Guohua Xu
State Key Laboratory of Crop Genetics and Germplasm Enhancement,
College of Resource and Environmental Sciences, Nanjing Agricultural University

Crop productivity relies heavily on nitrogen (N) fertilization. Production and application of N fertilizers consume huge amounts of energy, and excess is detrimental to the environment; therefore, increasing plant N use efficiency (NUE) is essential for the development of sustainable agriculture.
Plants take up both nitrate and ammonium as main nitrogen (N) sources. Although ammonium is the predominant form in anaerobic flooded paddy soil, rice and other wetland plants may take up significant amounts of nitrate formed by nitrification of ammonium in the rhizosphere. We proposed that high affinity nitrate transport system (HATS) contributed mainly by OsNAR2/OsNRT2 two components plays important role in rice N nutrition. We detected that OsNAR2.1 expressed mainly in the root epidermal cells and interacted with OsNRT2.1 and OsNRT2.2 for nitrate transport at relative higher affinity level and with OsNRT2.3a at relative lower affinity level. In contrast, OsNRT2.3b could function independently in nitrate transport mainly in leaf phloem at relative lower affinity level. OsNRT2.3a was located in the plasma membrane and mainly expressed in xylem parenchyma cells of the stele of nitrate-supplied roots. Knockdown of OsNRT2.3a suppressed long-distance translocation of nitrate from roots to shoots, but not root influx.
We found that root nitrate pools were not the indicators of N status in rice. It is interesting that alteration of the NAR2/NRT2 two component system could enhance nitrate influx, transport to shoot, xylem pH, phloem pH homeostasis, which further increased root ammonium uptake and accumulation of P and Fe in leaves. We showed that enhancing the ability to utilize nitrate for balancing ammonium nutrition in paddy field could increase yield and N use efficiency in rice.