Nitrate backflow from shoots to roots
Two nitrate transporters are related to nitrate flow from shoots to roots: NPF2.9/NRT1.9, and NPF1.1/NRT1.12. NPF2.9/NRT1.9 is expressed in root phloem. In NPF2.9/NRT1.9 mutants, the nitrate content in root phloem exudates is lower (by 20 to 30%) (Wang & Tsay, 2011); furthermore, downwards nitrate transport to root tips, when assayed with a compartmentalized root system, is reduced, suggesting that NPF2.9/NRT1.9 effectively plays a role in the backflow of nitrate to roots (Wang & Tsay, 2011). Unlike NPF1.2/NRT1.11, NPF1.1/NRT1.12 is expressed in roots (not only in shoots) and since it is involved in xylem-to-phloem nitrate transfer (Hsu & Tsay, 2013), it suggests that it can be involved in nitrate cycling to roots.
Taken as a whole, there is substantial evidence that some nitrate transporters allow nitrate transfer and circulation into the phloem either downwards (to roots) or upwards (to seeds and young leaves). In addition, although nitrate is not the major nitrogen form in the phloem, the strong growth retardation phenotype observed in theNPF1.2/NRT1.11 NPF1.1/NRT1.12 double mutant (Hsu & Tsay, 2013) suggests that phloem-mediated nitrate redistribution is essential.