The alleviation of ammonium toxicity in plants

Nitrogen(N)is an essential macronutrient for plants and profoundly affects crop yields and qualities.Ammonium(NH4+)and nitrate(NO3-)are major inorganic N forms absorbed by plants from the surrounding environments.Intriguingly,NH4+is usually toxic to plants when it serves as the sole or dominant N source.It is thus important for plants to coordinate the utilization of NH4+and the alleviation of NH4+toxicity.To fully decipher the molecular mechanisms underlying how plants minimize NH4+toxicity may broadly benefit agricultural practice.In the current minireview,we attempt to discuss recent discoveries in the strategies for mitigating NH4+toxicity in plants,which may provide potential solutions for improving the nitrogen use efficiency(NUE)and stress adaptions in crops.

Nitrate transporter NRT1.1 and anion channel SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity

Ammonium(NH_(4)^(+))and nitrate(NO_(3)^(-))are major inorganic nitrogen(N)sources for plants.When serving as the sole or dominant N supply,NH_(4)^(+)often causes root inhibition and shoot chlorosis in plants,known as ammonium toxicity.NO_(3)^(-) usually causes no toxicity and can mitigate ammonium toxicity even at low concentrations,referred to as nitrate-dependent alleviation of ammonium toxicity.Our previous studies indicated a NO_(3)^(-) efflux channel SLAH3 is involved in this process.However,whether additional components contribute to NO_(3)^(-)-mediated NH_(4)^(+)detoxification is unknown.Previously,mutations in NO_(3)^(-) transporter NRT1.1 were shown to cause enhanced resistance to high concentrations of NH_(4)^(+).Whereas,in this study,we found when the high-NH_(4)^(+) medium was supplemented with low concentrations of NO_(3)^(-),nrt1.1 mutant plants showed hyper-sensitive phenotype instead.Furthermore,mutation in NRT1.1 caused enhanced medium acidification under high-NH_(4)^(+)/Iow-NO_(3)^(-) condition,suggesting NRT1.1 regulates ammonium toxicity by facilitating H+uptake.Moreover,NRT1.1 was shown to interact with SLAH3 to form a transporter-channel complex.Interestingly,SLAH3 appeared to affect NO_(3)^(-) influx while NRT1.1 influenced NO_(3)^(-) efflux,suggesting NRT1.1 and SLAH3 regulate each other at protein and/or gene expression levels.Our study thus revealed NRT1.1 and SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity through regulating NO_(3)^(-) transport and balancing rhizosphere acidification.

Thymol ameliorates ammonium toxicity via repressing polyamine oxidase-derived hydrogen peroxide and modulating ammonium transporters in rice root

Background:Ammonium is an indispensable nutrient for crop growth,but anoxic conditions or inappropriate fertilizer usage result in the increase in ammonium content in soil.Excessive ammonium causes phytotoxicity.Thymol is a kind of natural phenolic compound with anti-microbial properties.However,little is known about the role of thymol in modulating plant physiology.Here we find the novel role of thymol in protecting rice from ammonium toxicity.Results:Thymol remarkably rescued rice seedlings growth from ammonium stress,which may resulted from the attenuation of reactive oxygen species(ROS)accumulation,oxidative injury,and cell death in both shoots and roots.Polyamine oxidase(PAO)metabolizes polyamines to produce ROS in plants in response to stress conditions.Thymol blocked ammonium-induced upregulation of a set of rice PAOs,which contributed to the decrease in ROS content.In rice seedlings upon ammonium stress,thymol downregulate the expression of ammonium transporters(AMT1;1 and AMT1;2);thymol upregulated the expression of calcineurin B-like interacting protein kinase 23(CIPK23)and calcineurin B-like binding protein 1(CBL1),two negative regulators of AMTs.This may help rice avoid ammonium overload in excessive ammonium environment.Correlation analysis indicated that PAOs,AMTs,and CBL1 were the targets of thymol in the detoxification of excessive ammonium.Conclusion:Thymol facilitates rice tolerance against ammonium toxicity by decreasing PAO-derived ROS and modulating ammonium transporters.Such findings may be applicable in the modulation of nutrient acquisition during crop production.

Estimation of Acute Toxicity of Ammonium Sulphate to the Fresh Water Catfish,Heteropneustes fossilis——Ⅰ.Analysis of LC_(50) Values Determined by Various Methods

The acute toxic effects of ammonium sulphate to fresh-water catfish, Heteropneustes fossilis (H.fossilis) have been studied by determining LC50 values with 95% confidence limits, by the graphic method, the logistic method, the Spearman and Karber method and the trimmed Spearman-Karber method. The trimmed Spearman-Karber method was found the most ideal for ammonium salt toxicity test. The flaws in the trimmed Spearman-Karber method are also discussed.

Ammonium affects cell viability to inhibit root growth in Arabidopsis

Ammonium （NH4^＋） is an important form of nitrogen nutrient for most plants, yet is also a stressor for many of them. However, the primary events of NH4^＋ toxicity at the cellular level are still unclear. Here, we showed that NH4^＋ toxicity can induce the root cell death in a temporal pattern which primarily occurs in the cells of root maturation and elongation zones, and then spreads to the ceils in the meristem and root cap. The results from the NH4^＋-hypersensitive mutant hsn1 further confirmed our findings. Taken together,NH4^＋ toxicity inhibits primary root growth by inhibiting cell elongation and division and inducing root cell death.