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Abstract

In plants, NH₄⁺ influx occurs via a saturable high-affinity, and a low-affinity transport system. Regulation of ¹³NH₄⁺ influx was studied by altering NH₄⁺ 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 pM to 10 mM NH₄⁺ , HATS influx of ¹³NH₄⁺ was rapidly down-regulated to < 20% within 72 h. Conversely, ¹³NH₄⁺ 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 NH₄⁺ assimilation was blocked by MSX treatment; however when either Gin or Asn was added together with MSX, NH₄⁺ influx declined rapidly. Root concentrations of NH₄⁺ and all 4 amino acids increased substantially during downregulation of NH₄⁺ influx and were reduced during the up-regulation. Pretreatments with Gin, 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 [Gin] declined accompanied with increased root [Asn], [Glu], and [Asp]. Transcript levels of OsAMTl.l in roots decreased several fold within 48 hours of transfer from 10 pM to 10 mM NH₄⁺ . Likewise when plants acclimated in 10 mM NH₄⁺ were transferred to 10 uM NH₄⁺ , there was an equally rapid up-regulation of OsAMTl.l and ¹³NH₄⁺ influx in the roots. Changes in transcript abundance following these treatments were in order OsAMTl.l > OsAMTl.l > OsAMTl.3. Amino acid pretreatments also reduced OsAMTl transcripts. MSX pretreatment slightly reduced OsAMTl.l transcripts in 12 h. However, additions of Gin or Asn with MSX caused a much greater reduction. OsAMTl.3 expression and ¹⁵NH₄⁺ influx increased approximately 3-fold late in the photoperiod, while OsAMTl.l and OsAMTl.2 exhibited only modest changes. Sucrose application during the dark period increased root sucrose concentration and OsAMTl.3 expression. Growth analysis and NH/ influx of transgenic lines, over-expressing OsAMTl.l in two cultivars were also undertaken. One over-expression line, 75-4, showed increased influx.