Is phloem sap nitrate a signaling molecule?
There is now considerable evidence that nitrate can play a signaling
role to control plant development (Kant, 2018) and nitrate redistributed
by the phloem seems to participate in this control (summarized in Fig.
2). Historically, an important step forward was the demonstration that
nitrate shoot content acts as a signal to regulate the shoot:root ratio.
In fact, manipulating nitrate reductase activity showed a correlation
between leaf nitrate concentration and shoot:root ratio, across
different levels of nitrate supply (Scheible, Lauerer, Schulze, Caboche,
& Stitt, 1997). Furthermore, in split‐root experiments, root growth is
inhibited by the accumulation of nitrate in the shoot, regardless of the
fact that one part of the root system was supplied with high nitrate and
the other part with low nitrate (Scheible et al., 1997). More recently,
split-root experiments have shown that there is a cytokinin-mediated
signaling from the root part under low N conditions, so as to promote
lateral root development in the root part under N-rich conditions
(Ruffel et al., 2011).
The signaling cascade associated with nitrate has been dissected
recently and shown to involve Ca2+-sensor protein
kinases and NIN-like proteins (NLP) transcription factors to control
gene expression, including genes encoding enzymes and transporters
involved in nitrogen assimilation (Chu et al., 2020; Liu et al., 2017).
In addition, at the plant scale, nitrate signaling involves small
peptides transited via phloem sap. The signaling peptide C-TERMINALLY
ENCODED PEPTIDE 1 (CEP1) is produced by roots under N deficiency, and
transported to the shoot via the xylem, leading to the induction of the
signaling peptides CEP DOWNSTREAM 1 (CEPD1) and CEPD2; these peptides
are then transported to roots via the phloem and upregulates the high
affinity nitrate transporter NRT2.1 (Gu et al., 2018; Ohkubo, Tanaka,
Tabata, Ogawa-Ohnishi, & Matsubayashi, 2017).
In such a molecular context, could phloem nitrate itself have a
signaling role? It might be so, in particular in root tips where
longitudinal root growth (increase in root length) occurs. It is worth
noting that in root tips, protophloem develops first, that is, before
protoxylem (Bauby, Divol, Truernit, Grandjean, & Palauqui, 2007;
Mähönen et al., 2000). Also, in root tips, nitrate absorption is very
small, compared to other root sections (Lazof, Rufty, & Redinbaugh,
1992). Therefore, root tip metabolism depends on the import of organic
N, not nitrate. We can thus speculate that the influx of nitrate via
protophloem could contribute to determine root tip nitrate concentration
which has in turn a regulatory role. In effect, in Legumes, nitrate
inhibits primary root growth at the root tip via NPF6.8-mediated
abscisic acid signaling and reactive oxygen species (Limami & Morère-Le
Paven, 2019; Pellizzaro et al., 2014; Zang et al., 2020). InArabidopsis , nitrate stimulates meristematic activity (primary
root growth) at the root tip via cytokinin signaling (Naulin et al.,
2020).
If nitrate transported by phloem sap plays effectively a signaling role,
it raises two questions: (i ) First, nitrate concentration in
phloem must be controlled to avoid both fluctuations and high
concentrations. That is, signal-carrying molecules have to be at low
concentration under steady-state conditions and their transient increase
can play the role of a signal. Unfortunately, relatively little is known
on possible fluctuations of nitrate concentration in phloem sap and
whether its appearance in sap is linked to specific circumstances
(although there is some correlation with xylem nitrate and other
nutrients, see below, Perspectives for N cycling in plants ).
(ii ) Second, unclear would be the reason to have both CEP- and
nitrate-based regulation of root growth. That said, while CEP-based
regulation seems to be associated with low N signaling, nitrate itself
could represent a more generic N status signal (not only low N
conditions). It is now well established that nitrate interacts with many
hormones such as cytokinins (Sakakibara, Takei, & Hirose, 2006), and
this aspect has been reviewed elsewhere recently (Vega, O’Brien, &
Gutiérrez, 2019). Taken as a whole, nitrate concentration itself could
participate in defining the signaling milieu and thus the flow of
nitrate from shoots to roots via the phloem could be important to
control root growth.