Phenotypical evidence of effective amelioration of ammonium-inhibited plant (root) growth by exogenous low urea

Ammonium and nitrate are major soil inorganic-nitrogen sources for plant growth, but many species cultivated with even low millimolar NH₄⁺ as a sole N form display a growth retardation. To date, critical biological components and applicable approaches involved in the effective enhancement of NH₄⁺ tolerance remain to be thoroughly explored. Here, we report phenotypical traits of urea-dependent improvement of NH₄⁺-suppressed plant/root growth. Urea at 0.1 mM was sufficient to remarkably stimulate NH₄⁺ (3 mM)-fed cotton growth, showing a 2.5∼4-fold increase in shoot- and root-biomass and total root-length, 20 % higher GS activity, 18 % less NH₄⁺-accumulation in roots, and a comparable plant total-N content compared to the control, implying a novel role for urea in cotton NH₄⁺detoxification. A similar phenomenon was observed in tobacco and rice. Moreover, comparisons between twelve NH₄⁺-grown Arabidopsis accessions revealed a great degree of natural variation in their root-growth response to low urea, with WAR and Blh-1 exhibiting the most significant increase in primary- and lateral-root length and numbers, and Sav-0 and Edi-0 being the most insensitive. Such phenotypical evidence suggests a common ability of plants to accommodate NH₄⁺-stress by responding to exogenous urea, providing a novel aspect for further understanding the process of urea-dependent plant NH₄⁺ tolerance.