Figure 4.

Concerted regulation of nitrate, ammonium, and potassium transport. Top, under nutrient sufficiency, the nitrate transporter NRT1.1 operates in the low-affinity mode, AMTs transport any NH₄ ⁺ present in the soil solution as the preferred N source, and AKT1 ensures K⁺ influx down its electrochemical gradient. Bottom, upon nutrient scarcity, the N-myristoylated and Ca²⁺-bound CBL1/9 recruit to the plasma membrane and activate the protein kinase CIPK23 that in turn phosphorylates and activates the high-affinity mode of transport of NRT1.1 and HAK5, and stimulates K⁺ influx through AKT1. CIPK23 is also able to inhibit AMTs when the NH₄ ⁺ is at toxic levels.