Nitrate redistribution via phloem sap circulation
Nitrate redistribution via phloem circulation has been recently suggested to occur in three physiological contexts (Fig. 1): remobilization from old leaves, development of reproductive structures and nitrate backflow from shoots to roots. In Arabidopsis , the nitrate transporter NPF5.5 is involved in controlling embryo N content: in knock-out mutants, embryos at the bent cotyledon stage have a 7-8% reduction in the N content suggesting that the import of nitrate to the developing seed has been impacted (Léran et al. , 2015). There is presently no direct evidence that NPF5.5 only participates in the import of nitrate into seeds but its implication in nitrate transfer from the phloem to developing tissues is likely. NPF2.12/NRT1.6 appear to be expressed intensively in funiculus vascular bundles, which are mostly made of phloem tissue in Arabidopsis (Almagro et al. , 2008), and mutants affected in this transporter have altered isotopic (13N) nitrogen transfer to reproductive structures (Babst et al. , 2019). Three nitrate transporters have been shown to be involved in nitrate transfer and/or redistribution to the phloem from xylem or source nitrate-containing tissues: NPF2.13/NRT1.7, NPF1.1/NRT1.12, and NRT1.2/NRT1.11 (reviewed in (Iqbal et al. , 2020)). In particular, it has been recently shown that NRT1.7 is essential for remobilization of nitrate from old leaves to deliver nitrate into the phloem, making it available for new, developing leaves and enhancing nitrogen use efficiency (Chen et al. , 2020). Also, in mutants affected in NRT1.9 (transporter expressed in root phloem), 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 NRT1.9 plays a role in the backflow of nitrate to roots (Wang & Tsay, 2011). Double mutants affected in both NRT1.11 and NRT1.12 have a strong growth phenotype suggesting that xylem-to-phloem nitrate transfer and/or phloem-mediated nitrate is essential for plant growth (Hsu & Tsay, 2013). Taken as a whole, recent data on nitrate transporters clearly show that phloem can carry nitrate upwards (developing sink organs in shoots) and downwards (back to roots).