Nitrite Reduces Cytoplasmic Acidosis under Anoxia

Abstract

The ameliorating effect of nitrate on the acidification of the cytoplasm during short-term anoxia was investigated in maize (Zea mays) root segments. Seedlings were grown in the presence or absence of nitrate, and changes in the cytoplasmic and vacuolar pH in response to the imposition of anoxia were measured by in vivo 31 P nuclear magnetic resonance spectroscopy. Soluble ions and metabolites released to the suspending medium by the anoxic root segments were measured by high-performance liquid chromatography and 1 H nuclear magnetic resonance spectroscopy, and volatile metabolites were measured by gas chromatography and gas chromatography-mass spectrometry. The beneficial effect of nitrate on cytoplasmic pH regulation under anoxia occurred despite limited metabolism of nitrate under anoxia, and modest effects on the ions and metabolites, including fermentation end products, released from the anoxic root segments. Interestingly, exposing roots grown and treated in the absence of nitrate to micromolar levels of nitrite during anoxia had a beneficial effect on the cytoplasmic pH that was comparable to the effect observed for roots grown and treated in the presence of nitrate. It is argued that nitrate itself is not directly responsible for improved pH regulation under anoxia, contrary to the usual assumption, and that nitrite rather than nitrate should be the focus for further work on the beneficial effect of nitrate on flooding tolerance.

Nitrogen metabolism in plants under low oxygen stress

“…This finding strengthens the hypothesis that nitrite rather than nitrate had a positive effect in hypoxic tissue (Horchani et al 2011). Since nitrite was effective at very low concentration, its role was suggested not to be restricted to the regeneration of NAD(P) + , but rather linked to a regulatory mechanism related to NO emission (Talwar et al 2012;Ma et al 2012;Zhang et al 2007;Libourel et al 2006).…”

“…However, nitrite is not only a byproduct of hypoxic N metabolism in the root, since it is thought that nitrite rather than nitrate per se is responsible of the beneficial effects of nitrate. Nitrite concentration in micromolar range was as effective as nitrate on the adjustment of cytosolic pH in maize roots exposed to anoxic conditions (Libourel et al 2006). In Medicago truncatula RNAi transgenic lines impaired for NR1 and NR2 gene expression or in wild-type plants treated with tungstate, the reduction of hypoxia-induced NO production in the nodules was partly reversed by the addition of nitrite.…”

“…These symptoms are characterized by an increase in the production of both ethanol and lactate accompanied by an accumulation of soluble sugar, an acidification of the cytosol, by plant epinasty chlorosis and a reduction in plant growth (Horchani et al 2010;Stoimenova et al 2003). Conversely, it was observed that maize seedlings supplied with nitrate during anoxia maintained a slightly higher cytosolic pH (Libourel et al 2006).…”

Nitrite Acts as a Transcriptome Signal at Micromolar Concentrations in Arabidopsis Roots

“…Normally, nitrite is restricted to cells in which it is made and is present at low micromolar levels compared to high (millimolar) nitrate levels; however, when roots experience hypoxia or anoxia, nitrite levels increase to levels where nitrite is secreted from roots (Botrel et al, 1996;Allegre et al, 2004;Libourel et al, 2006). It has been proposed that nitrite helps protect plants from anoxia by reducing cytoplasmic acidosis (Libourel et al, 2006) and supporting anaerobic metabolism (Stoimenova et al, 2003(Stoimenova et al, , 2007Igamberdiev et al, 2005). From our bioinformatics analysis and from inspection of the gene lists, we found no evidence for nitrite-specific effects on processes involved in anaerobic metabolism (having found only ion transport and general metabolism in the BioMaps analysis), but it is worth further study to test the idea that nitrite may regulate anoxic responses.…”

“…Such plants also produce high levels of nitric oxide (NO; Morot- Gaudry-Talarmain et al, 2002) because nitrite serves as a substrate for the formation of NO (Yamasaki et al, 1999;Rockel et al, 2002;Lea et al, 2004;Meyer et al, 2005;Planchet et al, 2005). It should be noted that nitrite can have a beneficial effect because it protects maize (Zea mays) roots from anoxia by reducing cytoplasmic acidosis (Libourel et al, 2006).…”