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

Kai Guo,Weimin Xu,Bingdong Pang,Zhiqi Shi,Ning Wang,Hongwu Bai,Li Zhang,Guangchi An,Jian Chen

doi:10.1186/s43014-021-00053-1

Abstract

BackgroundAmmonium 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.ResultsThymol 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.ConclusionThymol 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.Graphical abstract

Highlights

Ammonium (NH4+) is one of the major sources for crops acquiring nitrogen
Thymol attenuated growth stunt and oxidative injury in rice seedlings upon ammonium stress We first evaluated the toxic effect of excessive NH4Cl on rice seedlings by measuring the length of root and shoot, respectively, upon the exposure of NH4Cl at different concentrations (10-80 mM)
In time-course experiments, thymol began to significantly rescue the growth of root and shoot at 6 h, further giving rise to increased growth speed than that of NH4Cl treatment alone (Fig. 1E and F). These results suggested that thymol was able to recover the growth of rice seedlings from NH4Cl stress

Summary

Introduction

Ammonium (NH4+) is one of the major sources for crops acquiring nitrogen. Applying artificial fertilizer is the main approach to provide nitrogen for crops due to the decrease in agricultural soil fertility and low nitrogen use efficiency of crops. The inappropriate fertilizer usage with imbalanced nutrients, e.g. the prolonged usage of ammonium as sole nitrogen source, results in the increase in soil ammonium level. Soils at anoxic conditions (e.g. rice paddies) or low pH conditions are always rich in ammonium because of the high rate of ammonification. In these soils, applying other form of nitrogen (e.g. nitrate) can increase ammonium level (Chen et al 2013; Esteban et al 2016). Modifying nitrogen sources can affect plant endogenous polyamines level (Altman and Levin 1993), but little is known about whether ammonium triggers polyamines metabolism to produce ROS. We find the novel role of thymol in protecting rice from ammonium toxicity

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Results

Conclusion