Nitrogen is a major limiting factor for crop productivity. For most plants, nitrate is their primary nitrogen source. Nitrate taken into the plant can be assimilated right away in the root tissue, stored in the vacuole for future use, or transported to the leaf tissue and assimilated there. Characterization of nitrate transporter genes in the NRT1 (PTR) family revealed novel molecular and regulatory mechanisms for several critical steps of nitrate transport including uptake, storage, xylem loading and unloading, and remobilization.
Nitrate also serves as a signaling molecule regulating plant growth. Dual-affinity nitrate transporter CHL1 is switched between high- and low-affinity modes by phosphorylation and dephosphorylation at threonine residue 101. In addition to being a transporter, CHL1 also functions as a nitrate sensor. Using dual-affinity binding and phosphorylation switch, CHL1 can sense wide range of nitrate concentration changes in the soil, and induce different levels of transcriptional responses. Moreover, dynamic interactions between CHL1, kinases and protein phosphotase render nitrate to elicit temporal changes of the responses. This mechanism becomes a paradigm for how other nutrients are sensed by transporter.
N fertilizer production consumes 1% of global energy. Nevertheless, only 30-50% of N fertilizer applied is utilized by crops, and the reminder leads to production of green house gas and eutrophication of river and ocean. It is an urgent issue to enhance crop NUE. Several NRT1 genes, when mutated, showed nitrate-dependent growth defects, and therefore provide new tools of engineering crops to enhance NUE. In addition, NUE of different ecotypes of Arabidopsis will be compared and novel genes responsible for NUE will be accessed by genome-wide association mapping.
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