Nitrate confers rice adaptation to high ammonium by suppressing its uptake but promoting its assimilation

Nitrogen (N) is the most important macronutrient driving plant growth and development. For higher plants, inorganic N including nitrate (NO3−) and ammonium (NH4+) are predominant N sources ( Hu et al., 2023 Hu B. Wang W. Chen J. Liu Y. Chu C. Genetic improvement toward nitrogen-use efficiency in rice: Lessons and perspectives. Mol. Plant. 2023; 16: 64-74 Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar ). Nitrate needs to be firstly reduced into ammonium to implement its assimilation, thus requiring a higher energy consumption than ammonium, making ammonium more cost effective for plants. However, ammonium usually causes severe growth retardation of plants under high concentration, which is known as ammonium toxicity. The concentrations of nitrate and ammonium greatly vary in different soil environments. Nitrate is the major inorganic N form in dry land, while ammonium accounts for thehighest proportion of inorganic N in the paddy field, where nitrification is suppressed ( Haynes and Goh, 1978 Haynes R.J. Goh K.M. Ammonium and nitrate nutrition of plants. Biol. Rev. 1978; 53: 465-510 Crossref Google Scholar ). Although nitrogen is generally one of the most important contributing factors for yield increase, irrational fertilization strategies can cause negative effects. Especially, in the paddy field, when nitrogen fertilizer (in the form of urea) is applied in an excessive amount at one time, the local soil ammonium concentration can reach a toxic level for most plant species, i.e., 2–20 mM ( Britto and Kronzucker, 2002 Britto D.T. Kronzucker H.J. NH4+ toxicity in higher plants: a critical review. J. Plant Physiol. 2002; 159: 567-584 Crossref Scopus (1393) Google Scholar ; Savin et al., 2021 Savin M.C. Daigh A.L.M. Brye K.R. Norman R. Miller D. Vertical distribution of fertilizer nitrogen from surface water flooding of a silt loam and clay soil used for rice production. Soil Use Manage. 2021; 37: 406-417 Crossref Scopus (6) Google Scholar ). However, as a paddy-field-grown crop, rice can well adapt to the high-ammonium condition, thus being viewed as a typical "ammonium-tolerant" plant ( Britto and Kronzucker, 2002 Britto D.T. Kronzucker H.J. NH4+ toxicity in higher plants: a critical review. J. Plant Physiol. 2002; 159: 567-584 Crossref Scopus (1393) Google Scholar ). This feature ensures optimal growth and grain yield improvement of rice under increasing N fertilizer input, which constitutes one of the most important prerequisites of the "Green Revolution" for wide application of N fertilizers. Although several hypotheses on ammonium toxicity adaptation have been proposed in different plants ( Li et al., 2014 Li B. Li G. Kronzucker H.J. Baluška F. Shi W. Ammonium stress in Arabidopsis: signaling, genetic loci, and physiological targets. Trends Plant Sci. 2014; 19: 107-114 Abstract Full Text Full Text PDF PubMed Scopus (191) Google Scholar ), the molecular mechanism underlying the ammonium tolerance of rice is still elusive.