Ammonium (NH₄⁺) toxicity inhibits shoot growth in Arabidopsis, but the underlying mechanisms remain poorly characterized. Here, we show that a novel Arabidopsis mutant, ammonium tolerance 1 (amot1), exhibits enhanced shoot growth tolerance to NH₄⁺. Molecular cloning revealed that amot1 is a new allele of EIN3, a key regulator of ethylene responses. The amot1 mutant and the allelic ein3-1 mutants show greater NH₄⁺ tolerance than the wild type. Moreover, transgenic plants overexpressing EIN3 (EIN3ox) are more sensitive to NH₄⁺ toxicity The ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) increases shoot sensitivity to NH₄⁺, whereas the ethylene perception inhibitor Ag⁺ decreases sensitivity. NH₄⁺ induces ACC and ethylene accumulation. Furthermore, ethylene-insensitive mutants such as etr1-3 and ein3eil1 display enhanced NH₄⁺ tolerance. In contrast, the ethylene overproduction mutant eto1-1 exhibits decreased ammonium tolerance. AMOT1/EIN3 positively regulates shoot ROS accumulation, leading to oxidative stress under NH₄⁺ stress, a trait that may be related to increased expression of peroxidase-encoding genes. These findings demonstrate the role of AMOT1/EIN3 in NH₄⁺ tolerance and confirm the strong link between NH₄⁺ toxicity symptoms and the accumulation of hydrogen peroxide.

Plant growth and development depends on sufficient nutrient availability in soils. Agricultural soils are generally nitrogen (N) deficient and thus, soils need to be supplemented with fertilizers. Ammonium (NH₄ ⁺) is a major inorganic N source. However, at high concentrations ammonium nutrition becomes a stress situation that inhibits plant growth. The cause of ammonium stress or toxicity is multi-factorial but the interaction with other nutrients is among the main determinants of plants sensitivity towards high NH₄ ⁺ supply. In addition, NH₄ ⁺ uptake and assimilation provokes the acidification of the cell external medium (apoplast/rhizosphere), which has a clear impact on nutrients availability. This review summarizes current knowledge, at both physiological and molecular level, of ammonium nutrition interaction with essential mineral elements absorbed as cations: macronutrients (K ⁺, Ca ²⁺, Mg ²⁺) and micronutrients (Fe ^2+/3+, Mn ²⁺, Cu ^+/2+, Zn ²⁺, Ni ²⁺). We hypothesize that considering these nutritional interactions, and soil pH, when formulating fertilizers may be key in order to boost the use of NH₄ ⁺-based fertilizers, which have less environmental impact compared to the nitrate-based ones. In addition, we are convinced that better understanding these interactions will help to identify novel targets with potential to improve crop productivity.

A stunted root system is a significant symptom of iron (Fe) toxicity, yet little is known about the effects of excess Fe on lateral root (LR) development. In this work, we show that excess Fe has different effects on LR development in different portions of the Arabidopsis (Arabidopsis thaliana) root system and that inhibitory effects on the LR initiation are only seen in roots newly formed during excess Fe exposure. We show that root tip contact with Fe is both necessary and sufficient for LR inhibition and that the auxin, but not abscisic acid, pathway is engaged centrally in the initial stages of excess Fe exposure. Furthermore, Fe stress significantly reduced PIN-FORMED2 (PIN2)-green fluorescent protein (GFP) expression in root tips, and pin2-1 mutants exhibited significantly fewer LR initiation events under excess Fe than the wild type. Exogenous application of both Fe and glutathione together increased PIN2-GFP expression and the number of LR initiation events compared with Fe treatment alone. The ethylene inhibitor aminoethoxyvinyl-glycine intensified Fe-dependent inhibition of LR formation in the wild type, and this inhibition was significantly reduced in the ethylene overproduction mutant ethylene overproducer1-1. We show that Auxin Resistant1 (AUX1) is a critical component in the mediation of endogenous ethylene effects on LR formation under excess Fe stress. Our findings demonstrate the relationship between excess Fe-dependent PIN2 expression and LR formation and the potential role of AUX1 in ethylene-mediated LR tolerance and suggest that AUX1 and PIN2 protect LR formation in Arabidopsis during the early stages of Fe stress.