Ammonium (NH4+), an important source of nitrogen for many species, is frequently present in soils and in the atmosphere in significant quantities. A stunted root system is a significant symptom of excess NH4+ toxicity, but little is known about the effects of NH4+ on lateral root (LR) development.
The research group lead by Prof. Weiming Shi at Institute of Soil Science, Chinese Academy of Sciences, noted that the effect of NH4+ on Arabidopsis LR formation was dependent on the locus of NH4+ exposure. Shoot-supplied ammonium (SSA) reduced LR formation, and SSA inhibits LR number by interfering with AUX1-dependent auxin transport from shoot to root. This finding has been published in Plant Cell and Environment (2011, 34: 933-946).
To elucidate the question how the SSA inhibits auxin transport from shoot to root, we gave a further research by using pharmacological and genetic approaches. We found that NH4+ exposure of wild-type (Col-0) Arabidopsis led to pronounced inhibition of LR production chiefly in the distal portion root, and triggered ethylene evolution and enhanced activity of the ethylene reporter EBS:GUS in the shoot. It is shown that shoot contact with NH4+ is necessary to stimulate shoot ethylene evolution. The ethylene antagonists Ag+ and AVG mitigated LR inhibition under NH4+ treatment. Under NH4+ treatment, the decrease in LR number was more significant in the mutant lines that show enhanced ethylene synthesis (eto1-1 and xbat32) compared with wild-type seedlings; in contrast, the blocked ethylene signaling mutant (etr1-3) had less inhibition of LR number. Enhanced shoot ethylene synthesis and/or signalling blocked recovery of LR production when auxin was applied in the presence of NH4+ and negatively impacted shoot AUX1 expression. Negative regulation of shoot AUX1 expression by SSA-induced ethylene signalling and/or synthesis may trigger the inhibition of auxin transport from shoot to root. This finding has been published in Journal of Experiment Botany (2013, 64: 1413-1425).
These findings highlight the important role of shoot ethylene evolution in NH4+-mediated inhibition of LR formation and provide novel insight into how LRs are regulated in response to NH4+ stress. The findings are also expected to be useful to instruct foliar nitrogen fertilizer applications and crop growth management under excessive atmospheric NH4+ deposition in the future.
