High and low exogenous nitrate concentrations produce distinct calcium signatures in Arabidopsis roots

Calcium (Ca2+) acts as a secondary messenger in plant responses to many stimuli, including nitrate (NO -) signaling in roots. Nitrate uptake in plants occurs through two distinct transporter systems that have different affinities for the anion, which result in divergent downstream regulatory responses. However, it is unknown if the concentration dependent NO - response is linked to differential Ca2+ response. In this study, we used an Arabidopsis transgenic line expressing CBL1-mRuby2-GCaMP6s to investigate if there are unique Ca2+ signatures in intact root tissue in response to high and low NO - concentrations. A significant Ca2+ response was observed in root hairs in response to both high (5 mM) and low (0.25 mM) NO - concentrations. A distinct Ca2+ wave was observed in the low NO - treated roots, and an asynchronous Ca2+ response was captured in individual root epidermal cells. Cell-specific analysis revealed a distinct Ca2+ profile, comprising waves and spikes in response to NO -, and prominent in root hairs but not in non-root hair bearing epidermal cells. The findings from this study demonstrate that there are distinct cell-specific and NO - condition-specific Ca2+ signatures in Arabidopsis roots.