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.