12.4 Potential Cross Talk Between
NPF, NRT2 Transporters and
SNF
The functional characterization of the NPF and
NRT2 families in a legume plant offers the
opportunity to evaluate the potential roles played
by these transporters in (i) a specific root
organogenesis pathway occurring in response to
biotic and abiotic signals (e.g.,Rhizobiumand N
conditions), (ii) a specific tissue context repre-
sented by nodules, (iii) a specific functional
constrain devoted to the exchanges occurring
between the symbiotic partners.
A potential role of the NPF and NRT2 pro-
teins could be played in several early steps of
nodule formation. One possibility would be a
direct involvement in the control of nitrate uptake
rate and/or an indirect role through transporting
the endogenous CLE peptides. A regulatory
action may also reflect the involvement in the
local pathway that senses and transduces the
external nitrate signal to the root machinery
involved in nodule organogenesis. Furthermore,
as indicated in Table12.1, a direct auxin and
abscisic acid uptake capacity has been reported
in the cases ofAtNPF6.3, AtNPF4.1, and At-
NPF4.6 (Krouk et al. 2010 ; Kanno et al. 2012 ),
while a negative feedback loop between NRT2.1
expression and ethylene biosynthesis was dis-
covered (Zheng et al. 2013 ). These three hor-
mones have important and antagonistic actions
on epidermal and cortical cells responses in the
early steps of the nodule organogenesis program
(Ding and Oldroyd 2009 ). A dual transport
capacity for nitrate and hormones as in the cases
of ATNPF4.6 and ATNPF6.3 (Table12.1) would
also be functional for NPF proteins involved in
the regulation of nodule development. The high-
affinity nitrate transporter MtNIP/LATD plays a
role in nodule development, but its action
appears to be independent by its nitrate transport
function suggesting the involvement of a differ-
ent biochemical activity (Salehin et al. 2013 ).
Furthermore, the development and functioning
of nodules is based on a coordinated differentia-
tion of plant and bacterial cells, which produce a
mature organ with both infected and uninfected
plant cells. Gene expression studies reveal that
nodules have a distinct metabolic phenotype. A
complex network of transport and exchanges
takes place in nodules, which provides reduced
carbon and other nutrients from the plant to bac-
teroids andfixed nitrogen from bacteroids to the
plants. Many transporters must be involved in the
dynamic of metabolite exchanges, but at the
moment, the knowledge of their molecular basis
is still limited. The increased amount of data
indicating high transport plasticity for members
of the NPF family, including the reported
capacity of amino acids (Waterworth and Bray
2006 ) and dicarboxylic acids (Jeong et al. 2004 ),
makes these proteins candidates for playing
important roles in the control of this nutrient
traffic.
The nitrate itself plays also an important role
as regulator of legume nodules activity as it is
known that a few days after nitrate exposure,
nodule activity is almost completely lost and the
nodules become senescent (Matamoros et al.
1999 ), but the mechanisms through which this
action takes place is still controversial. The effect
of nitrate is mediated by significant changes at
the gene expression level occurring in nodules
(Cabeza et al. 2014 ). The analysis of the global
response of nodule transcriptome apparently
suggests that nitrate targets the very heart of the
N 2 reduction process, i.e., the formation of the
nitrogenase complex itself and ATP generation
(Cabeza et al. 2014 ). According to this, a nitrate
reductase-dependent nitric oxide (NO) synthesis
process, involved in the maintenance of the
energy status required for Nfixation under oxy-
gen-limiting conditions, has been reported
(Horchani et al. 2011 ). Therefore, the involve-
ment of nitrate transporters into this complicate
network of activities supporting the nodule
functioning must be taken in consideration.
A crucial drive for the understanding of the
NPF and NRT2 roles during the symbiotic
interaction will certainly come from the exploi-
tation of the recently released LORE1-tagged
collection that includes 40.000 LORE1L. japo-
nicus lines comprising more than 120.00012 Nitrate Transport and Signaling 133