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Mineral nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize.

ABSTRACT: Glutamine synthetase (GS) catalyzes the first step of nitrogen assimilation in plant cell. The main GS are classified as cytosolic GS1 and plastidial GS2, of which the functionality is variable according to the nitrogen sources, organs and developmental stages. In maize (Zea mays L.) one gene for GS2 and five genes for GS1 subunits are known, but their roles in root metabolism are not yet well defined. In this work, proteomic and biochemical approaches have been used to study root GS enzymes and nitrogen assimilation in maize plants re-supplied with nitrate, ammonium or both.The plant metabolic status highlighted the relevance of root system in maize nitrogen assimilation during both nitrate and ammonium nutrition. The analysis of root proteomes allowed a study to be made of the accumulation and phosphorylation of six GS proteins. Three forms of GS2 were identified, among which only the phosphorylated one showed an accumulation trend consistent with plastidial GS activity. Nitrogen availabilities enabled increments in root total GS synthetase activity, associated with different GS1 isoforms according to the nitrogen sources. Nitrate nutrition induced the specific accumulation of GS1-5 while ammonium led to up-accumulation of both GS1-1 and GS1-5, highlighting co-participation. Moreover, the changes in thermal sensitivity of root GS transferase activity suggested differential rearrangements of the native enzyme. The amino acid accumulation and composition in roots, xylem sap and leaves deeply changed in response to mineral sources. Glutamine showed the prevalent changes in all nitrogen nutritions. Besides, the ammonium nutrition was associated with an accumulation of asparagine and reducing sugars and a drop in glutamic acid level, significantly alleviated by the co-provision with nitrate.This work provides new information about the multifaceted regulation of the GS enzyme in maize roots, indicating the involvement of specific isoenzymes/isoforms, post-translational events and biochemical factors. For the first time, the proteomic approach allowed to discriminate the individual contribution of the GS1 isoforms, highlighting the participation of GS1-5 in nitrate metabolism. Moreover, the results give new insights about the influence of amino acid metabolism in plant C/N balance.

SUBMITTER: Prinsi B

PROVIDER: S-EPMC4393875 | biostudies-literature | 2015

REPOSITORIES: biostudies-literature

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Project description:Paenibacillus polymyxa WLY78, a N2-fixing bacterium, has great potential use as a biofertilizer in agriculture. Recently, we have revealed that GlnR positively and negatively regulates the transcription of the nif (nitrogen fixation) operon (nifBHDKENXhesAnifV) in P. polymyxa WLY78 by binding to two loci of the nif promoter according to nitrogen availability. However, the regulatory mechanisms of nitrogen metabolism mediated by GlnR in the Paenibacillus genus remain unclear. In this study, we have revealed that glutamine synthetase (GS) and GlnR in P. polymyxa WLY78 play a key role in the regulation of nitrogen metabolism. P. polymyxa GS (encoded by glnA within glnRA) and GS1 (encoded by glnA1) belong to distinct groups: GSI-α and GSI-β. Both GS and GS1 have the enzyme activity to convert NH4+ and glutamate into glutamine, but only GS is involved in the repression by GlnR. GlnR represses transcription of glnRA under excess nitrogen, while it activates the expression of glnA1 under nitrogen limitation. GlnR simultaneously activates and represses the expression of amtBglnK and gcvH in response to nitrogen availability. Also, GlnR regulates the expression of nasA, nasD1D2, nasT, glnQHMP, and glnS. IMPORTANCE In this study, we have revealed that Paenibacillus polymyxa GlnR uses multiple mechanisms to regulate nitrogen metabolism. GlnR activates or represses or simultaneously activates and inhibits the transcription of nitrogen metabolism genes in response to nitrogen availability. The multiple regulation mechanisms employed by P. polymyxa GlnR are very different from Bacillus subtilis GlnR which represses nitrogen metabolism under excess nitrogen. Both GS encoded by glnA within the glnRA operon and GS1 encoded by glnA1 in P. polymyxa WLY78 are involved in ammonium assimilation, but only GS is required for regulating GlnR activity. The work not only provides significant insight into understanding the interplay of GlnR and GS in nitrogen metabolism but also provides guidance for improving nitrogen fixation efficiency by modulating nitrogen metabolism.

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Mineral nitrogen sources differently affect root glutamine synthetase isoforms and amino acid balance among organs in maize.

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