid dioxygenase (NCED), the C
15
product xanthoxin
is converted to ABA via two steps in the cytosol. Since NCEDs are localized to
the stroma and/or thylakoids (Endo et al.
2008
; Tan et al.
2003
), xanthoxin has
to be translocated from the inside to the outside of chloroplasts. The first step in
the major ABA catabolism pathway is the hydroxylation of the 8′ position by the
CYP707A subfamily of cytochrome P450 (P450) enzymes (Cutler and Krochko
1999
; Kushiro et al.
2004
; Saito et al.
2004
). P450s are generally localized in the
endoplasmic reticulum (ER). Thus, ABA synthesized in the cytosol needs to be
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