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

Abstract

As an important nitrogen source, nitrate (NO₃ ^-) 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 NO₃ ^- in plants. NO₃ ^- accumulation within the vacuole depends on the activities of both tonoplast proton pumps and chloride channel (CLC) proteins, and less NO₃ ^- is stored in vacuoles when the activities of these proteins are reduced. The ratio of the distribution of NO₃ ^- in the cytoplasm and vacuole affects the long-distance transport of NO₃ ^-, which is regulated by the proteins NPF7.3 and NPF7.2 that play opposite but complementary roles. NPF7.3 is responsible for loading NO₃ ^- from the root cytoplasm into the xylem, whereas NPF7.2 regulates the unloading of NO₃ ^- from the xylem, thereby facilitating the long-distance transport of NO₃ ^- through the roots to the shoots. Vegetables, valued for their nutrient content, are consumed in large quantities; however, a high content of NO₃ ^- can detrimentally affect the quality of these plants. NO₃ ^- that is not assimilated and utilized in plant tissues is converted via enzyme-catalyzed reactions to nitrite (NO₂ ^-), which is toxic to plants and harmful to human health. In this review, we describe the mechanisms underlying NO₃ ^- distribution and transport in plants, a knowledge of which will contribute to breeding leafy vegetables with lower NO₃ ^- contents and thus be of considerable significance from the perspectives of environmental protection and food safety.

Keywords: NPF7.2; NPF7.3; chloride channel protein; health; nitrate; proton pumps; vegetable.