Reducing the Nitrate Content in Vegetables Through Joint Regulation of Short-Distance Distribution and Long-Distance Transport

As an important nitrogen source, nitrate (NO(3)(−)) absorbed by plants is carried throughout the plant via short-distance distribution (cytoplasm to vacuole) and long-distance transportation (root to shoot), the two pathways that jointly regulate the content of NO(3)(−) in plants. NO(3)(−) accumulation within the vacuole depends on the activities of both tonoplast proton pumps and chloride channel (CLC) proteins, and less NO(3)(−) is stored in vacuoles when the activities of these proteins are reduced. The ratio of the distribution of NO(3)(−) in the cytoplasm and vacuole affects the long-distance transport of NO(3)(−), which is regulated by the proteins NPF7.3 and NPF7.2 that play opposite but complementary roles. NPF7.3 is responsible for loading NO(3)(−) from the root cytoplasm into the xylem, whereas NPF7.2 regulates the unloading of NO(3)(−) from the xylem, thereby facilitating the long-distance transport of NO(3)(−) through the roots to the shoots. Vegetables, valued for their nutrient content, are consumed in large quantities; however, a high content of NO(3)(−) can detrimentally affect the quality of these plants. NO(3)(−) that is not assimilated and utilized in plant tissues is converted via enzyme-catalyzed reactions to nitrite (NO(2)(−)), which is toxic to plants and harmful to human health. In this review, we describe the mechanisms underlying NO(3)(−) distribution and transport in plants, a knowledge of which will contribute to breeding leafy vegetables with lower NO(3)(−) contents and thus be of considerable significance from the perspectives of environmental protection and food safety.