Ammonium and nitrate ions are important nitrogen sources for plants.
In aerobic agricultural soil, because nitrate is the major nitrogen source,
many crops prefer to take up nitrate. On the other hand in anaerobic agricultural
soil such as paddy field, ammonium is the major form of available nitrogen,
and rice prefers the uptake of ammonium rather than nitrate. Ammonium
is taken up from the soil into plant cells by ammonium transporters. Ammonium
transporters have been characterized in several plant species, including
Arabidopsis thaliana (Gazzarrini et al. 1999) and Lycopersicon
esculentum (Lauter et al. 1996).
In this study, we have cloned and characterized ammonium transporter cDNAs
of rice (Oryza sativa L.), OsAMT1;1 and OsAMT1;2.
These clones were identified by searching the rice database of expressed
sequence tags for homologs of AtAMT1;1. The OsAMT1;1 clone
is identical to that reported previously (von Wiren et al. 1997).
The open reading frames of OsAMT1;1 and OsAMT1;2 encode
499- and 497-amino acid residues with predicted molecular masses of 52.6
and 52.2 kD, respectively. Hydropathy analysis of the deduced amino acid
sequence suggests that OsAMT1;1 and OsAMT1;2 contain 10-11 putative transmembrane
domains. Sequence comparison shows that OsAMT1;1 and OsAMT1;2 are closely
related to Arabidopsis and tomato ammonium transporters (67-63%,
Following nitrogen starvation, hydroponically grown 3-week-old rice seedlings
were transferred to nutrient solutions containing no nitrogen, 0.3 mM
ammonium or 0.3 mM nitrate as nitrogen sources. After 2 hr of treatment,
the roots and shoots were sampled, and mRNA accumulation of OsAMT1;1
and OsAMT1;2 was analyzed. OsAMT1;1 transcripts were detectable
in both roots and shoots irrespective of the nitrogen source (Fig. 2),
while negligible amount of OsAMT1;2 transcripts were detectable
only in the root. The expression was largely enhanced by the ammonium
but not by the nitrate treatment (Fig. 2).
To study ammonium-inducibility of OsAMT1;1 and OsAMT1;2
in roots, hydroponically grown rice was exposed to nitrogen deficiency
for two weeks, and then transferred to nutrient solution containing 0.3
mM ammonium as the nitrogen source. After 0, 15, 30, 60, 90 and 120 min
of the treatment, mRNA accumulation of OsAMT1;1 and OsAMT1;2
in roots was analyzed. The expression of OsAMT1;1 was observed
in all the treatments, while an increased level of OsAMT1;2 transcripts
was detectable at 60 min after the ammonium treatment (data not shown).
To study ammonium-dependent expression of OsAMT1;1 and OsAMT1;2
in roots, following nitrogen starvation, hydroponically grown rice was
exposed to 0.3 mM ammonium
as the nitrogen source for 2 hr, and transferred
to a nitrogen-deficient solution for 0.5, 1, 2 and 4 hr. The expression
of OsAMT1;1 was enhanced by the ammonium treatment. However, even
when transferred to the nitrogen-deficient solution, expression of OsAMT1;1
was constantly detectable (Fig. 3). OsAMT1;2 transcripts disappeared
in the nitrogendeficient condition, indicating that the expression of
OsAMT1;2 is ammonium-dependent.
Consequently, while OsAMT1;1 was constitutively expressed (Figs.
2 and 3), accumulation of OsAMT1;2 transcripts was roots-specific
and ammonium-inducible (Figs. 2 and 3). We complemented a yeast mutant
to determine whether the protein encoded by the cDNA is a functional ammonium
transporter. Both cDNAs were able to functionally complement the mutant
which is deficient in ammonium uptake, indicating that OsAMT1;1
and OsAMT1;2 encodes functional ammonium transporters (data not
shown). Thus, OsAMT1;1 and OsAMT1;2 may have different physiological
roles in ammonium uptake in rice. There results imply that OsAMT1;1
is involved in uptake and/or salvage of the ammonium in vascular tissues,
and OsAMT1;2 in the uptake in root epidermis. Further experiments
will be needed to evaluate this hypothesis.
Gazzarrini S., L. Lejay, A. Gohon, O. Ninnemann, W.B. Frommer and N. von
Wiren, 1999. Three functional transporters for constitutive, diurnally
regulated, and starvation-induced uptake of ammonium into Arabidopsis
roots. Plant Cell 11: 937-947.
Lauter F.R., O. Ninnemann, M. Bucher, J.W. Riesmeier and W.B. Frommer,
1996. Preferential expression of an ammonium transporter and of two putative
nitrate transporters in root hairs of tomato. Proc. Natl. Acad. Sci. USA
von Wiren N., A. Bergfeld, O, Ninnemann and W.B. Frommer, 1997. OsAMT1;1:
a high-affinity ammonium transporter from rice (Oryza sativa cv.
Nipponbare). Plant Mol. Biol. 3: 681.