Figure 3.

The schematic diagram presents the multiple pathways regulating the root system architecture (RSA; lateral and primary root) response to the localized and high nitrate conditions in Arabidopsis. Only those pathways discussed in the present review are depicted. The green arrows indicate systemic transport and assimilation, the black arrows indicate positive signaling as a stimulatory effect, red lines indicate negative signaling as an inhibitory effect, the orange lines depict the unknown positive and negative signaling pathways, and dotted lines represent the unconfirmed nitrate-mediated signaling pathways. The low nitrate and severely low nitrate conditions have been reported to have a stimulatory and inhibitory effect on LR development, respectively, while high NO₃⁻ supply has an inhibitory effect on LR growth [24] (see text for further information). External NO₃ regulates primary root growth in Arabidopsis. The receptor for the external glutamate signal is shown as a glutamate-gated Ca²⁺ channel because these are known to be activated at root tips [79]. However, its specific role in this signaling pathway is unconfirmed (see text for further information).