Arabidopsis under ammonium over-supply: Characteristics of ammonium toxicity in relation to the activity of ammonium transporters

Because of highly intensive farming practices, crop plants could suffer relatively long terms of ammonium (NH₄⁺) excess stress introduced by overdose application of nitrogen fertilizers. However, the lack of sufficient understanding of plant responses to NH₄⁺ excess stress impairs the detection of effective solutions to this problem. The present work examined the biological influences of over-supplied NH₄⁺ in Arabidopsis thaliana using two mutant lines each with an ammonium transporter (AMT) gene (AtAMT1;1 or AtAMT1;3) knocked out. Our results indicated that lacking one of the major components of root NH₄⁺ -absorbing systems significantly alleviated the toxicity effects on Arabidopsis plants by reducing the accumulation of free NH₄⁺, suggesting that persistent absorption of NH₄⁺ through AMT was the main cause of excessive accumulation of free NH₄⁺ in the plants. Shading treatment led to a reduced transpirational driving force and thereby constrained the accumulation of toxic NH₄⁺ in the plants, finally resulting in higher NH₄⁺ -promoted growth in the wild type (WT). Under the shading treatment, the amt1;1 and amt1;3 mutant plants acquired insufficient NH₄⁺ and showed reduced growth when compared with the WT. Furthermore, the foliar application of sucrose notably alleviated the inhibitory effects on plant growth in the WT but had no effect on either the amt1;1 or amt1;3 mutant plants, indicating that carbon scarcity associated with NH₄⁺ excess is probably a major cause of NH₄⁺ toxicity in plants. Accordingly, increasing carbon source could be a potentially effective approach that alleviates the inhibition caused by NH₄⁺ excess and increases nitrogen use efficiency under NH₄⁺ over-supply.