Reassimilation of ammonium in Lotus japonicus

Abstract

This review summarizes the most recent results obtained in the analysis of two important metabolic pathways involved in the release of internal sources of ammonium in the model legume Lotus japonicus: photorespiratory metabolism and asparagine breakdown mediated by aparaginase (NSE). The use of photorespiratory mutants deficient in plastidic glutamine synthetase (GS2) enabled us to investigate the transcriptomics and metabolomic changes associated with photorespiratory ammonium accumulation in this plant. The results obtained indicate the existence of a coordinate regulation of genes involved in photorespiratory metabolism. Other types of evidence illustrate the multiple interconnections existing among the photorespiratory pathway and other processes such as intermediate metabolism, nodule function, and secondary metabolism in this plant, all of which are substantially affected in GS2-deficient mutants because of the impairment of the photorespiratory cycle. Finally, the importance of asparagine metabolism in L. japonicus is highlighted because of the fact that asparagine constitutes the vast majority of the reduced nitrogen translocated between different organs of this plant. The different types of NSE enzymes and genes which are present in L. japonicus are described. There is a particular focus on the most abundant K(+)-dependent LjNSE1 isoform and how TILLING mutants were used to demonstrate by reverse genetics the importance of this particular isoform in plant growth and seed production.

Transgenic poplar expressing the pine GS1a show alterations in nitrogen homeostasis during drought

“…In leaves, as ammonium from photorespiration is produced in mitochondria and re-assimilated mainly by GS2 in the chloroplast (Betti et al, 2014), we suggest that during drought in GS poplars, part of the ammonium released during photorespiration is re-assimilated in cytosol through enhanced cytosolic GS activity (Gallardo et al, 1999), contributing to its detoxification.…”

Redundancy and metabolic function of the glutamine synthetase gene family in poplar

“…In plants, cytosolic (GS1) and chloroplastic (GS2) glutamine synthetase isoenzymes have been identified and are found in different intracellular locations that are related to their specialized roles. The chloroplastic GS2 is coded by a single gene in most plant species and has been detected in photosynthetic tissues where it assimilates the ammonium released from photorespiration or nitrate/nitrite reduction [ 13 ]. Conversely, GS1 is coded by a small gene family which varies in number among species, and the different isoenzymes are found in different types of cells and tissues according to their different physiological functions [ 14 ].…”

The afterglow thermoluminescence band as an indicator of changes in the photorespiratory metabolism of the model legumeLotus japonicus

“…The Ljgln2‐2 mutant is able to grow in a similar manner to the wild‐type (WT) at high CO 2 conditions that suppress PR. However, it accumulates high levels of NH 4 + when transferred from high CO 2 to air conditions, as a consequence of the activation of PR and the lack of GS 2 (Orea et al , Pérez‐Delgado et al , Betti et al , Pérez‐Delgado et al ). Photorespiratory ammonium accumulation was also shown previously in barley GS 2 mutants (Wallsgrove et al ) and in GS 2 ‐antisense oilseed rape lines (Husted et al ).…”

“…In this work, we have analyzed the changes observed in the TL bands in L. japonicus plants cultivated under high CO 2 conditions that suppress PR and under air conditions that permit a normal flux through the PR pathway. We included in our study the Ljgln2‐2 mutant deficient in GS 2 activity that is required for photorespiratory ammonium reassimilation in the chloroplasts and consequently impaired in the photorespiratory cycle (Orea et al , Pérez‐Delgado et al , Betti et al , Pérez‐Delgado et al ). The results obtained in this work indicate that the I AG /I B ratio and the level of lipid peroxidation significantly increase under conditions that suppress PR.…”

“…The first GS photorespiratory mutants isolated from legume plants were identified several years ago in our laboratory from the model legume L. japonicus (Orea et al 2002, Márquez et al 2005). These mutants have been substantially characterized at the molecular and physiological levels (Orea et al 2002, Márquez et al 2005, Betti et al 2006, Pérez-Delgado et al 2013, Betti et al 2014, Pérez-Delgado et al 2016. One of the mutants, named Ljgln2-2, is deficient in plastidic GS (GS 2 ) but has normal levels of cytosolic GS (GS 1 ) (Orea et al 2002, Márquez et al 2005, Betti et al 2012.…”