Plant nitrate transporters: from gene function to application

Plant nitrate transporters were first identified and functionally characterized more than 20 years ago. They are encoded at least by four gene families, NRT1 (NPF), NRT2, CLC, and SLAC1/SLAH. In this review, we overview the functions of the nitrate transporters in relation to their potential use as targets for improving crop nitrogen use efficiency. These functions include their roles in root architecture and nutrient acquisition; vacuole nitrate and protein storage; nutrient allocation from source to sink; sensing both abiotic and biotic stresses; the ionic balance of nitrate with potassium, chloride and cellular pH; and the circadian clock-regulated carbon and nitrogen balance. We provide and discuss some examples of the use of nitrate transporter genes and their regulators in improving plant growth and development, nitrogen use efficiency, and resistance to some abiotic stresses. We propose several strategies for effectively using nitrate transporters to achieve higher crop yields and nitrogen use efficiency by using gene transformation or genome editing or molecular marker-assisted breeding.