Root energy availability controls the fast decrease in root nitrate uptake in response to light intensity decrease

Photosynthesis determines plant growth, influencing plant nitrogen demand, hence it is also crucial to the regulation of nitrate uptake. It remains unclear how fast plants adjust root nitrate uptake in response to change in photosynthesis. This research aimed to investigate the instantaneous response of root nitrate uptake to shoot photosynthesis. Arabidopsis grown hydroponically in a climate chamber under 175 μmol m^-2 s^-1 photosynthetically active radiation (PAR). On day 34, plants were transferred to three light intensities (75, 175, or 300 μmol m^-2 s^-1 PAR). Nitrate uptake, expression levels of four nitrate transporters, plant nitrogen (N) content, and plant mass were measured before transfer, 0.5 h, and 3 h after the transfer. CO₂ gas exchange was measured continuously. Photosynthesis rate immediately adjusted to new light intensity, reaching a steady state 20 min after the transfer. Nitrate influx and root sugar concentration both decreased in plants exposed to low-light intensity. Conversely, nitrate influx remained constant over the 3 h in plants transferred to high light, despite increased sugar concentration in the root. Only the expression level of NRT1.1 was upregulated in plants transferred to high light. Our findings highlight the importance of energy for nitrate uptake under low light condition. It also suggests that, within the initial 3 h following the light intensity transition, plants can maintain growth independent of nitrate uptake, likely through the assimilation of stored nitrogen. This demonstrates plants' ability to sustain growth without instance response in nitrate uptake and highlights the importance of energy availability in nitrate uptake.