Abstract
This paper was aimed to investigate the possible implications of the lack of plastidic glutamine synthetase (GS2) in phenolic metabolism during stress responses in the model legume Lotus japonicus. Important changes in the transcriptome were detected in a GS2 mutant called Ljgln2-2, compared to the wild type, in response to two separate stress conditions, such as drought or the result of the impairment of the photorespiratory cycle. Detailed transcriptomic analysis showed that the biosynthesis of phenolic compounds was affected in the mutant plants in these two different types of stress situations. For this reason, the genes and metabolites related to this metabolic route were further investigated using a combined approach of gene expression analysis and metabolite profiling. A high induction of the expression of several genes for the biosynthesis of different branches of the phenolic biosynthetic pathway was detected by qRT-PCR. The extent of induction was always higher in Ljgln2-2, probably reflecting the higher stress levels present in this genotype. This was paralleled by accumulation of several kaempferol and quercetine glycosides, some of them described for the first time in L. japonicus, and of high levels of the isoflavonoid vestitol. The results obtained indicate that the absence of GS2 affects different aspects of phenolic metabolism in L. japonicus plants in response to stress.
Highlights
Glutamine synthetase (GS, EC 6.3.1.2) is the key enzyme in charge of glutamine biosynthesis in nature
Comparative Transcriptomic Analysis of the Response of L. japonicus to Drought and Active Photorespiration In order to investigate the role of plastidic GS2 in the response to stress in L. japonicus, a mutant called Ljgln2-2 that lacks of this enzyme was submitted to two different stress treatments: drought or active PR
Different genes for isoflavonoid biosynthesis were induced by drought stress but exclusively in the mutant genotype. These results strongly suggest that the two types of stress situations analyzed in this paper may stimulate in different ways the production of phenolic compounds, FIGURE 3 | Graphical summary of the changes in the expression of genes for the biosynthesis of phenolic compounds under active photorespiratory conditions
Summary
Glutamine synthetase (GS, EC 6.3.1.2) is the key enzyme in charge of glutamine biosynthesis in nature This enzyme catalyzes the incorporation into one molecule of glutamate of the ammonium derived either from primary nitrogen assimilation (nitrate reduction, N2 fixation) or nitrogen reassimilation in the plants. GS1 is localized in the vascular tissue and plays an important role in the assimilation of external ammonium, the ammonia derived from N2 fixation and other sources of nitrogen, as well as in the remobilization of nitrogen during senescence. GS2 is Phenolic metabolism in Lotus japonicus predominantly expressed in green tissues, and it has been demonstrated that this particular isoform has an essential role in the reassimilation of the ammonium released by photorespiration and it has implications in the general metabolism of the plant, and in nodulation (Betti et al, 2014). Little is known yet on the implications of the different isoforms of GS in relation with other aspects of the stress responses in plants, phenolic metabolism
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