Phloem sap nitrate is a signaling molecule
There is now considerable evidence that nitrate can play a signaling role to control plant development (Kant, 2018). 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 et al. , 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 (Scheibleet al. , 1997). Molecular data have now demonstrated a link between nitrate and phytohormones, and this aspect has been reviewed recently elsewhere (Vega et al. , 2019). The signaling cascade associated with nitrate has also been dissected recently and shown to involve Ca2+-sensor protein kinases and NIN-like proteins (NLP) transcription factors to control gene expression, including gene encoding enzymes and transporters involved in nitrogen assimilation (Liu et al. , 2017; Chu et al. , 2020). The role played by nitrate as a signal from shoots to roots implies that its 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 (but see below, Perspectives for N cycling in plants ). It is worth noting that in root tips, nitrate absorption is small, compared to other root parts (Lazof et al. , 1992). In addition, root tips are the sites of longitudinal root growth (increase in root length), where protophloem develops first (i.e. before protoxylem) (Mähönen et al. , 2000; Bauby et al. , 2007). It is plausible that downwards transport of nitrate via (proto)phloem plays a role in controlling root tip nitrate concentration and thus root growth.