In Vivo Assay of Foliar Nitrate Reductase (NR) Activity & Substrate Inducibility
Nitrate Reductase (NR; EC 1.7.1.1) is the first and rate-limiting enzyme in the plant inorganic nitrogen assimilation pathway. Located in the cytosol, NR is a homodimeric enzyme containing three internal prosthetic electron carriers: FAD, heme ($b_{557}$), and a molybdenum-pterin (Moco) cofactor. It catalyzes the two-electron reduction of nitrate ($text{NO}_3^-$) to nitrite ($text{NO}_2^-$) utilizing cytosolic $text{NADH}$ as the electron donor:
$$text{NO}_3^- + text{NADH} + text{H}^+ xrightarrow{text{Nitrate Reductase (NR)}} text{NO}_2^- + text{NAD}^+ + text{H}_2text{O}$$
In living illuminated leaf tissue, nitrite is rapidly imported into chloroplasts and reduced to ammonium ($text{NH}_4^+$) by light-driven Nitrite Reductase (NiR) utilizing reduced ferredoxin ($Fd_{text{red}}$):
$$text{NO}_2^- + 6 Fd_{text{red}} + 8 text{H}^+ xrightarrow{text{NiR (Chloroplast)}} text{NH}_4^+ + 6 Fd_{text{ox}} + 2 text{H}_2text{O}$$
The In Vivo Assay Strategy: To measure NR activity quantitatively without purifying the labile enzyme:
1. Leaf discs are vacuum-infiltrated with an assay buffer containing $0.1text{ M KNO}_3$ (substrate) and $5% (v/v)text{ 1-propanol}$ (surfactant that transiently permeabilizes cellular membranes, allowing endogenous $text{NADH}$ and exogenous nitrate to reach cytosolic NR while facilitating leakage of produced $text{NO}_2^-$ into the surrounding medium).
2. The incubation is conducted in complete darkness and anaerobic conditions. Depriving chloroplasts of photosynthetic light completely shuts down nitrite reductase (NiR, which requires light-generated ferredoxin), preventing any consumption of nitrite. Consequently, all nitrite produced by NR accumulates quantitatively in the extracellular buffer.
Griess Diazotization Colorimetry: Nitrite ($text{NO}_2^-$) is quantified by the classical two-step Griess reaction:
1. Under acidic conditions ($1.0text{ M HCl}$), nitrite diazotizes sulfanilamide to form a reactive diazonium salt.
2. The diazonium intermediate couples with N-(1-naphthyl)ethylenediamine dihydrochloride (NEDD) to form an intensely pink water-soluble azo dye with a sharp absorption maximum at $lambda = 540text{ nm}$ ($epsilon_{540} = 40,000text{ M}^{-1}text{cm}^{-1}$).
- Plant Material: Fresh primary leaves from 14-day-old maize (*Zea mays*) seedlings, pre-treated with $+10text{ mM KNO}_3$ (Induced) vs $-N$ nutrient solution (Uninduced).
- In Vivo Incubation Buffer: $0.1text{ M Potassium Phosphate}$ ($text{pH } 7.5$), $0.1text{ M KNO}_3$, $5% (v/v)text{ 1-Propanol}$.
- Griess Reagent A: $1.0% (w/v)$ Sulfanilamide dissolved in $3.0text{ M HCl}$.
- Griess Reagent B: $0.02% (w/v)$ N-(1-naphthyl)ethylenediamine dihydrochloride (NEDD) in deionized water (stored in dark amber bottle at $4^circtext{C}$).
- Nitrite Standard Stock: $1.0text{ mM Sodium Nitrite}$ ($text{NaNO}_2$, $69.0text{ mg/L}$), diluted to working standards: $0$, $5$, $10$, $15$, $20$, $25 mutext{M}$.
- Equipment: Vacuum desiccator with vacuum pump, shaking water bath ($30^circtext{C}$), foil-wrapped culture tubes, spectrophotometer ($540text{ nm}$).
Punch 20 uniform leaf discs ($5.0text{ mm}$ diameter) from healthy maize blades using a paper punch. Weigh exactly $0.200text{ g}$ of leaf discs and place into a foil-wrapped tube containing $5.0text{ mL}$ of incubation buffer. Place tubes without caps in a vacuum desiccator and apply vacuum ($70text{ kPa}$) for 3 minutes to displace intercellular air. Release vacuum suddenly; discs should sink to the bottom, confirming complete infiltration.
Cap tubes tightly, wrap completely in two layers of aluminum foil to exclude all photons, and incubate in a dark shaking water bath at $30.0^circtext{C}$ ($60text{ rpm}$) for precisely 60 minutes. Prepare an identical uninduced control tube (leaves grown without nitrate) and a blank tube (incubation buffer without leaf tissue).
Terminate incubation after 60 min. Pipette $1.0text{ mL}$ of the incubation medium into an assay tube. Add $1.0text{ mL}$ of Griess Reagent A ($1%$ Sulfanilamide) and mix. Incubate at room temperature for 5 minutes. Add $1.0text{ mL}$ of Griess Reagent B ($0.02%$ NEDD) and mix thoroughly. A vibrant magenta/pink color develops immediately. Allow 20 minutes for color stabilization.
Measure absorbance at $540text{ nm}$ against the reagent blank. Simultaneously prepare a $text{NaNO}_2$ standard curve ($0–25 mutext{M}$ nitrite). Interpolate nitrite concentration ($mutext{M}$) and calculate specific foliar enzyme activity in $mutext{mol NO}_2^-cdottext{g}^{-1}text{ FW}cdottext{h}^{-1}$.
| Experimental Treatment | A₅₄₀ (OD) | Assay Medium [NO₂−] (μM) | Total NO₂− in 5 mL (μmol) | Foliar FW (g) | NR Activity (μmol/g FW/h) |
|---|---|---|---|---|---|
| Blank (Buffer Alone) | 0.000 | 0.00 μM | 0.000 μmol | — | 0.00 |
| Uninduced (-N Maize) | 0.065 | 2.50 μM | 0.0125 μmol | 0.200 g | 0.625 μmol · g−¹ · h−¹ |
| +KNO₃ Induced Maize | 0.505 | 19.42 μM | 0.0971 μmol | 0.200 g | 4.855 μmol · g−¹ · h−¹ |
Enzyme Activity Calculation:
Nitrite standard calibration equation: $A_{540} = 0.0260 cdot [text{NO}_2^- (mutext{M})] quad (R^2 = 0.9995)$.
For induced sample: $[text{NO}_2^-] = frac{0.505}{0.0260} = mathbf{19.423 mutext{M}} = 19.423 mutext{mol/L}$.
Total nitrite released in $5.0text{ mL}$ ($0.005text{ L}$) incubation buffer:
$$text{Total }text{NO}_2^- = 19.423 mutext{mol/L} times 0.005text{ L} = mathbf{0.0971 mutext{mol}}$$
Specific in vivo NR activity:
$$text{Activity} = frac{0.0971 mutext{mol}}{0.200text{ g FW} times 1.0text{ h}} = mathbf{4.855 mutext{mol NO}_2^-cdottext{g}^{-1}text{ FW}cdottext{h}^{-1}}$$
Nitrogen Induction Factor:
$$text{Induction Ratio} = frac{4.855}{0.625} = mathbf{7.77text{-fold Induction}}$$
Biological Conclusion: Nitrate reductase demonstrates classic substrate-inducible gene expression and post-translational activation in response to soil nitrate availability.
