Does energy cost constitute the primary cause of ammonium toxicity in plants?

Nitrate (NO 3 - ) and ammonium (NH 4 + ) are the main nitrogen (N) sources and key determinants for plant growth and development. In recent decades, NH 4 + , which is a double-sided N compound, has attracted considerable amounts of attention from researchers. Elucidating the mechanisms of NH 4 + toxicity and exploring the means to overcome this toxicity are necessary to improve agricultural sustainability. In this review, we discuss the current knowledge concerning the energy consumption and production underlying NH 4 + metabolism and toxicity in plants, such as N uptake; assimilation; cellular pH homeostasis; and functions of the plasma membrane (PM), vacuolar H + -ATPase and H + -pyrophosphatase (H + -PPase). We also discuss whether the overconsumption of energy is the primary cause of NH 4 + toxicity or constitutes a fundamental strategy for plants to adapt to high-NH 4 + stress. In addition, the effects of regulators on energy production and consumption and other physiological processes are listed for evaluating the possibility of high energy costs associated with NH 4 + toxicity. This review is helpful for exploring the tolerance mechanisms and for developing NH 4 + -tolerant varieties as well as agronomic techniques to alleviate the effects of NH 4 + stress in the field.