Ammonium tolerance and the regulation of two cytosolic glutamine synthetases in the roots of sorghum

Abstract

Tolerance to ammonium nutrition in plants can be related to their ability to detoxify ammonium via nitrogen assimilation in roots. Here, we report that sorghum-sudangrass (Sorghum bicolor L. Â S. bicolor var. sudanense) hybrids exhibited enhanced biomass production under high levels of inorganic nitrogen supply as well as increased capacity for nitrogen assimilation in roots. Glutamine synthetase (GS, EC 6.3.1.2) activity and protein accumulated in roots at increasing concentrations of either nitrate or ammonium, with particularly high levels of GS in ammonium-treated plants. Ammonium but not nitrate differentially regulated two distinct cytosolic GS (GS1) isoforms composed by polypeptides of similar size but different charge. The comparative analysis of GS gene sequences and the deduced GS1 polypeptides suggested that the two GS1 isoforms were the expression products of SbGln1.2 and SbGln1.3 genes. SbGln1.3 expression was shown to be upregulated by high levels of inorganic nitrogen supply, with a maximal abundance of SbGln1.3 transcripts in ammonium-grown plants. SbGln1.2 expression was uniform along the root axis meanwhile protein and transcript levels for SbGln1.3 were particularly abundant in the upper part of the axis where lateral roots are prominent. Kinetic analysis revealed that the two GS1 isoenzymes have relatively low-affinity for ammonium ions. The spatial distribution of low-affinity GS1 isoenzymes would provide a sustained glutamine biosynthesis at high levels of ammonium supply and may represent at the same time an efficient system of ammonium detoxification. Such a mechanism may prevent transport of ammonium to leaves alleviating symptoms of toxicity and therefore contributing to sorghum ammonium tolerance.

Glutamine Synthetase (GS): A Key Enzyme for Nitrogen Assimilation in The Macroalga Gracilariopsis lemaneiformis (Rhodophyta)

“…However, our these results are contrary to previous study in higher plants as GS1‐3 gene of barley Hordeum vulgare and sorghum Sorghum bicolor was up‐regulated under the ammonium medium (El Omari et al. , Goodall et al. ).…”

Cisgenic overexpression of cytosolic glutamine synthetase improves nitrogen utilization efficiency in barley and prevents grain protein decline under elevated CO2

“…In spite of increasing thousand kernel weight, the grain protein concentration in the cisgenic lines increased under elevated CO 2 (Figures c, c), suggesting that cisgenic GS1 overexpression partially was able to circumvent biochemical bottlenecks in N assimilation. This may be the case even though down‐regulation of GS1 expression under CO 2 enrichment has been reported for different TaGS1 isogenes in wheat (Buchner et al ., ; Vicente et al ., , ), possibly reflecting a decrease in NH 4 + (El Omari et al ., ; Funayama et al ., ; Goodall et al ., ; Guan et al ., ; Konishi et al ., ). The results for the cisgenic lines corroborate the importance of GS1 in maintaining grain protein concentration, as has also been highlighted through identification of QTLs in barley (Fan et al ., ) and via a positive correlation between GS activity and grain protein content in wheat cultivars (Nigro et al ., ; Zhang et al ., ).…”

Nitrogen use efficiency (NUE) in rice links to NH4 + toxicity and futile NH4 + cycling in roots

“…However, higher plants display widely differing responses to NH 4 + nutrition (Marschner 1995) and, accordingly, can be divided into tolerant and sensitive species . Based on a series of comparative studies, more than 18 kinds of plants or plant species, including eight kinds of wild plants, have been classified as highly adapted to NH 4 + as a nitrogen source Rios-Gonzalez et al 2002;Cruz et al 2006;Dominguez-Valdivia et al 2008;Omari et al 2010). Further, more than 22 kinds of plants or plant species, eight of which wild, have been classified as sensitive to the NH 4 + source Cruz et al 2006;Roosta et al 2009).…”