Plant Science

Plants use nitrate, nitrite, and ammonium as inorganic nitrogen (N) sources. These N compounds are included in plant tissues at various concentrations depending on the balance between their uptake and assimilation. Thus, the contents of nitrate, nitrite, and ammonium are physiological indicators of plant N economy. Here, we describe a protocol for measurement of these inorganic N species in A. thaliana shoots or roots.

Glutamate dehydrogenase (GDH) is an NAD(H) dependent enzyme that catalyzes, in vitro, the reversible amination of glutamate. Here we describe how to determine spectrophotometrically GDH activity monitoring NADH evolution. This protocol is described here for Arabidopsis thaliana (A. thaliana) although it is also valid for other plant species. GDH protein is a hexamer composed, in the case of Arabidopsis, of a combination of GDHα, GDHβ and GDHγ subunits. Every combination of subunits is possible; however, it is still barely known whether different combinations affect the enzymatic properties of the hexamers. In other species, hexamers are a combination of GDHα and GDHβ but it cannot be discarded the existence of other genes since for instance GDHγ subunit in Arabidopsis was described in Fontaine et al. (2012).

Glutamate + NAD⁺ + H⁺ → 2-Oxoglutarate + NADH + NH4⁺

This protocol is a simple colorimetric assay for internal ammonium quantification in aqueous extracts from plant tissues. The method is based on the phenol hypochlorite assay (Berthelot reaction):

NH₄⁺ + hypochlorite + OH^- + phenol → indophenol

The oxidation of indophenol caused by phenol oxidation is a blue dye that is quantified at 635 nm in a spectrophotometer. Per ammonium molecule one molecule of indophenol is formed. The protocol described here is for Arabidopsis thaliana (A. thaliana) leaves and roots, although it is also valid for other plants species.

Legumes play a vital role in global food supply because they are uniquely capable of fixing atmospheric nitrogen (N) through symbioses with root and stem nodule bacteria, collectively called the rhizobia. These commonly include bacteria in the genera Rhizobium, Mesorhizobium, Sinorhizobium (Ensifer), and Bradyrhizobium, although other genera of bacteria have now been shown to form root nodule symbioses with several legume species (Weir, 2012). The symbiotic interaction is important for agricultural productivity, especially in less developed countries where nitrogen fertilizer is expensive. However, nodulation ability and competitiveness have practical importance in agricultural production, because the inoculation of efficient rhizobia is often unsuccessful, due to large part to the presence of competitive populations of ineffective indigenous rhizobia in soils (Toro, 1996; Triplett and Sadowsky, 1992). This protocol allows one us to quantitatively evaluate the relative nodulation competitiveness of Sinorhizobium strains.

Despite its extensive use as a nitrogen fertilizer, the role of urea as a directly accessible nitrogen source for crop plants is still poorly understood. So far, the physiological and molecular aspects of urea acquisition have been investigated only in a few plant species highlighting the importance of a urea transporter in roots, DUR3 (Kojima et al., 2007; Wang et al., 2012; Zanin et al., 2014a). Regarding maize plants, a crop that needs a large amount of urea fertilizer, the capability to take up urea via an inducible and high-affinity transport system has been recently characterized (Zanin et al., 2014a; Zanin et al., 2014b). Here, we described a small-scale protocol suitable for the measurement of urea net high-affinity uptake in roots of intact maize plants.

Nitrogen-15 is a rare stable isotope of nitrogen. This isotope is often used in agricultural research. For example, Nitrogen-15 is used to trace mineral nitrogen compounds and translocate the nitrogen molecule in plants. This protocol is used to determine nitrate uptake and accumulation in rice seedlings by using Nitrogen-15.