Evidence that exogenous urea acts as a potent cue to alleviate ammonium‐inhibition of root system growth of cotton plant (<scp><i>Gossypium hirsutum</i></scp>)

Abstract

Abstract Many plants grown with low‐millimolar concentration of NH 4 + as a sole nitrogen source develop NH 4 + ‐toxicity symptoms. To date, crucial molecular identities and a practical approach involved in the improvement of plant NH 4 + ‐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 μM) to repress the growth inhibition of roots and whole plant cultivated in a NH 4 + ‐containing nutrient solution. A growth‐recovery assay revealed that the relief in NH 4 + ‐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 NH 4 + ‐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 NH 4 + ‐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 NH 4 + ‐suppressed root growth, providing a novel aspect for deeper exploring and understanding plant NH 4 + ‐tolerance.