NRT1.1-related NH4+ toxicity is associated with a disturbed balance between NH4+ uptake and assimilation

First author: Shaofen Jian; Affiliations: Hunan Agricultural University (湖南农业大学): Changsha, China

Corresponding author: Zhenhua Zhang (张振华)

A high concentration of ammonium (NH4+) as the sole source of nitrogen in the growth medium is often toxic to plants. The nitrate transporter NRT1.1 is involved in mediating the effects of NH4+ toxicity; however, the mechanism remains undefined. In this study, wild-type Arabidopsis (Arabidopsis thaliana, Col-0) and NRT1.1 mutants (chl1-1 and chl1-5) were grown hydroponically in NH4NO3 and (NH4)2SO4 media to assess the function of NRT1.1 in NH4+ stress responses. All the plants grew normally in medium containing mixed N sources, but Col-0 displayed more chlorosis, and lower biomass and photosynthesis than the NRT1.1 mutants in (NH4)2SO4 medium. Grafting experiments between Col-0 and chl1-5 further confirmed that NH4+ toxicity is influenced by NRT1.1. In (NH4)2SO4 medium, NRT1.1 induced the expression of NH4+ transporters, increasing NH4+ uptake. Additionally, the activities of glutamine synthetase (GS) and glutamate synthetase (GOGAT) in roots of Col-0 plants decreasedandsoluble sugaraccumulated significantly, whereas pyruvate kinase (PK)-mediated glycolysis was not affected, all of which contributed to NH4+ accumulation. By contrast, the NRT1.1 mutants showed reduced NH4+ accumulation and enhanced NH4+ assimilation through GS, GOGAT and GDH. Moreover, the up-regulation of genes involved in ethylene synthesis and senescence in Col-0 plants treated with (NH4)2SO4 suggests that ethylene is involved in NH4+ toxicity responses. The study showed that NH4+ toxicity is related to a nitrate-independent signaling function of NRT1.1 in Arabidopsis, characterized by enhanced NH4+ accumulation and altered NH4+ metabolism, which stimulates ethylene synthesis, leading to plant senescence.

铵态氮(NH4+-N)作为植物利用的一种无机氮源，对植物的生长发育起到重要作用，但是，生长环境中NH4+浓度过高又极易对植物造成毒害作用，抑制植物生长，降低作物产量。在农业生产中，由于过量施肥以及土壤渍害造成的土壤氧化还原电位下降，植物铵毒现象时有发生。日本学者Takushi Hachiya等早在2011年就发现，拟南芥NRT1.1突变体chl1-1和chl1-5对高浓度NH4+（10 mM）的耐受性增强，但是对NRT1.1参与调控植物铵毒的作用机制尚不清楚。本研究发现（模式图），在(NH4)2SO4环境下，NRT1.1通过信号调控诱导根中NH4+吸收转运蛋白的表达来增强对生长环境中NH4+的吸收；另外，根系NH4+的同化途径GS/GOGAT循环显著降低，而PK酶活性没有受到显著影响，造成植物体内NH4+大量累积，碳氮代谢失衡，并诱导乙烯的产生，促进植物衰老。nrt1.1突变体根中NH4+吸收转运蛋白没有受到(NH4)2SO4的显著诱导，GS/GOGAT循环也没有受到显著影响，而GDH的活性显著提高，通过对NH4+吸收与同化之间的协同作用，降低了植株体内NH4+的累积，缓解铵毒的发生。

通讯：张振华（http://zhxy.hunau.edu.cn/jsff/fjs/201606/t20160623_158213.html）

个人简介：2000-2004年，湖南农业大学，农业资源与环境，学士；2004- 2007年，湖南农业大学，植物营养学，硕士；2007-2010年，湖南农业大学，植物营养学，博士（省优博）；2008-2010年，菲律宾国际水稻研究所，博士论文研究（作物营养学），国家留学基金委公派留学基金项目；2013-2016年, 上海植生所，博后。

研究方向：稻油轮作系统粮油作物提质增效机制。

doi: https://doi.org/10.1104/pp.18.00410

Journal: Plant Physiology

First Published: 18 October, 2018

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