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.