Regulation of ammonium-mediated inhibition of root system development via local and systemic pathways
Baohai Li¹, Qing Li¹, Liming Xiong², Herbert J. Kronzucker³ and Weiming Shi^1,*

¹State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China.
²Plant Stress Genomics Research Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.
³Department of Biological Sciences, University of Toronto, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada.
*Corresponding Author：Tel: (+8625) 86881566; Fax: ((+8625) 86881000; E-mail: wmshi@issas.ac.cn

Abstract
The formation of root system architecture is greatly dependent on the environmental conditions such as nutrients. Ammonium (NH₄⁺) is an important nitrogen source for higher plants. However, excess NH₄⁺ shows a strong suppression on root system development. Here, with local nutrition supply device, we revealed that NH₄⁺-mediated inhibition of root elongation is primarily through root tip, whereas NH₄⁺-mediated reduction of lateral root number is a result of shoot contact with NH₄⁺. Furthermore, NH₄⁺ inhibition of root growth was shown to target primarily cell elongation, which was directly linked to excessive NH₄⁺ efflux in the root elongation zone, but not related to auxin pathway. This NH₄⁺ efflux is regulated by GDP-mannose pyrophosphorylase (GMPase). On the other hand, NH₄⁺-induced reduction of lateral root formation results from less lateral root emergence rather than lateral root initiation, which is caused by the decreased transport of shoot-derived auxin to roots, via the auxin influx carrier AUX1. Taken together, these results suggest regulations of primary root elongation and lateral root formation by NH₄⁺ is through local and systemic two independent pathways. Our findings provide a novel insight to regulations of root development by environmental nutrient factors.

Key Words: Ammonium; AUX1, VTC1, root system development