A feedback regulatory loop by MAPK-CCA1 engages auxin signalling to stimulate root foraging for nitrate

In eucaryotes, mitogen-activated protein kinase (MAPK) cascades are evolutionarily conserved signalling modules crucial for growth regulation and stress tolerance. However, the regulatory role of MAPK in nutrient sensing by plants remains largely unclear. Here we uncovered MEKK14 and its paralogue MEKK13 determine lateral root elongation via enhanced cell division and expansion. We further fine-mapped a naturally occurring histidine-to-glutamine substitution in MEKK14 that weakens protein kinase activity and attenuates lateral root growth and response to nitrate (NO₃^-). We further demonstrate that NO₃^- transcriptionally upregulates MEKK13/14 depending on NLP7 to activate a MKK3-MPK1/2/7/14 signalling module. Downstream of this signalling cascade, the core oscillator of the circadian clock CCA1 is phosphorylated and stabilized to feedback induce MEKK13/14 expression and to activate auxin signalling-dependent lateral root foraging for NO₃^-. Our findings reveal a positive-feedback phosphorylation-transcriptional regulatory loop in root NO₃^- foraging, extending the regulatory function of MAPK signalling in the nutrient sensing.