Molecular mechanisms underlying ammonium uptake and use in rice

Nitrogen is a major limiting nutrient in plant growth and productivity. Plants have evolved sophisticated mechanisms for efficient nitrogen acquisition and use, allowing them to cope with fluctuations in availability. In flooded paddy soils, wetland rice, an important mainstay crop, preferentially takes up ammonium as a nitrogen source. However, an ammonium supply at high concentration is toxic to many plants. Thus, rice has acquired mechanisms for highly efficient uptake and use of ammonium. External ammonium is absorbed by epidermal and exodermal cells at the root surface via ammonium transporters AMT1;1 and AMT1;2. Most of the absorbed ammonium is assimilated into glutamine via glutamine synthetase 1;2 (GS1;2) and NADH-dependent glutamate synthase 1 (NADH-GOGAT1). Glutamine is then transported radially inward from the surface cells to central vascular tissues and subsequently allocated to the shoot through the xylem, thus fulfilling the growth requirements of above-ground organs. GS1 and NADH-GOGAT are also important for nitrogen recycling from senescent organs via phloem to developing organs. I am trying to unravel these complex mechanisms including signal transduction in order to improve nitrogen use efficiency and growth of rice for achieving more productive and sustainable agriculture towards SDGs (Target 2.4).

• Molecular mechanism to prevent excessive uptake of an ammonium nutrient in rice

  Root AMT1s are inactivated via phosphorylation of the conserved threonine residue at the carboxyl-tail under elevated external ammonium. We identified the role of a protein kinase ACTPK1 in phosphorylation and inactivation of ammonium-induced AMT1;2 in ammonium-preferring rice grown under sufficient ammonium. Plant J. 93: 992 (2018). doi: 10.1111/tpj.13824

• Identification of a vacuolar glutamine efflux transporter in rice

  Little is known about the mechanism of glutamine storage in roots. We revealed the role of amino acid transporter ATL6 in exporting ammonium-derived glutamine from the cytosol into the vacuole for temporary vacuolar storage of excess glutamine, thereby influencing the root-to-shoot allocation of glutamine and the subsequent growth of rice seedlings under ammonium replete conditions. Plant J. 107: 1616 (2021). doi: 10.1111/tpj.15403