Evidence That Exogenous Urea Acts As A Potent Cue To Alleviate Ammonium-Inhibition Of Root System Growth Of Cotton Plant (Gossypium Hirsutum)

Many plants grown with low-millimolar concentration of NH4+ as a sole nitrogen source develop NH4+-toxicity symptoms. To date, crucial molecular identities and a practical approach involved in the improvement of plant NH4+-tolerance remain largely unknown. By phenotyping of upland cotton grown on varied nitrogen forms, we came across a phenomenon that caused sub-millimolar concentrations of urea (e.g., up 50 mu M) to repress the growth inhibition of roots and whole plant cultivated in a NH4+-containing nutrient solution. A growth-recovery assay revealed that the relief in NH4+-inhibited growth required only a short-term exposure (>= 12 h) of the roots to urea, implying that urea could elicit an internal signaling and be involved in antagonizing NH4+-sensitivity. Intriguingly, split-root experiments demonstrated that low urea occurrence in one root-half could efficaciously stimulate not only supplied root but also the root-half grown in NH4+-solution without urea, indicating the existence of urea-triggered local and systemic long-distance signaling. In the split-root experiment we also observed high arginase activity, strong arginine reduction and remarkable upregulation of polyamine biosynthesis-related genes (ADC1/2, SPDS and SPMS). Therefore, we suggest that external urea might serve as an effective cue (signal molecule) in an arginine-/polyamine-related process for ameliorating NH4+-suppressed root growth, providing a novel aspect for deeper exploring and understanding plant NH4+-tolerance.