Efficient NH₄⁺ uptake system could be an important complementary strategy for maize plants to absorb enough nitrogen

ObjectivesMaize is a major grain crop grown in dry land where nitrate is the dominant nitrogen (N) form available to the roots due to the strong nitrification. Therefore, efficient uptake and utilization of nitrate were thought to be critically important to the growth and the grain yield of maize. The present work characterizes in particular, the efficient ammonium uptake in maize roots that provides a comparative support to the growth of maize plants. The results seemingly present additional understanding on the strategies of efficient nitrogen nutrition in maize.

MethodsSeedlings of the maize cultivar Zhengdan 958 were subjected to hydroponic culture experiments with same strength of ammonium or nitrate as sole nitrogen source to evaluate the growth efficiencies of maize seedlings supported by either form of the nitrogen nutrition. The characteristics on NH₄⁺ uptake by maize roots was particularly determined using the Scanning Ion-selective Electrode Technique (SIET), a recently developed Non-invasive Mirco-Test techniques (NMT). The expression of root specific genes coding for ammonium transporter (AMT) proteins was analyzed by quantitative RT-PCR.

ResultsAmmonium, when supplied as a sole nitrogen source, was capable of supporting the seedling growth to be comparative to those supplied by nitrate, though the maize plants were thought to be a ‘nitrate-preferring’ plant. The NMT measurements indicated that ammonium uptake by maize roots showed a typical high-affinity (the apparent Km was 60 μmol/L), coinciding a process potentially mediated by the specific ammonium transporter (AMT) proteins. A N-starvation treatment resulted in 3- and 1-fold reduction of the Vmax and Km values for root ammonium uptake, respectively, as compared to the roots grown under sustained NH₄⁺ supplement. This phenomenon was different from the findings recorded in plant species such as rice, and might imply an absence of the substrate-mediated feedback inhibition during NH₄⁺ uptake in the maize plants. Additionally, the NH₄⁺ uptake rates were significantly reduced (> 20% inhibition) with the presence of NO₃^– in the measuring solution. In the micro-molar concentration range, the uptake rate of NH₄⁺ was significantly greater than that treated by same strength of NO₃^–. Real-time quantitative PCR analyses revealed that the transcription abundance of two root-specific AMT genes ZmAMT1;1a and ZmAMT1;3 was significantly induced in the presence of ammonium, partly supporting the elevated NH₄⁺ uptake capacity determined in the roots grown under sustained supply of NH₄⁺.

ConclusionsBoth NH₄⁺ and NO₃^– could be absorbed efficiently when maize is supplied with pure each of them. High NO₃^– supply will inhibit the absorption of NH₄⁺ when maize is supplied with both N sources. Our results suggest a possibility that efficient NH₄⁺ uptake system could be an important complementary strategy for maize plants to absorb enough N at low NO₃^– supply condition.