High-affinity ammonium transport in rice (Oryza sativa L.): Physiology, biochemistry and molecular biology

In plants, NH₄+ influx occurs via a saturable high-affinity, and a low-affinity transport system. Regulation of 13NH₄+ influx was studied by altering NH ₄+ supply and pretreatments with amino acids. Diurnal variations in NH₄+ influx were also studied. Increased N pools caused NH₄+ influx to decrease, whereas reduced N pools increased NH₄+ influx. When rice plants were transferred from 10 μM to 10 mM NH₄ +, HATS influx of 13NH₄+ was rapidly down-regulated to <0% within 72 h. Conversely, 13NH ₄+ influx exhibited a steep increase following transfer from 10 mM to 10 μM NH₄+. Upon pretreatments with amino acids, NH₄+ influx was reduced. There was no major decline in NH₄+ influx when NH ₄+ assimilation was blocked by MSX treatment; however when either Gln or Asn was added together with MSX, NH₄ + influx declined rapidly. Root concentrations of NH₄ + and all 4 amino acids increased substantially during down-regulation of NH₄+ influx and were reduced during the up-regulation. Pretreatments with Gln, Asn, Glu, or Asp led to increased root concentrations of these amino acids as well as that of NH₄+. When treated with MSX, root [NH₄+] increased several fold, while root Gln] declined accompanied with increased root [Asn], [Glu], and [Asp].; Transcript levels of OsAMT1.1 in roots decreased several fold within 48 hours of transfer from 10 μM to 10 mM NH₄ +. Likewise when plants acclimated in 10 mm NH₄ + were transferred to 10μM NH₄+, there was an equally rapid up-regulation of OsAMT1.1 and 13NH₄+ influx in the roots. Changes in transcript abundance following these treatments were in order OsAMT1.1 > OsAMT1.2 > OsAMT1.3. Amino acid pretreatments also reduced OsAMT1 transcripts. MSX pretreatment slightly reduced OsAMT1.1 transcripts in 12 h. However, additions of Gln or Asn with MSX caused a much greater reduction.; OsAMT1.3 expression and 15NH₄ + influx increased approximately 3-fold late in the photoperiod, while OsAMT1.1 and OsAMT1.2 exhibited only modest changes. Sucrose application during the dark period increased root sucrose concentration and OsAMT1.3 expression.; Growth analysis and NH₄+ influx of transgenic lines, over-expressing OSAMT1.1 in two cultivars were also undertaken. One over-expression line, 75-4, showed increased influx.