Nitrogen is a key
mineral nutrient playing a crucial role in plant growth and development. Understanding the mechanisms of
nitrate uptake from the soil and distribution through the plant in response to
nitrogen starvation is an important step on the way to improve
nitrogen uptake and utilization efficiency for better growth and productivity of plants, and to prevent negative effects of
nitrogen fertilizers on the environment and human health. In this study, we show that Arabidopsis
NITRATE TRANSPORTER 2.5 (NRT2.5) is a plasma membrane-localized high-affinity
nitrate transporter playing an essential role in adult plants under severe
nitrogen starvation. NRT2.5 expression is induced under
nitrogen starvation and NRT2.5 becomes the most abundant transcript amongst the seven NRT2 family members in shoots and roots of adult plants after long-term
starvation. GUS reporter analyses showed that NRT2.5 is expressed in the epidermis and the cortex of roots at the root hair zone and in minor veins of mature leaves. Reduction of NRT2.5 expression resulted in a decrease in high-affinity
nitrate uptake without impacting low-affinity uptake. In the background of the high-affinity
nitrate transporter mutant nrt2.4, an nrt2.5 mutation reduced
nitrate levels in the phloem of N-starved plants further than in the single nrt2.4 mutants. Growth analyses of multiple mutants between NRT2.1, NRT2.2, NRT2.4, and NRT2.5 revealed that NRT2.5 is required to support growth of
nitrogen-starved adult plants by ensuring the efficient uptake of
nitrate collectively with NRT2.1, NRT2.2 and NRT2.4 and by taking part in
nitrate loading into the phloem during
nitrate remobilization.