Rapid induction of NH4 + efflux in rice roots under high-ammonium stress and its association with varietal differences in ammonium tolerance and ammonium-utilization efficiency.

Ammonium (NH4 +) efflux is a pivotal mechanism underlying root responses to NH4 + toxicity, yet critical questions have remained unresolved, such as on the speed of onset and rectification of the NH4 +-efflux response to NH4 + stress, its association with ammonium-use efficiency (AUE) and NH4 + tolerance, and the identity of the transporter proteins or channel types mediating efflux. Here we survey the current state of the literature on the topic and combine the survey with new experiments in the leading crop species rice. We provide a pharmacological profile of NH4 + fluxes under transient high-NH4 + treatments in rice-root protoplasts and demonstrate that acute NH4 + stress induces immediate and sustained NH4 + efflux, likely mediated by aquaporins, ATP-Binding Cassette transporters, and potassium channels. We furthermore provide a screen of 99 cultivars that reveals a strong negative correlation (P < 0.01) between NH4 + efflux and both root-NH4 + tolerance and AUE, identifying efflux as a critical constraint on nitrogen-use efficiency. Probenecid-mediated inhibition of efflux is shown to boost NH4 + uptake by 23.65% in high-efflux cultivars, underscoring the regulatory role of NH4 + efflux. Our findings shine new light on plant adaptation to NH4 + toxicity and establish a molecular framework for improving nitrogen-use-efficiency of crops through modulation of NH4 +-efflux pathways.