Physiological and Molecular Responses of Nitrogen-starved Rice Plants to Re-supply of Different Nitrogen Sources

Rice grown under anaerobic waterlogged soil condition takes up ammonium as a major source of nitrogen (N). A substantial quantity of nitrate is taken up by rice when aeration conditions prevail. This work compared the physiological and molecular responses of N-depleted rice (Oryza sativa L. japonica ssp. cv. Wuyunjing) to re-supply of different N sources in a hydroponic system. Expression of six genes known to be related to N uptake and assimilation in roots was analyzed by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR). N starvation increased root growth but decreased shoot biomass and nitrate reductase (NR) activity. In comparison to nitrate as N source, ammonium tended to inhibit root growth but significantly improved shoot growth, total N uptake, and, particularly, the translocation of N to the above ground parts. Replacing half of the ammonium by nitrate in the single ammonium solution increased glutamine synthetase (GS) activity and total N uptake. This replacement enhanced expression of OsAMT1;3 but decreased the expression of OsAMT1;2 in the roots. Likewise, replacing half of the nitrate by ammonium in the single nitrate solution, increased NR activity and expression of an NR gene (OsNR1) but suppressed expression of OsNRT2 in the roots. Expression of both OsAMT1;1 and a GS gene (OsGS1;1) was relatively constant and not affected by the N form or starvation in the rice roots. Predominant beneficial effects of dual supply of the two forms of N in comparison with a single form of N either as ammonium or as nitrate might be due to improvement of ammonium uptake due to nitrate presence near the roots and enhanced reduction of nitrate in leaves by ammonium.