Formation of unidentified nitrogen in plants: an implication for a novel nitrogen metabolism

Abstract

Plants take up inorganic nitrogen and store it unchanged or convert it to organic forms. The nitrogen in such organic compounds is stoichiometrically recoverable by the Kjeldahl method. The sum of inorganic nitrogen and Kjeldahl nitrogen has long been known to equal the total nitrogen in plants. However, in our attempt to study the mechanism of nitrogen dioxide (NO(2)) metabolism, we unexpectedly discovered that about one-third of the total nitrogen derived from (15)N-labeled NO(2) taken up by Arabidopsis thaliana (L.) Heynh. plants was converted to neither inorganic nor Kjeldahl nitrogen, but instead to an as yet unknown nitrogen compound(s). We here refer to this nitrogen as unidentified nitrogen ( UN). The generality of the formation of UN across species, nitrogen sources and cultivation environments for plants has been shown as follows. Firstly, all of the other 11 plant species studied were found to form the UN in response to fumigation with (15)NO(2). Secondly, tobacco ( Nicotiana tabacum L.) plants fed with (15)N-nitrate appeared to form the UN. And lastly, the leaves of naturally fed vegetables, grass and roadside trees were found to possess the UN. In addition, the UN appeared to comprise a substantial proportion of total nitrogen in these plant species. Collectively, all of our present findings imply that there is a novel nitrogen mechanism for the formation of UN in plants. Based on the analyses of the exhaust gas and residue fractions of the Kjeldahl digestion of a plant sample containing the UN, probable candidates for compounds that bear the UN were deduced to be those containing the heat-labile nitrogen-oxygen functions and those recalcitrant to Kjeldahl digestion, including organic nitro and nitroso compounds. We propose UN-bearing compounds may provide a chemical basis for the mechanism of the reactive nitrogen species (RNS), and thus that cross-talk may occur between UN and RNS metabolisms in plants. A mechanism for the formation of UN-bearing compounds, in which RNS are involved as intermediates, is proposed. The important broad impact of this novel nitrogen metabolism, not only on the general physiology of plants, but also on plant substances as human and animal food, and on plants as an integral part of the global environment, is discussed.

Nitrogen Dioxide at an Ambient Level Improves the Capability of Kenaf (Hibiscus Cannabinus) to Decontaminate Cadmium

“…The seedlings of both groups were grown for 10 d more and irrigated with 50 ml of 0.1% Hyponex containing 20 µM CdCl 2 every 4 d. Plants were then harvested, washed with pure water, and separated into leaves, stems, and roots. Harvested organs were separately lyophilized, powdered, and stored in a desiccator until use (Morikawa et al, 2004;Takahashi et al, 2001).…”

Burning effects on the distribution of organic N compounds in a 15N labelled forest soil

“…1A) was mainly explained by the important increase of the hydrolyzable unidentified N (HU-N, 66.3%), which was likely due to the recent incorporation of plant phytomass because: (1) this fraction comprises a substantial proportion of total N in many plant species (Morikawa et al, 2004), and (2) most of this unidentified N from plant materials is extracted with H1 (González- Prieto et al, 1995a, b). The augment in HU-N, and to a lesser extent that in amino acid N (AA-N, 11.2%), counterbalanced the significant reduction observed in the amide (AM-N, 33.2%) and the amino sugar fractions (AS-N, 68.0%) (Fig.…”

Light-triggered selective nitration of PsbO1 in isolated Arabidopsis thylakoid membranes is inhibited by photosynthetic electron transport inhibitors

“…27 We also reported that some NO 2 -derived nitrogen is converted into oxidized nitrogen compounds, such as nitro and nitroso compounds. 28,29 Therefore, nitrated proteins might play a role in the cellular signaling underlying NO 2 -regulated plant growth.…”