Low-affinity nitrate transporter members share
sequence similarity with peptide transporters
(PTR) forming a superfamily that includes 53
and 80 members in A. thaliana and rice,
respectively. Nitrate and/or di/tripeptides trans-
port activity has been demonstrated for eighteen
Arabidopsis members, and in most of the cases,
these proteins were reported to act as proton-
coupled transporters (Table 12.1) (Liu et al.
1999 ; Chiang et al. 2004 ). However, a bio-
chemical approach to determine the transported
substrate specificity is crucial for members of this
superfamily, as this cannot be argued by
sequence data alone. Several reports indicate
they encompass proteins capable of transporting
different substrates such as nitrate, di/tripeptides,
amino acids, glucosinolates, malate, auxin, and
ABA (Frommer et al. 1994 ; Liu et al. 1999 ;
Jeong et al. 2004 ; Waterworth and Bray 2006 ;
Krouk et al. 2010 ; Kanno et al. 2012 ; Nour-Eldin
et al. 2012 ). In a few cases, a dual transport
capacity for different substrates was also found
on the same member (Table12.1). For this rea-
son, a new unified nomenclature based exclu-
sively on phylogenetic relationship has been very
recently established (Leran et al. 2014 ). Proteins
of the nitrate/peptide transporters family (NPF)
from 33 fully sequenced plant genomes were
analyzed and eight unambiguous clades (sub-
families) identified. Therefore, NPF members are
now identified by a two number code, indicating
subfamily and relative position within this,
respectively (Leran et al. 2014 ).
NRT2 proteins form small families of plant
transporters in plants, including seven and four
members in Arabidopsis and rice, respectively. In
contrast to NPF genes, NRT2 members can be
identified by sequence data and all the NRT2
proteins identified in higher plants transport
nitrate. The NRT2 proteins are not functional
alone, as an additional component, called NAR2/
NRT3, is required for their nitrate transport
activities in plants (Tsay et al. 2007 ). It is
believed that the NRT2s are also proton-coupled
transporters and four out of the seven NRT2
genes found in Arabidopsis show a nitrate-rela-
ted phenotype when mutated (Lin et al. 2007 ;
Chopin et al. 2007 ; Kiba et al. 2012 ). Recently,
the AtNRT2.1 and AtNRT2.6 proteins have also
been involved in the plant response to bacterial
pathogen infection, asnrt2.1andnrt2.6mutants
show a reduced susceptibility toPseudomonas
syringae pv tomato and Erwinia amylovora
bacteria (Camanes et al. 2012 ; Dechorgnat et al.
2012 ).12.2 Nitrate Effects on Root
Architecture and Nodulation
ProgramsNitrate may trigger the signaling pathways con-
trolling lateral root development either systemi-
cally or locally (Zhang and Forde 1998 ). In
Arabidopsis, increasing evidence indicates a role
for NPF and NRT2 proteins in signaling trans-
duction pathways. AtNPF6.3 (old name At-
NRT1.1) plays a dual nutrient transporter/sensor
role (transceptor) in the perception of external
nitrate concentrations (Ho et al. 2009 ; Gojon
et al. 2011 ). A switch between HATS and LATS
affinity in AtNPF6.3 is controlled by phosphor-
ylation at the T101 residue (Liu and Tsay 2003 ).
Moreover, an activity of AtNPF6.3 as an auxin
transport facilitator, allowing the definition of a
functional linkage between nitrate and auxin
signaling, controlling secondary root elongation,
was reported (Krouk et al. 2010 ). The high-
affinity complex NRT2.1–NAR2.1 also partici-
pates in regulating lateral root development,
independently of uptake functions (Little et al.
2005 ). In legumes, theM. truncatulaMtNPF1.7/
NIP-LATD, recently characterized as a high-
affinity nitrate transporter (Bagchi et al. 2012 ), is
involved in lateral root and nodule development
and primary root meristem maintenance (Bright
et al. 2005 ).
As for secondary root developmental control,
the nitrate effect on nodulation is exerted through
both local and systemic controls (Carroll and
Gresshoff 1983 ; Day et al. 1989 ; Carroll
and Mathews 1990 ; Fujikake et al. 2003 ; Omrane
and Chiurazzi 2009 ; Jeudy et al. 2010 ). High
nitrate concentration in the growth medium
([1 mM) inhibits nodule formation, and old
observations indicate that this effect does not126 V.T. Valkov and M. Chiurazzi